U.S. patent application number 12/463491 was filed with the patent office on 2009-10-22 for methods and apparatus for obtaining low-dose imaging.
This patent application is currently assigned to Dexela Limited. Invention is credited to Edward Bullard, Martin Stanton, Alexander Stewart.
Application Number | 20090262893 12/463491 |
Document ID | / |
Family ID | 38024015 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090262893 |
Kind Code |
A1 |
Stewart; Alexander ; et
al. |
October 22, 2009 |
METHODS AND APPARATUS FOR OBTAINING LOW-DOSE IMAGING
Abstract
In one aspect, a method of obtaining projection data of an
object from a plurality of view angles with respect to the object
is provided. The method comprises acts of providing radiation, at
each of the plurality of view angles, to an exposure area in which
the object is positioned, controlling a radiation energy of the
radiation provided at each of the plurality of view angles such
that the respective radiation energy is different for at least two
of the plurality of view angles, and detecting at least some of the
radiation passing through the exposure area at each of the
plurality of view angles to obtain the projection data.
Inventors: |
Stewart; Alexander;
(Waltham, MA) ; Stanton; Martin; (Concord, MA)
; Bullard; Edward; (London, GB) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, P.C.
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
Dexela Limited
London
GB
|
Family ID: |
38024015 |
Appl. No.: |
12/463491 |
Filed: |
May 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11595664 |
Nov 9, 2006 |
7545907 |
|
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12463491 |
|
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60735140 |
Nov 9, 2005 |
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Current U.S.
Class: |
378/62 |
Current CPC
Class: |
G01N 2223/612 20130101;
G01N 23/046 20130101; A61B 6/4233 20130101; A61B 6/02 20130101;
A61B 6/482 20130101; A61B 6/542 20130101; A61B 6/405 20130101; A61B
6/466 20130101; A61B 6/4291 20130101; G01N 2223/419 20130101; A61B
6/583 20130101; A61B 6/4085 20130101; A61B 6/4035 20130101; A61B
6/502 20130101; G01N 2223/423 20130101 |
Class at
Publication: |
378/62 |
International
Class: |
G01N 23/04 20060101
G01N023/04 |
Claims
1. A method of obtaining projection data of an object from a
plurality of view angles with respect to the object, the method
comprising acts of: providing radiation, at each of the plurality
of view angles, to an exposure area in which the object is
positioned; controlling a radiation energy of the radiation
provided at each of the plurality of view angles such that the
respective radiation energy is different for at least two of the
plurality of view angles; and detecting at least some of the
radiation passing through the object at each of the plurality of
view angles to obtain the projection data.
Description
RELATED APPLICATIONS
[0001] This Application is a continuation of and claims the benefit
under 35 U.S.C. .sctn.120 of U.S. application Ser. No. 11/595,664
entitled "METHODS AND APPARATUS FOR OBTAINING LOW-DOSE IMAGING,"
filed on Nov. 9, 2006, which claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Application Ser. No. 60/735140, entitled
"PLANAR IMAGING METHODS AND TECHNIQUES," filed on Nov. 9, 2005,
each of which is herein incorporated by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to radiation imaging, and more
particularly, to obtaining projection data of an object by exposing
the object to radiation from a plurality of view angles.
BACKGROUND OF INVENTION
[0003] Imaging apparatus that utilize relatively high energy
radiation such as x-ray and gamma rays are widely used to obtain
images of subject matter more or less opaque to electromagnetic
energy in the visual spectrum. For example, x-ray imaging
technology has been employed in a wide range of applications from
medical imaging to detection of unauthorized objects or materials
in baggage, cargo or other containers. X-ray imaging typically
includes passing high energy radiation (i.e., x-rays) through an
object to be imaged. X-rays from a source passing through the
object interact with the internal structures of the object and are
altered according to various characteristics of the material (e.g.,
transmission, scattering and diffraction characteristics, etc.)
which the x-rays encounter. By measuring changes in the X-ray
radiation (e.g., attenuation, modifications to the energy spectrum,
scatter angle, etc.) that exits the object, information related to
characteristics of the material, such as the density distribution,
may be obtained.
[0004] Computer tomography (CT) techniques involve capturing
transmitted x-ray information from numerous angles about an object
being imaged, to reconstruct a three-dimensional (3D) volume image
of the object. The data obtained from each view angle is referred
to as projection data or view data and is indicative of the
absorption characteristics of the object in directions related to
the respective view angle. However, CT imaging often involves
obtaining hundreds or thousands of projections to form a 3D
reconstruction of the projection data, thus requiring the object to
be exposed to relatively large doses of x-ray radiation and/or over
relatively long exposure times. Such large doses may not be
suitable for certain imaging applications having particular safety
and/or time constraints. For example, when imaging human tissue,
and/or when the imaging procedure is performed on a routine or
frequent basis (such as is often the case in mammography), dose
levels and/or exposure times used in conventional CT imaging may
exceed that which is more desirable.
[0005] To reduce a patient's exposure during breast imaging
procedures (e.g., imaging of the human female breast), conventional
mammography is often performed by obtaining only a pair of
two-dimensional (2D) radiographic images of the breast (i.e., each
image is reconstructed from a single projection of the breast),
typically acquired at approximately complementary angles to one
another. However, the superposition of structure within the breast
that occurs when 3D structure is projected onto two dimensions
often obscures the true nature of the structure. This superposition
of structure may make it difficult to identify or detect tissue
anomalies. For example, distinct structure in 3D that overlaps in
2D may make it difficult to distinguish cancerous subject matter
from benign subject matter within the breast.
[0006] In conventional mammography, the inability to ascertain the
true nature of breast structure may result in both significant
false negative and false positive rates, leading to potential
missed early stage cancers in the case of the former, or
unnecessary trauma to the patient and/or unnecessary hospital
visits, surgical procedures, etc., in the case of the latter.
Similarly, other imaging procedures that solve radiation dose
and/or time considerations by acquiring only a limited number of 2D
radiographic images are vulnerable to the same risks of
misdiagnosis.
SUMMARY OF THE INVENTION
[0007] Some embodiments according to the present invention include
a method of obtaining projection data of an object from a plurality
of view angles with respect to the object, the method comprising
acts of providing radiation, at each of the plurality of view
angles, to an exposure area in which the object is positioned,
controlling a radiation energy of the radiation provided at each of
the plurality of view angles such that the respective radiation
energy is different for at least two of the plurality of view
angles, and detecting at least some of the radiation passing
through the exposure area at each of the plurality of view angles
to obtain the projection data.
[0008] Some embodiments according to the present invention includes
an apparatus for obtaining projection data of an object from a
plurality of view angles with respect to the object, the apparatus
comprising a radiation source adapted to provide radiation to an
exposure area in which the object may be positioned, the radiation
source being moveable to provide the radiation to the exposure area
from each of the plurality of view angles, an exposure controller
coupled to the radiation source, the exposure controller adapted to
control a radiation energy of the radiation provided by the
radiation source at each of the plurality of view angles such that
the respective radiation energy is different for at least two of
the plurality of view angles, and at least one detector positioned
to detect at least some of the radiation passing through the
exposure area at each of the plurality of view angles to obtain the
projection data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a geometric illustration aid in explanation of
Applicant's insight that variations in object thickness may result
in variation in subject dose when projection data is obtained from
a plurality of view angles of a non-uniform object;
[0010] FIG. 2 is a flow chart illustrating a method for obtaining
projection data from a plurality view angles using variable
radiation energy to compensate for variations in object
thicknesses, in accordance with one embodiment of the present
invention;
[0011] FIGS. 3A and 3B are diagrammatic illustrations of respective
exemplary view angle configurations, in accordance with embodiments
of the present invention;
[0012] FIG. 4 is a partially block, partially diagrammatic
illustration of an imaging system capable of performing methods in
accordance with the various aspects of the present invention;
[0013] FIG. 5 is an illustration of a method of generating
radiation using an x-ray tube and an electron beam (e-beam) focused
on a target responsive to the e-beam; and
[0014] FIG. 6 is an illustration of a typical radiation spectrum
obtained as a function of the voltage applied to an x-ray tube.
DETAILED DESCRIPTION
[0015] As discussed above, conventional approaches to providing
generally low-dose radiation imaging suffer from images that
provide confusing representations of internal structures of an
object due, at least in part, to the projection of
three-dimensional structure onto one or more two-dimensional
images. The resulting superposition of distinct structure located
at different levels in 3D makes discerning the actual structure in
a 2D representation difficult, rendering conventional imaging
procedures vulnerable to errors in diagnosis. Applicant has
appreciated that a number of projections obtained using variable
radiation doses may be appropriate for acquiring projection data
that may be reconstructed to form 3D images, while still respecting
a relatively low dose budget (e.g., dose budgets suitable for
mammography or other tissue exposures that are generally dose
limited due to safety concerns). U.S. Pat. No. 6,744,848
(hereinafter the '848 patent), which is herein incorporated by
reference in its entirety, describes various methods and apparatus
for obtaining 3D images in a relatively low dose environment.
[0016] Applicant has appreciated that, in some circumstances, the
subject dose received by a patient may be varied with respective to
the view angle at which projection data was obtained, particularly
when the object being imaged is non-uniform in shape. The term
"subject dose" refers herein to the amount of radiation absorbed by
an object being exposed to radiation, and may be an important
component in assessing the potential harm of the x-ray imaging
process, In particular, radiation that merely passes through tissue
is significantly less harmful to the subject than radiation
absorbed by the tissue. Accordingly, reducing the amount of
radiation absorbed by tissue is a significant concern in medical
imaging, and particularly in procedures that are performed
regularly and/or are sensitive to radiation dose (e.g., breast
imaging).
[0017] The absorption of radiation is an exponential function of
the thickness of material that the radiation must penetrate (and a
function of the material properties). For example, the attenuation
of radiation through matter can be described generally according to
the characteristic attenuation function:
I=I.sub.0e.sup.-.beta.z (1).
where I.sub.0 is the intensity of the radiation emitted from a
radiation source before penetrating the material, I is the
intensity of radiation having penetrated the material through a
thickness z, and .beta. is a coefficient incorporating both a
material specific linear absorption coefficient .mu. and the energy
of the radiation .lamda. (which is related to wavelength). The term
"intensity," with respect to radiation, refers to the amount of
radiation present at a given surface or passing through a given
volume during a given interval of time, and is thus a measure of
radiation flux. Thus, the difference between I and I.sub.0 is
indicative of the subject dose (i.e., I.sub.0-I is indicative of
how much radiation the object absorbed). Inspection of Equation 1
reveals that as z increases, the difference between I and I.sub.0
likewise increases, That is, when all other variables are held
constant, the subject dose increases as the thickness of the
material being penetrated increases.
[0018] Accordingly, in imaging procedures that expose an object at
multiple angles, the relative amount of radiation absorbed by the
object will vary with view angle for non-uniform objects, resulting
in increased subject dose levels at view angles wherein impinging
radiation must penetrate increased thicknesses of material.
Applicant has appreciated that, due at least in part to the
increased transmissivity of higher energy radiation, the subject
dose received by a non-uniform object may be maintained or
decreased by varying the radiation energy (not radiation intensity)
of the radiation to which the object is exposed. In particular, as
the thickness of an object increases at certain view angles, the
radiation energy may be increased to maintain the amount of
absorption within a suitable range, while maintaining image
quality. As one result, the data desired for calculating a 3D
representation of the object may be obtained while maintaining or
decreasing the subject dose of the object. It should be appreciated
that the term "radiation energy" and "radiation intensity" refer to
separate and distinct properties of the radiation. The term
radiation energy is related to the wavelength of the photons of the
radiation, while the term intensity is related to the radiation
flux.
[0019] In one embodiment, projection data is obtained from a
plurality or view angles (i.e., at a plurality of different
orientations with respect to an object). At each of the plurality
of view angles, a radiation energy is selected based, at least in
part, on a thickness of the object in a direction related to the
respective view angle. In this manner, as the thickness of the
object changes, the radiation energy may be varied to control the
absorption of radiation while substantially maintaining image
quality, thus limiting the subject dose received to desired
levels.
[0020] In general, radiation provided for the purpose of obtaining
projection data of an object is polychromatic. That is, rather than
the radiation having a single energy (monochromatic), radiation
emitted from a radiation source will have an energy distribution
comprising multiple energies. For example, radiation used in
imaging exposures is often generated by directing an electron beam
(e-beam) to strike a target surface. Common target materials
include tungsten, molybdenum, rhodium, etc. The interaction of the
e-beam with the target surface results in the emission of radiation
comprised of multiple energies that are dependent on the target
material type and the energy of the e-beam. That is, each target
material will emit a characteristic energy distribution, or
spectrum, in response to an impinging e-beam.
[0021] An e-beam used to bombard a target to generate radiation is
often itself generated in a vacuum tube, and has an energy
proportional to a voltage potential between a cathode and anode
(the target material) of the vacuum tube. As the energy in the
e-beam increases, so does the energy of the radiation emitted by
the target, as discussed in further detail below. Thus, a target
material will emit radiation having a characteristic spectrum that
depends both on the material type being used and on the energy in
the e-beam impinging on its surface. In the context of e-beam
technology, varying the radiation energy typically involves varying
the energy level of the e-beam to correspondingly vary the
characteristic spectrum.
[0022] As a general matter, varying the radiation energy refers to
varying the energy content of the energy distribution (i.e.,
varying the content of the energy spectrum). For example,
increasing the radiation energy typically involves shifting the
distribution of energies to higher frequencies. For example, if the
energy in the e-beam is increased, the energy distribution of the
radiation emitted by the target in response to the impinging e-beam
will experience a corresponding shift to higher frequencies. That
is, portions of the energy spectrum of radiation emitted from a
target, increases in intensity and shifts toward higher frequencies
(shorter wavelengths) when the energy of the e-beam used to bombard
the target is increased. It should be appreciated that the energy
distribution may not shift uniformly (e.g., characteristic energy
peaks for various target materials may increase in intensity but
not shift in frequency as a function of e-beam energy), but the sum
of the energies in the energy distribution will vary in proportion
to the e-beam energy.
[0023] Accordingly, the term "radiation energy" refers generically
to an indicator of the energy distribution as a whole. For example,
with monochromatic radiation, the radiation energy may indicate the
wavelength of the emitted radiation. With polychromatic radiation,
the radiation energy may indicate the energy distribution in any
characteristic manner, for example, the radiation energy may refer
to a mean energy, a median energy, a predominant energy, a sum of
the energy distribution, or other indicator characteristic of the
energy distribution. Therefore, increasing or decreasing the
radiation energy for polychromatic radiation refers to increasing
or decreasing one or more characteristics of the energy
distribution and/or increasing or decreasing the energy
distribution as a whole.
[0024] It should be appreciated that while the terms radiation
intensity and radiation energy refer to different characteristics
of radiation, they are not entirely unrelated, In particular,
increasing the intensity of radiation while maintaining the same
radiation energy will increase the total energy of the radiation.
That is, providing more radiation at any particular radiation
energy will naturally increase the total energy of the radiation.
Accordingly, the term "total energy" refers to the energy present
in radiation emitted over some interval of time, such as during an
exposure, and takes into consideration both the radiation energy
and the radiation intensity of the radiation. It should be
appreciated that the radiation intensity may be decreased and the
radiation energy increased while maintaining the same total energy
in the radiation provided for an exposure (and likewise for an
increase in radiation intensity and a decrease in radiation
intensity), as the total energy is comprised of both components.
Accordingly, the total energy incident on an object may be
increased while the subject dose actually decreases, due to the
increased transmissivity of the object at the higher energy.
[0025] As discussed above, projection data may be obtained from an
object by exposing the object to relatively high energy radiation
and detecting the radiation that passes through the object. By
comparing the known intensity of the radiation incident on the
object with the detected intensity of the radiation exiting the
object, information about the absorption characteristics (e.g., the
density) of the tissue may be computed. To obtain images having
suitable contrast, the difference in intensity between radiation
entering and exiting an object must be such that it carries enough
information to discriminate between various tissue types, For
example, if too much or too little radiation exits the object with
respect to the amount of provided radiation, the penetrating
radiation will carry insufficient information to distinguish tissue
types, resulting in relatively low contrast images.
[0026] In particular, considering the limiting cases, if no
radiation exits the object because, for instance, the thickness of
the material is such that all of the radiation is absorbed and/or
the radiation energy is insufficient to fully penetrate the
material, no information about the density distribution of the
material is obtained. Likewise, if the radiation exits the object
substantially unchanged because, for example, the material
thickness is such that no appreciable amount of radiation is
absorbed and/or the radiation energy is too high, no information
about the density distribution of the material is obtained. Between
the two extremes, a ratio between impinging and exiting radiation
exists that produces optimal contrast information in the exiting
radiation.
[0027] Higher energy radiation is generally more transmissive than
lower energy radiation, thus penetrating matter more readily with a
lesser probability of being absorbed. However, when absorbed,
higher energy radiation deposits more energy in the absorbing
matter, incurring more subject dose per absorbed unit (i.e., per
absorbed photon). Fortunately, an appropriate radiation energy for
an appropriate material thickness usually may be determined to
ensure that exiting radiation carries sufficient information to
contrast the different absorption characteristics of the object,
while satisfying subject dose requirements, However, a change in
the material thickness may disturb the ratio and have a deleterious
effect on the contrast information carried by exiting radiation.
Thus, in addition to increasing the subject dose of an exposure,
increased material thickness may also reduce the information
content of radiation exiting the object. Applicant has appreciated
that by varying the radiation energy according to view angle, the
ratio between impinging and exiting radiation may be maintained to
optimize contrast.
[0028] Following below are more detailed descriptions of various
concepts related to, and embodiments of, methods and apparatus
according to the present invention. It should be appreciated that
various aspects of the invention described herein may be
implemented in any of numerous ways. Examples of specific
implementations are provided herein for illustrative purposes only.
In addition, the various aspects of the invention described in the
embodiments below may be used alone or in any combination, and are
not limited to the combinations explicitly described herein.
[0029] FIG. 1 illustrates a method and apparatus for obtaining
projection data of an object from multiple view angles under
relatively low-dose conditions, in accordance with one type of
embodiment of the present invention. In FIG. 1, an object 110 is
being exposed to radiation at multiple view angles. Object 110 is
non-uniform, e.g., object 110 is larger in one dimension than in
another. In particular, object 110 has a larger extent in the
x-direction than the y-direction. Object 110 may be, for example, a
breast that has been compressed generally in the y-direction in
preparation for a breast imaging procedure. As a result, when
radiation source 120 is positioned generally in alignment with the
y-axis (e.g., at position A), radiation entering object 110 has
less subject matter to penetrate before exiting the object than it
does when the radiation source is positioned generally in alignment
with the x-axis (e.g., at position C).
[0030] For example, ray 125a emitted by radiation source 120 at
position A must penetrate a distance d.sub.1 before exiting the
object, while ray 125b provided by radiation source 120 at position
B must penetrate a greater distance d.sub.2 to exit the object.
Thus, photons along ray 125b have a greater likelihood of being
absorbed by object 110. As the radiation source rotates about the
object from position A to position C, the amount of subject matter
rays must penetrate before exiting the object increases (i.e.,
distance d.sub.1<d.sub.2<d.sub.3). As a result, if the
radiation energy in the radiation is held constant, less of the
radiation impinging on object 110 will exit at each increasing
angle .theta. with respect to the y-axis. The view-angle-dependent
absorption in FIG. 1 impacts the imaging process in a number of
ways. First the object will be subjected to an increase in subject
dose as the radiation source rotates from alignment with the y-axis
to alignment with the x-axis. In addition, the information contrast
in the radiation that does exit the object to impinge on detector
130 will be reduced as a function of increasing angle .theta..
Applicant has appreciated that the effects of variations in
material thickness can be substantially counter-balanced by varying
the radiation energy.
[0031] As illustrated in Equation 1 above, the relationship between
the intensity of radiation impinging on an object (I.sub.0) and the
intensity of radiation exiting an object (I) is a function of the
thickness z of material through which the radiation penetrates. In
addition, the relationship between I.sub.0 and I is also a function
of radiation energy. In particulars .beta. is a function of
radiation energy .lamda. and a material-specific linear absorption
coefficient .mu. that may incorporate various absorption and
scattering effects including Thompson scattering, Compton
scattering, photoelectric (PE) absorption, pair production,
photodisintegration, etc. The radiation energy effects the rate of
the exponential decay of the radiation intensity as a function of
thickness z. Thus, the intensity of radiation having higher
energies will be less effected by increased material thicknesses
than the intensity of radiation having lower energies. This energy
dependence can be approximated by the expression,
I(.lamda.)=I.sub.0(.lamda.)e.sup.-f(.lamda.,.mu.)z (2).
[0032] Accordingly, the photon energy of the x-ray radiation
impacts the relationship between I and I.sub.0 as a function of z.
Therefore, approximate or measured values of the material
properties of an object being imaged, for example, the known
material properties of breast material, may be used to compute
radiation energies for various thicknesses z that maintain the
difference I.sub.0-I and/or the ratio I/I.sub.0 within an
acceptable range at respective view angles. Stated differently,
variations in subject dose and contrast information caused by
changes in material thickness may be reduced and/or eliminated by
correspondingly varying the radiation energy.
[0033] Accordingly, in FIG. 1, the radiation energy of the
radiation provided to the object may be varied as a function of the
view angle of the radiation source with respect to the object to
account for the increase in object thickness. It should be
appreciated that an object being imaged may not vary as uniformly
or be as homogeneous as object 110. Thus, as a general matter, the
radiation energy applied to an object being imaged may be varied as
a function of a thickness of the object in a direction related to
the corresponding view angle. In particular, as the amount material
in which the radiation must penetrate to exit the object increases,
the lo energy of the radiation may be correspondingly increased to
maintain an acceptable level of radiation absorption and/or to
maintain sufficient information contrast in the exiting
radiation.
[0034] The appropriate energy levels at different view angles or
thicknesses may be determined either analytically using modeled
absorption characteristics, or computed empirically by testing
various sample or reference objects, as described in further detail
below. In addition, information obtained by monitoring the ratio of
I to I.sub.0 on successive projections may be used to appropriately
adjust the radiation energy in a subsequent exposure at the next
view angle. I.sub.0 may be measured using the same detector
apparatus arranged to collect the projection data, or additional
detectors may be arranged to perform measurements on and/or
otherwise monitor I.sub.0. Alternatively, I.sub.0 may be
approximated from knowledge of the operating parameters of the
radiation source. Measuring and/or otherwise determining the ratio
of I to I.sub.0 may be achieved in other ways, as the aspects of
the invention are not limited in this respect.
[0035] FIG. 2 illustrates a method of performing x-ray imaging
using x-ray energy that varies as a function of the view angle of
the radiation source with respect to an object being imaged, in
view of variations in object thickness in directions related to the
corresponding view angle, in accordance with one embodiment of the
present invention. Method 200, for example, may be suitable for
x-ray imaging procedures having a limited dose budget due to
sensitivity of the object (e.g., biological tissue) and/or
regularity of the procedure (e.g., routine breast
examinations).
[0036] In act 210, x-ray radiation is generated and provided to an
object being imaged, the x-ray radiation is directed at a first
view angle with respect to the object. The x-ray radiation may be
provided to form a radiation field such as, for example, a beam
that propagates out from a source point in the general shape of a
cone to expose an exposure area to the radiation. As such, various
rays within the radiation may be propagating in different
directions (i.e., depending on the location of the ray within the
cone beam). Accordingly, the view angle of the radiation may
generally be viewed as the angular orientation of a reference line
from the source point through a reference point of the object being
imaged (e.g., a center point of the object).
[0037] The x-ray radiation at the first view angle is generated at
a first radiation energy, the first radiation energy being based,
at least in part, on a thickness of the object in a direction
related to the first view angle. The first energy may be chosen,
for example, such that the amount of x-rays absorbed by the object
is below a desired subject dose threshold in view of the particular
imaging procedure. In addition, the first energy may be selected
such that the ratio of generated x-rays to penetrating x-rays
exiting the object provides for sufficient contrast in the
resulting image. The x-rays exiting the object may then be detected
to acquire information about the material characteristics of the
object (act 220). That is, projection data of the object may be
acquired corresponding to the first view angle. The projection data
may be stored for use in computing one or more images of the
internal structure of the object.
[0038] In act 230, x-ray radiation is generated and provided to the
object at a next view angle and a next energy is selected for the
x-ray radiation provided at the next view angle based, at least in
part, on the thickness of the object in a direction related to the
next view angle. In particular, the next x-ray energy may be varied
to account for an increase or decrease in the thickness of the
object so as to ensure that the subject dose received by the object
does not exceed desired levels and/or to maintain adequate
information contrast in the detected x-rays exiting the object. The
x-rays penetrating and exiting the object at the next energy may
then be detected to acquire projection data related to the material
characteristics of the object at the next view angle (act 240). The
projection data may be stored for use in computing the one or more
images of the internal structure of the object.
[0039] In act 250, a determination is made as to whether x-ray
radiation is to be generated at another view angle and energy. If
an exposure at an additional view angle and energy is desired, then
acts 230 and 240 may be repeated. It should be appreciated that
x-ray radiation may be provided at any number of view angles and at
any number of energies, as the aspects of the invention are not
limited in this respect. In addition, while various embodiments may
use x-ray radiation, radiation in other portions of the
electromagnetic spectrum (e.g., gamma rays) may be used, as the
aspects of the invention are not limited in this respect.
[0040] If no further exposures are desired, the projection data
obtained from the plurality of view angles and energies may be used
to compute one or more images of the structure of the object (act
250). For example, the projection data may be reconstructed to form
a three dimensional (3D) image of the structure of the object. The
projection data may be used in reconstructing an image according to
any of various reconstruction methods and algorithms, as the
aspects of the invention are not limited in this respect. For
example, a 3D image may be computed according to any of the various
reconstruction methods described in the '848 patent, or any other
suitable reconstruction algorithm configured to transform
projection data into image data.
[0041] It should be appreciated that various aspects of the
invention may be used in combination with techniques described in
the '848 patent. For example, varying the radiation as a function
of view angle (e.g., to compensate for varying thicknesses of a
non-uniform object) may be combined with varying the radiation
intensity as a function of view angle to assist in optimizing the
exposure procedures. In particular, the radiation intensity and
radiation energy may be varied in view of each other to provide an
exposure that satisfies both subject dose constraints and desired
contrast in the projection data. Other techniques described in the
'848 patent and disclosed herein may be used in any combination, as
the aspects of the invention are not limited in this respect. For
example, projection data obtained using the various techniques
described in the '848 patent may be reconstructed in view of the
variable radiation energies used at different view angles about the
object.
[0042] In reconstructing projection data obtained using variable
radiation energy at different exposures, it should be appreciated
that, since the energy distribution of the radiation flux varies
between exposures, the relationship between the fraction of
radiation intensity absorbed (and scattered) by the object and the
line integrals of the object density between the radiation source
and the radiation detector will change between exposures because
the mass attenuation coefficients of substances depend upon
radiation energy (e.g., as expressed in Equation 2).
[0043] The relationship between the projection data and the
radiation intensity incident on the radiation detector may also
change between exposures of differing radiation energy (e.g.,
between exposures having different energy distributions or spectra)
due to the characteristics of the detector with regard to radiation
energy distribution (and the angle of incidence of the radiation
upon the detector if this changes between exposures).
[0044] In order to process projection data from multiple exposures
taken using varying radiation energy distributions, the
relationship between projected density and recorded radiation
intensity at the detector (equation 2) should be adjusted for each
exposure to use the effective mass attenuation coefficient given
the radiation energy emitted by the radiation source and the
elemental composition of the object (e.g., the atomic make-up of
the object).
[0045] Methods for processing projection radiation data taking into
account the radiation energy distribution to iteratively estimate
the mass attenuation coefficient for the volume elements of a
reconstruction are described in I A Elbakri, J A Fessler, entitled
"Segmentation-free Statistical Image Reconstruction for
Polyenergetic X-ray Computed Tomography with Experimental
Validation," Phys. Med. Biol., 48(15):2543-78, August 2003. Various
of the methods described in the above identified literature may be
used to reconstruct projection data obtained using polychromatic
radiation energy. However, it should be appreciated that other
reconstruction methods may be used, as the aspects of the invention
are not limited in this respect.
[0046] For some objects of interest, such as soft body tissues like
the breast, the elemental composition is comprised mainly of
relatively light elements (e.g., of elements having an atomic
number <10) distributed fairly uniformly (e.g., the object is of
relatively homogenous density). In such cases, the relation between
mass attenuation coefficient and radiation energy may be similar
throughout the object and one effect of using a higher average
radiation energy is that the attenuation coefficients are reduced
at higher energy. In this case only a multiplicative correction for
the calculated line integrals of object density may need to be
applied to correct for the effect of a change in radiation energy
distribution between exposures. This may be calculated from the
mass-fraction weighted integrals of the elemental mass attenuation
coefficients over the radiation energy distributions of each
exposure, or calibration images may be taken with a known
calibration object of similar elemental composition and thickness
to the object (e.g., Plexiglas sheets) covering the range of usable
energy distributions for the apparatus, over the range of view
angles.
[0047] One of these calibration images can be selected for each
exposure to give the projected calibration data for a similar
thickness at similar radiation energy distribution and source and
detector positioning. Since the projected radiation intensity data
from the calibration object divided by the radiation intensity data
observed or calculated at corresponding locations without an object
present gives attenuation measurements, and the line integrals of
density may be calculated for the known calibration object density,
the effective attenuation coefficients may be calculated using
equation 2. It should be appreciated that reconstructing an image
from projection data obtained using polychromatic radiation, and/or
obtained using different radiation energies for different view
angles may be performed in other ways, as the aspects of the
invention are not limited in this respect.
[0048] As discussed in the '848 patent, radiation exposures may be
performed at a number of non-uniformly distributed view angles. For
example, the change in angle from one view angle to another may
increase as the angle from a reference view angle (e.g., position A
in FIG. 1) increases. That is, as a radiation source is rotated
about an object from a reference position, the angle between
successive exposures may be increased. However, the various view
angles selected also may be uniformly distributed, as the aspects
of the invention are not limited in this respect.
[0049] In FIG. 3A, the plurality of view angles used to obtain
projection data of object 310 are distributed with non-uniform
angular offsets with respect to one another. For example, as the
view angles are rotated away from a reference view angle at
.theta..sub.0=0.degree. in both the clockwise and counterclockwise
directions, the angle between each successive view angle increases.
In particular, in the clockwise direction
(.theta..sub.1-.theta..sub.0)<(.theta..sub.2-.theta..sub.1)&-
lt;(.theta..sub.3-.theta..sub.2), etc. Similarly, in the
counterclockwise direction,
(.theta..sub.1'-.theta..sub.0)<(.theta..sub.2'-.theta..sub.1')<(.th-
eta..sub.3'-.theta..sub.2'). As discussed in the '848 patent,
performing exposures at non-uniform angles may facilitate obtaining
optimal projection data for a given dose budget.
[0050] At each of the plurality of view angles, the radiation
energy may be varied to compensate for changes in the material
thickness of object 310 in directions related to the respective
view angle. It should be appreciated that the number and
distribution illustrated in FIG. 3A are merely exemplary, Any
number of view angles may be used at any desired distribution, as
the aspects of the invention are not limited in this respect.
Moreover, the view angles need not be distributed symmetrically
with respect to the reference view angle, as any desired
distribution may be used with the various aspects of the
invention.
[0051] In FIG. 3B, the angular offsets are distributed essentially
uniformly about object 310. For example, as the view angles are
rotated away from the reference view angle at
.theta..sub.0=0.degree. in both the clockwise and counterclockwise
directions, the angle between each successive view angle remains
essentially the same. In particular, in the clockwise direction,
(.theta..sub.1-.theta..sub.0)=(.theta..sub.2-.theta..sub.1)=(.theta..sub.-
3-.theta..sub.2), etc. Similarly, in the counterclockwise
direction,
(.theta..sub.1'-.theta..sub.0)=(.theta..sub.2'-.theta..sub.1')=(.theta..s-
ub.3'-.theta..sub.2'). At each of the plurality of view angles, the
radiation energy may be varied to compensate for changes in the
material thickness of object 310 in directions related to the
respective view angle. Accordingly, any number of view angles may
be distributed in any fashion; uniformly or non-uniformly,
symmetric or asymmetric, etc., as the aspects of the invention are
not limited in this respect. In addition, the angular range over
which projection data is obtained need not be 180.degree. as
illustrated in FIGS. 3A and 3B, but may cover a range greater than
or less than 180.degree..
[0052] As discussed above, the relationship in Equation 2 may be
used to compute an appropriate radiation energy to be used as a
function of material thickness to maintain subject dose within a
desired range and/or to maintain sufficient contrast information in
radiation exiting the object. Alternatively, the set of radiation
energies used for a particular procedure may be determined
empirically from information obtained from prior procedures using
similar objects, or by taking measurements using phantom objects
having material properties similar to the object being imaged. For
example, in mammography, experience from prior breast imaging
procedures may be used to gain an understanding of the appropriate
energy levels for various thicknesses of the breast. Alternatively,
phantom objects having material properties similar to the breast
may be imaged to determine the appropriate energy levels for
various thicknesses through the phantom object.
[0053] Once the relationship between thickness and radiation energy
has been established for a particular type or class of objects,
(either empirically or analytically), measurements of a target
object to be imaged may be obtained to design an appropriate
exposure plan (i.e., a plan for the number and distribution of view
angles and the corresponding radiation energy to be used at each
successive view angle) for the target object. Alternatively, the
target object need not be measured before performing an imaging
procedure on the object, but merely categorized by inspection. For
example, the object may be considered to be a member of a
particular class of which the approximate thickness of the object
at various view angles is known a priori, for example, through
prior measurements or knowledge of the object. In mammography,
prior knowledge and/or measurements of breast dimensions may be
used in place of performing actual measurements on a subject
breast. In addition, the apparatus performing the imaging may
obtain measurements of the object being imaged.
[0054] In one embodiment, an exposure plan (i.e., a plan for
exposing the object at various view angles and radiation energies)
is determined for multiple categories of objects, for example,
small, medium and large breasts and selected and performed on
target breasts according to which category the target breast falls
within. Accordingly, a discrete set of exposure plans corresponding
to each of the defined categories may be programmed, and the
appropriate exposure plan selected to image a particular target
object. However, the exposure plan may be optimized for each object
via measurements or use of other knowledge.
[0055] In another embodiment, information obtained from an exposure
of an object at a first view angle may be used to determine the
radiation energy to be used at a subsequent view angle. For
example, analysis of the projection data obtained at one view angle
may be used to bootstrap one or more subsequent exposures. Each
successive exposure may provide additional information to guide the
radiation energy to be used at the next view angle. Thus, the
exposure plan may be determined and/or modified during the exposure
procedure. The exposure plan for a particular object may be
determined in any suitable manner, as the aspects of the invention
are not limited in this respect.
[0056] FIG. 4 illustrates one embodiment of an imaging system
suitable for obtaining projection data and performing imaging
procedures in accordance with various aspects of the present
invention. Imaging system 400 includes a radiation source 420, a
detector 430, a motion controller 440, a resolution controller 450
and an image processor 460. The imaging system 400 can be used to
image a single object 410 or a plurality of objects located within
an exposure area 414. The exposure area 414 defines generally the
region of space between the radiation source 420 and the detector
430, and is located in the path of the radiation provided by
radiation source 420 in the direction of detector 430. The exposure
area 414 may be the entire region of space located in the path of
the radiation passing from the radiation source 420 to the detector
430, or only a predetermined portion of the space.
[0057] Radiation source 420 may be any component or combination of
components capable of emitting radiation such as x-ray or gamma
radiation. In imaging system 400, radiation source 420 is
positioned to emit radiation toward exposure area 414 such that,
when object 410 is present in exposure area 414, at least some of
the radiation impinges on object 410. In particular, the radiation
source 420 is adapted to emit radiation to form a radiation field
416, which may be of any shape or size. In a preferred embodiment,
radiation field 416 is formed by a cone beam that substantially
encloses object 410 within a cone of x-rays during exposures.
However, radiation field 416 may form other shapes such as a fan
beam, pencil beam, etc., and may be arranged to expose any portion
of object 410, as the aspects of the invention are not limited in
this respect.
[0058] Radiation source 420 is capable of being rotated about
object 410 such that radiation may be directed at object 410 from a
plurality of angular positions, i.e., a plurality of view angles
with respect to object 410 (e.g., as illustrated in FIGS. 3A and
3B). Detector 430 is positioned to receive at least some of the
radiation that passes through the exposure area 414, and in
particular, radiation that has penetrated and exited object 410.
Detector 430 may be a single detector, or a detector array disposed
continuously or at a plurality of discrete locations. Detector 430
may be of any type responsive to radiation generated by radiation
source 420. The signals generated by detector 430 carry information
about the absorption characteristics of object 410 to form, at
least in part, projection data of object 410.
[0059] Detector 430 may be configured to rotate in correspondence
with the radiation source 420 to detect radiation exiting object
410 from the plurality of view angles. Motion controller 440 may be
coupled to radiation source 420 and detector 430 to cause the
rotational movement of the radiation source/detector apparatus such
that, as the apparatus rotates about the object, the object remains
positioned within the exposure area between the source and
detector. Motion controller 440 may be capable of being programmed
to move the radiation source and detector to any desired view angle
with respect to object 410. Together, the radiation source 420,
detector 430 and motion controller 440 permit projection data of
object 410 to be obtained from any set of view angles. In some
embodiments, motion controller 440 may be programmed to control the
position of the radiation source and detector independently. For
example, the motion controller may move the radiation source and
detector along different paths as projection data is obtained from
the different view angles, as the aspects of the invention are not
limited in this respect.
[0060] Exposure control 450 is coupled to radiation source 420 to
control the radiation energy of the radiation emitted by the
radiation source. As discussed above, the radiation energy emitted
from the radiation source may be varied by any suitable means. For
example, radiation source 420 may include an x-ray tube adapted to
generate an e-beam and direct the e-beam to a target capable of
converting e-beam energy to x-ray energy. Exposure control 450 may
be coupled to the x-ray tube to control the voltage potential used
to generate the e-beam, the magnitude of the voltage being
proportional to the energy in the e-beam and, thus, the radiation
energy emitted by the target. By controlling the voltage level,
exposure control 450 may vary the radiation energy generated by
radiation source 420.
[0061] Alternatively, radiation source 420 may include one or more
filters capable of selectively filtering desired energies from
radiation emitted by the radiation source. Accordingly, exposure
control 450 may control the one or more filters to vary the energy
in the radiation passing through exposure area 414 as desired.
Exposure control 450 may also be capable of selecting different
materials for the target arranged to convert e-beam energy to x-ray
energy. For example, different materials, in response to an
impinging e-beam, may convert the e-beam energy to x-ray radiation
of differing energy levels. That is, the energy distribution may be
a function of target material. Thus, by selectively controlling the
target material, exposure control 450 may vary the radiation energy
provided by radiation source 420. It should be appreciated that
exposure control 450 may be adapted to control the radiation energy
emitted by radiation source 420 by any of the above methods, either
alone or in any combination. In addition, exposure control 450 may
be adapted to control and vary the radiation energy by others
means, as the aspects of the invention are not limited in this
respect.
[0062] Detector 430 may be coupled to image processor 460 to
provide the image processor with the projection data generated by
exposing the object to radiation at various view angles. Image
processor may then reconstruct the projection data into an image of
the internal structure of object 410. In one embodiment, a 3D image
is computed by image processor 460 from the projection data
provided by detector 430. Image processor 460 may include one or
more processors and a storage medium capable of storing one or more
programs to be executed on the one or more processors. The programs
may instruct image processor to process the projection data
according to any of various reconstruction algorithms to form
images of the structure of the object. Any reconstruction algorithm
may be used, as the aspects of the invention are not limited in
this respect. For example, image processor 460 may perform any of
the reconstruction methods described in the '848 patent, or any
other suitable reconstruction scheme capable of transforming
projection data into image data. In one embodiment, the
relationship in Equation 2 is used to perform reconstruction. Other
more complicated relationships may also be used, as the aspects of
the invention are not limited in this respect.
[0063] In another embodiment, the detector 430 remains stationary
as the radiation source is moved about the object. For example, if
the detector 430 is sufficiently large (e.g., a flat panel
two-dimensional detector array) and/or if the angular range over
which projection data is obtained is sufficiently small (e.g., the
angular range is limited to a range between 5.degree. and
45.degree. both clockwise and counterclockwise from a reference
view angle), a single position for the detector 430 may be
sufficient to capture projection data from each of the desired view
angles. In addition, in embodiments where detector 430 remains
stationary, the object may be positioned in direct contact with the
detector.
[0064] Imaging system 400 may be used to implement methods
according to various aspects of the present invention. For example,
the method described in connection with FIG. 2 may be performed on
imaging system 400. In particular, motion controller 440 may be
configured to rotate radiation source 420 and detector 430 (if
appropriate) about object 410 to expose the object to radiation at
a plurality of view angles. At each of the plurality of view
angles, exposure control 450 may be adapted to select a desired
energy level for the radiation emitted by radiation source 420.
Accordingly, imaging system 400 may be configured to obtain
projection data according to any desired exposure plan.
[0065] In other embodiments, radiation may be provided from
multiple view angles by a plurality of radiation sources
distributed uniformly or non-uniformly about an exposure area to
expose an object to be imaged. When it is desired to provide
radiation from a particular view angle, the corresponding radiation
source may be activated, while the other radiation sources are
non-operational. The plurality of radiation sources could be
appropriately switched to obtain projection data from view angles
defined by the respective stationary positions of the plurality of
radiation sources. In such embodiments, the motion controller may
be replaced with a controller that successively activates the
plurality of radiation sources at desired view angles to obtain
projection data of the object positioned in the exposure area.
[0066] In embodiments in which a plurality of stationary radiation
sources are provided, a corresponding plurality of detector or
detector arrays may be positioned to detect radiation emitted by
the respective radiation sources. Alternatively, a single detector
or detector array positioned to detect radiation emitted by each of
the plurality of radiation sources may be used. Other
configurations and components capable of providing radiation at a
plurality of view angles may be used, as the aspects of the
invention are not limited for use on any particular device, or any
particular configuration and arrangement of components.
[0067] FIG. 5 illustrates an exposure control for a radiation
source that produces radiation using e-beam technology, in
accordance with one embodiment of the present invention. Radiation
source 520 generates an e-beam 523 by forming a voltage potential
between a cathode (electron emitting filament 521) and an anode
(target material 526). The e-beam propagates through a x-ray tube
to impinge on a surface of target 526. As discussed above, the
energy in the e-beam is proportional to the voltage potential
between the cathode and anode. Radiation in the form of cone beam
525 is emitted as the e-beam strikes the target. Radiation will be
emitted having an energy spectrum characteristic of the type of
material used and of the energy of the impinging e-beam. In
particular, bombarding tungsten with an e-beam will result in a
characteristic energy distribution different than bombarding
molybdenum, and will vary as a function of the e-beam energy.
[0068] FIG. 6 illustrates the characteristic spectrum of tungsten
at a number of energy levels of an e-beam. In particular, FIG. 6
includes a first spectrum 605a, a second spectrum 605b, a third
spectrum 605c, and a fourth spectrum 605d resulting from bombarding
a tungsten target with an e-beam generated at a voltage potential
of 80 kilo-volts (kV), 100 kV, 120 kV and 140 kV, respectively, The
energy spectra illustrated include two components, the
Bremsstrahlung radiation, and the energy peaks characteristic of
the target material. The Bremsstrahlung radiation is characterized
by the generally continuous distribution of radiation that
increases in intensity and shifts toward higher frequencies
(shorter wavelengths) when the energy of the e-beam is increased.
The energy peaks are the characteristic bands shown by the spikes
at, for tungsten, approximately 59 kilo-electron volts (keV) and 69
keV. As illustrated, increased e-beam energy increases the
intensity of the peaks, but does not shift the peaks to higher
frequencies.
[0069] As illustrated, the energies in the resulting spectrum of
radiation emitted by the target may be controlled, at least in
part, by controlling the e-beam energy, which in turn may be
controlled via the voltage potential of the x-ray tube.
Accordingly, exposure controller 560 may provide a voltage control
signal 563 indicative of a desired voltage level of the vacuum
tube. The voltage control signal may be received by power and
timing control circuitry 522. Power and timing control circuitry
522 may comprise any type of circuitry adapted to modify or
establish the voltage potential of the x-ray tube according to
voltage control signal 563, thus increasing the energy in e-beam
523. As a result, exposure control 560 is capable of controlling
the radiation energy of the radiation emitted from radiation source
520. As indicated by spectrums 605a-605d, the e-beam energy also
effects the intensity of the radiation emitted by the target.
Accordingly, voltage control signal 563 may also be used to vary
the intensity of the radiation provided by radiation source
520.
[0070] As discussed above, the radiation intensity of such
radiation is related to the number of photons passing through a
given volume in a given interval of time (i.e., radiation intensity
is a measure of flux). Therefore, the longer radiation source 120
is operated, the more radiation is produced, increasing the
radiation intensity (i.e., the more photons that are emitted by the
radiation source). Accordingly, radiation intensity may be
controlled, at least in part, by controlling how long the radiation
source is operated (i.e., by controlling the length of the
exposure). Exposure control 560 may provide a timing control signal
565 that indicates the duration of an exposure. For example, timing
control signal 565 may be a toggle control that turns the radiation
source on and off. Alternatively, timing control signal 565 may be
a signal indicative of a duration during which the radiation source
should be operated.
[0071] The timing control signal 565 may be received by power and
timing circuitry 522 to control the length of a given exposure.
Power and timing circuitry 522 may include any circuitry adapted to
energize and de-energize the radiation source (e.g., power and
timing circuitry 522 is capable of turning the radiation source on
and off). As a result, exposure control 560 is adapted to control
the radiation intensity of radiation emitted by radiation source
520 via the voltage control signal 563, the timing control signal
565, or both.
[0072] Exposure control 560 may include one or more processors and
a memory capable of storing one or more programs to be executed by
the one or more processors. As a result, exposure control may be
programmed to control the radiation intensity and radiation energy
according to any desired exposure plan. Specifically, exposure
control 560 may be programmed to control the exposure parameters of
a number of exposures taken from a plurality of view angles to
control the subject dose and/or maintain a desired ratio between
emitted and exiting radiation that provides suitable contrast.
[0073] Radiation source 120 may also include a filter 528 designed
to block radiation of particular energies. For example, a filter
may be used to suppress one or more of the characteristic peaks of
a given target material, or may block some band of energies such
that radiation having a more desirable energy spectrum enter an
exposure area to impinge on a object being imaged. Filter 528 may
be used to further control the energy and/or intensity of the
radiation emitted from the radiation source. It should be
appreciated that a filter is not required, as the aspects of the
invention are not limited in this respect.
[0074] As discussed above, various aspects of the present invention
may be well suited to performing exposures in relatively low-dose
environments. Referring again to FIG. 4, in one embodiment, object
410 is a breast to be imaged in a mammography procedure. When the
breast is compressed for imaging, the thickness of the breast
material may vary depending on the view angle from which it is
viewed. To avoid substantially increasing the subject dose received
by the breast at view angles where the thickness of the breast is
larger, imaging system 400 may be configured to vary the radiation
energy depending on the thickness of the breast at each of a
plurality of view angles. As a result, higher energy radiation may
be provided to the breast at view angles where increased
thicknesses of breast material must be penetrated. As a result,
acceptable image quality may be maintained over the range of view
angles while limiting the subject dose to safe levels.
[0075] In one embodiment, object 410 is a breast that has been
compressed in the y-direction in preparation for a mammography
procedure. Radiation source 420 may include an x-ray tube and a
target, wherein the x-ray tube is adapted to generate an e-beam and
direct the e-beam to impinge on a target. Electrons incident on the
target, for example, a tungsten target, result in the emission of
x-ray radiation. The radiation energy emitted from radiation source
420 may be related to a voltage of the x-ray tube used to generate
the electron beam from which the x-ray radiation is formed.
According to one exemplary exposure plan, the radiation energy
emitted into the exposure area is generated by operating the x-ray
tube at a voltage between 20 and 40 Kev when the radiation source
is located in substantial alignment with the y-axis (e.g., at
position A in FIG. 1) and a voltage between 30 and 90 Kev when the
radiation source is located in substantial alignment with the
x-axis (e.g., at position C is FIG. 1).
[0076] At intermediate angles between position A and position B,
the x-rays may be provided at energies resulting from operating the
x-ray tube at voltages in between the voltage ranges mentioned
above to cause the radiation energy to increase as a function of
view angle. It should be appreciated that the above voltage levels
are exemplary, and the aspects of the invention are not limited for
use with any particular x-ray tube voltage and/or resulting
radiation energy distribution. In addition, the radiation energy
may be varied in any suitable manner. For example, the x-ray energy
may varied by changing the voltage applied to the x-ray tube as
discussed above, using different anode material (i.e., providing a
target of different materials, each adapted to generate x-ray
radiation having different radiation distributions), and/or the use
of filters arranged between the source and the object to
selectively provide radiation at desired frequencies to the object.
The radiation energy may be varied as a function of view angle by
any amount and by any means, as the aspects of the invention are
not limited in this respect.
[0077] In addition, the absorption ratio (i.e., the proportion of
radiation impinging on an object that exits the object) may be
substantially maintained such that the projection data has contrast
sufficient to distinguish the different tissue characteristics
within the breast. For example, the ratio of radiation emitted from
the radiation source to the radiation penetrating and exiting the
breast should be sufficient to distinguish between normal healthy
breast tissue and tissue anomalies, particularly with respect to
potential cancerous growths in early stages of development.
Accordingly, various aspects of the invention facilitate obtaining
relatively high contrast volume images, while simultaneously
limiting the subject dose received by a patient.
[0078] In conventional digital mammography, it was believed
necessary to use a nearly flawless detector which covers the entire
breast, since flaws or small gaps might contribute to the failure
to capture information indicative of a tissue anomaly, for example,
an early stage cancer. However, large area flawless detectors are
relatively expensive compared to forming a detector to cover a
similar area using a number of smaller and/or flawed detectors. In
addition, large area detectors are typically flat. Applicant has
appreciated that smaller and/or flawed detectors may be combined in
an array in a planar or non-planar configuration. Provided that
each part of the exposure volume of an object is projected onto a
functional part of one of the detectors (i.e., a surface of one of
the detectors responsive to the radiation) in at least one exposure
(i.e., from at least one view angle), satisfactory projection data
may be obtained.
[0079] In one embodiment, multiple detectors may be tiled together
having small gaps between them to form a detector array (e.g., the
detector 430 in FIG, 4). Another embodiment includes forming a
detector array using multiple detectors having some insensitive
areas that do not provide usable data, but which project to
different regions of tissue on each exposure (i.e., at each
different view angle). Another embodiment includes disposing the
detector array in a non-planar configuration. For example, the
multiple detectors can overlap and be arranged in a tilted fashion
such that the object (e.g., a breast) may be partially surrounded
by the array. However, any number of detectors, in any arrangement,
and of any quality may be used, as the aspects of the invention are
not limited in this respect.
[0080] As discussed in the '848 patent, the subject dose received
by a patient may be prevented from exceeding a desired amount by
varying the intensity of the radiation emitted by the radiation
source with respect to view angle. Applicant appreciated that 3D
images could be obtained while operating within a specific dose
budget by obtaining relatively high resolution projection data at
view angles deemed more critical for the 3D reconstruction, and
obtaining lower resolution data at view angles deemed less critical
in the 3D reconstruction. For example, in FIG. 3A, the highest
radiation intensity may be used at the view angle at
.theta.=0.degree.. As the radiation source rotates about the object
in one or both of the clockwise and counterclockwise direction, the
radiation intensity may be reduced for each successive view
angle.
[0081] Applicant has appreciated that benefits of varying the
radiation intensity as a function of view angle may also be
achieved using exposure plans wherein the view angles are
distributed uniformly about the object. For example, variable
radiation intensity exposures may be performed using the
configuration illustrated in FIG. 3B. In particular, a highest
radiation intensity may be used when exposing object 310 from the
view angle at .theta.=0.degree.. As the radiation source rotates
about the object in one or both of the clockwise and
counterclockwise direction, the radiation intensity may be reduced
for each successive view angle distributed in equiangular offsets
from one another. It should be appreciated, however, that variable
intensity exposure plans may be used in connection with any number
of view angles in any distribution, uniform or non-uniform, as the
aspects of the invention are not limited in this respect.
[0082] Applicant has further appreciated that concepts related to
variable radiation intensity and variable radiation energy
exposures may be combined to obtain projection data facilitating
the reconstruction of 3D images, while respecting dose budgets
designed for imaging sensitive objects such as human tissue and/or
for routine imaging procedures where limiting dose may be desirable
or even necessary, In particular, a exposure plan may be designed
to expose an object at a desired number of view angles. From one
view angle to the next one or both of the radiation intensity and
radiation energy may be varied to obtain dose limited projection
data. It should be appreciated that the above described concepts
may be used in other combinations, as the aspects of the invention
are not limited in this respect.
[0083] Applicant provides below additional embodiments of and
various concepts related to aspects of the present invention. It
should be appreciated that the aspects of the invention are not
limited by the embodiments described below. In addition, the
various embodiments described below may be used alone or in
combination with any of the concepts, features, and/or embodiments
described in the foregoing.
[0084] For many applications, including diagnostic applications, it
is desirable to obtain the best possible quality three dimensional
(3D) image of an object while minimizing the exposure of the
object. For example, in x-ray screening mammography, it would be
desirable to obtain 3D images of the breast, but because exams may
be performed on an annual basis, it is desirable that the x-ray
dose provided to a patient be minimized so as not to substantially
increase the patient's risk of developing breast cancer during that
patient's lifetime.
[0085] Methods have been disclosed for obtaining generally low-dose
images of an object, including the methods described in U.S. Pat.
No. 6,744,848 ('848) entitled "Method and System for Low-dose
Three-dimensional Imaging of a Scene" and U.S. Pat. No. 5,872,828
('828) entitled "Tomosynthesis System for Breast Imaging," both of
which are herein incorporated by reference in their entirety. In
some of the methods described, a small number of two dimensional
(2D) images (e.g., between 9 and 30 images) are taken of the object
from different view angles. In the '828 patent, each of the images
are taken at equal x-ray doses (e.g., obtained at equal energy and
flux) and the different view angles are equally spaced from one
another. In the '848 patent it was disclosed that the images do not
need to be equally spaced and that the provided x-ray flux can be
varied between images acquired from the different view angles.
[0086] Applicant describes herein additional methods to obtain
images (e.g., 3D images) of an object at relatively low dose. These
methods include, but are not limited to: methods for varying the
x-ray energy; the use of density fiducials; and methods to
determine the relationship between observed x-ray intensity and
specimen density. While any combination of these methods can be
used to obtain a high quality 3D image, each of these methods can
also be used independently. These methods may be used either alone
or in any combination with methods described in either the '848 or
'828 patents.
[0087] I. X-Ray Intensity to Density Relationship
[0088] In transmission x-ray tomographic data processing, a three
dimensional specimen density volume is calculated from measurements
of the transmitted x-ray intensity along paths from an x-ray
source, thru the specimen volume, to an x-ray detector (x-ray
projection measurements, referred to herein as projection data),
Various methods disclosed below may more accurately model the
density from x-ray intensity data (e.g., from the projection data
obtained by the x-ray detector.
[0089] To reconstruct a three dimensional specimen volume from the
x-ray projection measurements, the intensity of the x-ray
projection measurements may need to be associated with the density
of the specimen. For un-scattered monochromatic x-rays this
relationship may be modeled by exponential absorption along the
x-ray path. This relatively simple relationship between projection
measurements and specimen density is complicated, however, by a
number of factors including: spectrum of the x-ray source;
beam-hardening by the specimen; and x-ray scatter by the
specimen.
[0090] X-ray Source Energy Distribution. The x-ray source in most
imaging applications is typically not monochromatic, but instead
has a more complicated polychromatic spectrum. The source can also
be filtered to preferentially remove x-rays, for example, the low
energy x-rays, to more closely approximate a monochromatic x-ray
source. Further complicating the intensity-density relationship is
the fact that the x-ray dose (i.e., by varying the x-ray flux)
provided by the x-ray source may be intentionally varied as a
function of the view angle as described, for example, in the '848
patent.
[0091] X-ray scatter. Measured x-ray intensity values for a given
x-ray path is both reduced by scatter away from the initial x-ray
paths from the x-ray source and increased by scatter from other
x-ray paths to a given detection point.
[0092] Beam Hardening. Beam hardening refers to effect of the
spread of energy in a polychromatic x-ray source. In particular, as
a polychromatic x-ray beam travels through a specimen, lower energy
x-rays will be preferentially absorbed by the specimen. As a
result, density at the proximal side of the specimen will receive a
different x-ray energy distribution than density at the distal
side.
[0093] Together, scattered x-rays and beam hardening may be
substantial causes of inaccurate projected density calculations in
conventional 3D imaging method (for example the Tomosynthesis
method described in the '828). These inaccurate projected density
calculations cause error artifacts in the reconstructed 3D images.
Both scattering and beam-hardening increase the intensity recorded
at the detector for any given projected density, although the
effects differ for higher density projections. Some aspects of the
invention are directed to accounting, at least in part, for these
effects.
[0094] In one embodiment a set of projections of a breast is
obtained using either a same or different radiation energy without
using density fiducials. An initial 3D reconstruction is calculated
using, for example, the methods described in the '848 patent and/or
the '828 patent. This initial reconstruction is then used to
provide a density model of the breast to calculate beam scatter and
absorption based on the known x-ray energy distribution for each
projection as well as the absorption/scatter characteristics of the
breast. A second reconstruction is then performed utilizing this
information.
[0095] In another embodiment, a subset of the projections is
collected with an anti-scatter grid, while the remaining
projections are collected without the anti-scatter grid. The
resulting sets of projection data may (or may not) be at the same
angular positions and may (or may not) be at the same x-ray
exposure. In one example, low dose pre-exposures of the first,
middle and last angles in the tomographic sequence are acquired
with an anti-scatter grid. By comparing a projection data obtained
with an anti scatter grid with the projection data acquired for the
tomographic reconstruction, the ratio of scatter to transmitted
x-rays can be estimated over the whole of the image. By doing this
for multiple source positions of the tomographic sequence, such as
the extremes of the angular range and the mid-position, the ratio
of scatter to transmitted x-ray intensity can be interpolated for
other angles in the sequence.
[0096] In another embodiment, the relationship between x-ray
intensity and density can be estimated from the different projected
tissue thickness from rays at different angles. Because of beam
hardening and intentional variation of the x-ray energy between
exposures, the same volume of density will be exposed to a
different x-ray energy distribution as a function of the view
angle. By comparing the derived density updates from data collected
at different view angles (and therefore different penetration
depths) the intensity-density ratio can be further defined.
[0097] In a further embodiment, the relationship between x-ray
intensity and density may be estimated from projection data where
exposures from some view angles are taken more than once, repeated
exposures being performed using different x-ray energies. The
comparison of the projection data taken with different energies can
be used to determine the relative absorption coefficients of the
tissue for the various x-ray energies. Since the scattering of the
tissue varies with energy, the comparison can also be used to
estimate the contribution of scattered x-rays to the detected
intensity. The exposure for the comparison energies need not be the
same, since the images can be scaled. In a variant on the method,
very low dose images can be used for these comparisons.
[0098] II. Density Fiducials
[0099] In another embodiment, projection data is obtained from a
plurality of exposures having one or more density fiducials
arranged in an exposure area during respective exposures, In one
application of density fiducials, the fiducials are used to provide
information useful in calculating specimen density from observed
x-ray intensity information, This information can be used alone to
facilitate characterizing scatter and/or beam hardening effects, or
in combination with methods described above as a check of the
corresponding method. Preferably, information provided by one or
more density fiducials is also used to refine the reconstruction
either during or post calculation of one or more images.
[0100] In one embodiment, one or more density fiducials arranged in
the exposure area are selected from a range of densities and x-ray
absorption properties similar to the object being imaged. In one
example, for a breast imaging application, the one or more density
fiducial has a density in a range of densities spanning from 75% to
200% of the density of the prevalent fatty tissue. The one or more
density fiducials may be placed such that, in each exposure, there
will be a set of fiducials on both sides of the object (i.e.,
arranged proximal and distal relative to the x-ray source.) In one
breast imaging example, one ore more fiducials can be attached to
modified compression paddles used to compress and generally
immobilize the breast.
[0101] In another embodiment, one or more fiducials may be placed
above or below the compression paddles such that the shadow of the
one or more fiducials moves across the specimen as a function of
the imaging position. Since radiation will encounter a fiducial
located proximally relative to the source before interacting with
the object being imaged such that the fidicial will not absorb
x-rays scattered by the object that are directed towards the
detector, the intensity variation due to the shadow from a fiducial
proximal to the x-ray source will differ from the shadow caused by
a similar fiducial placed distally with respect to the radiation
source. In particular, the component of intensity due to scatter
from breast tissue outside of the shadow of the fiducial will be
unaffected by the fiducial proximal to the source. Thus, the
difference between the two shadows may be used to measure the
contribution of the scattered x-rays to the intensity measured by
the detector.
[0102] Shadows from fiducials that do not intersect with the object
may be used to calibrate the relationship between absorption and
projected density by direct measurement; for this purpose the
projection of the fiducial must be taken into account; spherical
fiducials are useful since the projected density is dependent only
on the angle of x-ray incidence on the detector. In the case that
the x-ray energy is varied between exposures, this direct
information is may be particularly useful.
[0103] In another embodiment, one or more density fiducials
arranged in an exposure area are opaque or substantially opaque to
the x-ray beam. In this case, x-ray intensity observed under the
shadow of the one or more fiducials can be considered to be from
x-ray scatter. This allows the direct determination of the effect
of x-ray scatter. If opaque fiducials are employed, it may be
desirable to position the fiducials between the specimen and the
x-ray source and substantially above the specimen such that the
shadow of one or more fiducials does not occlude the same region of
the specimen as the specimen is imaged from different view angles.
As an alternative, the position of the one or more fiducials may be
moved between exposures at the different view angles. It should be
appreciated that in the above examples, the fiducials may either be
moved relative to the specimen, for example, by moving the
fiducials with the x-ray source and/or detector, or may be located
at a fixed position relative to specimen.
[0104] In evaluating if one or more fiducials should be placed
proximal or distal to the specimen, consideration may be given to
the fact that fiducials placed proximal to the specimen provide
information without x-ray exposure to the specimen, whereas
fiducials placed distal to the specimen interrupt at least some
x-rays that have already traveled through the specimen.
[0105] The proportionate reduction in intensity by a density
fiducial can be measured by comparing the mean of the intensity in
pixels shadowed by the fiducial with the mean intensity of nearby
pixels not shadowed by the fiducial. For a fiducial of known
density variation, the intensity variation may be measured as a
function of the projected fiducial density. Alternatively, the edge
of the fiducial shadow can be examined; the median or mean ratio of
x-ray intensity change across the edge of the shadow of the density
fiducial can be measured.
[0106] While scatter affects, amongst other things, the high
density asymptote of the density-intensity relationship, beam
hardening affects, amongst other things, the curvature of the
relationship. The proportionate difference in intensity for a small
increment in density of a density fiducial provided information
about the local slope of the intensity-density relationship,
whereas the effect of a density fiducial with a high increment in
density provides information about the intensity recorded for high
densities, which will differ according to the location of the
density fiducial with respect to the specimen (as discussed
above).
[0107] In another application of the use of fiducials, the observed
location of the fiducials can be used to refine the geometry of the
imaging apparatus. For a variety of reasons, including user error
and the uncertainty in the mechanical positioning of the apparatus,
the imaging geometry might not be precisely known from a simple
readout of the component positions. Information from one or more
fiducials can be used as a method to check and to refine the
imaging geometry.
[0108] III. Identification of Specimen Boundaries
[0109] The accuracy of the calculation of the density distribution
may be enhanced by the identification of the specimen boundaries.
In many cases, a portion of the surface boundaries can be
determined from the imaging geometry. For example, in breast
imaging, the breast is often constrained by a compression device.
This allows precise determination of the regions of the breast that
are in contact with the compression device. In this example,
however, a significant portion of the surface boundary can not be
determined in this manner. If this surface is not correctly
determined, the reconstruction algorithms may inappropriately place
density either inside or outside the specimen boundaries. This may
lead to reconstruction errors at the skin line in iterative and
non-iterative algorithms. In one embodiment of the present
invention, additional x-ray exposures may be taken from source
positions further out from the chest wall. In another embodiment,
additional x-ray exposures are performed at higher angles. In both
the previous embodiments, the additional x-ray exposures may be
taken at respective lower x-ray doses than taken in the one or more
main exposures.
[0110] In another embodiment skin surface information is collected
using x-ray absorbing markers placed on the skin surface. These
x-ray absorbing markers could also serve as density fiducials as
mentioned above. The markers could be placed on the skin either
directly (e.g. using a mesh garment or tape), or indirectly using
an inflatable pillow (or pillows) attached to the compression
paddle, the pillows could be on either the proximal paddle or the
distal paddle, or both paddles. In evaluating if such pillows
should be placed proximal or distal to the specimen, consideration
may be given to the fact that a pillow placed proximal to the
specimen provides information without x-ray exposure to the
specimen, whereas a pillow placed distal to the specimen will be
interrupting x-rays that have already traveled through the
specimen. Filling such pillows with fluid with an x-ray density
similar to breast tissue may mitigate the effect of the surface
boundary upon the reconstruction algorithms.
[0111] In another embodiment, the pillow could be filled with fluid
and have a surface with structures such as corrugations or capsules
which hold spaces filled with lower density material (e.g. air)
next to the skin. These spaces could serve as markers for the skin
surface. Pillows may be inflated either before or after compression
of the breast is established. Inflation after compression may help
to prevent the pillow from interfering with the compression
procedure.
[0112] In another embodiment, specimen boundary information may be
collected optically. For example, in 3D breast imaging by
projecting a grid pattern onto the skin between the compression
paddles, and imaging the pattern with one camera close to the x-ray
source, and one positioned close to the detector. In another
example, two or more optical images of the specimen may be taken
and standard surface mapping techniques that employ the use of
feature recognition and/and texture mapping can be used to identify
the surface boundaries. Once the specimen boundaries are
identified, the reconstruction algorithms can assign regions
outside the specimen the correct background density (typically 0
for air).
[0113] IV. Pre-Exposures to Estimate Imaging Properties
[0114] In addition to the uses described above, low dose
pre-exposures may be used to estimate various imaging properties
and to assist in the identification of problems that occur during
the collection of image data, Low dose pre-exposures can be used
for determining specimen or subject motion by comparing the images
with the actual tomographic image acquisition. Such pre exposures
also allow for automatic energy and dose exposure compensation,
since the dose used for the actual tomographic image acquisition
can be adjusted on the basis of the pre exposure images. A common
problem in mammography is poor patient positioning. By using data
such as the height, weight, age of the patient in combination with
the angles and thickness from the compression system, low dose
images can be examined to ascertain the position of the pectoral
muscle and the extent of coverage of breast tissue. This
information can be automatically used to alert the operator to the
need to reposition the patient. Determining inadequate positioning
before the main exposure sequence is often valuable in locations
where the cases are not evaluated immediately, since a patient call
back for a repeated exam might be avoided. Furthermore, in
combination with the techniques described above for detecting the
skin line of the specimen, the common problem in which a part of
the specimen overlaps the edge of the detector can be anticipated
and the operator automatically alerted.
[0115] V. Detecting Specimen Motion
[0116] Specimen motion can be identified based on two or more
images taken at the same position (e.g., from the same view angle).
Preferably, images are taken at the start and end of the imaging
procedure. Well known image mathematics (in particular,
cross-correlation or image difference methods) can be employed to
detect or determine any specimen motion. This information can be
automatically used to alert the operator to the need to repeat the
exposure, or to omit or correct some data during the computational
reconstruction process. In various embodiments described above, two
or more images are acquired at the same view angle (e.g., the
exposures may be repeated at the same view angle) to allow an
improved estimate of the intensity to density relationship. When
those methods are employed, it is possible to use resulting images
to detect specimen motion. It should be appreciated, though, that
repeating an exposure at a particular view angle may be repeated
solely for the purposes of detecting specimen motion, or in
combination with other techniques.
[0117] VI. Display Resolution
[0118] The resolution of reconstructed volumes in typically not
uniform, and differs according to the orientation of features in
the tissue. In addition, there are artifacts due to the
reconstruction algorithms which can be misleading. Because of these
issues it may be important to only display tissue structures that
are well determined by the data. In one embodiment of this
invention, the plane to plane resolution of the reconstructed image
is limited for display purposes to a predetermined resolution
(e.g., from 3 to 5 mm) between planes to assist in preventing
spurious structures from being displayed to the radiologist, while
still displaying the fall resolution within the plane. In another
embodiment of the present invention, the display allows tilting the
displayed volume, but limits this tilt to a range of angles less
than the angular range used for the x-ray source motion.
[0119] Displaying Information on a Display Device with Lower
Resolution than the Data
[0120] The medical community often uses very high resolution
monitors to ensure that every pixel from the data may be displayed.
It is sometimes necessary to produce lower resolution images from
higher resolution data. Often averaging is used to combine the
information from multiple data pixels into a single display pixel.
However, for some imaging applications such as mammography, this
may be inadequate. In particular, mammographic images may contain
sharp features such as microcalcifications which may be
diagnostically important. Averaging techniques may render these
features less conspicuous. Applicant has recognized that combining
the pixels by recording a maximum pixel value instead of the
average pixel value may improve the conspicuity of such small,
bright features. Alternatively, Applicant has recognized that
averaging techniques may be used after applying a function to
adjust the relative influence of pixels with different intensities.
After averaging, the inverse function may be applied to recover the
original pixel value scaling.
[0121] Displaying Information from Multiple Planes
[0122] In order to display information from more than a single
plane simultaneously, maximum intensity projection has been
suggested ("Tomographic mammography using a limited number of
low-dose cone-beam projection images," Wu et al, Med Phys 30 (3),
March 2003). Maximum intensity projection has the disadvantage that
features can be completely bidden by slightly denser features which
project to the same displayed pixels. To mitigate this effect,
Applicant has appreciated that the radiologist is typically not
interested in visualizing the densities pertaining to fat, the
projection can be calculated as a standard average along rays but
applying a function to the density data before projection. The
function adjusts the relative influence of pixels with different
values projecting to the same average pixel. After averaging, the
inverse function may be applied to recover the original pixel value
scaling. A further aspect of this invention is that such
projections be rapid in order to enable interactive control by the
user.
[0123] Another issue in displaying multiple planes in projection is
that features which fall outside of the range of planes projected
for display may be clinically important. An example of this is the
clustering of microcalcifications. If there is a sharp boundary to
the range of planes projected for display then some
microcalcifications may be invisible, and the clustering may be
misunderstood. To avoid this effect, in one embodiment, the
contribution from planes at and near the boundary of the range of
planes projected for display is gently tapered instead of having a
sharp cutoff. This tapering causes features such as
Microcalcifications to brighten and dim with distance in a more
intuitive manner as the projection range of planes projected for
display is swept through the 3D data.
[0124] It should be appreciated that while various embodiments
described herein have focused primarily on x-ray imaging, the
methods disclosed here can also be applied to other imaging
modalities, such as optical imaging, IR imaging, PECT, SPECT and
gamma imaging, and NMR and ultrasound imaging. Further, while many
of the examples were provided for breast imaging, these methods may
also be employed for any imaging applications (including
non-medical imaging applications) where it is useful to minimize
either the x-ray dose provided to the specimen or the image
acquisition time. Further, the methods used to evaluate the
intensity-density relationship may be applied to conventional
imaging modalities, such as the various forms of both high-dose and
low-dose CT imaging.
[0125] Following below are descriptions of various embodiments of
methods and apparatus according to the present invention. It should
be appreciated that various aspects of the invention described
herein may be implemented in any of numerous ways. Examples of
specific implementations are provided herein for illustrative
purposes only. In addition, the various aspects of the invention
described in the embodiments below may be used alone or in any
combination, and are not limited to the combinations explicitly
described herein.
[0126] One embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
angular positions using a plurality of different x-ray energies,
detecting radiation transmitted through the object at each angular
position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of angular positions, and producing a
three-dimensional image of the object based on said two-dimensional
x-ray data.
[0127] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
non-uniformly distributed angular positions using a plurality of
different x-ray energies, detecting radiation transmitted through
the object at each angular position, producing two-dimensional
transmission data representative of the radiation transmitted
through the object at each of the plurality of non-uniformly
distributed angular positions, and producing a three-dimensional
image of the object based on said two-dimensional x-ray data.
[0128] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly distributed angular positions using a plurality of
different x-ray doses, detecting radiation transmitted through the
object at each angular position, producing two-dimensional
transmission data representative of the radiation transmitted
through the object at each of the plurality of uniformly
distributed angular positions, and producing a three-dimensional
image of the object based on said two-dimensional x-ray data.
[0129] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a preliminary three-dimensional image of the object based
on said two-dimensional x-ray data, using said preliminary
three-dimensional image of the object to derive a model of x-ray
scatter from the object, and calculating a final three-dimensional
image of the object based on the two-dimensional x-ray and the
model of the x-ray scatter.
[0130] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a preliminary three-dimensional image of the object based
on said two-dimensional x-ray data, using said preliminary
three-dimensional image of the object to derive a model of x-ray
beam hardening from the object, and calculating a final
three-dimensional image of the object based on the two-dimensional
x-ray and the model of the x-ray beam hardening.
[0131] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a preliminary three-dimensional image of the object based
on said two-dimensional x-ray data, using said preliminary
three-dimensional image of the object to derive a model of x-ray
beam hardening and x-ray scattering from the object, and
calculating a final three-dimensional image of the object based on
the two-dimensional x-ray and the model of the x-ray beam hardening
and x-ray scatter.
[0132] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, wherein a
subset of said angular positions employ an anti-scatter grid,
detecting radiation transmitted through the object at each angular
position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data in which
differences between the images collected with and without the
anti-scatter grid is used to incorporate x-ray scatter
corrections.
[0133] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a three-dimensional image of the object based on said
two-dimensional x-ray data in which differences between the images
collected at each of the plurality positions, each which passes
through a different thickness of the object, is used to calculate
the effect of beam hardening on the x-ray intensity to density
relationship.
[0134] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, wherein a
subset of the angular positions are imaged two or more times, each
time at a different x-ray energy, detecting radiation transmitted
through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which differences between the images
collected at the same position is used to refine the intensity to
density relationship.
[0135] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the x-ray
source and the object, irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0136] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the object
and the x-ray detector, irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0137] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials both between the
object and the x-ray source and between the x-ray detector,
irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0138] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the x-ray
source and the object, wherein the density of said fiducials spans
the range from 75% to 150% of the predominant density in the
object, irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0139] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the object
and the x-ray detector, wherein the density of said fiducials spans
the range from 75% to 150% of the predominant density in the
object, irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0140] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials both between the
object and the x-ray source and between the x-ray detector, wherein
the density of said fiducials spans the range from 75% to 150% of
the predominant density in the object, irradiating the object from
a plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data in which
information derived from the density fiducials is used to refine
the x-ray intensity to density relationship.
[0141] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the x-ray
source and the object, wherein the density of said fiducials is
greater than 150% of the predominant density in the object,
irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0142] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the object
and the x-ray detector, wherein the density of said fiducials is
greater than 150% of the predominant density in the object,
irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the x-ray intensity to density
relationship.
[0143] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials both between the
object and the x-ray source and between the x-ray detector, wherein
the density of said fiducials is greater than 150% of the
predominant density in the object, irradiating the object from a
plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data in which
information derived from the density fiducials is used to refine
the x-ray intensity to density relationship.
[0144] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the x-ray
source and the object, wherein the density of said fiducials spans
the range from 75% to 150% of the predominant density in the
object, irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the imaging geometry.
[0145] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the object
and the x-ray detector, wherein the density of said fiducials spans
the range from 75% to 150% of the predominant density in the
object, irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the imaging geometry.
[0146] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials both between the
object and the x-ray source and between the x-ray detector, wherein
the density of said fiducials spans the range from 75% to 150% of
the predominant density in the object, irradiating the object from
a plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data in which
information derived from the density fiducials is used to refine
the imaging geometry.
[0147] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the x-ray
source and the object, wherein the density of said fiducials is
greater than 150% of the predominant density in the object,
irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the imaging geometry.
[0148] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials between the object
and the x-ray detector, wherein the density of said fiducials is
greater than 150% of the predominant density in the object,
irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
density fiducials is used to refine the imaging geometry.
[0149] Another embodiment includes a method of imaging an object
comprising the acts of placing density fiducials both between the
object and the x-ray source and between the x-ray detector, wherein
the density of said fiducials is greater than 150% of the
predominant density in the object, irradiating the object from a
plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data in which
information derived from the density fiducials is used to refine
the imaging geometry.
[0150] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
collecting two or more optical images of the object, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
two or more optical images is used to determine the object
boundaries.
[0151] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
collecting two or more optical images of the object, wherein
optically identifiable features are projected onto the object,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
two or more optical images is used to determine the object
boundaries.
[0152] Another embodiment includes a method of imaging an object
comprising the acts of placing x-ray opaque markers on the surface
of the object, irradiating the object from a plurality of uniformly
or non-uniformly distributed angular positions, detecting radiation
transmitted through the object at each angular position, producing
two-dimensional transmission data representative of the radiation
transmitted through the object at each of the plurality of
uniformly or non-uniformly distributed angular positions, producing
a three-dimensional image of the object based on said
two-dimensional x-ray data in which information derived from the
x-ray opaque markers is used to determine the object
boundaries.
[0153] Another embodiment includes a method of imaging an object
comprising the acts of placing the object within a flexible
material that conforms to the boundaries of the object and has a
known outer boundary, wherein the flexible material has a density
less than the object, irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, detecting
radiation transmitted through the object at each angular position,
producing two-dimensional transmission data representative of the
radiation transmitted through the object at each of the plurality
of uniformly or non-uniformly distributed angular positions,
producing a three-dimensional image of the object based on said
two-dimensional x-ray data.
[0154] Another embodiment includes a method of imaging an object
comprising the acts of placing the object within a flexible
material that conforms to the boundaries of the object and has a
known outer boundary, wherein the flexible material has a density
approximately equal to that of the object, irradiating the object
from a plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data.
[0155] Another embodiment includes a method of imaging an object
comprising the acts of placing the object within a flexible
material that conforms to the boundaries of the object and has a
known outer boundary, wherein the flexible material has a density
greater than that of the object, irradiating the object from a
plurality of uniformly or non-uniformly distributed angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data.
[0156] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, wherein a
set of pre-exposures are collected at a subset of the angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data, wherein
differences between the pre-exposure data and the exposure data is
used to determine if the object moved as the set of images was
being collected.
[0157] Another embodiment includes a method of imaging an object
comprising the acts of irradiating the object from a plurality of
uniformly or non-uniformly distributed angular positions, wherein a
set of pre-exposures are collected at a subset of the angular
positions, detecting radiation transmitted through the object at
each angular position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data, wherein
differences between the pre-exposure data and the exposure data is
used to refine the x-ray intensity to density model.
[0158] Another embodiment a method of imaging an object comprising
the acts of irradiating the object from a plurality of uniformly or
non-uniformly distributed angular positions, wherein a set of
pre-exposures are collected at a subset of the angular positions,
detecting radiation transmitted through the object at each angular
position, producing two-dimensional transmission data
representative of the radiation transmitted through the object at
each of the plurality of uniformly or non-uniformly distributed
angular positions, producing a three-dimensional image of the
object based on said two-dimensional x-ray data, wherein gaps or
missing data in said two-dimensional x-ray data are identified and
data from these regions are not used in the calculation of the
three-dimensional image.
[0159] Another embodiment a method of displaying a
three-dimensional image of an object in which the display
resolution is limited to the imaging resolution generated by the
three-dimensional imaging method.
[0160] Another embodiment a method of displaying a
three-dimensional image of an object in which only the maximum
pixel value from each sub-region of the three-dimensional image is
displayed.
[0161] Another embodiment includes a method of displaying a
three-dimensional image of an object in which a scaled difference
of the maximum pixel value and the average pixel value from each
sub-region of the three-dimensional image is displayed.
[0162] As mentioned above, the foregoing additional embodiments of
and various concepts related to aspects of the present invention.
The additional embodiments may be used alone or in combination with
any other method, technique or embodiment described herein, as the
aspects of the invention are not limited to the particular
combination described herein.
[0163] The above-described embodiments of the present invention can
be implemented in any of numerous ways. For example, the
embodiments may be implemented using hardware, software or a
combination thereof. When implemented in software, the software
code can be executed on any suitable processor or collection of
processors, whether provided in a single computer or distributed
among multiple computers. It should be appreciated that any
component or collection of components that perform the functions
described above can be generically considered as one or more
controllers that control the above-discussed function. The one or
more controller can be implemented in numerous ways, such as with
dedicated hardware, or with general purpose hardware (e.g., one or
more processor) that is programmed using microcode or software to
perform the functions recited above.
[0164] It should be appreciated that the various methods outlined
herein may be coded as software that is executable on one or more
processors that employ any one of a variety of operating systems or
platforms. Additionally, such software may be written using any of
a number of suitable programming languages and/or conventional
programming or scripting tools, and also may be compiled as
executable machine language code.
[0165] In this respect, it should be appreciated that one
embodiment of the invention is directed to a computer readable
medium (or multiple computer readable media) (e.g., a computer
memory, one or more floppy discs, compact discs, optical discs,
magnetic tapes, etc.) encoded with one or more programs that, when
executed on one or more computers or other processors, perform
methods that implement the various embodiments of the invention
discussed above. The computer readable medium or media can be
transportable, such that the program or programs stored thereon can
be loaded onto one or more different computers or other processors
to implement various aspects of the present invention as discussed
above.
[0166] It should be understood that the term "program" is used
herein in a generic sense to refer to any type of computer code or
set of instructions that can be employed to program a computer or
other processor to implement various aspects of the present
invention as discussed above. Additionally, it should be
appreciated that according to one aspect of this embodiment, one or
more computer programs that when executed perform methods of the
present invention need not reside on a single computer or
processor, but may be distributed in a modular fashion amongst a
number of different computers or processors to implement various
aspects of the present invention.
[0167] Various aspects of the present invention may be used alone,
in combination, or in a variety of arrangements not specifically
discussed in the embodiments described in the foregoing and is
therefore not limited in its application to the details and
arrangement of components set forth in the foregoing description or
illustrated in the drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. In particular, the various concepts related to
variable radiation energy and variable radiation intensity may be
used in any way, either alone or in any combination, as the aspects
of the invention are not limited to the specific combinations
described herein. Accordingly, the foregoing description and
drawings are by way of example only.
[0168] Use of ordinal terms such as "first", "second", "third",
etc., in the claims to modify a claim element does not by itself
connote any priority, precedence, or order of one claim element
over another or the temporal order in which acts of a method are
performed, but are used merely as labels to distinguish one claim
element having a certain name from another element having a same
name (but for use of the ordinal term) to distinguish the claim
elements.
[0169] Also, the phraseology and terminology used herein is for the
purpose of description and should not be regarded as limiting. The
use of "including," "comprising," or "having," "containing",
"involving", and variations thereof herein, is meant to encompass
the items listed thereafter and equivalents thereof as well as
additional items.
* * * * *