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Claims  |
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What is claimed is:
1. A system for controlling the direction of a fan beam of X-radiation,
produced at the focal spot of an X-ray source of a tomography system and
directed toward a detector array of X-ray detectors of the tomography
system, said system comprising:
(a) fan beam defining means including a fan beam collimator, positionable
between the focal spot and at least some of the detectors of the array,
for defining said fan beam; and
(b) means for defining a secondary beam of X-radiation, produced at the
focal spot of the X-ray source at the same time as the fan beam, and
directed transversely to the fan beam;
(c) means, responsive to said secondary beam of X-radiation, for detecting
a shift of the focal spot in a direction parallel to the Z-axis of the
tomography system and for generating a control signal as a function of the
amount of said shift; and
(d) means, responsive to the control signal, for moving the fan beam
collimator so as to maintain an alignment between said fan beam and said
detectors.
2. A system according to claim 1, wherein said means for detecting a shift
of the focal spot and for generating the control signal comprises (a)
target means for receiving X-radiation from said secondary beam, and for
generating a signal representative thereof, and (b) means for defining a
secondary collimator, positionable between the focal spot and said target
means, for defining said secondary beam of radiation, said secondary beam
being directed from the focal spot onto the target means so that movement
of said focal spot in a direction parallel to the Z-axis shifts said
secondary beam of radiation on said target means.
3. A system according to claim 2, wherein said target means comprises a
second array of detectors, positionable relative to said focal spot so
that shifting of said focal spot in a direction parallel to the Z-axis
shifts said secondary beam of radiation transversely to said second array.
4. A system according to claim 3, wherein said second array of detectors
are positionable relative to said focal spot so that shifting of said
focal spot in a direction parallel to the Z-axis shifts said secondary
beam of radiation perpendicularly to said second array.
5. A system according to claim 3, wherein the detectors of the second array
each generates a detection signal as a function of the radiation portion
of the secondary beam detected by the detector, and said means for
detecting a shift of the focal spot and for generating a control signal
further comprises means for processing the detection signals received from
at least some of said detectors of said second array so as to determine
the position of the center of said secondary beam and for generating said
control signal as a function of said position.
6. A system according to claim 3, wherein the detectors of the second array
each generates a detection signal as a function of the radiation portion
of the secondary beam detected by the detector, and said means for
detecting a shift of the focal spot and for generating a control signal
further comprises means for processing the detection signals received from
at least some of said detectors of said second array so as to determine a
centroid value of the detection signals as a function of the relative
position of each of said detectors and the value of each detection signal,
and for generating said control signal as a function of said centroid
value.
7. A system according to claim 6, wherein said means for generating said
control signal includes means for processing the detection signals through
a plurality of channels, and the centroid value is determined by the
following equation:
##EQU3##
where C represents the centroid value of the beam,
N.sub.c is the channel number,
V.sub.c is the signal value for the channel, and
n is the number of channels receiving signal information.
8. A system according to claim 3, wherein said second array of detectors
includes two detectors disposed at opposite ends of the array for
providing a failure signal should either of said two detectors detect at
least a portion of the secondary beam.
9. An improved tomography system comprising:
a source for generating radiation from a focal spot;
means for rotating said source about a Z-axis during a tomographic scan;
a primary array of detectors for receiving radiation from the source during
a tomographic scan;
primary collimating means, positioned relative to the focal spot, for
collimating radiation emanating from the focal spot so as to define a
primary beam of radiation propagating from the focal spot onto the
detectors of said primary array during a tomographic scan so as to define
a fan beam;
means for defining a secondary beam of radiation propagating from the focal
spot at the same time as the fan beam, and directed transversely to the
fan beam;
means, responsive to a secondary beam of radiation, for sensing the
position of the focal spot and for generating a position signal as a
function of the position of said focal spot; and
means, responsive to the position signal, for moving said primary
collimating means so as to maintain an alignment of said primary beam of
radiation and the detectors of the primary array.
10. A system according to claim 9, wherein said means for sensing the
position of the focal spot and for generating a position signal comprises
target means for receiving X-radiation from said secondary beam and for
generating a signal representative thereof, and said means for defining a
secondary beam includes a secondary collimator, positioned between the
focal spot and said target means, for defining said secondary beam of
radiation, said secondary beam being directed from the focal spot onto the
target means so that shifting of said focal spot shifts said secondary
beam of radiation on said target means.
11. A system according to claim 10, wherein the primary beam of radiation
is disposed substantially within and defines the scanning plane of the
tomography system wherein the Z-axis is substantially normal to said
scanning plane, and said secondary beam extends along an axis transverse
to said scanning plane.
12. A system according to claim 9, wherein said means for sensing the
position of the focal spot and for generating the position signal
comprises target means for receiving X-radiation from said secondary beam
and for generating a signal representative thereof, and said means for
defining a secondary beam includes a secondary collimator, positionable
between the focal spot and said target means, for defining a secondary
beam of radiation, said secondary beam being transverse to the scanning
plane of the tomography system and being directed from the focal spot onto
the target means so that movement of said focal spot in a direction
parallel to the Z-axis shifts said secondary beam of radiation on said
target means.
13. A system according to claim 12, wherein said target means comprises a
second array of detectors, positionable relative to said focal spot so
that shifting of said focal spot in a direction parallel to the Z-axis
shifts said secondary beam of radiation transversely to said second array.
14. A system according to claim 13, wherein said second array of detectors
are positioned relative to said focal spot so that shifting of said focal
spot in a direction parallel to the Z-axis shifts said secondary beam of
radiation perpendicularly to said second array.
15. A system according to claim 13, wherein the detectors of the second
array each generates a detection signal as a function of the radiation
portion of the secondary beam detected by the detector, and said means for
detecting a shift of the focal spot and for generating a control signal
further comprises means for processing the detection signals received from
at least some of said detectors of said second array so as to determine
the position of the center of said secondary beam and for generating said
control signal as a function of said position.
16. A system according to claim 13, wherein the detectors of the second
array each generates a detection signal as a function of the radiation
portion of the secondary beam detected by the detector, and said means for
detecting a shift of the focal spot and for generating a control signal
further comprises means for processing the detection signals received from
at least some of said detectors of said second array so as to determine a
centroid value of detection signals as a function of the relative position
of each of said detectors and the value of each detection signal, and for
generating said control signal as a function of said centroid value.
17. A system according to claim 16, wherein said means for generating said
control signal includes means for processing the detection signals through
a plurality of channels, and the centroid value is determined by the
following equation:
##EQU4##
where C represents the centroid value of the beam,
N.sub.c is the channel number,
V.sub.c is the signal value for the channel, and
n is the number of channels receiving signal information.
18. A system according to claim 13, wherein said second array of detectors
includes two detectors disposed at opposite ends of the array for
providing a failure signal should either of said two detectors detect at
least a portion of the secondary beam.
19. A system according to claim 12, wherein said target means includes: a
plurality of detector elements arranged in a linear array extending along
a longitudinal axis, and said plurality of detector elements are arranged
relative to said focal spot so that movement of said focal spot in a
direction parallel to the Z-axis shifts said secondary beam substantially
parallel to said longitudinal axis.
20. A system according to claim 19, wherein each said detector element
includes a detecting surface and each said detecting surface is arranged
in a linear array along said longitudinal axis and lies in a common plane
transverse to said secondary axis and said secondary beam.
21. A system according to claim 20, wherein each said detecting element has
an output for transmitting a corresponding position signal to said means
for moving said primary collimator means, and said linear array has two
ends and the outputs of the detecting elements at each said end of said
linear array are summed together and the outputs of adjacent pairs of
detecting elements between said ends are summed together.
22. A system according to claim 20, wherein said detecting means includes
sixteen detecting elements in said linear array.
23. A system according to claim 22, wherein each said detecting element has
an output for transmitting said position signals to said means for moving
said primary collimating means, and the linear array has two ends and the
outputs of the detecting elements at each said end of said linear array
are summed together and the outputs of adjacent pairs of detecting
elements between said ends are summed together so as to form eight output
channels.
24. A system according to claim 9, wherein said means, responsive to the
position signal, for moving the primary collimating means so as to
maintain the alignment of said primary beam of radiation and the detectors
of the primary array includes processing means, including a lookup table
means, for storing data relating to the value of said position signal with
the position of said primary collimating means.
25. A system according to claim 9, wherein said primary collimator means
includes a plurality of slits for defining a corresponding number of
different primary beams of radiation, and said means for moving said
primary collimating means includes means for selectively moving each of
said slits into alignment with said focal spot and primary array of
detectors. |
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Claims |
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Description |
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RELATED APPLICATION
This application is related to U.S. Ser. No. 08/343,248, contemporaneously
filed with the present application in the names of John Dobbs and Hans
Weedon, entitled X-ray Image Data Normalization in a CT Scanner, and
assigned to the present assignee (Attorney's Docket Number ANA-60).
1. Field of the Invention
The invention relates generally to tomography systems, and more
particularly to an apparatus for maintaining the alignment of a collimated
beam of radiation in a tomography system so as to (a) compensate for
Z-axis focal spot shift and (b) maintain the positional stability of the
beam on detectors of the tomography system.
2. Background of the Invention
Tomography systems have been used for many years to create images of
cross-sectional slices through objects, and are particularly useful as a
diagnostic aid. The most commonly known form is the CT (computerized
tomography) scanner. The third generation type of CT scanner includes a
disk or drum, rotatably supported in a gantry support frame and supporting
an X-ray source and an X-ray detector array. Typically, the radiation
source provides either periodic X-ray pulses or continuous-wave (CW)
X-rays. The X-rays emanate from a point usually referred to as the "focal
spot". A collimator is provided to control the shape and the direction of
the X-ray beam from the focal spot. The detector array, usually comprising
gas or solid state detectors for counting X-ray photons, is disposed on
the disk at a predetermined position diametrically opposite to the source
on opposite sides of an opening in the disk (through which the object
being scanned is disposed) so as to define a corresponding plurality of
X-ray paths from the source and collimator to the respective detectors
within a common plane of rotation of the disk. The ray paths between the
focal spot and the detectors resembles a fan, and hence the term "fan
beam" is sometimes used to refer to the shape of the beam.
In the fourth generation type CT scanner, the rotatable disk supports the
X-ray source. However, the detectors are disposed on the gantry support
frame, which rotatably supports the disk. The detectors are distributed
around the support frame at predetermined angular spacings relative to the
center of the disk so as to define a corresponding plurality of X-ray
paths from the source to the respective detectors within a common plane of
rotation of the disk. As the source rotates about the rotation axis of the
disk, the ray paths from the rotating source to each detector resembles a
fan beam with respect to each detector in which the point of convergence
of the fan beam is the detector.
In either type of CT scanner, the disk is normally adapted to rotate
through at least a full 360 degree rotation about a rotation axis,
sometimes referred to as the "Z-axis", so that the source (and in the case
of third generation machines the detectors) rotates through a plurality of
incremental positions where a corresponding series or set of readings
(called "projection views") by the detectors are made. The number of
photons absorbed along the various ray paths through the object, during
each sampling period defining each projection view, is a function of the
absorption characteristics of the portions of the object along each path
during each set of readings. Thus, a plurality of projection views are
taken through the portion of an object disposed within the common plane of
rotation of the X-ray paths (hereinafter the "scanning plane"). The
detectors generate a corresponding plurality of analog information signals
representative of X-ray flux detected by the detectors during each
sampling period or projection view.
The output analog information signals of the X-ray detectors acquired from
all of the projection views of the 360 degree rotation, i.e., through all
of the incremental angular positions of the 360 degree rotation within the
plane of rotation, are processed, typically through a back projection
processing technique, so as to create a reconstructed image of the
interior structure of the object exposed to the X-rays, typically in the
form of a two-dimension image of a thin slice.
Great success has been achieved in the development of components, i.e.,
sources and detectors, for providing data based upon accurately measured
X-ray flux during each projection view, as well as improved data
acquisition systems (DASs) for processing the analog information signals
provided from the detectors. The benefits of these developments include
reduced electrical artifact-producing errors, such as gain error,
systematically-related radial or displacement-related errors, electrical
noise caused by electrical and encoding quantization noise, offset and
gain variations, differential non-linearity of encoding devices and
dielectric absorption effects, among others.
However, even employing such elaborate electrical compensation schemes,
errors can still arise due to mechanical misalignments which occur during
a scan, or over a number of scans. These errors can be critical when one
considers that the image generation through CT scanning by the back
projection method is based upon mathematical relationships first developed
by Johann Radon. The standard CT scanning system, based upon these
mathematical relationships, assumes that the components of the system,
especially the source and detectors, always remain perfectly aligned
relative to one another during a scan, both throughout each scan and over
the course of many scans, and that the components move about an axis of
rotation, the Z-axis, so as to circumscribe a "perfect" circle in the
scanning plane, concentric with the intersection point of the rotation
axis with the rotation plane, so as to define a "phantom image" of all
mass disposed in the circle in the scanning plane.
The back projection process of reconstructing the phantom image is based on
reconstructed values derived as a function of the measured data values
obtained during each projection view of the scan. Thus, the image data
acquired during the scan of the circular phantom image can be used to
reconstruct the phantom image including the portion of any object in the
flux path within the plane of rotation. It should be clear that the X-ray
flux measurements themselves must be made extremely accurate relative to
each other throughout each scan. Otherwise, systematic or random errors
can prevent the determination of the reconstructed values to the degree of
accuracy necessary to achieve a reconstructed image to the desired level
of quality.
Consequently, any mechanical misalignments and lateral movement of the
various tomography components during a scan, or those occurring over
several scans (which are not correctable using current electronic error
correcting techniques) can cause major inaccuracies in reconstructed
images. Such images are misleading in their apparent information content
and could lead to improper diagnosis by a physician relying on the data.
These types of errors can destroy the integrity of the reconstructed image
so as to make proper analyses very difficult, if not impossible.
One particular cause of misalignment is the source of the radiation itself.
For example, in one type of X-ray source, the X-rays are generated by
directing an electron beam at a tungsten anode. To aid in heat dissipation
and to avoid melting, the anode, in the form of a disk, is continuously
rotated through the path of the electron beam such that only a small
portion of the disk is exposed to the electron beam at anyone instant of
time. It is this small portion of the disk where the disk is exposed to
the election beam that defines the focal spot or emanation of the high
energy photons. A collimator containing a beam defining slit is typically
positioned near the focal spot and relative to the detectors so that the
X-ray beam defined by the slit will be aligned with the detector array.
However, as the rotating disk anode heats up, thermal expansion causes the
focal spot to shift, thus causing the path of the X-ray beam defined by
the collimator slit to positionally drift with respect to the collimator
and detectors. Typically, the rotational axis of the anode disk is
perpendicular to the scanning plane and parallel to the Z-axis, and
therefore any shift in the anode disk, hereinafter called "Z-axis beam
shifting", causes a drifting or shift of the focal spot in a direction
parallel to the Z-axis so that the X-ray beam shifts relative to the
Z-axis. This shift means that not all of the X-ray flux emanating from the
focal spot and passing through the slit of the collimator will necessarily
be detected by each detector of the detector array, both because of the
resulting spatial misalignment of the focal spot relative to the slit and
detector array, and because of variations in the sensitivity of the
detectors of the detector array in the Z-axis direction. This shift
therefore can detract from the integrity of the image data and can cause
major inaccuracies in the reconstructed image.
OBJECTS OF THE INVENTION
Accordingly, it is a general object of the present invention to provide an
improved tomography system.
It is another, more specific object of the present invention to provide an
improved apparatus for providing a more positionally stable beam of high
energy X-ray photons used in a tomography system.
Another object of the present invention is to provide an improved X-ray
radiation source assembly for use in a tomography system adapted to
compensate for shifting of the focal spot of the X-ray source.
A further object of the present invention is to provide an improved
tomography system which compensates for misalignment of the source
relative to the collimator and detectors due to Z-axis beam shifting so as
to provide more accurate scan data.
And another object of the present invention is to provide an improved
source of X-rays which automatically compensates for the shifting of the
focal spot of the radiation source so as to maintain alignment of the
focal spot with respect to the collimator and detectors of a tomography
system.
Yet another object of the present invention is to compensate for Z-axis
focal spot shift and maintain the positional stability of the beam on the
detector array in order to reduce errors in X-ray flux measurement due to
variation in sensitivity of the detectors of the detector array in the
Z-axis direction.
SUMMARY OF THE INVENTION
These and other objects of the present invention are achieved by a system
for controlling the direction of the X-ray beam of a tomography system
relative to collimator and the X-ray detector array of the tomography
system so as to compensate for any shift of the focal spot in a direction
parallel to the Z-axis so that the focal spot, collimator and the
detectors remain aligned with one another.
In accordance with the preferred embodiment, the system includes a source
of radiation for defining a focal spot from which a beam of radiation
emanates, a monitoring device for detecting a shift of the focal spot in a
direction parallel to the Z-axis, and an actuating device, preferably
including a primary X-ray collimator, for adjusting the direction of the
beam in response to a shift of the focal spot along an axis parallel to
the Z-axis, hereinafter the "Z'-axis", as determined by the monitoring
device so as to maintain the focal spot, primary collimator and detector
array of the tomography system aligned.
This is preferably achieved by defining at least two beams of radiation
from the focal spot of the source. Both beams preferably emanate from the
same focal spot of the X-ray source. A primary beam of radiation is used
to perform the scanning function of the tomography system, and is directed
along a primary axis in the scanning plane through the object being
scanned to the primary detector array of the tomography system. A
secondary beam of radiation is directed at an angle with respect to the
first beam, preferably in a direction transverse to the scanning plane, to
a secondary monitoring device which monitors the position of the focal
spot of the source of the beam along the Z'-axis. The secondary monitoring
device preferably includes means for generating a position signal
representative of the position of the focal spot along the Z'-axis. The
monitoring device determines, in response to the position signal, if the
focal spot has shifted along the Z'-axis, and sends any necessary control
signals to the actuating device so as to move the primary collimator and
adjust the direction of the primary beam in order to insure that the
primary beam is correctly aligned with the primary X-ray detector array.
In a typical third generation tomography system, the primary beam is
defined by the primary collimator, usually in the form of an X-ray
impervious plate having a narrow elongated slit so as to define a fan beam
emanating from the focal spot and increasing in width as the distance from
the focal spot increases. The thickness of the slice defined by the beam
to which the scanned object is exposed where the beam intersects the
primary detector array is determined by (i) the width of the narrow slit
of the primary collimator, and (ii) the spacings between the focal spot,
primary collimator and the detector array. The width of the slit is much
smaller than both the length of the slit and the diameter of the circle
circumscribed by the X-ray source as it rotates about the Z-axis of
rotation of the disk, e.g., the ratio of the slit length: slit width can
vary from about 40 to 200. Typically, the thickness of the beam is less
than about 10 mm, although this can vary.
The preferred system also includes a second collimator which forms the
shape of the secondary beam and directs it onto the secondary monitoring
device. As the focal spot shifts along the Z'-axis, the position of the
secondary beam on the secondary monitoring device also shifts. The
secondary monitoring device produces an output control signal
representative of the amount and direction of the shift along the Z'-axis.
The actuating element moves the primary collimator, in response to the
control signal, to maintain the alignment of the focal spot, primary
collimator, and the detector array of the tomography system, and thus,
compensate for Z-axis beam shifting.
As described in copending U.S. Ser. No. 08/343,248, contemporaneously filed
with the present application in the names of John Dobbs and Hans Weedon,
entitled X-ray Image Data Normalization in a CT Scanner, and assigned to
the present assignee (Attorney's Docket Number ANA-60), the secondary
monitoring device can also be used to monitor the level of X-ray flux
generated by the X-ray source, so that the secondary monitor device can be
used to control the output of the radiation source and/or to provide
compensation data when reconstructing the phantom image.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects of this invention, the various features
thereof, as well as the invention itself, may be more fully understood
from the following description, when read together with the accompanying
drawings in which:
FIG. 1 is an axial view of a CT scanning apparatus employing the preferred
embodiment of the present invention;
FIG. 2 is an isometric view of a typical source of high energy X-ray
photons and the fan beam created by the source and defined by the primary
collimator of the apparatus of FIG. 1;
FIG. 3 is a diagrammatic view of the preferred embodiment of the focal spot
movement compensation system of the present invention for use in the
apparatus shown in FIG. 1;
FIG. 4 is an isometric view of one secondary collimator and the detector
array of the preferred secondary X-ray monitoring device of the focal spot
movement compensation system shown in FIG. 3 and arranged in accordance
with the present invention;
FIG. 5 is a diagrammatic view of the preferred embodiment of the secondary
monitoring device; and
FIG. 6 is a graph illustrating the relationship between the signal output
of the secondary monitoring device and the position of the secondary X-ray
beam on the detector array of the focal spot movement compensation system
of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a third generation type CT scanning apparatus 12, modified in
accordance with the present invention. Apparatus 12 includes a support
structure comprising a cart 14 for supporting a gantry assembly 16
including a cylindrical or annular support frame 18 and a rotatable disk
20 rotatably supported in the frame 18 for rotation about the Z-axis,
indicated at 28. The support frame 18 can be pivotally mounted on the
sides of cart 14 so as to permit adjustment of the orientation of the
support frame 18 about the pivot axis 22. A suitable drive system,
including drive rollers and idler rollers indicated at 24, for supporting
and rotating the disk 20, is shown and described in greater detail in U.S.
patent application Ser. No. 08/193,783, entitled X-Ray Tomographic
Scanning System filed Feb. 8, 1994 in the name of Gilbert W. McKenna, and
assigned to the present assignee (Attorney's Docket Number ANA-30). The
disk 20 includes a circular opening 26 large enough for the torso of a
patient 30, supported on a patient pallet 32, to pass through. The disk 20
also carries a source 40 of high energy X-ray photons and defining a focal
spot 42 from which the photons emanate. The source is positioned on one
side of the opening 26, while an array of primary detectors 50 are
positioned opposite the source 40 for detecting X-ray photons received
from source 40. The array of detectors are arranged in an arc having a
center of curvature coincident with the focal spot 42. The disk can also
carry energy sources and other support systems (not shown) necessary for
performing the scanning function, as is well known, and as described for
example, in U.S. patent application Ser. No. 08/193,696, entitled
Tomographic Scanner Having Center of Rotation for All Physics filed Feb.
8, 1993 in the names of Ronald E. Swain and Gilbert W. McKenna, and
assigned to the present assignee (Attorney's Docket Number ANA-31).
In order to define the fan beam 44 of X-rays photons emanating from the
focal spot 42 of the source 40, a primary collimator 60 is disposed
between the source 40 and the array of primary detectors 50. The
collimator 60 is made of a material impervious to X-rays, such as lead or
a lead alloy, and is provided with a narrow slit 62. The length of the
slit 62 (the longer dimension) defines in part the angle .alpha. subtended
by the fan beam around the focal spot 42 (the angle .alpha. also being a
function of the distance the collimator 60 is placed relative to the focal
spot 42), while the width of the slit defines the depth or thickness of
the beam passing through the object being scanned, e.g., patient 30, and
coincident on the array of primary detectors 50. Ideally, the slit is
dimensioned so that the beam is entirely intercepted by the detectors,
with the maximum surface area of the detectors 50 being exposed to the
X-ray photons.
As shown in FIG. 2, the fan beam 44 is produced by an X-ray source 40 and
is defined by the slit 62 of the primary collimator 60. The fan beam
intersects the detectors of the array so as to define ray paths, with the
ray path extending from the focal spot 42 through the Z-axis 28 defining
the path of the "center ray". For convenience of exposition, and to
facilitate understanding and establish convention, the center ray will be
referred to as the "primary axis" or "Y-axis" of the scanning plane,
indicated at 70 in FIG. 2, while the X-axis of the scanning plane extends
normal to the Y-axis and Z-axis and is identified at 72 in FIG. 2. It
should be appreciated that the Y-axis 70 and X-axis 72, which define the
scanning plane, to the extent they are used to define the fan beam rotate
with the disk 20 of FIG. 1 about the Z-axis 28, and that the Figs. show
only one possible rotational position of the disk.
As shown in FIG. 3, the X-ray source 40 includes an electron beam generator
122, which directs a beam of electrons 132 at an anode 124. Preferably the
anode 124 is a tungsten disk, rotatably mounted on a shaft 126, supported
by bearings 128 and rotatably driven by motor 130. The anode 124 is
rotated to facilitate cooling so as to prevent it from overheating and
prevent the electron beam 132 from damaging the anode 124. Typically, the
X-ray source 40 is enclosed in an evacuated housing 118 as shown by dashed
lines in FIG. 3 and positioned so that the rotation axis 134 of the shaft
126 is parallel to the Z-axis. In the preferred embodiment, the housing
118 is composed of glass and further enclosed in an oil filled aluminum
housing (not shown) to facilitate cooling.
Referring still to FIG. 3, the X-ray source 40 produces a primary beam of
X-rays that emanate from the focal spot 42 through a port in the housing
118. The slit 62 of primary or slice defining collimator 60 defines and
shapes the beam cross-section and directs the primary beam 44 along the
primary or Y-axis 70. To the extent thus far described with reference to
FIGS. 1-3, the tomography apparatus 12 is known in the prior art. In the
prior art the shaft 126 of the source 40 tends to thermally expand due to
heat generated by the source as it produces X-rays, and thermally contract
when the source stops producing the X-rays and the heat is dissipated.
Such thermal expansion and contraction causes the focal spot 42 to shift
along an axis, Z'-axis 136, parallel to the Z-axis 28 and the axis 134,
resulting in the focal spot 42 moving relative to the slit 62 of the
primary collimator 60 as well as the detectors 50, as indicated by the
bidirectional arrow 138 shown in FIG. 3. This can produce errors in
reading the X-rays with the detectors 50 if the movement of the shaft 126
occurs during a scan, or over the course of many scans since not all of
the flux emanating from the focal spot 42 and passing through the slit 62
of the collimator 60 will necessarily be detected by each detector 50 of
the detector array because (1) of the resulting spatial misalignment of
the focal spot relative to the slit and detector array and (2) of
variations in the sensitivity of the detectors 50 in the Z-axis direction.
In accordance with the present invention, a focal spot movement
compensation system, indicated generally at 150, is provided to compensate
for shifts of the focal spot 42 in a direction parallel to the Z-axis 28
and the Z'-axis 136. The system 150 includes both the primary collimator
60 and a secondary collimator 160 having a slit or aperture 162 for
defining, shaping and directing a secondary beam 170 along a secondary
axis 172 onto the secondary monitoring device 180 for monitoring the beam
170. As described hereinafter, the secondary monitoring device 180
preferably includes a detector array for providing an output signal as a
function of the position of the beam 170. The output of the device 180 is
connected to a control system or controller 184 for providing a correction
signal as a function of the amount of shift of the focal spot 42 as
detected by the device 180. The control system is connected to an actuator
device 190 mechanically connected to the primary collimator 60 and adapted
to move the collimator so as to move the slit 62 in a direction parallel
to the Z-axis 28 and Z'-axis 136, as indicated by the bidirectional arrow
192 shown in FIG. 3. The primary collimator 60 directs the primary beam 44
along the axis 70 through the patient 30 onto the primary detector array
50, as in the prior art, except that the collimator is adapted to
bidirectionally move as indicated by arrow 192 so as to maintain the
relative alignment of the focal spot 42, slit 60 and detectors 50.
Optionally, a precollimator 194 having a slit 196 can be provided to
precollimate the primary beam 44 along the axis 70, although it should be
appreciated that the slit 196 should be wide enough to allow for movement
of the focal spot 42, and therefore the beam 44, and the primary
collimator 60 so that the beam 44 is aligned with the primary detector
array 50.
In the preferred embodiment, the primary collimator 60, is similar to the
primary collimator used in the prior art, and for example comprises a
plate of X-ray impervious material, such as a tantalum, on the order of 1
mm thick, having at least one substantially rectangular opening or slit 62
so as to define the fan beam 44 to which the primary detector array 50 is
exposed. The primary or slice defining collimator 60 can be adjustable to
control the width or "fan out" of the beam and adjustable to control the
thickness (Z dimension) of the beam. In the preferred embodiment, the slit
62 of the primary collimator 60 imparts a substantially rectangular shape
on the cross-section of the beam 44 in which the width (X dimension) of
the beam is several orders of magnitude greater than the thickness (Z
dimension) of the beam, particularly at greater distances from the focal
spot 42 along the primary axis 70. In this embodiment, it is desirable for
the thickness of the beam 44 to be as small as possible to minimize
unnecessary X-ray exposure, and large enough to provide sufficient X-rays
for a tomographic scan. One method of controlling the thickness of the
beam includes sandwiching together tantalum or lead plates to form a
shutter and sliding them relative to each other by any suitable means to
affect the thickness of the beam. The width or "fan out" of the beam, i.e.
angle ".alpha." (shown in FIG. 1), may be controlled in a similar manner.
The primary collimator 60 preferably includes a movable plate 196 provided
with a slit 198, and a second plate 200 having single slot 202 fixed
relative to the Z-axis, the plates being arranged relative to one another
so as to function as a shutter. The slot 202 is much wider than the slit
198 and is sufficiently wide so that the slit can move in either direction
of arrow 192 between two extreme locations so that once assembled for
operation, the slice of the beam will always be the same and can always be
aligned with the detector array 50 regardless of the position of the focal
spot 42. The moveable plate 196 is secured in a suitable manner, such as a
follower nut of a screw-follower arrangement, which follows a screw,
indicated at 204 driven by actuating device 190 so that rotation of the
screw causes the plate 196 to translate in a direction parallel to the
Z-axis 28, without rotating about the screw. Device 190 preferably
includes a motor 186, controlled by a microprocessor in the control system
184. In the preferred embodiment, the actuating device 190 includes a
stepping motor where each step corresponds to a different position of the
movable plate 196 in one of the directions indicated by bidirectional
arrow 192. The total travel of the primary collimator is set to be
approximately 0.9375 inches, which corresponds to 2880 steps of the
stepping motor, providing translational movement of the plate 196 of only
about 0.0003255 inches per step, although these values can vary. Since
actual movement of the focal spot in a typical system requires only a
small amount of correction by the primary collimator, e.g. about 10 steps
of stepping motor 186, or about 0.003255 inches, the plate 198 can be
provided with more than one slit 198, such as shown 198A, so that fan
beams, i.e., slices of different thicknesses (in the Z-dimension) can be
easily provided by moving the desired slit into alignment with the slot
202 with the actuating device 190. The slits are designated by their
respective thicknesses of the fan beams they are specified to produce,
with the lengths of the slits being all the same in order to expose all of
the primary detectors of array 50, and the actual width of each slit being
slightly less than its nominal designation due to the spreading of the
beam 44. In the preferred embodiment, for example, the slits are 0.1297",
0.0648", 0.0389", and 0.0258" wide to produce 10 mm, 5 mm, 3 mm and 2 mm
thick beams, respectively.
In addition to the stepping motor 186, the actuating device 190 also
preferably includes a counter 188 for keeping track of the steps, and thus
the position of the plate 196. The counter is connected to and
communicates with the microprocessor provided in the control system 184.
As shown generally in FIG. 3, and in greater detail in FIGS. 4 and 5, the
secondary beam 170 is directed through the secondary collimator 160 onto
the secondary monitoring device 180. The secondary collimator 160 is also
made of an X-ray impervious material | | |
