U.S. patent application number 10/958972 was filed with the patent office on 2005-03-17 for method and system for creating task-dependent three-dimensional images.
Invention is credited to Webber, Richard L..
Application Number | 20050059886 10/958972 |
Document ID | / |
Family ID | 26790258 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050059886 |
Kind Code |
A1 |
Webber, Richard L. |
March 17, 2005 |
Method and system for creating task-dependent three-dimensional
images
Abstract
A system and method for producing three-dimensional
representations of an object. A first series of projected images of
the selected object is recorded in a first projection plane and a
second series of projected images of the selected object is
recorded in a second projection plane. The first and the second
series of projected images are rendered at a common magnification.
The first set of projected images is then integrated into a first
three-dimensional volume and the second set of projected images is
integrated into a second three-dimensional volume. The
three-dimensional representation of the object is then produced by
combining one projected image from the first set of projected
images with one projected image from the second set of projected
images. Alternatively, the three-dimensional representation is
produced by merging the first three-dimensional volume with the
second three-dimensional volume.
Inventors: |
Webber, Richard L.;
(Winston-Salem, NC) |
Correspondence
Address: |
DANN, DORFMAN, HERRELL & SKILLMAN
1601 MARKET STREET
SUITE 2400
PHILADELPHIA
PA
19103-2307
US
|
Family ID: |
26790258 |
Appl. No.: |
10/958972 |
Filed: |
October 5, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10958972 |
Oct 5, 2004 |
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10414439 |
Apr 14, 2003 |
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6801597 |
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10414439 |
Apr 14, 2003 |
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09561376 |
Apr 28, 2000 |
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6549607 |
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09561376 |
Apr 28, 2000 |
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09252632 |
Feb 19, 1999 |
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6081577 |
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60095463 |
Jul 24, 1998 |
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Current U.S.
Class: |
600/426 |
Current CPC
Class: |
G01N 23/046 20130101;
G01N 2223/419 20130101; A61B 6/548 20130101; A61B 6/5258 20130101;
Y10S 378/901 20130101 |
Class at
Publication: |
600/426 |
International
Class: |
A61B 005/05; A61B
006/00 |
Claims
1. A system for synthesizing an image of an object at a selected
slice position through the selected object comprising: a. an
identifiable fiducial reference located at a fixed position
relative to the selected object, the fiducial reference providing
constraint for a number of degrees of freedom correlated to a
number of degrees of freedom of the system; b. a source of
radiation for irradiating the selected object and the fiducial
reference; c. a recording medium for recording a first set and a
second set of projected images of the fiducial reference and a
region of interest of the selected object; and d. an image
synthesizer for determining overlapping regions of the projected
images by identifying fiducial reference points common to the
overlapping regions and for bringing the first and second sets of
projected images into alignment based on the identified reference
points to reconstruct a tomographic slice from the object images;
whereby the fiducial reference, the source of radiation, and the
recording medium are arbitrarily positioned relative to one
another.
2. The system according to claim 1, wherein the recording medium
includes a CCD device.
3. The system according to claim 1, wherein the fiducial reference
comprises at least five identifiable reference markers in a fixed
geometry relative to each other.
4. The system according to claim 3, wherein at least four of the
reference markers are co-planar.
5. The system according to claim 4, wherein a maximum of any two of
the four co-planar reference markers are co-linear.
6. The system according to claim 5, wherein a fifth reference
marker is not co-planar with the four co-planar reference
markers.
7. The system according to claim 1, wherein the source of radiation
comprises an electron source for irradiating the selected object
and the fiducial reference with electrons.
8. The system according to claim 1, wherein the source of radiation
comprises a visible light source for irradiating the selected
object and the fiducial reference, and the recording medium
comprises a photographic recording medium for recording
photographic images of the selected object and fiducial
reference.
9. The system according to claim 8, wherein the source of radiation
and the recording medium comprise a video camera.
10. A system according to claim 1, wherein the fiducial reference
comprises a rectangular parallelepiped having at least six
reference markers, each face of the parallelepiped comprising at
least one reference marker, the parallelepiped comprising at least
two bars disposed at intersecting edges of the parallelepiped such
that the bars provide at least one additional reference marker
disposed at the intersection of the bars.
11. A system for synthesizing an image of an object according to
claim 1, wherein the fiducial reference comprises at least two
identifiable reference markers in a fixed geometry relative to each
other.
12. A system for synthesizing an image of an object according to
claim 11, wherein the the markers differ in opacity from one
another.
13. A system for synthesizing an image of an object according to
claim 1, wherein the image synthesizer is adapted to extrapolate
registration and calibration of the sets of projected images.
14. A method for synthesizing an image slice through a selected
object at a selected slice position through the object from a
plurality of projected images of the object comprising the steps
of: a. providing a fiducial reference to provide a total number of
degrees of freedom associated therewith greater or equal to the
total number of degrees of freedom of the system; b. recording
projected images of a region of interest of the selected object and
the fiducial reference on a recording means at different arbitrary
relative positions between (1) a source of radiation, (2) the
selected object and fiducial reference, and (3) the recording
means; and c. synthesizing an image slice of the selected object at
a selected slice position by determining overlapping regions of the
projected images by identifying fiducial reference points common to
the overlapping regions and bringing the first and second sets of
projected images into alignment based on the identified reference
points.
15. A method for synthesizing an image slice according to claim 14,
comprising the step of extrapolating registration and calibration
of the sets of projected images.
16. A method according to claim 14, wherein the region of interest
comprises a subvolume in which the magnification of the projected
images is substantially constant.
17. A method according to claim 14, wherein the fiducial reference
comprises identifiable reference markers.
18. A method according to claim 17, comprising the steps of
associating each reference marker projected image with the
corresponding reference marker and measuring the position and size
of each reference marker image.
19. A method according to claim 18, comprising the step of
determining the magnification of the projected image of the
reference marker.
20. A method according to claim 19, wherein the image of the
reference marker comprises a minor diameter and the step of
determining the magnification uses the minor diameter to determine
the magnification.
21. A method according to claim 19, comprising the step of
generating a projected transformation matrix.
22. A method according to claim 21, wherein the step of generating
the projected transformation matrix comprises mapping the position
of the reference marker image in the projected image onto a
corresponding position of the reference marker in a virtual
projection plane.
23. A method according to claim 22, comprising the steps of
synthesizing a plurality of image slices and generating a
three-dimensional representation of the selected object from the
plurality of image slices.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of co-pending
application Ser. No. 09/252,632, entitled "Method And System For
Creating Task-Dependent Three-Dimensional Images," filed on Feb.
19, 1999, such application being incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and system for
creating three-dimensional displays or images from a multiplicity
of two-dimensional projected images and, more specifically, to a
method and system for producing task-dependent radiographic images
of an object of interest which are substantially free of blurring
artifacts.
BACKGROUND OF THE INVENTION
[0003] A variety of three-dimensional imaging modalities has been
developed for medical applications, as well as for use in
non-destructive testing of manufactured parts. In particular, a
wide range of tomosynthetic imaging techniques has previously been
demonstrated to be useful in examining three-dimensional objects by
means of radiation. These imaging techniques differ in the size and
configuration of the effective imaging aperture. At one extreme,
the imaging aperture approaches zero (i.e., a pinhole) and the
resulting display is characterized by images produced from a single
transmission radiograph. This yields an infinitely wide depth of
field and therefore no depth information can be extracted from the
image. At the other extreme, the aperture approaches a surrounding
ring delimiting an infinite numerical aperture resulting in
projection angles orthogonal to the long axis of the irradiated
object. This yields an infinitely narrow depth of field and hence
no information about adjacent slices through the object can be
ascertained. It therefore follows that a "middle ground" approach,
which provides the ability to adapt a sampling aperture to a
particular task, would be highly advantageous.
[0004] The key to achieving the full potential of diagnostic
flexibility lies in the fact that perceptually meaningful
three-dimensional reconstructions can be produced from optical
systems having any number of different aperture functions. That
fact can be exploited since any aperture can be approximated by
summation of a finite number of appropriately distributed point
apertures. The key is to map all incrementally obtained projective
data into a single three dimensional matrix. To accomplish this
goal, one needs to ascertain all positional degrees of freedom
existing between the object of interest, the source of radiation,
and the detector.
[0005] In the past, the relative positions of the object, the
source, and the detector have been determined by fixing the
position of the object relative to the detector while the source of
radiation is moved along a predetermined path, i.e. a path of known
or fixed geometry. Projective images of the object are then
recorded at known positions of the source of radiation. In this
way, the relative positions of the source of radiation, the object
of interest, and the detector can be determined for each recorded
image.
[0006] A method and system which enables the source of radiation to
be decoupled from the object of interest and the detector has been
described in U.S. Pat. No. 5,359,637, that issued on Oct. 25, 1994,
which is incorporated herein by reference. This is accomplished by
fixing the position of the object of interest relative to the
detector and providing a fiducial reference which is in a fixed
position relative to the coupled detector and object. The position
of the image of the fiducial reference in the recorded image then
can be used to determine the position of the source of radiation.
In addition, a technique for solving the most general application
wherein the radiation source, the object of interest, and the
detector are independently positioned for each projection has been
described by us in co-pending U.S. patent application Ser. No.
09/034,922, filed on Mar. 5, 1998, which is also incorporated
herein by reference.
[0007] Once the relative positions of the radiation source, the
object, and the detector are determined, each incrementally
obtained projective image is mapped into a single three-dimensional
matrix. The mapping is performed by laterally shifting and summing
the projective images to yield tomographic images at a selected
slice position through the object of interest. A three-dimensional
representation of the object can be obtained by repeating the
mapping process for a series of slice positions through the object.
However, the quality and independence of the tomographic images is
compromised by blurring artifacts produced from unregistered
details located outside the plane of reconstruction.
[0008] In addition, quantitative information has traditionally been
difficult to determine from conventional tomography. Although many
questions of medical interest are concerned with temporal changes
of a structure (e.g., changes in the size and shape of a tumor over
time), the ability to compare diagnostic measurements made over
time is complicated by the fact that factors other than the
parameter of diagnostic interest often contribute to the measured
differences. For example, spatial variations produced from
arbitrary changes in the observational vantage point(s) of the
radiation source create differences between the measurements which
are unrelated to temporal changes of the object being investigated.
In addition, conventional X-ray sources produce radiation that
varies with changes in tube potential, beam filtration, beam
orientation, tube current, distance form the focal spot, and
exposure time. The fluctuations in the output of radiation sources
is therefore another factor that limits the ability to derive
quantitative information from conventional tomography.
[0009] In light of the foregoing, it would be highly beneficial to
provide a method for producing a three-dimensional representation
of an object that is substantially free of blurring artifacts from
unregistered details. In addition, the method should enable
quantitative information related to temporal changes associated
with the object to be measured.
SUMMARY OF THE INVENTION
[0010] The present invention relates to a system and a method for
synthesizing an image slice through a selected object from a
plurality of projected radiographic images of the selected object.
The system comprises a radiation source for irradiating the object.
The preferred radiation source depends upon the particular
application. For example, the present invention may be practiced
using x-rays, electron microscopy, ultrasound, visible light,
infrared light, ultraviolet light, microwaves, or virtual radiation
simulated by manipulation of magnetic fields (magnetic resonance
imaging (MRI)). In one embodiment of the present invention, the
position of the radiation source within a plane parallel to an
image plane is determined from projected images of two object
points associated with a fiducial reference which is maintained in
fixed position relative to the selected object. Once the projected
images are compensated for differences in magnification, the
relative position of the radiation source within the plane parallel
to the image plane is determined from an estimate of the actual
distance between the two object points obtained from a sinusoidal
fit of the distances between the projected images of the object
points.
[0011] A recording medium or radiation detector is used to record a
series of projected images of the selected object. The recording
medium may be in the form of a photographic plate or a
radiation-sensitive, solid-state image detector such as a
charge-coupled device (CCD), or any other system capable of
producing two-dimensional projections or images suitable for
digitization or other analysis.
[0012] An image synthesizer is provided for transforming the series
of projected images of the selected object into an image slice. The
image slice consists of an array of pixels with each pixel having
an associated attenuation value and corresponds to a
cross-sectional slice through the selected object at a selected
slice position. A three-dimensional representation of the object
can be obtained by repeating the transformation at a series of
slice positions through the object.
[0013] In addition, an optional source comparator is provided for
adjusting the radiation source to enable meaningful quantitative
comparisons between projected images recorded either at different
times and/or using different radiation sources. The source
comparator is positionable between the radiation source and the
radiographic medium for producing a gradient image indicative of
characteristics associated with the output from the radiation
source. In operation, the source comparator is used to record a
first gradient image using a first radiation source at the same
time that a first projected image or series of projected images is
recorded. When a second projected image or series of projected
images are to be recorded, the source comparator is used to record
a second gradient image. The second gradient image is compared to
the first gradient and differences between the two gradient images
are noted. The beam energy, filtration, and beam exposure
associated with the radiation source used to record the second
gradient image are then adjusted to minimize the differences
between the first gradient image and the second gradient image.
[0014] In one embodiment, the source comparator comprises two
wedges or five-sided polyhedrons of equal dimension having a
rectangular base and two right-triangular faces. The triangular
faces lie in parallel planes at opposite edges of the base such
that the triangular faces are oriented as mirror images of each
other. As a result, each wedge has a tapered edge and provides a
uniformly increasing thickness from the tapered edge in a direction
parallel to the plane of the base and perpendicular to the tapered
edge. The wedges are arranged with the base of one wedge adjacent
to the base of the other wedge such that the tapered edges of the
two wedges are at adjacent edges of the base. One wedge is formed
from a uniform high attenuation material while the other wedge is
formed from a uniform low attenuation material. Accordingly, when
the source comparator is irradiated from a radiation source
directed perpendicularly to the bases of the wedges, the resulting
image will be a quadrilateral having an intensity gradient that is
maximized in a particular direction.
[0015] In operation, the system of the present invention is used to
produce an image slice through the selected object that is
substantially free of blurring artifacts from unregistered details
located outside a plane of reconstruction. The radiation source and
recording medium are used to record a series of two-dimensional
projected images of the selected object. The series of
two-dimensional projected images are then shifted by an amount and
in a direction required to superimpose the object images of the
two-dimensional images. The shifted two-dimensional images can then
be combined in a non-linear manner to generate a tomosynthetic
slice through the selected object. In one embodiment, the
two-dimensional images are combined by selecting details from a
single projection demonstrating the most relative attenuation at
each pixel. Alternatively, a different non-linear operator could be
used wherein the two-dimensional images are combined by selecting
details from a single projection demonstrating the least relative
attenuation at each pixel in the reconstructed image. Optionally, a
series of reconstructed images at varying slice positions through
the selected object are determined to create a three-dimensional
representation of the selected object.
[0016] Alternatively, the system of the present invention is used
to synthesize a three-dimensional reconstruction of the object from
as few as two projected images of the object. A first projected
image of the object is recorded in a first projection plane and a
second projected image is recorded in a second projection plane.
Each of the first and the second projected images are then rendered
at a common magnification. Using a known angle between the first
and the second projection planes, the first and the second
projected images are transformed to occupy the same volume. The
transformed first and second projected images are then combined
into a three-dimensional representation of the selected object.
Additional projected images are optionally combined with the
three-dimensional representation to refine the three-dimensional
representation.
[0017] In yet another embodiment, the system of the present
invention is used to synthesize a three-dimensional representation
of the selected object from two or more sets of projected images of
the selected object. The first and second sets of projected images
are tomosynthetically transformed into a series of contiguous
slices forming a first and a second three-dimensional volume,
respectively, using previously disclosed methods (e.g., U.S. Pat.
No. 5,668,844) or those in the public domain (e.g., tomosynthesis).
The first and second three-dimensional volumes are then rendered at
a common magnification. The second three-dimensional volume is then
rotated by an angle corresponding to the angular disparity between
the first and the second three-dimensional volumes. The rotated
second three-dimensional volume is then merged with the first
three-dimensional volume to produce a three-dimensional
representation of the selected object.
[0018] Alternatively, the system of the present invention can be
used to determine temporal changes in the selected object. The
radiation source and recording medium are used to record a first
series of two-dimensional projected images of the selected object.
At some later time, the radiation source and recording medium are
used to record a second series of two-dimensional projected images
of the selected object. Both series are tomosynthetically converted
into a series of slices via previously disclosed methods
(TACT.RTM.) or those in the public domain (tomosynthesis). Each
slice of the first series is then correlated with a corresponding
slice of the second series to form pairs of correlated slices. Each
pair of slices is then aligned to maximize the overlap between
homologous structures. Each pair of correlated slices is then
subtracted to produce a difference image. Each difference image is
then displayed individually. Alternatively, all of the difference
images can be overlapped to yield a complete difference image
corresponding to the volumetric difference associated with the
entire tomosynthetically reconstructed volume.
[0019] When a three-dimensional representation of the selected
object is produced, the three-dimensional representation can be
viewed holographically using a display in accordance with the
present invention. The display comprises stereoscopic spectacles
which are worn by an observer and a target operatively associated
with the spectacles. Accordingly, as the observer changes his or
her vantage point, movement of the spectacles translates into a
corresponding movement of the target. A detector is operatively
associated with the target for tracking movement of the target. The
detector is connected to a monitor such that the monitor receives a
signal from the detector indicative of movement of the target. In
response to the signal from the detector, the monitor displays an
image pair of the three-dimensional representation which, when
viewed through the spectacles produces a stereoscopic effect. The
image pair which is displayed is changed to compensate for changes
in the vantage point of the observer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The foregoing summary, as well as the following detailed
description of the preferred embodiments of the present invention,
will be better understood when read in conjunction with the
accompanying drawings, in which:
[0021] FIG. 1 is a schematic representation of a system for
creating; three-dimensional radiographic displays using computed
tomography in accordance with the present invention;
[0022] FIG. 2 is a flow chart showing the steps involved in
creating three-dimensional radiographic displays using computed
tomography in accordance with the present invention;
[0023] FIG. 3 is a flow chart showing details of a method of
projectively warping or transforming a projected image from an
actual plane of projection onto a virtual projection plane;
[0024] FIG. 4 is a schematic representation of a system having nine
degrees of freedom in which a source is shifted and displaced
relative to an original projection plane and in which a projection
plane of a recording medium is shifted, rotated, displaced, and
tilted relative to the original projection plane;
[0025] FIG. 5 is a schematic representation showing an arrangement
of reference markers in accordance with the an embodiment of the
present invention, wherein five spherical reference markers are
positioned at five of the eight vertices of a cube;
[0026] FIG. 6 is a schematic representation of a system having
seven degrees of freedom in which an infinite point source is
shifted relative to an original projection plane and in which a
projection plane of a recording medium is shifted, displaced, and
tilted relative to the original projection plane;
[0027] FIG. 7 is a schematic representation of a system having four
degrees of freedom in which an infinite point source is shifted
relative to an original projection plane and in which a projection
plane of a recording medium is shifted relative to the original
projection plane;
[0028] FIG. 8 is an exploded, schematic representation of a
charge-coupled device (CCD) for use as a recording medium with
intrinsic components enabling automated determination of projective
geometry;
[0029] FIG. 9 is a schematic representation of an embodiment of the
present invention wherein the recording medium is smaller than the
projected image of the object;
[0030] FIG. 10 is a schematic representation of an embodiment of
the present invention wherein the source is a hand-held X-ray
source with a laser a ming device;
[0031] FIG. 11 is a schematic representation of an embodiment of
the present invention wherein the reference markers of the fiducial
reference are positioned at the vertices of a square pyramid;
[0032] FIG. 12 is a schematic representation of an embodiment of
the present invention wherein the source is a hand-held X-ray
source which is constrained relative to the recording medium by a
C-arm;
[0033] FIG. 13 is an enlarged schematic representation of the
object of interest and the recording medium depicted in FIG.
14;
[0034] FIG. 14 is a schematic representation of an embodiment of
the present invention wherein the reference markers of the fiducial
reference are positioned at the centers of the faces of a
parallelpiped;
[0035] FIG. 15 is a schematic representation of an embodiment of
the present invention wherein the corners of a frame define four
reference markers;
[0036] FIG. 16 is a schematic representation of a reference image
cast by a spherical reference marker showing the resulting
brightness profile;
[0037] FIG. 17 is a schematic representation of the parameters
associated with a system comprising three spherical, non-collinear
reference markers wherein the orthogonal distance between the
radiation source and the recording medium is fixed at a distance
short enough so that the images cast by the reference markers are
magnified relative to the size of the actual reference markers;
[0038] FIG. 18 is a schematic representation of the relevant
parameters associated with a reference image associated with a
spherical reference marker;
[0039] FIG. 19 is a schematic representation of an embodiment of
the present invention wherein the fiducial reference comprises a
radiopaque shield with a ring-like aperture;
[0040] FIG. 20 is a schematic, perspective view of an embodiment of
the present invention, wherein the detector comprises a
charge-coupled device (CCD) and the fiducial reference comprises a
frame, shown with the front and a section of the top removed;
[0041] FIG. 21 is a sectional view of the embodiment depicted in
FIG. 20 taken along the 23-23 line;
[0042] FIG. 22 is an alternate embodiment of a laser aiming device
in accordance with the present invention;
[0043] FIG. 23 is a flow chart showing the steps involved in a
method for task-dependent tomosynthetic image reconstruction in
accordance with the present invention;
[0044] FIGS. 24A and B are schematic representations of a linear
tomosynthetic reconstruction and a non-linear tomosynthetic
reconstruction in accordance with the present invention;
[0045] FIG. 25 is a flow chart showing the steps involved in a
method for determining temporal changes in accordance with the
present invention;
[0046] FIG. 26 is a schematic representation of a source comparator
used for matching X-ray sources;
[0047] FIG. 27 is a flow chart showing the steps of a method for
using the source comparator of FIG. 26;
[0048] FIG. 28 is a schematic representation of a
pseudo-holographic image display;
[0049] FIG. 29 is a tomosynthetic slice through a human breast
reconstructed using a linear summation of projected images;
[0050] FIG. 30 is a tomosynthetic slice through the human breast
reconstructed using a linear summation of projected images
augmented by a deconvolution filter; and
[0051] FIG. 31 is a tomosynthetic slice through the human breast
reconstructed using a non-linear reconstruction scheme;
[0052] FIG. 32 is a flow chart showing the steps of a method for
creating nearly isotropic three-dimensional images from a single
pair of arbitrary two-dimensional images;
[0053] FIG. 33 is a flow chart showing the steps of a method for
creating a three-dimensional image from two series of
two-dimensional images;
[0054] FIG. 34 is a flow chart showing the steps of a method for
producing a three-dimensional representation of a stationary object
from multiple plane projections recorded by an arbitrarily
positionable camera;
[0055] FIG. 35 is a schematic representation of a three-dimensional
scaling calibration for determining the relative position of a
camera in two planes orthogonal to the projection plane of the
camera;
[0056] FIG. 36 is a schematic representation of a remote-controlled
mobile radiation source;
[0057] FIG. 37 is a schematic representation of an embodiment of
the present invention wherein a camera is used to record
overlapping sets of projected images;
[0058] FIG. 38 is a schematic representation of an embodiment of
the present invention wherein laser light sources provide fiducial
reference points; and
[0059] FIG. 39 is a schematic representation of a three-dimensional
image of an object produced from a single pair of arbitrary
two-dimensional images.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] The present invention generally relates to a system 20, as
depicted schematically in FIG. 1, for synthesizing an image of an
object 21 at a selected slice position 35 through the object 21
from a plurality of radiographic projected images 38 of the
selected object 21. A fiducial reference 22 is held in a fixed
position relative to the selected object 21, for example, by
directly attaching the fiducial reference 22 to the object 21. The
fiducial reference comprises two finite sized, identifiable
reference markers, 23 and 123, which are maintained coupled
together in a fixed geometry relative to each other by a
radiolucent bar 24. However, the fiducial reference 22 may comprise
various numbers and arrangements of reference markers 23.
Alternatively, as shown in FIG. 38, the reference markers, 23 and
123, are provided by the reflection of laser light from the surface
of the selected object. A radiation source 27 is provided to
irradiate the object 21 along with the fiducial reference 22.
Irradiation of the object 21 casts a projected image 38 onto a
recording medium 31. The projected image 38 comprises an object
image 40 of the object 21 and reference images, 39 and 139, of the
reference markers, 23 and 123, respectively.
[0061] In general, the pattern of source 27 positions does not need
to be in any fixed geometry or position. Indeed, the position of
the source 27 may be totally arbitrary in translation and
displacement relative to the object 21. Likewise, the recording
medium 31 may also be arbitrarily movable relative to the object 21
by translation, displacement, tilting, or rotation. The only
requirement is that for every degree of freedom in the system
resulting from movement of the source 27 or the recording medium 31
relative to the object 21, the fiducial reference 22 must include
sufficient measurable or defined characteristics, such as size,
shape, or numbers of reference markers 23, to account for each
degree of freedom.
[0062] The minimum number of reference markers required to
completely determine the system depends on the constraints, if any,
imposed on the relative positions of (1) the radiation source, (2)
the object and fiducial reference, and (3) the recording medium.
The system may have a total of nine possible relative motions (2
translations and 1 displacement for the radiation source relative
to a desired projection plane and 2 translations, 1 displacement, 2
tilts, and 1 rotation for the recording medium relative to the
desired projection plane). Each of these possible relative motions
must be capable of analysis either by constraining the system and
directly measuring the quantity, by providing a sufficient number
of reference markers to enable the quantity to be determined, or by
estimating the value of the quantity. Each unconstrained relative
motion represents a degree of freedom for the system. For a system
to be completely determined, the total number of degrees of freedom
in the system must be less than or equal to the total number of
degrees of freedom associated with the fiducial reference.
[0063] More than the minimum number of reference markers can be
used. In such cases, the system is overdetermined and least squares
fitting can be used to improve the accuracy of the resulting image
slices. If, however, less than the minimum number of reference
markers is used, then the system is underdetermined and the unknown
degrees of freedom must either be estimated or measured
directly.
[0064] Although the reference markers can be essentially any size
and shape, spherical reference markers of known diameter may be
used. When using spherical reference markers of a finite size, a
single reference marker can account for up to five degrees of
freedom. When a spherical reference marker is projected obliquely
onto the recording medium, the reference image cast by the
spherical reference marker is elliptical and is independent of any
rotation of the reference marker. Determining the position of the
reference image in the projection plane (X- and Y-coordinates) and
the magnitudes of the major and minor diameters of the elliptical
image accounts for four degrees of freedom. Further, when the
distance between the radiation source and the reference marker is
sufficiently short, the reference image will be magnified relative
to the actual size of the reference marker, thereby accounting for
an additional degree of freedom. In contrast, only two degrees of
freedom (the X- and Y-coordinates) are typically associated with
the reference image of a point-size reference marker.
[0065] The most complex, yet most generally applicable, arrangement
is depicted in FIG. 4, wherein the radiation source 27 and the
recording medium 31 are completely unconstrained and uncoupled from
the selected object 21. In this arrangement, there are nine degrees
of freedom: 2 translational (.DELTA.X and .DELTA.Y) and 1
displacement (.DELTA.Z) degrees of freedom for the radiation source
27 relative to an original or desired projection plane 37 and 2
translational (.DELTA.X' and .DELTA.Y'), 1 displacement
(.DELTA.Z'), 2 tilting (.DELTA..gamma. and .DELTA..PHI.), and 1
rotational (.DELTA..PSI.) degree of freedom for the recording
medium 31 relative to the original or desired projection plane.
Accordingly, a fiducial reference system sufficient to solve a
projection system having nine degrees of freedom is needed to
completely determine the system.
[0066] One embodiment of the present invention that permits this
general arrangement to be realized conveniently involves
two-dimensional projected images from a system comprised of a
fiducial reference having five point-size or finite reference
markers. This approach conveniently facilitates three-dimensional
reconstructions when exactly four reference markers are coplanar
and no three or more reference markers are collinear. Under these
conditions, only the projection from the non-coplanar marker need
be distinguished from the other four because the projections from
the latter always bear a fixed sequential angular arrangement
relative to each other which simplifies identification of
homologous points in all projections. For example, the reference
markers can be placed at five contiguous vertices of a cube as
shown in FIG. 5. Fiducial reference 122 comprises five reference
markers, 23, 123, 223, 323, 423, positioned contiguously at five
vertices of a cube. The object 121 is preferably positioned within
the cube. The four co-planar reference markers, 23, 123, 223, and
323, then can be used for projectively warping or transforming the
projected images onto a desired projection plane while the
remaining reference marker 423 serves as the alignment marker
required to determine the normalized projection angle as described
in U.S. Pat. No. 5,359,637.
[0067] The most general reconstruction task requiring information
sufficient to determine all nine possible degrees of freedom
requires computation of separate projective transformations for
each projected image in each and every slice. However, by limiting
the region of interest to a subvolume constrained such that the
magnification across and between its slices may be considered
constant, it is possible to generate veridical three-dimensional
images within the volume much more efficiently. The increase in
efficiency under these conditions results from the fact that all
projections within this region can be mapped by a single fixed
transformation, and that associated slice generation can be
accomplished by simple tomosynthetic averaging of laterally shifted
projections as described in U.S. Pat. No. 5,359,637.
[0068] Another useful arrangement of the fiducial reference
comprising five reference markers is shown in FIG. 11, wherein a
fiducial reference 222 employing a pyramidal distribution of
reference markers 323 is used. The fiducial reference 222 comprises
five reference markers 23, 123, 223, 323, and 423, which are held
in a fixed relationship relative to each other and to the object
221. As was the case in FIG. 5, four of the reference markers, 23,
123, 223, and 323, lie in a plane that can be used to establish the
desired projection plane. Here, they define the four corners of the
base of a pyramid. The fifth reference marker 423 is positioned to
define the apex of the pyramid and serves as the means for
determining the projection angles relative to the desired
projection plane as described in U.S. Pat. No. 5,359,637. In use,
the fiducial reference 222 may be attached or fixed relative to the
object 221 such that the base of the pyramid is proximate to the
recording medium and the apex of the pyramid is proximate to the
source.
[0069] In FIG. 15, a fiducial reference 322 having an alternative
arrangement of reference markers in a pyramidal distribution is
shown. In this arrangement, the fiducial reference 322 comprises a
radiopaque frame 25 having a radiolucent central window. The four
inside corners of the radiopaque frame 25 define four reference
markers, 23, 123, 223, and 323, at the base of the pyramid. The
fifth reference marker 423 is positioned at the apex of the
pyramid. Preferably, the object 321 is positioned between the frame
25 and the reference marker 423.
[0070] In FIG. 14, a fiducial reference 422 which is also useful
for solving a system with nine degrees of freedom is shown.
Fiducial reference 422 comprises a rectangular parallelpiped 33
with radiopaque reference markers, 23, 123, 223, 323, 423, and 523,
centered on each of the six faces of the parallelpiped 33. The
reference markers, 23, 123, 223, 323, 423, and 523, are marked with
distinguishable indicia, such as X, Y, Z, {circle over (X)},
{circle over (Y)}, and {circle over (Z)} so that the reference
images cast by the markers, 23, 123, 223, 323, 423, and 523, can be
identified easily and distinguished from one another. Alternatively
or additionally, two or more of the edges of the parallelpiped 33
may be defined by radiopaque bars 26 such that the intersections of
the bars 26 provide additional reference markers, such as reference
marker 623 located at the intersection of the three bars labeled 26
in FIG. 14.
[0071] Alternatively, the six degrees of freedom for the radiation
source 27 relative to the desired projection plane 37 (two
translational, one displacement, two rotational, and one tilting
degree of freedom) can be determined independently from the use of
the fiducial reference when the orientation of the detector is
fixed or known relative to either the object of interest or the
radiation source. For example, the position of the radiation source
27 can be determined from multiple plane projections recorded from
an arbitrarily positioned camera provided that the lens aperture is
adjusted such that the entire object always appears in focus. The
three relative angles associated with each projection are
determined by attaching three orthogonally oriented angle sensing
devices, such as gyroscopes, to the camera. The displacement of the
radiation source relative to the object is determined using a range
finder associated with the camera. Since the position of the camera
within a plane parallel to the camera's projection plane is used
only to determine the three-dimensional geometric relationships
underlying the disparity observed between object images, the
remaining degrees of freedom need only be measured relative to one
another and, therefore, can be fixed from a geometric analysis of
paired point projections. Referring to FIG. 35, it can be seen
that, when the arbitrary camera positions are compensated for
displacement and projection orthogonality, the projected distances,
D1, D2, and D3, between the paired points P1 and P2 are a
sinusoidal function of the corrected projection angle. Hence, the
actual distance between P1 and P2 can be estimated from a
non-linear curve fit to the observed projection distances.
[0072] A method for determining the position of the radiation
source relative to the object using an arbitrarily positionable
camera in accordance with the present invention is depicted in FIG.
34. At step 1000, angle sensors attached to the camera are
initialized in order to eliminate possible drift in accuracy. The
object is then roughly centered within the viewfinder of the camera
and an object image, the nominal displacement of the camera from
the object, and the angle data are recorded at step 1002. An
intrinsic range finder associated with the camera is used to
determine the nominal distance from the camera to the object of
interest and the angle sensors are used to determine the angle
data.
[0073] At step 1004, it is determined whether additional object
images are desired. If additional object images are desired, the
camera is repositioned at step 1005 and the process returns to step
1002. It should be appreciated that a minimum of three object
images is required to produce a meaningful sinusoidal regression,
as discussed in detail below. If no additional object images are to
be recorded, the recorded object images and data is optionally
stored in a computer readable format and the process proceeds to
step 1007.
[0074] Each of the object images is then individually scaled to
render all of the object images at the same magnification at step
1009. The scaling is possible using the range recorded for each
object image because the linear magnification is inversely
proportional to the range. By scaling the object images, an
effective displacement between the camera and the object can be
defined.
[0075] At step 1011, a first object point, visible on all of the
projected object images, is selected. A representative object image
is then selected at step 1013. The representative object image
should be the object image which best approximates the orientation
to which desired reconstructed tomosynthetic slices are to be
parallel.
[0076] Each object image is then rotated and translated, at step
1015, so that all of the object images are brought into
tomosynthetic registration. Specifically, each object image is
rotated by an amount sufficient to adjust the rotational
orientation of the camera about an axis perpendicular to the
projection plane to match that of the representative object image.
Rotational adjustment of the object images assures that the
registrations which follow will not exclude a second reference
point, whose selection is discussed below. Each rotated object
image is then translated both vertically and horizontally by an
amount which causes superposition of the projected image of the
first object point within each object image with the projected
image of the first object point within the representative object
image.
[0077] At step 1017, a second object point visible on all of the
scaled, rotated, and translated object images is selected. The
distance between the projected images of the second object point
and the first object point is measured, at step 1019, for each of
the object images. If the relative change in distance does not
exceed a task-dependent threshold value and produce a
well-distributed range of values, the accuracy of the subsequent
non-linear regression may be compromised. Accordingly, at step
1021, it is determined whether the measured distances exceed the
task-dependent threshold. If the threshold is not exceeded, a new
second object point is selected at step 1017. If the threshold is
exceeded, the process proceeds to step 1023.
[0078] At step 1023, the actual distance between the first object
point and the second object point is estimated from the measured
distance separating the projected images of the first and second
object points in the recorded object images. The estimate of the
actual distance is determined using the effective displacement of
the camera from the object and a sinusoidal curve fitting
procedure, as well as the projection angle defined by a line
connecting the first and second object points and the plane of the
representative object image.
[0079] Using affine projection geometry, the recorded angle data,
and the recorded displacement data, each object image is remapped
onto the plane defined by the representative object image selected
above at step 1025. The remapping is performed using the first
object point as the common point of superposition. At step 1027,
the object images are then tomosynthetically reconstructed using
the second object point as a disparity marker. The distances
between object images is then calibrated, at step 1029, using the
estimate for the distance between the first and second object
points and trigonometrically correcting the object images for
foreshortening caused by variations in the projection angle.
[0080] Referring to FIG. 36, one arrangement for unconstraining and
uncoupling the radiation source from the selected object is
depicted. As shown in the figure, a radiation source 1050 is
mounted on a mobile carriage 1052. The carriage 1052 is controlled
remotely using a transmitter 1054 which transmits a signal to the
carriage 1052 through an antenna 1056. In operation, the
transmitter 1054 is operated to maneuver the carriage 1052, and
thereby the radiation source, 1050, to move around a selected
object 1058 to enable projected images of the object 1058 to be
recorded on a detector 1060 at a variety of relative positions of
the radiation source 1050, the object 1058 and fiducial reference
1062, and the detector 1060. In order to provide essentially
complete freedom in positioning the radiation source 1050 relative
to the object 1058 and fiducial reference 1062, the elevation and
angle of tilt of the radiation source 1050 relative to the object
1058 and fiducial reference 1062 is also controllable through the
transmitter 1054.
[0081] Reducing the uncertainty of the projection geometry through
the constraint of one or more degrees of freedom reduces the
complexity of the resulting reconstruction. An arrangement of the
system of the present invention which is somewhat constrained is
depicted in FIGS. 12 and 13, wherein a hand-held X-ray source is
provided such that the orthogonal distance between the radiation
source 127 and the recording medium 131 is fixed by a C-arm 129 at
a distance short enough so that the image cast by the fiducial
reference 122 is magnified relative to the size of the actual
fiducial reference 122. Preferably, the C-arm 129 is connected to
the recording medium 131 by a concentric swivel collar 149 to allow
the C-arm 129 to be rotated relative to the recording medium 131. A
disposable and crushable radiolucent foam cushion 130 may be
attached to the surface of the recording medium 131 to permit
comfortable customized stable adaptation of the detector 131 to the
object 121. The other end of the C-arm 129 is attached to a potted
X-ray source 145 so that radiation emanating from the potted X-ray
source 145 impinges upon the recording medium 131. A trigger 146 is
provided for operating the source 127. The source 127 optionally
comprises a circular beam collimator 147 for collimating radiation
emanating from the source 127. The collimator 147 may provide a
relatively long focal-object distance to provide nearly affine
projection geometries. Preferably, a handle 148 is also provided to
enable the operator to more easily maneuver the source 127. The
hand-held X-ray source 127 is connected to a computer/high voltage
source 128 for controlling operation of the device. In addition, a
disposable plastic bag 132 can be positioned around the detector
131 for microbial isolation. The source 127 can optionally comprise
a rotatable transparent radiopaque plastic cylinder 119 and a
transparent radiopaque shield 152 to protect the operator from
scattered radiation. In this arrangement, there are 3 degrees of
freedom (two translational and one displacement for the radiation
source 127). Accordingly, a fiducial reference compensating for at
least three degrees of freedom is necessary to completely describe
or analyze the system. One convenient embodiment for solving the
system depicted in FIGS. 12 and 13 employs a fiducial reference 122
comprising a single radiopaque sphere of finite diameter. Under
those conditions, the length of the minor axis of the resulting
elliptical shadow plus two translational measurements are
sufficient to define the projection geometry completely.
[0082] The computational steps involved in synthesizing a
three-dimensional image using three spherical, non-linear reference
markers in a system wherein the orthogonal distance between the
radiation source and the recording medium is fixed at a distance
short enough so that the images cast by the reference markers are
magnified relative to the size of the actual reference markers
(i.e., a system with eight degrees of freedom as depicted in FIGS.
12 and 13) can be derived with reference to FIGS. 17 and 19. In the
drawings, c is the fixed distance between the source and the
projection plane; PS is the orthogonal projection of the source
onto the projection plane; B, M, and T are the reference markers; r
is the radius of the reference markers; a.sub.p is the distance
from the center of a reference marker to the source; .theta. is the
angle subtended by the center of a reference marker relative to a
line orthogonal to the projection plane through the source; .phi.
is the angle at the apex of an isosceles triangle having a base of
length r and a height of length a.sub.p; B.sub.s, M.sub.s, and
T.sub.s are the reference images associated with the reference
markers; a (or, alternatively, d.sub.p) is the major diameter of
the reference images; b is the minor diameter of the reference
images; x is the length of a section of an arc associated with a
reference image measured from the projection of the center of the
corresponding reference marker onto the projection plane along the
major diameter, b, in a direction toward P.sub.s; y is the length
of an arc associated with a reference image through the projection
of the center of the corresponding reference marker onto the
projection plane and parallel to the minor diameter of the
reference image; and d.sub.s is the major diameter of a reference
image in a virtual projection plane.
[0083] In FIG. 6, another arrangement of the system of the present
invention is depicted wherein the radiation source 27 is located at
a fixed distance from the selected object 21 and sufficiently far
so that magnification is not significant. However, the recording
medium 31 is allowed to be shifted, displaced, and tilted relative
to the selected object 21 and an original or desired projection
plane 37. In this arrangement, there are seven degrees of freedom
(two translational degrees of freedom for the radiation source 27
and 2 translational, 1 displacement, and 2 tilting degrees of
freedom for the recording medium 31). Therefore, a fiducial
reference having at least seven degrees of freedom is needed to
solve the system. Accordingly, a fiducial reference comprising at
least four point-size reference markers can be used to determine
the position of the radiation source relative to the selected
object 21 and the recording medium 31.
[0084] In FIG. 7, yet another arrangement of the system of the
present invention is depicted wherein the distance between the
object 21 and the radiation source 27 is sufficiently large so that
magnification can be ignored and wherein the recording medium 31 is
free to shift laterally relative to the object 21 and the desired
or original projection plane 37. In this arrangement, there are
four degrees of freedom (two translational degrees of freedom for
the radiation source 27 and two translational degrees of freedom
for the recording medium 31). Therefore, a fiducial reference
having at least four degrees of freedom is necessary to completely
determine the system. Accordingly, a fiducial reference comprising
at least two point-size reference markers can be used to determine
the position of the radiation source relative to the selected
object 21 and the recording medium 31. This relatively constrained
system may be useful in three-dimensional reconstructions of
transmission electron micrographs produced from video projections
subtending various degrees of specimen tilt and exhibiting various
amounts of arbitrary and unpredictable lateral shift due to
intrinsic instability associated with the instrument's electron
lenses.
[0085] Referring to FIG. 1, the radiation source 27 may be either a
portable or a stationary X-ray source. However, the radiation
source 27 is not limited to an X-ray source. The specific type of
source 27 which is utilized will depend upon the particular
application. For example, the present invention can also be
practiced using magnetic resonance imaging (MRI), ultrasound,
visible light, infrared light, ultraviolet light, or
microwaves.
[0086] In the embodiment shown in FIG. 10, the source 227 is a
hand-held X-ray source, similar to that described above in
reference to source 127, except that a low power laser aiming
device 250 and an alignment indicator 251 are provided to insure
that the source 227 and the recording medium 231 are properly
aligned. In addition, a radiolucent bite block 218 is provided to
constrain the detector 231 relative to the object 221, thereby
constraining the system to three degrees of freedom (two
translational and one displacement for the radiation source 227
relative to the object 221 and detector 231). Consequently, the
fiducial reference 222 can be fixed directly to the bite block 218.
When the source 227 is properly aligned with the recording medium
231, radiation emanating from the aiming device 250 impinges on the
recording medium 231. In response to a measured amount of radiation
impinging on the recording medium 231, a signal is sent to activate
the alignment indicator 251 which preferably produces a visible
and/or auditory signal. With the alignment indicator 251 activated,
the X-ray source 245 can be operated at full power to record a
projected image. In addition, the source 227 can optionally
comprise a collimator 247 to collimate the radiation from the X-ray
source and/or a transparent scatter shield 252 to protect the
operator from scattered radiation. In lieu of the scatter shield
252, the operator can stand behind a radiopaque safety screen when
exposing the patient to radiation from the source 227. A handle 248
and trigger 246 may be provided to facilitate the handling and
operation of the source 227. The source 227 is connected to a
computer/high voltage source 228 and an amplifier 260 for
controlling operation of the device.
[0087] In one embodiment, the aiming device 250 comprises an X-ray
source operated in an ultra-low exposure mode and the projected
image is obtained using the same X-ray source operated in a
full-exposure mode. Alternatively, a real-time ultra-low dose
fluoroscopic video display can be mounted into the handle 248 of
the source 227 via a microchannel plate (MCP) coupled to a CCD. The
video display switches to a lower gain (high signal-to-noise) frame
grabbing mode when the alignment is considered optimal and the
trigger 246 is squeezed more tightly.
[0088] An alternate embodiment of an aiming device in accordance
with the present invention is shown in FIG. 22. The aiming device
850 comprises a laser source 857 and a radiolucent angled mirror
858 which produces a laser beam, illustrated by dashed line 859,
which is concentric with the radiation emanating from the source
827. The alignment indicator 851 comprises a radiolucent spherical
surface 861 which is rigidly positioned relative to the detector
831 by a C-arm 829 that is plugged into the bite block 818. When
the aiming device 850 is aimed such that the laser beam 859
impinges upon the spherical surface 861, the specular component of
the laser beam 859 is reflected by the spherical surface 861.
Accordingly, proper alignment of the source 827, the object 821,
and the detector 831 is obtained when the reflected portion of the
laser beam 859 is within a small solid angle determined by the
position of the aiming device 850. Direct observation of the
reflected portion of the laser beam 859 by a detector or observer
862 can be used to verify the alignment. As shown in the figure,
the fiducial reference 822 comprises a radiolucent spacer
containing a fiducial pattern that is affixed to the detector 831.
Further, a central ring area 863 can be designated at the center of
the spherical surface 861 such that aiming the laser beam 859 at
the central ring area 863 assures an essentially orthogonal
arrangement of the source 827 and the detector 831. In addition,
replacing the concentric laser source 857 with a laser source that
produces two laser beams that are angled relative to the radiation
emanating from the source 827 permits the distance between the
source 827 and the detector 831 to be set to a desired distance,
provided that the two laser beams are constrained to converge at
the spherical surface 861 when the desired distance has been
established.
[0089] Referring again to FIG. 1, the recording medium 31 is
provided for recording the projected object image 40 of the
selected object 21 and the projected reference images, 39 and 139
of the reference markers 23 and 123. The recording medium 31 may be
in the form of a photographic plate or a radiation-sensitive,
solid-state image detector such as a radiolucent charge-coupled
device (CCD).
[0090] In one particular embodiment depicted in FIG. 8, the
recording medium 331 comprises a CCD having a top screen 200, a
bottom screen 206 positioned below the top screen 200, and a
detector 210 positioned below the bottom screen 206. The top screen
200 is monochromatic so that a projected image projected onto the
top screen 200 causes the top screen 200 to fluoresce or
phosphoresce a single color. In contrast, the bottom screen 206 is
dichromatic, so that the bottom screen 206 fluoresces or
phosphoresces in a first color in response to a projected image
projected directly onto the bottom screen 206 and fluoresces or
phosphoresces in a second color in response to fluorescence or
phosphorescence from the top screen 200. The detector 210 is also
dichromatic so as to allow for the detection and differentiation of
the first and the second colors. The recording medium 331 may also
comprise a radiolucent optical mask 202 to modulate the texture and
contrast of the fluorescence or phosphorescence from the top screen
200, a radiolucent fiber-optic spacer 204 to establish a known
projection disparity, and a radiopaque fiber-optic faceplate 208 to
protect the detector 210 from radiation emanating directly from the
radiation source.
[0091] Yet another embodiment is depicted in FIGS. 20 and 21,
wherein the detector 731 comprises a phosphor-coated CCD and the
fiducial reference 722 comprises a radiopaque rectangular frame
725. Both the detector 731 and the fiducial reference 722 are
contained within a light-tight package 756. The detector 731 and
fiducial reference 722 are preferably positioned flush with an
upper, inner surface of the package 756. The dimensions of the
frame 725 are selected such that the frame 725 extends beyond the
perimeter of the detector 731. Phosphor-coated strip CCDs 754 are
also contained within the package 756. The strip CCDs 754 are
positioned below the frame 725 such that radiation impinging upon
the frame 725 castes an image of each edge of the frame 725 onto
one of the strip CCDs 754. The positions of the frame shadow on the
strip CCDs 754 is used to determine the projection geometry.
[0092] In the embodiment shown in FIG. 9, the recording medium 431
is smaller than the projected image of object 521. Provided that
the reference images, 39 and 139, corresponding to the reference
markers, 23 and 123, can be identified on all the projected images,
image slices extending across the union of all the projected images
can be obtained. This is illustrated schematically in FIG. 9,
wherein the reference images, 39 and 139, are taken with the source
27 and the recording medium 431 in the image positions indicated by
the solid lines. Similarly, the dashed images, 39' and 139', are
taken with the source 27' and the recording medium 431' in the
positions indicated by the dashed lines. Accordingly, image slices
of an object which casts an object image that is larger than the
recording medium 431 can be synthesized. Further, by using multiple
fiducial references spaced in a known pattern which are all linked
to the object of interest, additional regions of commonality can be
identified between multiple overlapping projection geometries, so
that a region of any size can be propagated into a single, unified
reconstruction. Thus, it is possible to accommodate an object much
larger than the recording medium used to record individual
projection images.
[0093] Similarly, as depicted in FIG. 37, regions of overlap
between two or more sets of projected images recorded can be used
as a basis for extrapolating registration and calibration of the
sets of projected images. As shown, a first set of projected images
is recorded using an X-ray camera configured to provide a first
aperture. A second set of projected images is then recorded using
the camera configured to provide a second aperture. The first and
second sets of projected images are then brought into alignment by
identifying fiducial reference points that are common to the
overlapping regions of the projected images.
[0094] The present invention also relates to a method for creating
a slice image through the object 21 of FIG. 1 from a series of
two-dimensional projected images of the object 21, as shown in FIG.
2. The method of synthesizing the image slice starts at step 45.
Each step of the method can be performed as part of a
computer-executed process.
[0095] At step 47, a fiducial reference 22 comprising at least two
reference markers, 23 and 123, is selected which bears a fixed
relationship to the selected object 21. Accordingly, the fiducial
reference 22 may be affixed directly to the selected object 21. The
minimum required number of reference markers 23 is determined by
the number of degrees of freedom in the system, as discussed above.
When the fiducial reference 22 comprises reference markers 23 of a
finite size, the size and shape of the reference markers 23 are
typically recorded.
[0096] The selected object 21 and fiducial reference 22 are exposed
to radiation from any desired projection geometry at step 49 and a
two-dimensional projected image 38 is recorded at step 51.
Referring to FIG. 1, the projected image 38 contains an object
image 40 of the selected object 21 and a reference image, 39 and
139, respectively, for each of the reference markers 23 and 123 of
the fiducial reference 22.
[0097] At step 53, it is determined whether additional projected
images 38 are desired. The desired number of projected images 38 is
determined by the task to be accomplished. Fewer images reduce the
signal-to-noise ratio of the reconstructions and increase the
intensities of component "blur" artifacts. Additional images
provide information which supplements the information contained in
the prior images, thereby improving the accuracy of the
three-dimensional radiographic display. If additional projected
images 38 are not desired, then the process continues at step
60.
[0098] If additional projected images 38 are desired, the system
geometry is altered at step 55 by varying the relative positions of
(1) the radiation source 27, (2) the selected object 21 and the
fiducial reference 22, and (3) the recording medium 31. The
geometry of the system can be varied by moving the radiation source
27 and/or the recording medium 31. Alternatively, the source 27 and
recording medium 31, the selected object 21 and fiducial reference
22 are moved. When the radiation source and recording medium
produce images using visible light (e.g., video camera), the
geometry of the system must be varied to produce images from
various sides of the object in order to obtain information about
the entire object. After the system geometry has been varied, the
process returns to step 49.
[0099] After all of the desired projected images have been
recorded, a slice position is selecteu at step 60. The slice
position corresponds to the position at which the image slice is Lo
be generated through the object.
[0100] After the slice position has been selected, each projected
image 38 is projectively warped onto a virtual projection plane 37
at step 65. The warping procedure produces a virtual image
corresponding to each of the actual projected images. Each virtual
image is identical to the image which would have been produced had
the projection plane been positioned at the virtual projection
plane with the projection geometry for the radiation source 27, the
selected object 21, and the fiducial reference 22 of the
corresponding actual projected image. The details of the steps
involved in warping the projection plane 37 are shown in FIG. 3.
The process starts at step 70.
[0101] At step 72, a virtual projection plane 37 is selected. In
most cases it is possible to arrange for one of the projected
images to closely approximate the virtual projection plane
position. That image can then be used as the basis for
transformation of all the other images 38. Alternatively, as shown
for example in FIG. 4, if the fiducial reference 22 comprises more
than two co-planar reference markers 23, a plane which is parallel
to the plane containing the co-planar reference markers 23 can be
selected as the virtual projection plane 37. When the virtual
projection plane 37 is not parallel to the plane containing the
co-planar reference markers 23, although the validity of the slice
reconstruction is maintained, the reconstruction yields a slice
image which may be deformed due to variations in magnification. The
deformation becomes more prominent when the magnification varies
significantly over the range in which the reconstruction is carried
out. In such cases, an additional geometric transformation to
correct for differential magnification may be individually
performed on each projected image 38 to correct for image
deformation.
[0102] One of the recorded projected images 38 is selected at step
74 and the identity of the reference images 39 cast by each
reference marker 23 is determined at step 76. In the specialized
case, such as the one shown in FIG. 1, where spherical reference
markers 23 of the same radius are used and the relative proximal
distance of each reference marker 23 to the radiation source 27 at
the time that the image 38 was recorded is known, assignment of
each elliptical image 39 to a corresponding reference marker 23
call be accomplished simply by inspection. Under such conditions,
the minor diameter of the elliptical image 39 is always larger the
closer the reference marker 23 is to the radiation source 27. This
is shown most clearly in FIG. 17 wherein the minor diameter of
reference image B.sub.s corresponding to reference marker B is
smaller than the minor diameter of reference image T.sub.s
corresponding to reference marker T. Alternatively, when applied to
radiation capable of penetrating the fiducial reference 22 (i.e.,
X-rays), spherical reference markers 23 which are hollow having
different wall thicknesses and hence, different attenuations can be
used. Accordingly, the reference image 39 cast by each spherical
reference marker 23 can be easily identified by the pattern of the
reference images 39. Analogously, spherical reference markers 23 of
different colors could be used in a visible light mediated
system.
[0103] The position of each reference image 39 cast by each
reference marker 23 is measured at step 78. When a spherical
reference marker 23 is irradiated by source 27, the projected
center 41 of the reference marker 23 does not necessarily
correspond to the center 42 of the reference image 39 cast by that
reference marker 23. Accordingly, the projected center 41 of the
reference marker 23 must be determined. One method of determining
the projected center 41 of the reference marker 23 is shown in FIG.
16. The variation in intensity of the reference image 39 associated
with reference marker 23 along the length of the major diameter of
the reference image 39 is represented by the brightness profile 43.
The method depicted in FIG. 16 relies on the fact that the
projected center 41 always intersects the brightness profile 43 of
the reference image 39 at, or very near, the maximum 44 of the
brightness profile 43. Accordingly, the projected center 41 of a
spherical reference marker 23 produced by penetrating radiation can
be approximated by smoothing the reference image 39 to average out
quantum mottle or other sources of brightness variations which are
uncorrelated with the attenuation produced by the reference marker
23. An arbitrary point is then selected which lies within the
reference image 39. A digital approximation to the projected center
41 is isolated by performing a neighborhood search of adjacent
pixels and propagating the index position iteratively to the
brightest (most attenuated) pixel in the group until a local
maximum is obtained. The local maximum then represents the
projected center 41 of the reference marker 23.
[0104] Returning to step 78 of FIG. 3, when the fiducial reference
22 comprises reference markers 23 of finite size, the sizes of each
image 39 cast by each reference marker 23 are also recorded. For
example, the lengths of the major and minor diameters of elliptical
reference images cast by spherical reference markers 23 can be
measured. Computerized fitting procedures can be used to assist in
measuring the elliptical reference images 39 cast by spherical
reference markers 23. Such procedures, which are well-known in the
art, may be used to isolate the elliptical reference images 39 from
the projected image 38 and determine the major and minor diameters
of the reference images 39.
[0105] Because the attenuation of a spherical reference marker 23
to X-rays approaches zero at tangential extremes, the projected
minor diameter of resulting elliptical reference images 39 will be
slightly smaller than that determined geometrically by projection
of the reference marker's actual diameter. The amount of the
resulting error is a function of the energy of the X-ray beam and
the spectral sensitivity of the recording medium 31. This error can
be eliminated by computing an effective radiographic diameter of
the reference marker 23 as determined by the X-ray beam energy and
the recording medium sensitivity in lieu of the actual
diameter.
[0106] One method of obtaining the effective radiographic diameter
is to generate a series of tomosynthetic slices through the center
of the reference marker 23 using a range of values for the
reference marker diameter decreasing systematically from the actual
value and noting when the gradient of the reference image 39 along
the minor diameter is a maximum. The value for the reference marker
diameter resulting in the maximum gradient is the desired effective
radiographic diameter to be used for computing magnification.
[0107] Further, each projected image can be scaled by an
appropriate magnification. For fiducial references 22 comprising
spherical reference markers 23, the minor diameter of the reference
image 39 is preferably used to determine the magnification since
the minor diameter does not depend on the angle between the source
27 and the recording medium 31. Accordingly, the magnification of a
spherical reference marker 23 can be determined from the measured
radius of the reference marker 23, the minor diameter of the
reference image 39 on the recording medium 31, the vertical
distance between the center of the reference marker 23 and the
recording medium 31, and the vertical distance between the
recording medium 31 and the virtual projection plane 37.
[0108] Returning to FIG. 3 with reference to FIG. 1, a projection
transformation matrix, representing a series of transformation
operations necessary to map the selected projected image 38 onto
the virtual projection plane 37, is generated at step 80. The
projection transformation matrix is generated by solving each
projected image 38 relative to the virtual projection plane 37. In
one embodiment, the positions of the co-planar reference markers 23
are used to determine the transformation matrix by mapping the
position of the reference images 39 cast by each co-planar
reference marker 23 in the projected image onto its corresponding
position in the virtual projection plane. For example, when the
fiducial reference comprises a radiopaque frame 25, the positions
of the reference images 39 cast by the reference markers 23 formed
at the corners of the frame 25 are mapped to a canonical rectangle
having the same dimensions and scale as the frame 25. This approach
also serves to normalize the projective data. Depending on the
number of degrees of freedom, the transformation operations range
from complex three-dimensional transformations to simple planar
rotations or translations. Once the projective transformation
matrix has been generated, the matrix is used to map the projected
image 38 onto the virtual projection plane 37 at step 82.
[0109] At step 84, it is determined whether all of the projected
images 38 have been analyzed. If all of the projected images 38
have not been analyzed, the process returns to step 74, wherein an
unanalyzed image 38 is selected. If no additional projected images
38 are to be analyzed, then the process proceeds through step 85 of
FIG. 3 to step 90 of FIG. 2.
[0110] After each image has been warped onto the virtual projection
plane, an image slice through the object 21 at the selected slice
position is generated at step 90. An algorithm, such as that
described in U.S. Pat. No. 5,359,637, which is incorporated herein
by reference, can be used for that purpose. The position of the
reference image cast by the alignment marker or markers 23 in each
projected image 38 are used as the basis for application of the
algorithm to generate the image slices.
[0111] By generating image slices at more than one slice position,
a true three-dimensional representation can be synthesized.
Accordingly, it is determined whether an additional slice position
is to be selected at step 92. If an additional slice position is
not desired, the process proceeds to step 94. If a new slice
position is to be selected, the process returns to step 60.
[0112] If image slices at multiple slice positions have been
generated, the entire set of image slices is integrated into a
single three-dimensional representation at step 94. Alternative
bases for interactively analyzing and displaying the
three-dimensional data can be employed using any number of
well-established three-dimensional recording and displaying
methods. Additionally, the three-dimensional representation can be
displayed using the display device depicted in FIG. 28 in order to
produce a holographic-type display. The display device comprises a
pair of stereoscopic eyeglasses or spectacles 1080 which are worn
by an observer 1082. The eyeglasses 1080 contain lenses which are
either cross-polarized or which pass complementary colored light.
In addition, a target 1084 is positioned on the eyeglass frame
1080. A color computer monitor 1086 and video camera or detector
1088 are provided in association with the eyeglasses 1080. The
color monitor 1086 is used to display complementary-colpred or
cross-polarized stereoscopic image pairs 1090 of the
three-dimensional representation. The video camera 1088 is used to
track the target 1084 as the observer's head is moved. When the
observer's head is moved to a different position, the video camera
1088 relays information either directly to the color monitor 1086
or to the color monitor 1086 through computer-related hardware. The
information relayed by the video camera relates to the angle
subtended by the target 1084 relative to the video camera 1088. The
relayed information is then used to alter the angular disparity
associated with the stereoscopic image pairs 1090 being displayed
on the color monitor 1080 in quasi-realtime, so that the resulting
display is adjusted to correlate with the movement of the
observer's head and appears holographic to the observer.
[0113] Instead of creating a slice image or a three-dimensional
representation from one or more series of two-dimensional images, a
nearly isotropic three-dimensional image can be created from a
single pair of two-dimensional projections as depicted in FIG. 39.
As shown, the two-dimensional images are combined and overlap to
produce a three-dimensional image. Since only one two-dimensional
image is utilized to reconstruct each slice image, the method
depicted in FIG. 39 represents a completely degenerate case wherein
the slice image is infinitely thick. When the slice image is
infinitely thick, the slice image is indistinguishable from a
conventional two-dimensional projection of a three-dimensional
object.
[0114] The steps of a method for producing a three-dimensional
image of an object from a single pair of two-dimensional
projections is shown in FIG. 32. At step 1100, a three-dimensional
fiducial reference is functionally associated with an object of
interest. The association need only be complete enough to permit
the location of all of the details in the object to be determined
relative to the position of the object. The fiducial reference must
occupy a volume and be defined spatially such that a minimum of six
points can be unequivocally generated and/or identified
individually. For example, the object may be encased inside a cubic
reference volume wherein the corners of adjacent faces are rendered
identifiable by tiny, spherical fiducial markers.
[0115] A first projected image is then produced on a first
projection plane at step 1102. The relative positions of the
object, the radiation source, and the detector are then altered so
that a second projected image can be recorded on a second
projection plane at step 1104. The second projection plane must be
selected so that it intersects the first projection plane at a
known angle. However, for the resultant three-dimensional
representation to be mathematically well conditioned, the angle
should be or approach orthogonality.
[0116] At step 1106, a projective transformation of each projected
image is performed to map the images of the fiducial reference on
each face into an orthogonal, affine representation of the face.
For example, when a cubic fiducial reference is used, the
projective transformation amounts to converting the identifiable
corners of the image of fiducial reference corresponding to a
projected face of the fiducial reference into a perfect square
having the same dimensions as a face of the fiducial reference.
[0117] Each of the transformed projected images is then extruded,
at step 1108, such that both projected images occupy the same
virtual volume. The extrusion step is equivalent to the creation of
a virtual volume having the same dimensions as the fiducial
reference containing the sum of the transformed projected images.
At step 1110, an optional non-linear filtering technique is used to
limit visualization of the three-dimensional representation to the
logical intersection of the transformed projected images.
[0118] The three-dimensional representation can be refined by
optionally recording additional projected images. At step 1112, it
is determined whether additional projected images are to be
recorded. If additional projected images are desired, the process
returns to step 1104. However, if additional projected images are
not desired, the three-dimensional representation is displayed at
step 1114.
[0119] The present invention also relates to a method for reducing
distortions in the three-dimensional representation. Tomosynthesis
uses two-dimensional image projections constrained within a limited
range of angles relative to the irradiated object to produce a
three-dimensional representation of the object. The limited range
of angles precludes complete and uniform sampling of the object.
This results in incomplete three-dimensional visualization of
spatial relationships hidden in the resulting undersampled shadows
or null spaces. Another limiting factor which interferes with
artifact-free tomosynthetic reconstruction is the change in slice
magnification with depth caused by the relative proximity of the
source of radiation. These distortions can be reduced by merging
independently generated sets of tomosynthetic image slices, as
shown in FIG. 33.
[0120] At step 1120, a fiducial reference is functionally
associated with the object and at least two independent sets of
image slices are recorded. The angular disparity between the sets
of image slices is noted. For example, the first set of image
slices may comprise multiple anterior-posterior projections while
the second set of image slices comprises multiple lateral
projections. The sets of image slices are then integrated to create
a first and a second three-dimensional tomosynthetic matrix volume
at step 1122.
[0121] At step 1124, the resulting three-dimensional matrix volumes
are affinized to counteract the effects of having a finite
focal-object distance. Affinization is accomplished by first
identifying the reference images of the appropriate reference
markers of the fiducial reference. Once the reference images have
been identified, the three-dimensional matrix volumes are shifted
and scaled in order to correct for geometrical and surface
imperfections. The transformation of the first three-dimensional
matrix volumes is carried out in accordance with the following
equation:
A=CA
[0122] where A is the first three-dimensional matrix volume, A' is
the shifted and scaled first three-dimensional matrix volume, and C
is the affine correction matrix for the first three-dimensional
matrix volume. The affine correction matrix C is determined by the
number of slices comprising the three-dimensional matrix volume,
the correlation angle (i.e., the greatest angle of the projection
sequence in the range 1 [ - .PI. 4 -> .PI. 4 ]
[0123] measured from an axis normal to the detector surface), and
the correlation distance (i.e., the apex-to-apex distance created
by the intersection of the most disparate projections of the
sequence). The transformation of the second three-dimensional
matrix volume is analogously determined in accordance with the
following equation:
L'=DL
[0124] where L is the second three-dimensional matrix volume, L' is
the shifted and scaled second three-dimensional matrix volume, and
D is the affine correction matrix for the second three-dimensional
matrix volume.
[0125] At step 1126, the second three-dimensional matrix volume is
rotated by an angle .phi.. The angle .phi. is defined as the
angular disparity between the first and the second
three-dimensional matrix volumes. Specifically, the shifted and
scaled second three-dimensional matrix volume, L', is rotated in
accordance with the following equation:
L"=R .sub..psi.L'
[0126] where L" is the rotated, shifted, and scaled second
three-dimensional matrix volume and R.sub.100 is the rotational
transform matrix.
[0127] The transformed matrix volumes, A' and L", are then merged
using matrix averaging at step 1128. The matrix averaging is
accomplished in accordance with the following equation: 2 M = 1 2 (
A ' + L " )
[0128] where M is the averaged matrix of the two component
transformed matrix volumes, A' and L". Alternatively, a non-linear
combination of the transformed matrix volumes, A' and L", is
performed.
[0129] The present invention further relates to a method for
generating tomosynthetic images optimized for a specific diagnostic
task. A task-dependent method for tomosynthetic image
reconstruction can be used to mitigate the effects of ringing
artifacts from unregistered details located outside the focal plane
of reconstruction, which are intrinsic to the tomosynthetic
reconstruction process. The production and elimination of blurring
artifacts is depicted schematically in FIG. 24. As shown, a first
radiopaque object 1140 within the focal plane 1141 and a second
radiopaque 1142 object above the focal plane are irradiated from
two different source positions 1144 to produce two distinct data
images. The first data image 1146 contains an image of the first
radiopaque object 1140 at relative position C and an image of the
second radiopaque object 1142 at relative position B. The second
data image 1148 contains an image of the first radiopaque object
1140 at relative position F and an image of the second radiopaque
object 1142 at relative position G. When a linear combination of
the first and second data images is performed, the image intensity
at the same relative position of both data images is averaged. For
example, relative position B in one data image corresponds to
relative position E in the other data image and, therefore, the
corresponding relative position in the tomosynthetic image is
assigned an intensity equal to the average of the intensity
measured at relative position B and relative position E (i.e.,
(B+E)/2). As a result, the tomosynthetic image 1150 is marked by a
blurring of the image produced by the first radiopaque object 1140.
However, when a non-linear combination of the first and second data
images is performed, both data images are compared and, for
example, only the minimum intensity at each relative position is
retained. For example, relative position B in one data image
corresponds to relative position E in the other data image and,
therefore, the corresponding relative position in the tomosynthetic
image is assigned an intensity equal to the lesser of the
intensities measured at relative position B and relative position E
(i.e., B or E). As a result, the blurring shadows are eliminated
from the tomosynthetic image 1152.
[0130] The non-linear tomosynthetic approach in accordance with the
present invention is beneficial when, for example, physicians want
to know with relative assurance whether a lesion or tumor has
encroached into a vital organ. When viewing a linear tomosynthetic
reconstruction of the general region in three dimensions, the
ringing artifacts tend to blur the interface between the lesion or
tumor and the surrounding tissues. However, since tumors are
typically more dense than the tissues that are at risk of invasion,
the non-linear tomosynthetic reconstruction can be employed such
that only the relatively radiopaque tumor structures of interest
are retained in the reconstructed image. Similarly, a different
non-linear operator could be used such that only relatively
radiolucent structures of interest are retained in the
reconstructed image to determine whether a lytic process is
occurring in relatively radiopaque tissues.
[0131] The use of non-linear operators to reduce the affects of
ringing artifacts is effective because images of many structures of
radiographic interest have projection patterns determined almost
entirely by discrete variations in mass or thickness of relatively
uniform materials. Under these conditions, changes in radiographic
appearance map closely with simple changes in either material
thickness or density. In other words, complicating attributes
associated with visual images, such as specular reflections,
diverse energy-dependent (e.g., color) differences, etc., do not
contribute significantly to many diagnostic radiographic
applications. This simplification assures that many tissues can be
identified easily by their position in a monotonic range of X-ray
attenuations. Accordingly, selection of only projections yielding
maximum or minimum attenuations when performing tomosynthetic
reconstructions derived from such structures assures that resulting
image slices yield results characterized by only extremes of a
potential continuum of display options. Such displays make sense
when the diagnostic task is more concerned with specificity (i.e.,
a low likelihood of mistaking an artifact for a diagnostic signal)
than sensitivity (i.e., a low likelihood of missing a diagnostic
signal).
[0132] A method for task-dependent tomosynthetic image
reconstruction is depicted in the flow chart of FIG. 23. The method
begins at step 900 and proceeds to step 902 where a series of
projected images are acquired. In one embodiment, the projected
images are acquired in the same manner already described in
connection with steps 49-55 of FIG. 2. At step 904, the projected
images are shifted laterally, in the plane of the projection, by
amounts required to produce a desired tomosynthetic slice where all
the images are then superimposed, in a manner identical to the
method described in connection with steps 60 and 65 of FIG. 2.
[0133] Once the projected images have been acquired and
appropriately shifted, the type and degree of task-dependent
processing is chosen. At step 906, it is determined whether only
those features characterized by a relatively high attenuation are
to be unequivocally identified. If only features having a high
attenuation are to be identified, a pixel value corresponding to a
desired minimum attenuation is selected. The selected pixel value
is used as a minimum threshold value whereby each projected image
is analyzed, pixel by pixel, and all pixels having an associated
attenuation value below the selected pixel value are disregarded
when an image slice is generated.
[0134] If however, at step 906, it is determined that features
having a low attenuation are to be identified or that the entire
range of attenuating structures are to be identified, then it is
determined at step 910 whether only features characterized by a
relatively low attenuation are to be unequivocally identified. If
only features having a low attenuation are to be identified, a
pixel value corresponding to a desired maximum attenuation is
selected. The selected pixel value is used as a maximum threshold
value whereby each projected image is analyzed, pixel by pixel, and
all pixels having an associated attenuation value above the
selected pixel value are disregarded when an image slice is
generated.
[0135] If it is determined at step 910 that features having a low
attenuation are not to be identified or that the entire range of
attenuating structures are to be identified, then it is determined
at step 916 whether an unbiased estimate of the three-dimensional
configuration of the entire range of attenuating structures is to
be identified. If the entire range of attenuating structures is to
be identified, then conventional tomosynthesis is performed at step
918, whereby the attenuation values from all of the projected
images are averaged.
[0136] If the features having a high attenuation, the features
having a low attenuation, and the features covering the entire
range of attenuations are not to be identified, then it is
determined at step 920 whether the user desires to restart the
selection of features to be identified. If the user wants to
restart the identification process, then the method returns to step
906. If the user decides not to restart the identification process,
then the method ends at step 922.
[0137] Once it has been determined which features are to be
identified, then an image slice is generated at a selected slice
position at step 924. The process for generating the image slice at
step 924 is essentially the same as discussed previously in
connection with step 90 of FIG. 2. However, when only features
having either a high attenuation or a low attenuation are to be
identified, the image generation process is performed only on the
non-linearly selected images, instead of on all of the projected
images as initially acquired. Once the image slice has been
generated, the image slice is displayed at step 926 and the method
ends at step 922.
[0138] In another aspect of the present invention, a method is
provided for determining temporal changes in three-dimensions. The
me hod enables two or more sets of image data collected at
different times to be compared by adjusting the recorded sets of
image data for arbitrary changes in the vantage points from which
the image data were recorded. The method takes advantage of the
fact that a single three-dimensional object will present a variety
of different two-dimensional projection patterns, depending on the
object's orientation to the projection system. Most of this variety
is caused by the fact that a three-dimensional structure is being
collapsed into a single two-dimensional image by the projection
system. Limiting projection options to only two-dimensional slices
precludes this source of variation. The result is a much reduced
search space for appropriate registration of the images required to
accomplish volumetrically meaningful subtraction.
[0139] A flow chart showing the steps involved in the method for
determining temporal changes in three-dimensions of the present
invention is depicted in FIG. 25. A first set of image slices is
generated at step 1180. After the desired time period to be
assessed for changes has passed, the object is positioned in
roughly the same position as it was when the first set of image
slices was produced and a second set of image slices is generated,
at step 1182, using a similar exposure protocol.
[0140] At step 1184, the first set of image slices is spatially
cross-correlated with the second set of image slices. The
cross-correlation is accomplished by individually comparing each
image slice comprising the first set of image slices with the
individual image slices comprising the second set of image slices.
The comparison is performed in order to determine which image slice
in the second set of image slices corresponds to a slice through
the object at approximately the same relative position through the
object as that of the image slice of the first set of image slices
to which the comparison is being made.
[0141] After each of the image slices in the first set of image
slices is correlated to an image slice in the second set of image
slices, each of the correlated pairs of image slices are
individually aligned at step 1186. The alignment is performed in
order to maximize the associated cross-correlations by maximizing
the overlap between the image slices comprising the correlated
pairs of image slices. The cross-correlations are maximized by
shifting the image slices relative to one another until the
projected image of the object on one image slice is optimally
aligned with the projected image of the object on the other image
slice. Once each correlated pair of image slices has been aligned,
the image slices from one set of image slices is subtracted from
the image slices from the other set of image slices at step 1188 to
form a set of difference images.
[0142] At step 1190, the difference images are displayed. The
difference images can be presented as a series of individual
differences corresponding to various different slice positions.
Alternatively, the individual difference images can be integrated
to yield a composite difference representing a three-dimensional
image of the temporal changes associated with the selected
object.
[0143] The present invention further relates to a source comparator
and a method for matching radiation sources for use in quantitative
radiology. Meaningful quantitative comparisons of different image
data can be made only when the radiation source or sources used to
record the image data is very nearly unchanged. However,
conventional radiation sources produce radiation that varies with
changes in tube potential, beam filtration, beam orientation with
respect to the radiation target, tube current, and distance from
the focal spot. The source comparator and method of the present
invention enable the radiation output from one radiation source to
be matched to that of another radiation source or to that of the
same radiation source at a different time.
[0144] The source comparator 1200 for matching radiation sources in
accordance with the present invention is depicted in FIG. 26. The
source comparator 1200 comprises two wedges or five-sided
polyhedrons, 1202 and 1204, of equal dimension having a rectangular
base and two right-triangular faces. The triangular faces lie in
parallel planes at opposite edges of the base such that the
triangular faces are oriented as mirror images of each other. As a
result, each wedge, 1202 and 1204, has a tapered edge and provides
a uniformly increasing thickness from the tapered edge in a
direction parallel to the plane of the base and perpendicular to
the tapered edge. The wedges, 1202 and 1204, are arranged with the
base of one wedge 1202 adjacent to the base of the other wedge 1204
such that the tapered edges of the two wedges are at adjacent edges
of the base. One wedge is formed from a uniform high attenuation
material while the other wedge is formed from a uniform low
attenuation material to differentially attenuate the relative
proportion of high and low energy photons in the output from the
radiation source. Accordingly, when the source comparator 1200 is
irradiated from a radiation source directed perpendicularly to the
bases of the wedges, the resulting image will be a quadrilateral
having an intensity gradient that varies uniformly in a single
direction with the angle of the gradient being determined by the
distribution of high and low energy photons in the output from the
radiation source.
[0145] The source comparator 1200 of FIG. 26 is used in the method
of matching radiation sources in accordance with the present
invention as shown in FIG. 27. At step 1220, the source comparator
is positioned between a radiation source and a detector. An
original gradient image is then recorded by exposing the source
comparator to radiation from the radiation source at the source
settings to be used for recording a first set of data images. The
first set of data images is then recorded.
[0146] When a second set of data images is to be recorded, the
source settings for the radiation source to be used to record the
second set of data images are adjusted to match the settings used
for recording the first set of data images. At step 1222, the
source comparator is positioned between the radiation source and
the detector and a first gradient image is recorded. The source
comparator is then rotated perpendicularly to the detector by an
angle of 180.degree. and a second gradient image recorded at step
1224. The first and second gradient images are compared and the
source comparator oriented to produce the smaller gradient at step
1226. By so doing, it is assured that the source comparator bears
the same relative relationship to the radiation source for both
sets of data and, thereby, eliminates the potential for confounding
the data by spatial variations in the cross-sectional intensity of
the output from the radiation source.
[0147] The individual settings on the radiation source are then
iteratively adjusted. At step 1230, the beam energy is matched by
adjusting the kVp on the radiation source so that the measured
gradient value approaches the gradient value of the original
gradient image. The beam quality is then matched at step 1232 by
adjusting the filtration of the radiation source so that the angle
of the maximum gradient relative to the edge of the source
comparator approaches that of the original gradient image. The beam
exposures are then estimated by integrating the detector response
across a fixed region of the source comparator and matched at step
1234 by adjusting the mAs of the radiation source so that the
exposure approaches that of the original gradient image. At step
1236 it is determined whether the gradient image is substantially
the same as the original gradient image. If the two images are
significantly different, the beam energy, beam quality, and
exposure are readjusted. If, however, asymptotic convergence has
been reached and the two gradient images are substantially the
same, the radiation sources are matched and the process ends at
step 1238. Once the radiation sources have been matched, the second
set of data images can be recorded and quantitatively compared to
the first set of data images.
[0148] In the embodiment shown in FIG. 19, the source 627 is an
unconstrained point source and the detector 631 is completely
constrained relative to the object 621. Accordingly, the system has
three degrees of freedom (two translational and one displacement
for the radiation source 627 relative to the object 621 and
detector 631). A beam collimator 647 can be positioned between the
source 627 and the object 621 to collimate the radiation from the
source 627. The detector 631 comprises a primary imager 632 and a
secondary imager 634 positioned a known distance below the primary
imager 632. In one embodiment, both the primary and secondary
imagers, 632 and 634, are CCD detectors. The fiducial reference 622
comprises a radiopaque shield 633 with a ring-shaped aperture 636
of known size positioned between the primary imager 632 and the
secondary imager 634.
[0149] Radiation from the source 627 passes through collimator 647,
irradiates object 621, and produces an object image on the primary
imager 632. In addition, radiation from the source 627 which
impinges upon the radiopaque shield 633 passes through the aperture
636 to produce a ring-shaped reference image of the aperture 636 on
the secondary imager 634. Since the secondary imager 634 is not
used to record object images, the secondary imager 634 can be a low
quality imager such as a low resolution CCD. Alternatively, a lower
surface of the primary imager 632 can be coated with a
phosphorescent material 635, so that radiation impinging upon the
primary imager 632 causes the phosphorescent material 635 to
phosphoresce. The phosphorescence passes through the aperture 636
to produce the reference image on the secondary imager 634.
[0150] In operation, the reference image produced using the system
depicted in FIG. 19 can be used to determine the position of the
source 627 relative to the object 621 and the detector 631. A
circle, or ellipse, is fitted to the projected reference image. By
fitting a circle, or ellipse, to the reference image, the effect of
dead areas and/or poor resolution of the secondary imager 634 can
be eliminated by averaging. The position of the center of the
fitted circle, or ellipse, relative to the known center of the
aperture 636 is determined. The angle .alpha. of a central ray 637
radiating from the source 627 relative to the object 621 and the
detector 631 can then be determined. In addition, the length of the
minor diameter of the projected reference image is determined and
compared to the known diameter of the aperture 636 to provide a
relative magnification factor. The relative magnification factor
can then be used to determine the distance of the source 627 from
the object 621.
[0151] The center of the fitted circle can be determined as
follows. A pixel or point on the secondary imager 634 that lies
within the fitted circle is selected as a seed point. For
convenience, the center pixel of the secondary imager 634 can be
selected, since the center point will typically lie within the
fitted circle. A point R is determined by propagating from the seed
point towards the right until the fitted circle is intersected.
Similarly, a point L is determined by propagating from the seed
point towards the left until the fitted circle is intersected. For
each pixel along the arc L-R, the average of the number of pixels
traversed by propagating from that pixel upwardly until the fitted
circle is intersected and the number of pixels traversed by
propagating from that pixel downwardly until the fitted circle is
intersected is determined. Any statistical outliers from the
averages can be discarded and the average of the remaining values
calculated. This average represents the row address of the fitted
circle's center. To obtain the column address, the entire reference
image is rotated by 90.degree. and the process is repeated. The row
address and column address together represent the position of the
center of the fitted circle.
[0152] Although the above embodiments have been described in
relation to projected images of objects produced using X-rays, the
present invention is equally applicable to images produced using a
variety of technologies, such as visible light, ultrasound, or
electron microscopy images. Specifically, intermediate voltage
electron microscope (IVEM) images can be used to provide
quantitative three-dimensional ultrastructural information.
Further, the present invention can also be used to reconstruct
three-dimensional images of objects which either emit or scatter
radiation.
[0153] When IVEM images are used, the present invention allows
cellular changes to be detected and quantified in an efficient and
cost-effective manner. Quantitation of three-dimensional structure
facilitates comparison with other quantitative techniques, such as
biochemical analysis. For example, increases in the Golgi apparatus
in cells accumulating abnormal amounts of cholesterol can be
measured and correlated with biochemically measured increases in
cellular cholesterol.
[0154] When photographic images are used, it is possible to create
a true three-dimensional model of a diffusely illuminated fixed
scene from any number of arbitrary camera positions and angles. The
resulting three-dimensional image permits inverse engineering of
structural sizes and shapes, and may be expressed as a series of
topographic slices or as a projective model that can be manipulated
interactively. This capability is particularly useful in
retrofitting existing structures or quantifying three-dimensional
attributes using non-invasive methods. In addition, the present
invention can be applied to construct topological images of
geological structures by recording images of the structure created
by the sun.
EXAMPLES
[0155] Representative lumpectomy specimens containing cancer from
human breasts were radiographed using a digital mammographic
machine (Delta 16, Instrumentarium, Inc.). Exposure parameters were
regulated by an automatic exposure control mechanism built into the
unit. Seven distinct projections of each specimen were made using a
swing arm containing the tube head that swept across each specimen
in a single arched path. This resulted in mammographic projections
having angular disparities of 15, 10, 5, 0, -5, -10, and -15
degrees from vertical. These data were processed to yield a series
of tomosynthetic slices distributed throughout the breast tissues
in three ways: 1) conventional linear summation of all seven
appropriately shifted projections (FIG. 29), 2) identical linear
summation augmented by the application of an interactive
deconvolution filter known to minimize tomographic blur (FIG. 30),
and 3) a nonlinear tomosynthetic reconstruction scheme based on
selection of only the projection(s) yeilding the minimum brightness
at each pixel (FIG. 31). Notice the lack of "ringing" artifacts
caused by the wire used to locate the lesion in FIG. 31
corresponding to the nonlinear reconstruction method. Five
board-certified radiologists compared tomographic displays of these
tissues produced from all three methods and ranked them in terms of
their perceived interpretability with regard to cancer recognition
and relative freedom from apparent tomosynthetic artifacts. A
related exercise involved having a different set of eight observers
estimate the relative depths of a series of seven holes bored in a
solid Lucite block exposed under comparable conditions.
[0156] All five radiologists preferred the nonlinearly generated
tomosynthetic mammograms over those produced conventionally (with
or without subsequent blurring via interactive deconvolution). A
similar statistically significant result (p<0.05) was produced
when the performance of the hole-depth experiment was objectively
determined.
[0157] This approach is very efficient: it is simpler to implement
than conventional tomosynthetic back-projection methods; and it
produces sharp-appearing images that do not require additional
computationally intensive inverse filtering or interative
deconvolution schemes. Therefore, it has the potential for
implementation with full-field digital mammograms using only modest
computer processing resources that lie well within the current
state of the art. For certain tasks that are unduly compromised by
tomosynthetic blurring, a simple nonlinear tomosynthetic
reconstruction algorithm may improve diagnostic performance over
the status quo with no increase in cost or complexity.
[0158] Although the above discussion has centered around computed
tomography, it will be appreciated by those skilled in the art that
the present invention is useful for other three-dimensional imaging
modalities. For example, the present invention is also intended to
relate to images obtained using magnetic resonance imaging (MRI),
single photon emission computed tomography (SPECT), positron
emission tomography (PET), conventional tomography, tomosynthesis,
and tuned-aperture computed tomography (TACT), as well as
microscopic methods including confocal optical schemes.
[0159] It will be recognized by those skilled in the art that
changes or modifications may be made to the above-described
embodiments without departing from the broad inventive concepts of
the invention. It should therefore be understood that this
invention is not limited to the particular embodiments described
herein, but is intended to include all changes and modifications
that are within the scope and spirit of the invention as set forth
in the claims.
* * * * *