U.S. patent application number 10/262987 was filed with the patent office on 2003-02-13 for x-ray computer tomography scanning system.
Invention is credited to Cheng, Chin-An.
Application Number | 20030031300 10/262987 |
Document ID | / |
Family ID | 26949492 |
Filed Date | 2003-02-13 |
United States Patent
Application |
20030031300 |
Kind Code |
A1 |
Cheng, Chin-An |
February 13, 2003 |
X-ray computer tomography scanning system
Abstract
An X-ray computer tomography scanning system for imaging the
internal structure of a human being including a source that
projects a substantially conically shaped X-ray beam along a path
on to a subject such that at least a portion of said beam is
transmitted through said subject and a detector array that detects
the portion of the beam transmitted through the subject and
operative to generate electronic signals in response to the beam
transmitted through the subject that projects on to said detector
array. The system also including a patient support structure for
supporting the subject and rotating said subject about an axis
intersecting with and substantially perpendicular to the path of
the beam, wherein said subject is constrained such that the subject
remains in the path of said beam and an airbag restraining
mechanism attached to the support structure for holding the subject
in its position. The system includes an imaging mechanism operative
to generate an image responsive to the electronic signals generated
by the detector.
Inventors: |
Cheng, Chin-An; (Santa
Clara, CA) |
Correspondence
Address: |
OPPENHEIMER WOLFF & DONNELLY
P. O. BOX 10356
PALO ALTO
CA
94303
US
|
Family ID: |
26949492 |
Appl. No.: |
10/262987 |
Filed: |
October 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10262987 |
Oct 1, 2002 |
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09905614 |
Jul 13, 2001 |
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6470068 |
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60262841 |
Jan 19, 2001 |
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Current U.S.
Class: |
378/177 ;
378/4 |
Current CPC
Class: |
A61B 6/032 20130101;
A61B 6/0478 20130101; A61B 6/0421 20130101 |
Class at
Publication: |
378/177 ;
378/4 |
International
Class: |
G21K 001/12 |
Claims
What is claimed is:
1. A computer tomography scanning system comprising: a source that
projects a substantially conically shaped X-ray beam along a path
on to a subject such that at least a portion of said beam is
transmitted through said subject; a detector array that detects the
portion of the beam transmitted through the subject and operative
to generate electronic signals in response to the beam transmitted
through the subject that projects on to said detector array; a
patient support structure for supporting the subject and rotating
said subject about an axis intersecting with and substantially
perpendicular to the path of the beam, wherein said subject is
constrained such that the subject remains in the path of said beam;
an airbag restraining mechanism attached to the support structure
for holding the subject in its position; and an imaging mechanism
operative to generate an image responsive to the electronic signals
generated by the detector.
2. The computer tomography scanning system as recited in claim 1,
further comprising: an aperture that limits the amount of
unnecessary radiation projected on the subject by controlling the
peripheral configuration of the beam, said aperture being located
in the path of the beam between the source and the subject, thereby
allowing the beam to irradiate only the portion of the subject to
be imaged.
3. The computer tomography scanning system as recited in claim 1,
further comprising: an energy filter for reducing low energy
radiation and second order effects of the X-ray beam, said energy
filter being located in the path of the X-ray beam, between the
source and the detector.
4. The computer tomography scanning system as recited in claim 3,
wherein the energy filter further comprises: a filter plate for
normalizing the X-ray beam intensity, said filter plate having a
varied thickness corresponding to the intensity of the X-ray beam
intersecting each portion of said filter plate, thereby causing the
X-ray beam intensity to be nearly uniform across its projecting
area.
5. The computer tomography scanning system as recited in claim 2,
wherein the aperture further comprises: a plurality of plates, each
being substantially opaque to radiation; and a positioning
mechanism for controlling the peripheral configuration of the beam
by positioning the plates so as to constrain the periphery of the
beam, said plates intersecting in such a way as to form an opening
through which the constrained beam projects.
6. The computer tomography scanning system as recited in claim 1,
wherein the airbag restraining mechanism further comprises: a
securing mechanism for securing the airbag restraining mechanism
about the subject such that the subject is secured to the support
structure during rotation.
7. The computer tomography scanning system as recited in claim 6,
wherein the airbag restraining mechanism further comprises: an
inflation mechanism capable of inflating the airbag to a sufficient
pressure to hold the subject against the support structure during
rotation.
8. The computer tomography scanning system as recited in claim 1,
wherein the subject includes a head and wherein the airbag
restraining mechanism further comprises: a head restraint for
restraining the head of the imaging subject during rotation.
9. The computer tomography scanning system as recited in claim 8,
wherein the head restraint further comprises: a head support
attached to the support structure for supporting the head of the
imaging subject; and an airbag attached to said head support for
holding the head of the subject against the head support during
rotation, said airbag enveloping at least a portion of the head,
thereby gently holding the head of the subject against the head
support during rotation.
10. The computer tomography scanning system as recited in claim 9,
wherein the head restraint further comprises: an inflation
mechanism for inflating the airbag to a sufficient pressure to
restrain the head of the subject against the head support during
rotation, thereby gently restraining the head of the imaging
subject during rotation.
11. The computer tomography scanning system as recited in claim 1,
wherein the support structure further comprises: a seat for the
subject to sit on during rotation, the seat being removable to
allow the imaging subject to stand during rotation.
12. The computer tomography scanning system as recited in claim 1,
wherein the detector further comprises: a two dimensional
scintillation screen operative to generate a light beam responsive
to the portion of the X-ray beam transmitted through the subject
that projects on said scintillation screen; and a two dimensional
array of photosensitive devices operative to generate electronic
signals responsive to said light beam, said array being positioned
so as to intercept the light beam generated by the scintillation
screen.
13. The computer tomography scanning system as recited in claim 12,
wherein the detector further comprises: an optical mechanism for
focusing the light beam generated by the scintillation screen upon
the array of photosensitive devices, said optical mechanism being
positioned so as to intersect the light beam generated by the
scintillation screen and focus the light beam upon the array of
photosensitive devices.
14. The computer tomography scanning system as recited in claim 1,
wherein the detector further comprises: a two dimensional array of
X-ray detectors operative to generate electronic signals responsive
to the portion of the X-ray beam transmitted through the subject
that projects on said array of X-ray detectors.
15. The computer tomography scanning as recited in claim 1, further
comprising: a source support mechanism for supporting the source at
one of several height positions, said source support mechanism
holding said source rigidly at any one of said height positions;
and a detector support mechanism for supporting the detector at one
of several height positions corresponding to the height positions
of the source support mechanism, said detector support mechanism
holding the detector rigidly at any one of said height positions,
thereby allowing for imaging at different heights with the source
and detector remaining stationary relative to each other.
16. The computer tomography scanning system as recited in claim 1,
wherein the medical imaging system further comprises: an
auto-calibration mechanism for normalizing X-ray beam energy
distribution by fixing the position of the source relative to the
detector and adjusting the image generated by the imaging mechanism
to compensate for variations in X-ray beam intensity and detector
sensitivity.
17. The computer tomography scanning system as recited in claim 1,
wherein the imaging mechanism further comprises a 3D imaging device
operative to generate three dimensional images responsive to the
electronic signals generated by the detector.
18. A method of generating an image, comprising: restraining a
subject from unnecessary movement relative to a support structure
using an airbag restraining mechanism; rotating the support
structure; and projecting a substantially conically shaped X-ray
beam along a path intersecting with and substantially perpendicular
to the axis of rotation of said support structure, on to said
subject such that a portion of the X-ray beam transmits through the
subject and projects on to a detector; measuring the portion of the
X-ray beam that projects on to said detector; generating an image
responsive to the measurements of the X-ray beam transmitted
through the subject that projects on to the detector.
19. The method of claim 18, further comprising: reducing the
imaging subjects exposure to unnecessary radiation by controlling
the peripheral configuration of the X-ray beam projected on to said
subject.
20. The method of claim 19, wherein controlling the peripheral
configuration of the X-ray beam further comprises: providing a
plurality of plates, which are substantially opaque to radiation;
and positioning said plates directly between the source and the
subject in such a way as to form an opening through which only a
portion of the X-ray beam may pass to irradiate the subject.
21. The method of claim 20, wherein restraining the subject further
comprises: inflating the airbag restraining mechanism such that the
subject is held against the support structure to preventing
unnecessary movement of the subject as the support structure
rotates, thereby gently restraining the subject during
rotation.
22. The method of claim 18, wherein the imaging subject includes a
head and wherein restraining the imaging subject further comprises:
restraining the head of the imaging subject.
23. The method of claim 22, wherein restraining the head of the
imaging subject further comprises: providing a head support
attached to the support structure for supporting the head of the
subject during rotation; placing the head of the imaging subject
against the support structure; securing an airbag to the head
support such that the airbag encloses at least a portion of the
head against the head support; and inflating the airbag such that
the head of the subject is restrained from unnecessary movement as
the subject is rotated, thereby gently restraining the head of the
subject during rotation.
24. The method of claim 18, wherein scanning an imaging subject
includes auto-calibration comprising: providing a source for a
three dimensional X-ray beam; providing a detector array for
detecting a three dimensional X-ray beam; fixing the position of
the source; fixing the position of the detector array; projecting a
three dimensional X-ray beam on the detector array from the source;
measuring the X-ray beam with the detector array; calculating the
variations in measurements across the detector array; and
normalizing the sensitivity of the detector array to compensate for
the variations in the measurements across the detector array,
thereby minimizing the effects of beam intensity variation and
detector sensitivity variation on imaging as well as reducing
interference from a restraining mechanism on imaging.
25. The method of claim 18, wherein generating an image further
comprises: generating a three dimensional image responsive to the
measurements of the X-ray beam transmitted through the subject that
project on to the detector.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made and priority claimed from U.S. Provisional
Application Serial No. 60/262,841, filed Jan. 19, 2001, entitled
"X-Ray Computer Tomography Scanning System." This is a continuation
application of U.S. patent application Ser. No. 09/905,614, filed
Jul. 13, 2001, entitled :X-Ray Computer Tomography Scanning
System."
BACKGROUND OF INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to an apparatus and
process for medical imaging. More particularly, the present
invention relates to medical X-ray imaging systems such as an X-ray
computer tomography scanner that includes a subject restraining
mechanism and a process for operating thereof.
[0004] 2. Description of the Prior Art
[0005] Computerized tomography is a modem non-invasive technique
developed for revealing internal organs and tissues of a human body
in cross-section to aid in medical diagnosis, examination, surgery,
etc. Essentially an X-ray beam is passed through an imaging
subject, e.g. human body, at one location. Part of the X-beam is
attenuated by the tissue of the imaging subject, with the remainder
of the X-ray beam transmitting through the subject and projecting
onto a detector. The detector measures the intensity of the beam
transmitting through the imaging subject. The detector then sends
electronic signals responsive to the X-ray beam to a computer which
calculates and stores the beam intensity for the purpose of
reconstructing a graphic image of the subject.
[0006] For a measurement at a different location on the subject,
the X-ray beam is rotated in one plane to a different angular
position and similar measurements are made and recorded. This
process is continued for 360 degrees, at which time the computer
utilizes the combined measurements recorded to reconstruct a two
dimensional image of a cross-sectional slice of the imaging
subject. The X-ray beam is then transversely moved by moving the
subject and the process repeated to develop another picture of a
new cross-sectional slice of the subject. By taking a plurality of
such transversely spaced slices and stacking them one on top of the
other, a three dimensional image of a portion of the subject could
be constructed by the computer.
[0007] The earliest computerized axial tomography used a pencil
beam with a single detector. The beam and the detector were
simultaneously rotated and then linearly translated to develop an
appropriate scan of the subject. To reduce scanning time a
fan-shaped beam replaced the pencil beam and multiple detectors
arrayed in an arc were used instead of a single detector. Various
detector arrays and detector-beam movements were subsequently
developed to increase the speed of the scanning process.
[0008] More recent developments in computer tomography employ a
source which generates a cone-shaped X-ray beam, instead of the
fan-shaped beam, and a two dimensional detector array, instead of a
linear array of detectors. This configuration allows the scanning
of multiple slices at once. In related industrial applications of
computerized tomography disclosed in U.S. Pat. No. 5,023,895,
issued to McCroskey, et. al., the use of a cone beam in computer
tomography is further improved by rotating a turntable where an
object is placed.
[0009] None of these systems provide a simple, inexpensive device
for scanning the internal organs of a human being. None of these
systems provide an acceptable means for restraining a person to
prevent movements during the scanning process.
[0010] The present invention provides a relatively inexpensive and
compact computer tomography system and a process of using the same
that rapidly and accurately creates an image of the section of a
person under examination.
SUMMARY OF THE INVENTION
[0011] One object of the invention is to provide an improved
medical computer tomographic system which can rapidly produce
accurate images of medical subjects.
[0012] Another object of the invention is to provide an improved
medical computer tomographic system which can produce accurate
images of medical subjects inexpensively.
[0013] Another object of the invention is to provide an improved
medical computer tomographic system which can produce accurate
images of medical subjects without unnecessary discomfort to the
subject.
[0014] Another object of the invention is to provide an improved
medical computer tomographic system which can produce accurate
images of medical subjects without performing extensive complex
calibration to the x-ray source and detector in the conventional
computer tomographic systems.
[0015] Yet another object of the invention is to provide an
improved medical computer tomographic system which can produce
accurate images of medical subjects requiring less complexity of
equipment and operation than existing systems.
[0016] Briefly, the preferred embodiment includes a source, a
detector array, a support structure, a mechanism for rotating the
support structure, a restraining mechanism, and an imaging
mechanism. The source projects a roughly cone shaped beam of X-ray
radiation on to a subject such that a portion of the beam transmits
through the subject and projects on to the detector array. The
support structure supports the subject and rotates the subject
about an axis perpendicular to and intersecting with the X-ray beam
such that the X-ray beam continues to project on the subject
throughout the support units rotation. The restraining mechanism is
attached to the support structure and restrains the subject from
unnecessary movement during rotation. The portion of the X-ray beam
that transmits through the subject projects on to the detector
array. The detector array detects the X-ray beam transmitted
through the subject and is operative to generate an electronic
signals responsive to the beam transmitted through the subject. The
imaging mechanism receives the electronic signals from a detector
within the detector array and is operative to generate an image
responsive to the electronic signals.
[0017] One advantage of the invention is its simple design which
requires less equipment than other medical computer tomography
scanners and, hence, less maintenance. The system of the present
invention does not require the bulky and expensive equipment needed
to rotate the detector and X-ray source nor the equipment necessary
to feed the subject through the tomography scanner.
[0018] Another advantage is that the invention scans the subject in
one revolution, whereas prior art systems require multiple
rotations due to their use of fan-shaped x-ray beams as opposed to
cone-shaped x-ray beams. By using a cone-shaped x-ray beam the
present invention eliminates the need to feed the subject through
the scanning system.
[0019] Yet another advantage of the invention is low cost. The
system does not require the expensive mechanism necessary to move
the detectors and x-ray source, nor does it require as much
calibration as systems that rotate the x-ray source and
detector.
[0020] Yet another advantage of the invention is the simplified
calibration process required to calibrate imaging to compensate for
variation produced by the movement of the source and detector. The
invention does not require a complex calibration process and is
therefore likely to produce sharper images than conventionally
calibrated systems. Also the invention will be more reliable
because it has fewer components to fail and performs fewer
operations reducing the possibility of errors and, hence, requires
less maintenance.
[0021] The foregoing and other objects, features and advantages of
the invention will be apparent from the following detailed
description of the preferred embodiments which make reference to
the several figures of the drawing.
IN THE DRAWING
[0022] FIG. 1 is a generalized block diagram of a computer
tomography system according to an embodiment of the present
invention;
[0023] FIG. 2 is a generalized diagram of the scanning system
depicting the rotating subject relative to the X-ray beam and the
detector from a top view according to one embodiment of the present
invention;
[0024] FIG. 3a shows an exemplary embodiment of the aperture of the
preferred embodiment of the present invention;
[0025] FIG. 3b shows the aperture from the view of the plane of
line A-A of FIG. 3a;
[0026] FIG. 4 is a diagram illustrating the projection of the x-ray
beam through a scanning subject and onto the detector array
according to one embodiment of the present invention;
[0027] FIG. 5 depicts one configuration of the support structure
and airbag restraining mechanism as claimed in the present
invention;
[0028] FIG. 6 depicts one configuration of the head restraint
relative to the support structure as claimed in an embodiment of
the present invention;
[0029] FIG. 7 illustrates one configuration for the source support
mechanism and detector support mechanism relative to the support
structure as claimed in one embodiment of the present invention;
and
[0030] FIG. 8 is a block diagram illustrating a process flow for
generating images in one embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] The X-ray imaging system of the present invention is an
X-ray computer tomography system for imaging the internal organs of
a human being or an animal. FIG. 1 is a block diagram illustrating
an X-ray computer tomography system 20, according to one embodiment
of the present invention for imaging the internal organs of a
subject 30. Subject 30 may include the organs of a human being or
an animal. The computer tomography system 20 includes a source 22
for projecting a substantially conically shaped X-ray beam 28.
[0032] The source 22 is of the type used in medical X-ray imaging
as known in the art. The term X-ray may also refer to gamma
radiation or other penetrating radiation. The source 22 is
positioned such that the beam 28 projected from the source 22,
projects along a path onto the subject 30.
[0033] In the preferred embodiment, the source 22 is controlled by
a computer system 40, such that the source 22 only projects the
beam 28 when necessary for imaging. This is done to avoid
unnecessary exposure of the subject 30 to harmful radiation.
[0034] The path the X-ray beam 28 travels to project onto the
subject 30 passes through an aperture 24 which is placed in front
of the source 22 between the source 22 and the subject 30. The
aperture 24 positions and constricts the X-ray beam 28 by blocking
a portion of the beam 28 that is not necessary for imaging the
subject 30. The aperture 24 is capable of controlling the size of
the beam 28 as well as what part of the subject 30 the beam 28
projects on. By minimizing the beam 28 to a size necessary for
imaging the subject 30, the aperture 24 reduces the amount of
radiation projects on to the subject 30 and, hence, minimizes the
subject 30 exposed under the harmful radiation.
[0035] In the preferred embodiment the X-ray beam 28 projects on to
the subject 30 after passing through the aperture 24 that
constricts the X-ray beam 28. A laser targeting device (not shown)
mounted to the source 22 is used to determine the exact path of the
X-ray beam 28. The X-ray beam is of sufficient intensity to
transmit through the subject 30 and project on to a scintillation
screen 34. In the preferred embodiment the computer system 40
controls the intensity of the beam 28 so that it is sufficient to
penetrate the subject 30. Alternatively, the beam 28 may be preset
to a desired intensity. The intensity used for this type of medical
imaging is well known in the art.
[0036] The scintillation screen 34 is positioned in front of an
array of photodiodes 38 between the array of photodiodes 38 and the
subject 30 such that the X-ray beam 28 intersects with the
scintillation screen 34. The scintillation screen 34 is positioned
such that the path of the X-ray beam 28 is substantially
perpendicular to the plane of the scintillation screen 34. The
scintillation screen 34 is a rectangularly shaped solid sheet of
glass coated with scintillation crystal material. A suitable
scintillation crystal would be cesium iodide doped with thalium.
The scintillation screen 34 converts X-ray radiation energy into a
light energy 84. This technology is well established in the
art.
[0037] In the preferred embodiment the light energy 84 is
transmitted to a photo sensor or a detector (not shown) such as a
high-resolution lens system and a charge coupled device (CCD). The
light energy 84 is focused by the high-resolution lens system onto
the CCD. The CCD, which has a plurality of tiny light-sensitive
fields that absorb the light energy 84 after it has passed through
the high-resolution lens system, produces electronic signals 58
responsive to the light energy 84 detected by it. An opaque housing
covers every surface of the CCD camera 38 except the surface
adjacent to the scintillation screen 34 to prevent ambient light
from creating false images in the CCD camera 38.
[0038] The computer system 40 analyzes the electronic signals 58 to
create a shadow image of the subject 30 by analyzing the amount of
light energy impinging upon each light sensitive field of the CCD.
The more tissue of subject 30 the beam 28 passes through before
intersecting with the scintillation screen 34, the less light that
is converted by the scintillation screen 34 and the less light that
impinges upon a particular light sensitive field of the CCD. The
density of the tissue the X-ray beam 28 passes through has a
similar affect on the light energy impinging upon a particular
light sensitive field of the CCD. The greater the density of tissue
the beam 28 passes through, the less light energy that reaches the
CCD. The computer stores this shadow image for later
manipulation.
[0039] A support structure 26 rotates about a vertical axis of the
support structure 26 in step increments. This rotation is
controlled by the computer system 40 through a control cable 56,
such that a shadow image is created by the computer after each
increment of rotation. Enough shadow images are created to
sufficiently image the subject 30.
[0040] The support structure 26 supports the subject 30 during
rotation such that the subject 30 remains in the path of the X-ray
beam 28.
[0041] An airbag restraining mechanism 32 is attached to the
support structure 26 with one or more hinges 42 such that the
airbag restraining mechanism 32 is capable of rotating about its
hinged end to enclose a portion of subject 30 within the support
structure 26. The airbag restraining mechanism 32 has a latch 46 on
the end opposite its' hinged end, which is secured to the support
structure 26 after the subject 30 is enclosed by the restraining
mechanism 32. The airbag restraining mechanism 32 serves to
restrain the subject 30 from unnecessary movement during the
rotation 76 of the support structure 26. Movement of the subject 30
during scanning can reduce image quality by altering subject
geometry from one position or angle to another. The computer system
40 creates a tomographic image by compiling data from the shadow
images created for each angular increment of rotation. If the
geometry of the subject 30 varies significantly between shadow
images the computer system 40 will be unable to create an accurate
tomographic image. By restraining the subject 30 during the
scanning process, the airbag restraining mechanisms 32 improves
image accuracy. In alternative embodiment, other restraining
mechanism such as a belt may be substituted for the airbag.
[0042] A rate table 36 contains an electric motor which rotates the
support structure 26 about the vertical axis. This motor is
controlled by the computer system 40 through the control cable 56.
After each shadow image is captured the computer system 40 sends a
signal through control cable 56 to the rate table 36 to advance the
motor to rotate the rate table 36 and the patient support structure
26 by another increment. This process continues until completing
the entire image capturing process.
[0043] The computer system 40 controls the timing of projecting
X-ray beam 28 generated by the source 22. In an alternative
embodiment, the aperture 24 may be manually adjusted. The computer
system 40 uses the electronic signals 58 from the array of
photodiodes 38 to generate an image. The computer system 40
subsequently stores the image in a data storage device (not shown),
such as a magnetic hard disk drive or a semiconductor memory, in
the computer system 40. The computer system 40 then rotates the
subject 30 by sending electrical pulses to the motor in the rate
table 36, projects another X-ray beam 28 and creates another image.
The computer system 40 continues this process until enough shadow
images have been stored to create a complete tomographic image
through a method of back projection.
[0044] The computer system 40 uses all of the images stored to
create a tomographic image by a process of back projection image
reconstruction. The process of back plane image reconstruction is
well known in the art. The difference between the present invention
and the prior art for the medical use of X-ray tomographic back
plane projection imaging is that the present invention effectively
scans multiple slices at once compared to the prior art. Prior art
medical devices used a fan shaped X-ray beam with a linear detector
array while the present invention uses a conical shaped beam 28
with a two dimensional detector array 38. Each row 93 (See FIG. 4)
scanned by the present invention is the equivalent of a slice taken
in the conventional process. This arrangement in the present
invention allows it to scan a subject 30 in one rotation instead of
multiple rotations combined with the feeding of the patient through
the scanning device as required by the prior art technologies. The
present invention uses a method of back projection similar to the
one described in U.S. Pat. No. 5,023,895, which is incorporated by
reference herein.
[0045] The computer system 40 performs calibration for the scanning
system by storing an image with a calibration unit in place of the
subject 30 without the rotation of the subject. The computer system
40 then corrects each image of the calibration unit taken at each
increment of rotation using the stored image. This correction
consists of subtracting anomalous images contained in the image
taken at each increment of rotation without the subject 30 from
each image taken at each rotation with the subject 30.
[0046] The computer system 40 transmits the tomographic image to a
display 44 in the form of an electronic data signal 78. The display
44 is a computer monitor and/or printer. Display output may be
further processed in the form of two dimensional slice images or
three dimensional images.
[0047] FIG. 2 illustrates an alternative embodiment of the present
invention, in which an optical system is used to scan the subject
30. In the alternative embodiment the source 22 projects an X-ray
beam 28 through a first filter plate 19 positioned between the
source 22 and an aperture 24. A first filter in the first filter
plate reduces the X-ray energy level by absorbing low energy
portions of the X-ray beam. The first filter plate 19 further
serves to normalize the X-ray beam energy distribution with a
varying thickness corresponding to the predetermined energy
distribution of the X-ray beam 28. Areas of the first filter plate
19 intercepting higher energy parts of the X-ray beam 28 are of
greater thickness to absorb more energy so that the X-ray beam 28
transmitting through first filter plate 19 is of a more even energy
distribution across its projected area. The first filter plate 19
could also be positioned between the aperture 24 and the subject
30. After passing through the first filter plate 19, the X-ray beam
28 is focused by the aperture 24 on to the subject 30. The X-ray
beam 28 is projected such that a portion of the beam 28 is absorbed
by the tissue of the subject 30 and a portion of the beam 28
transmits through the subject 30 and impinges upon a second filter
plate 21 which serves to reduce the affects of scatter radiation by
absorbing low energy portions of the X-ray beam. The second filter
plate 21 is mounted in front of the scintillation screen 34 between
the scintillation screen 34 and the subject 30 and is perpendicular
to the path of the X-ray beam 28.
[0048] After passing through the second filter plate 21 the X-ray
beam 28 impinges upon the scintillation screen 34. The
scintillation screen 34 converts the X-ray beam 28 to a visible
light beam 84, and projects the visible light beam 84 through a
lens 48 and onto a photo sensing array 50 such as a CCD in a
digital camera described above. The images recorded by the digital
camera are then processed by the computer system 40 as if they were
the electronic signals 58 of the preferred embodiment as shown in
FIG. 1. Optical systems are made-up of three basic components,
which may vary greatly from system to system: the scintillation
screen 34, which converts x-rays to a visible image; an optical
transfer mechanism (usually a lens and mirror, or a fiber optic
coupler) 48, which transfers the image to a recording device, with
as little loss of information as possible; and a photo sensing
array 50 (usually a CCD in a digital camera).
[0049] FIG. 3a shows an exemplary embodiment of the aperture 24 of
the preferred embodiment of the present invention. The source 22
projects an X-ray beam 28 through an aperture 24.
[0050] FIG. 3b shows the aperture 24 from the view of the plane of
line A-A of FIG. 3. The aperture 24 comprises a first plate 52 and
a second plate 54, which are opaque to radiation. Such plates would
typically be made of lead or other dense material. First plate 52
and second plate 54 intersect such that they form an opening 60
through which the X-ray beam 28 passes before impinging upon the
subject 30. The shape of the X-ray beam 28 is thus defined by the
shape of the intersection of the first plate 52 and second plate
54. To facilitate changes in the size and focus of the X-ray beam,
a positioning mechanism is attached to first plate 52 and second
plate 54. Either first plate 52 or second plate 54 can be adjusted
horizontally or vertically to define the size and position of the
opening 60.
[0051] FIG. 4 illustrates the projection of the x-ray beam 28
(FIGS. 1 and 2) through a scanning subject 30 (FIGS. 1 and 2) and
onto the photo sensing array 50 according to one exemplary
embodiment of the present invention. The area occupied by the
subject 30 throughout rotation is shown as a solid object 85. The
X-ray beam 28 has a peripheral configuration 82 which is simply the
border of the projected area of the X-ray beam 28. The peripheral
configuration 82 of the X-ray beam 28 is shown as it projects
through the volume 85 occupied by the subject 30 and impinges upon
the photo sensing array 50. The peripheral configuration 82 of the
X-ray beam 28 strikes a plurality of rows 93 and columns 95 in the
photo sensing array 50 effectively scanning a plurality of rows
(slices) 93 simultaneously.
[0052] FIG. 5 shows the patient support structure 26 of the
preferred embodiment of the present invention. The patient support
structure 26 is constructed of material substantially transparent
to X-ray radiation. Such materials include low density plastic
structures or other low density materials. The patient support
structure 26 is of a semi-cylindrical shape approximately five feet
in height.
[0053] The inside surface of the patient support structure 26 which
the back of the subject 30 rests against is shaped to conform to
the shape of the back of the subject 30, such that pressing the
subject 30 against the back of the patient support structure 26
restrains the subject from unnecessary movement during
rotation.
[0054] The inside surface of the patient support structure 26 is
padded with material substantially transparent to X-ray radiation
such that a subject 30 is comfortably supported during the scanning
process. Such materials include low density foam polymer such as
neoprene or foam rubber.
[0055] A removable, adjustable seat 62 is attached to the patient
support structure 26 by being inserted into slots in the patient
support structure 26. The patient support structure 26 contains
several slots at various heights to accommodate imaging at
different heights. The slots are semi-circular and horizontal
occupying the inner surface of the patient support structure 26.
The slots are slightly thicker than the edge of the seat 62 such
that the seat 62 can be inserted without excessive force and remain
in place when occupied by a subject 30. The seat 62 is padded for
comfort.
[0056] The airbag restraint mechanism 32 is made up of an outer cup
60 and an inner airbag 88. The cup 60 is composed of a material
substantially transparent to X-ray radiation. The cup 60 is
semi-cylindrical in shape and hinged through a plurality of hinges
42 to the patient support structure 26 on one edge of the cup 60.
The hinges 42, allow the cup 60 to rotate about the hinges 42 such
that the edge of the cup 60 opposite the hinged edge of the cup 60
contacts the patient support structure 26 and the two form a closed
circle. The edge of the cup 60 opposite the hinges 42 can be
secured to the patient support structure 26 with a latch 46. Both
the hinges 42 and the latch 46 can be attached at different heights
on the patient support structure 26 thereby allowing for restraint
at different vertical positions.
[0057] The airbag 88 is attached to the inner surface of the cup 60
such that when the cup 60 encloses the subject 30, the airbag 88 is
located between the subject 30 and the cup 60. The airbag 88 is
constructed of a material substantially transparent to X-ray
radiation such as a low density plastic such as polyurethane or
nylon. Other low density materials such as latex or neoprene would
also be suitable.
[0058] When imaging is about to commence the airbag 88 is inflated
to a pressure sufficient to restrain the subject 30 from
unnecessary movement during rotation. Any substance behaving
substantially as a fluid is suitable to inflate the airbag 88, air
being the safest and most cost effective.
[0059] In an alternative embodiment, the airbag 88 may be replaced
with other restraining mechanism means, such as a belt, a strap or
a fastener.
[0060] The support structure 26 is mounted on a rate table 36 which
can be constructed of any rigid material. The rate table 36
contains an electric motor (stepper, servo or DC motor) for
rotating the patient support structure 26 about the center of the
rate table 36 to precisely preset positions. The electric motor is
controlled by the computer system 40 through control cable 56 (FIG.
1). The patient support structure 26 is attached to the rate table
36 such that the two form a single unit.
[0061] FIG. 6 shows a head restraint included in one embodiment of
the present invention comprising a head support structure 78 which
is attached to the support structure 26 with fasteners such that it
can be easily removed. The head support structure 78 is constructed
of a material substantially transparent to radiation such as a low
density plastic or cloth. The head support structure 78 extends
above the patient support structure 26 as shown in FIG. 6.
[0062] The top portion of the head support structure 78 forms a
semi-cylindrical shell with similar function to the patient support
structure 26. The top portion of the head support structure 78 is
padded similarly to patient support structure 26 such that the head
of the subject is restrained from unnecessary motion when pressed
against the head support structure 78.
[0063] A strap 92 is used to secure the forehead of the subject 30
against the head support structure 78 such that the subject's head
is restrained from unnecessary movement during rotation and
scanning processes.
[0064] Alternatively a device similar in operation to the airbag
restraining mechanism 32 could be employed to secure the head of
the subject 30 to the head support structure 78. Such a device
would have to constructed such that the airbag contained therein
did not overly pressure the soft tissue of the face.
[0065] FIG. 7 shows an exemplary embodiment of the present
invention utilizing height adjustment for scanning at different
height positions. The source 22 is mounted on a source support
mechanism 67. The source support mechanism 67 comprises two
cylindrical rails 66, 68 on which the source 22 can move vertically
72.
[0066] A manual or automated device is operative to elevate and
lower the source 22 and a second such device is operative to
elevate and lower the detector array 50. Such elevator devices are
commercially available through many sources. The source support
mechanism 67 uses a mechanism to hold the x-ray source in position
on the rails at the appropriate height for scanning.
[0067] The detector array 50 is mounted to a detector support
mechanism 74. The detector support mechanism 74 functions similarly
to the source support mechanism 67, except that the detector array
50 is locked into positions instead of the source 22. An elevator
device is also used to move the scintillation screen 34 vertically
70. The height positions of the detector support mechanism 74
correspond precisely to the height positions of the source support
mechanism 67, such that the source 22 remains stationary relative
to the detector array 50. This structure reduces the amount of
calibration that is unnecessary artifacts normally introduced in a
conventional computer tomography system to reposition the source
22.
[0068] Auto-calibration is preformed by fixing the relationship
between the X-ray source 22 and detector array 50 constant. A laser
targeting device(not shown) mounted to the source 22 is used to
align the detector array 50. The distribution of X-ray energy is
predetermined and normalized between the different detectors within
the array 50. In addition, the detector sensitivity among the
detectors within the array 50 can also be predetermined and
normalized. Each shadow image created is corrected with the
predetermined image corrections to account for anomalies due to the
relative position of the source 22 and photo sensing array 50.
[0069] FIG. 8 is a block diagram illustrating a process flow for
generating images in one embodiment of the present invention.
Electronic signals 58 are read by the computer system 40 as image
data 94 representing X-ray beam 28 intensity at each row 93 and
column 95 and angular increment of rotation 91 (see FIG. 4). The
image data 94 undergoes calibration 96 by correcting the image data
94 to compensate for aberrant measurements by comparing it to
pre-calibrated data 106. Pre-calibrated data 106 are intensity
measurements taken before a subject 30 is introduced. This data 106
is compared to the image data 94 to compensate for uneven energy
distribution in the X-ray beam 28. The corrected image data 94 is
then stored electronically.
[0070] After all necessary corrected image data 94 has been stored,
the compiled data 94 is combined and subjected to back projection
102 to create a tomographic image. Back projection 102 is an
established method of constructing tomographic images. The
tomographic image then undergoes array to pixel transformation 110
and is converted to a pixel image which can portray the tomographic
image as either a two dimensional cross section or a three
dimensional object. The pixel image is then displayed on a display
112 such as a color computer monitor or cathode ray tube.
[0071] Although the present invention has been described in terms
of specific embodiments, it is anticipated that alterations and
modifications thereof will no doubt become apparent to those
skilled in the art. It is therefore intended that the following
claims be interpreted as covering all such alterations and
modification as fall within the true spirit and scope of the
invention.
* * * * *