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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to apparatus and methods for flooding a nuclear
imaging device with radiation from a radiation source; for correcting for
non-uniformities in count density in nuclear imaging sources; for
providing uniform flooding of a nuclear imaging device by a standardized
source (solid or liquid); and in one of its aspects is directed to such
apparatus and methods for single photon emission computed tomography
("SPECT") and in another of its aspects is directed to such apparatus and
methods for cobalt-57 sources.
2. Description of the Prior Art
In nuclear imaging, compensation can be made for various errors such as
errors attributable to detection systems and items such as collimators. In
compensating for these errors it is known to require a flood-field image
of a standard radiation-emitting uniform source and to correct the images
produced by a camera after it has been calibrated with the uniform source
image. But in so calibrating nuclear imaging devices, a source with
non-uniform radiation emission count density can cause errors for which
compensation cannot be made. Especially in single photon emission computer
tomography, source count density uniformity is a necessity; but sources,
even certified standard sources, do exhibit varying degrees of count
density non-uniformity. These source non-uniformities cannot be
compensated for ("masked") as can deficiencies in the detection system
(e.g. crystals or photo multiplier tubes) or deficiencies in a collimator.
Commercially available standard nuclear imaging sources, including solid
sources and refillable liquid standards, exhibit some degree of
non-uniformity. Refillable liquid standards such as liquid phantoms are
subject to deformation, poor mixing, absence of retrospective
verification, accidental contamination and substantial handling
requirements. Increased handling required by refillable phantoms, such as
a refillable liquid technetium-99m phantom, subjects technologists to
increased radiation exposure.
Commercially available cobalt-57 solid sheet source standards generally
fail to provide an adequate uniform count density. Such standards with a
stated certified non-uniformity of .+-.1% may actually have a much higher
non-uniformity if standard National Electrical Manufactures Association
("NEMA") procedures are applied, since certified non-uniformity is based
on a sampling of counts from the surface of the standard rather than on a
total surface radiography. Also, since non-uniformity is quantified from
counts in only two smoothed pixels (the maximum and the minimum), the
method is potentially dependent on small flaws.
One method proposed for compensating for source non-uniformity involves a
tedious manual re-positioning of a source for mapping in multiple
positions. See "Uniformity Correction for SPECT Using a Mapped Cobalt-57
Sheet Source," Volume 26, No. 4, The Journal of Nuclear Medicine, April
1985.
In accordance with .sctn.1.56 of 37 C.F.R., the following are disclosed:
U.S. Pat. No. 4,697,075 Image evaluation apparatus for quantitative and
qualitative evaluation of an optical image generated by an x-ray imaging
calibration using projection means
U.S. Pat. No. 4,649,561 Test phantom for x-ray imaging systems
U.S. Pat. No. 4,646,341 Calibration standard for x-ray fluorescence
thickness measurement gauges
U.S. Pat. No. 4,663,772 Bone mineral analysis phantom for medical
diagnostic imaging
U.S. Pat. No. 4,613,754 Tomographic calibration apparatus
U.S. Pat. No. 4,571,491 Method for obtaining an atomic number image of an
unknown material
U.S. Pat. No. 4,517,460 Method of calibrating a gamma camera which samples
the output signal of a photo-multiplier tube/amplifier combination
U.S. Pat. No. 4,506,375 Radiometric calibration of an x-ray detector
U.S. Pat. No. 4,331,869 A dynamic phantom system for cardiac monitoring
U.S. Pat. No. 4,321,471 Monitoring the speed of rotation of a target of an
x-ray source
U.S. Pat. No. 4,172,978 Computerized tomographic apparatus which permits
evaluation of differences in performance of multiple detectors
U.S. Pat. No. 4,071,760 Radiography apparatus with photo cell drift
compensating means
U.S. Pat. No. 4,066,902 Radiography with detector compensating means
U.S. Pat. No. 4,014,109 Test phantom for evaluating the scan of a nuclear
imaging device
U.S. Pat. No. 3,715,588 Bone mineral analyzer with a radioactive photon
source
U.S. Pat. No. 2,374,280 X-ray photographic timer tester
U.S. Pat. No. 2,258,593 Calibration method for x-ray machines
U.S. Pat. No. 1,589,833 Measuring device for measuring short-wave limit of
x-ray spectrum and voltage of x-ray tube
U.S. Pat. No. 1,531,620 Device for measuring the percentage of transmission
and absorption of x-rays passing through a given medium
Publications
Uniformity Correction for SPECT Using a Mapped Cobalt-57 Sheet Source,
Oppenheim and Appledorn, The Journal of Nuclear Medicine, Volume 26,
Number 4, Apr. 1985.
Perspectives on Tomography, Keyes, The Journal of Nuclear Medicine, Volume
23, Number 7, 1982.
Physical Attributes of Single-Photon Tomography, Budinger, The Journal of
Nuclear Medicine, Volume 21, Number 6, 1980.
Single Photon Emission Computed Tomography (SPECT) Principles and
Instrumentation, Jaszczak and Coleman, Investigative Radiology, Dec. 1985.
NEMA A Guide to Revised Standards for Performance Measurements of
Scintillation Cameras, National Electrical Manufacturers Association,
1986.
There has long been a need for efficient and effective apparatuses and
methods for correcting for count density non-uniformity in nuclear
calibration sources. There has long been a need for apparatuses and
methods for uniformly flooding a nuclear imaging device such as a gamma
camera with radiation from a standard radiation source. There has long
been a need for them particularly for SPECT. The present invention
recognizes, addresses, and satisfies these needs.
SUMMARY OF THE INVENTION
The present invention teaches a method for uniformly flooding a nuclear
imaging device with radiation from an imaging source and for correcting
for the non-uniformity in count density of nuclear imaging sources and
apparatus for carrying out the methods. The method includes the steps of
positioning a nuclear imaging flood source beneath or adjacent a nuclear
imaging device such as a gamma camera having a plurality of radiation
sensing elements; moving the source in a controlled motion; thereby
blurring or diminishing the effects of any non-uniformities in the source,
i.e., insuring that each sensor or detector in the camera receives
radiation from more than one radiation-emitting area of the source so that
each sensor receives a more similar amount of light (radiation) than if
the source was not moved. The movement of the source can be accomplished
over a long period of time, in a pattern, in a reproducible pattern,
continuously, in a complex motion, or repetitively.
Apparatus for moving the source includes a support for the source and a
means for moving the source adjacent to or beneath a camera. Circular
motion may be achieved by connecting the support to means for rotating it,
such as a motor drive shaft, or by manually rotating it. Other simple
motion may be achieved by means which move the source about a simple
curve. If complex motion is desired, one embodiment of the apparatus
includes a plurality of geared shafts interconnected between the source
support and a drive motor. The resultant motion is a complex patterned
motion which presents many points on the source to each of a plurality of
sensors or detectors in the camera, thus diminishing the effects of
non-uniformities and local flaws in the source. In one embodiment the
source can be a phantom or a refillable liquid phantom; but a solid source
is preferred, particularly in cases requiring calibration for the common
tracers technetium-99m and iodine-123, a solid cobalt-57 sheet flood
source such as those which are commercially available. Other solid sources
which are suitable include (but are not limited to) a solid radioactive
gold source in calibrating for the heart tracer thallium-201.
It is therefore an object of the present invention to provide a method for
correcting for non-uniformities in count density in a nuclear imaging
source.
Another object of the present invention is the provision of apparatus for
such correction.
Yet another object is the provision of such correction for single photon
emission computed tomography.
A further object is the provision of such correction for solid and liquid
sources and phantoms.
An additional object is the provision of such correction for solid sources,
including but not limited to radioactive gold and cobalt-57 sources.
Another object is the provision of such correction which employs a method
which includes, and apparatuses which produce, the continuous motion of
the source with respect to a gamma camera or other nuclear imaging device.
Yet another object is the provision of such apparatus and method in which
the motion is complex.
A further object of the invention is the provision of apparatuses and
methods for uniformly flooding a nuclear imaging device with radiation
from a source, particularly a standardized solid source.
To one of skill in this art who has the benefit of this invention's
teachings, other and further objects, features, and advantages will be
apparent from the following description of presently preferred embodiments
of the invention, given for the purpose of disclosure, when taken in
conjunction with the accompanying drawings. Although this disclosure is
detailed to insure adequacy and aid understanding, this is not intended to
prejudice that purpose of a patent which is to protect the invention no
matter how others may later disguise it by variations in form or additions
or further improvements. The claims at the end hereof are intended as an
aid toward this purpose.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross sectional view of a typical prior art nuclear
imaging device, a gamma camera, disposed above a source of gamma
radiation.
FIG. 2 is a side view of a gamma camera disposed above a human patient.
FIG. 3 is a front perspective view of apparatus according to the present
invention disposed beneath a gamma camera.
FIG. 4 is a side perspective view of the apparatus of FIG. 3.
FIG. 5 is a front perspective view of the apparatus of FIG. 3 without the
gamma camera or source.
FIG. 6 is a side cross sectional view of the shaft and gear assembly of the
apparatus of FIG. 3.
FIG. 7 is a schematic view of the motor-shaft linkage of the apparatus of
FIG. 3.
FIG. 8 is a reproduction of an actual trace of a pencil held against a
piece of paper mounted on the support of the apparatus of FIG. 3 and
rotated thereby.
FIG. 9 is a copy of a flood image using a large stationary flood source
with three small radiating disks placed on the large source.
FIG. 10 is a copy of a flood image obtained by subjecting the source of
FIG. 9 to simple (circular) motion according to the present invention.
FIG. 11 is a copy of a flood image obtained by subjecting the source of
FIG. 9 to complex motion according to the present invention.
FIG. 12 is a copy of a computer-produced representation of the complex
motion of a point on the source support of the apparatus of FIG. 3.
FIG. 13 is a schematic view of a motor-shaft combination for providing
circular motion for the apparatus of FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 illustrates a typical commercially available prior art nuclear
imaging device, a gamma camera 1, having a collimator 2 through which
gamma radiation from a source 8 passes to a scintillation crystal 3,
through a light pipe 4 to photo multiplier tubes 5 from which signals go
to an electronics section 6 for position and energy analysis resulting in
output signals 7. FIG. 2 illustrates the gamma camera 1 disposed above a
human patient 9 who has received a radiation emitting radioisotope.
FIGS. 3-7 illustrate an apparatus 10 according to the present invention for
moving a standard commercially available flood source 20 beneath a gamma
camera 56. The apparatus 10 includes a cart 11 formed of various support
members 12 welded together with a cart bottom shelf 13 extending between
the support members and a cart top shelf 14 extending between the support
members above the bottom shelf 13. For convenience of handling and
movement the cart is mounted on wheels 15 and has a cart handle 24
extending between two of the support members above the top shelf 14.
An electric motor 16 is mounted to a motor mount 55 which is secured to the
bottom shelf 13. The motor used was a model no. 544 from Bodine Electric
Company. A speed control 17 is mounted to and beneath top shelf 14 with a
power cord 22 extending therefrom. A power cord 23 extends from the speed
control to the motor 16.
A plurality (four) of support rods 19 extend through and are secured to the
top shelf 14 and the bottom shelf 13. A ball bearing 21 is mounted at the
top of each support rod 19 and a source support plate 18 is movably
disposed on the ball bearings 21. Experience has shown that casters (not
shown) work as well as, but more quietly than, the ball bearings.
Between the motor 16 and the source support plate 18 is a gear-shaft
assembly 30 which is shown in detail in FIG. 6. As shown in FIG. 7 the
motor 16 has a gear box 25 from which a motor shaft 26 extends. A small
sprocket 27 is secured to the shaft 26. A chain wraps around the small
sprocket 27 and around a large sprocket 28 which is secured to a bottom
shaft 31 of the gear-shaft assembly 30. The driven movement of the
gear-shaft assembly 30 produces a complex motion pattern for the source 20
beneath the gamma camera 56. It is within the scope of this invention to
move the source along a simple curve or along a complex curve. Circular
motion can be achieved by simply driving a top shaft 33 by the motor
directly.
As shown in FIG. 6, the bottom shaft 31 is disposed in and through and is
rotatable within a bearing 38 which is secured to the top shelf 14. The
shaft 31 is also disposed in and through and is rotatable within a bottom
shaft gear 34. A bottom steel cuff 45 is secured to the bottom shaft 31 by
a lock screw 41 in a recess 57 in the bottom steel cuff 45, the lock screw
41 engaging the bottom shaft 31.
A middle shaft 32 has a first middle shaft gear secured to it by a lock
screw 40. The first middle shaft gear mates with and turns with the bottom
shaft gear 34. The middle shaft 32 extends through and is rotatable within
a bearing 47, the bottom steel cuff 45, and a bearing 48. A lock collar 50
secured to the middle shaft 32 by a lock screw 44 maintains the bearings
47, 48, a gear 36, and the cuff 45 in position about the middle shaft 32.
The middle shaft 32 extends into and is secured to a top steel cuff 46 by
a lock screw 41. A second middle shaft gear 36 is secured to and partially
within the bottom steel cuff by a lock screw 42. The bearing 48 extends
through and is rotatable within the second middle shaft gear 36.
A top shaft 33 extends through and is rotatable within bearings 49. The
bearings 49 are partially disposed within the top steel cuff 46 which
itself is tightly embraced about the bearings 49. A top shaft gear 37
secured to the top shaft 33 by a lock screw 43 mates with and turns with
the second middle shaft gear 36. A lock collar 51 secured to the top shaft
33 by a lock screw 43 mates with and turns with the second middle shaft
gear 36. A lock collar 51 secured to the top shaft 33 maintains the
bearings 49 and the top steel cuff 46 in position above the top shaft gear
37. A top lock collar 52 is secured to the top shaft 33 by lock screws 44.
A flange 53 welded to the top stop collar 52 supports and is secured to
the source support plate 18 (see FIG. 5).
The bottom shaft 31 is driven by the sprocket 28 and rotates at a fixed
axis with respect to the top shelf 14. The middle shaft 32, driven by the
bottom shaft gear 34 through the first middle shaft gear 35, moves in a
circle around the bottom shaft 31. The gears 34, 35 control the speed of
the middle shaft 32.
The middle shaft 32 pulls the second middle shaft gear 36 along with it
when it moves. The top steel cuff 46 moves with the middle shaft 32 since
it is secured to the shaft. When the top steel cuff 46 moves, it pulls the
top shaft 33 with it. As it does so, the top shaft gear 37 moves with the
top shaft 33 and, in so doing, the top shaft gear 37 drives the second
middle shaft gear 36. The speed of the top shaft 33 is dependent upon the
gear ratio of gear 37 to gear 36 (in combination with the gear ratio of
bottom shaft gear 34 to first middle shaft gear 35). The speed of the
middle shaft 32 is dependent on the gear ratio of the bottom shaft gear 34
to the first middle shaft gear 35.
In the apparatus as shown in FIGS. 3-7 the gear ratio of gears 37, 36 was
3:1 and that of gears 34, 35 was 1.6:2.5. Of course any desired and
appropriate ratios could be used. The chain 29 had a 1/4 inch pitch; the
small sprocket 27 had an effective diameter of 0.996 inches; and the large
sprocket 28 had an effective diameter of 4.777 inches. The shaft 26 had a
maximum speed of 72 revolutions per minute, but it was preferred to run it
at about one-half that speed. Best results were obtained when the bottom
shaft 31 was initially aligned with the top shaft 33 in order to align the
source support plate beneath the nuclear imaging device. A change in the
source support plate's motion path [to effect motion of the source along
other simple or complex curves in a plane illustrated by line A--A in FIG.
3 which is parallel to a plane in which individual sensors (not shown) of
the gamma camera are disposed (such as the plane of line B--B in FIG. 3]
can be effected by a variety of changes including, but not limited to,
changes in shaft diameter, gear diameter, gear ratios, and cuff size.
The controlled continuous complex motion achieved by the apparatus 10 is
shown in two ways. A pencil trace 54 reproduced in FIG. 8 was obtained by
placing a piece of paper on the source support plate 18 and holding a
pencil above and in contact with the paper while the plate was moved by
the apparatus 10. The trace of FIG. 8 shows the path that one detector in
the camera takes over the source, i.e., it indicates the areas on the
source detected by one detector in the camera. A motion trace 68 shown in
FIG. 12 was produced by a computer simulation of the motion of the
apparatus 10 and represents the path followed by a point on the support
plate 18 about three inches outward from the top shaft 33. (Of course, the
actual path is smooth and continuous.) This trace is that which would be
produced on a piece of photographic film above the source if a point light
source was placed on the source support plate and then the plate was moved
by the apparatus 10.
As will be discussed in more detail below, the images shown in FIGS. 9-11
demonstrate the use of the present invention's methods and apparatus when
a standard source has additional small radioactive areas. FIG. 9
illustrates a gamma radiation source image 60 which includes hot spots or
flaws 61, 62, 63 created by the addition of three cobalt-57 discs to the
standard source. FIG. 10 shows a gamma radiation source image achieved by
subjecting the source which produced image 60 to simple circular motion
according to this invention. This produced a more uniform image, but also
produced undesirable ring disc image tracks 64, 65, 66. Subjecting the
source which produced image 60 to complex motion with the apparatus 10
achieved the relatively more uniform image 67 shown in FIG. 11. Simple
circular motion of the source support plate 18 can be achieved by simply
mounting a shaft beneath the source support plate and rotating it either
by hand or with a motor.
As shown in FIG. 13 the motor 16 can have its drive shaft 26 connected
directly to the support plate 18 to produce circular motion of a source
placed on the support plate.
The effective non-uniformity of several flood correction approaches was
quantified. Stationary approaches involved a commercially available
standard 1% cobalt-57 flood source (ST) and a refillable Tc-99m
pertechnetate flood source phantom (RE). The ST approach utilized a 10 mCi
Co-57 solid sheet flood source certified as less than 1% non-uniform. The
RE source was charged with 20 mCi Tc-99m pertechnetate, agitated, allowed
to stand for 6 hours, and again agitated to achieve adequate mixing of
activity throughout the source. Particular care was taken to avoid
deformation of the phantom due to the volume of the Tc-99m solution.
Motion was introduced first by circularly (CI) rotating ST at 25
revolutions per minute. Complex motion (CX) was achieved with an apparatus
such as the apparatus 10 of FIG. 3. The continuously moving source
repeated the pattern diagrammed in FIG. 12 every 17 minutes.
Non-uniformity was quantitated by autoradiography. Autoradiography was
performed by attaching a cardboard cassette loaded with high resolution
radiographic film to a fixed surface. For ST and RE determinations, the
sources were placed beneath the loaded cassette for 60 and 18 hours,
respectively. For CI and CX, the sources were moved beneath the loaded
cassette for 60 hours. Exposed films were then developed.
The developed film was divided into one centimeter squares. A
microdensitometer was used to measure the optical density of each square
within the central (75%) field of view (CFOV) of each flood image.
Multiple readings of arbitrary squares were taken to establish a
reliability value. Optical density matrices were thus compiled for each
flood. After appropriate smoothing according to NEMA procedures, the
nonuniformity of each matrix was computed using the usual formula: the
difference between the maximum and minimum smoothed pixel density divided
by their sum. The standard deviation for optical densities measured in
each square was also computed for each CFOV.
Since the purpose of these tests was to optimize the flood field matrix
used to provide uniformity correction for SPECT data acquisition, each
approach (ST, CI, CX and RE) was used to acquire 30,000,000 counts into a
64 by 64 digital matrix. The sources were imaged through a slightly
defective low energy all purpose collimator to simulate the environment
often encountered clinically. Each matrix was then analyzed using
commercially available quality assurance software to compute differential
and integral non-uniformity by standard methods.
Another experiment introduced local flaws, in the form of small Co-57
sources, to the flood source (see FIGS. 9-11). Flood images using the ST,
CI and CX approaches were acquired for 30,000,000 counts. The same
analytic methods were then applied as described in the paragraph above.
Autoradiographic analysis established the moving flood sources, CI and CX,
as both more uniform than the stationary cobalt flood source, ST, and
comparable to RE (Table I). The precision of the autoradiography was
determined by repeating the reading of 112 regions. The optical density
measured during the two readings was 1.211.+-.0.018 and 1.217 .+-.0.020
(mean .+-. standard deviation). A single pixel was remeasured 90 times.
The optical density was 1.120.+-.0.004 (mean .+-. standard deviation).
The semiautomatic analysis using 30,000,000 count images | | |
