Image shift for gamma camera

A gamma camera includes first and second detectors which face an examination region. The detectors are rotatable about the examination region and translatable in a direction tangential to the examination region. Translation of the detectors is coordinated with the rotation of the detectors about the examination so as to increase the effective field of view of the detectors. In a first embodiment, the detectors are translated in the transverse direction when the detectors are located at each of a plurality of positions about the examination region. In a second embodiment, translation of the detectors is coordinated such that, for a given projection angle, the first detector is used to detect radiation data from a subset of the region of interest.

BACKGROUND OF THE INVENTION 
The present invention relates to the art of diagnostic imaging. It finds 
particular application in conjunction with nuclear or gamma cameras and 
will be described with particular reference thereto. It is to be 
appreciated, however, that the present invention will also find 
application in other non-invasive investigation techniques and imaging 
systems such as single photon planar imaging, whole body nuclear scans, 
positron emission tomography (PET) and other diagnostic modes. 
In diagnostic nuclear imaging, one or more radiation detectors are mounted 
on a movable gantry to view an examination region which receives a subject 
therein. Typically, one or more radiopharmaceuticals or radioisotopes 
capable of generating emission radiation are injected into the subject. 
The radioisotope preferably travels to an organ of interest whose image is 
to be produced. The detectors scan the subject along a selected path or 
scanning trajectory and radiation events are detected on each detector. 
Typically, the detector includes a scintillation crystal that is viewed by 
an array of photomultiplier tubes. The relative outputs of the 
photomultiplier tubes are processed and corrected, as is conventional in 
the art, to generate an output signal indicative of (1) a position 
coordinate on the detector head at which each radiation event is received, 
and (2) an energy of each event. The energy is used to differentiate 
between various types of radiation such as multiple emission radiation 
sources, stray and secondary emission radiation, transmission radiation, 
and to eliminate noise. A two-dimensional image representation is defined 
by the radiation data received at each coordinate. The radiation data is 
then reconstructed into an image representation of a region of interest. 
Detecting radiation with two detector heads oppositely disposed from each 
other improves the resolution and data collection efficiency, particularly 
for whole body imaging. For other studies, particularly cardiac studies, 
it is advantageous to position the detector heads orthogonally to each 
other. This enables a complete 180 degree data set to be collected by 
rotating the pair of detector heads only 90 degrees relative to the 
subject. Still other gamma cameras have three heads placed at 120 degree 
intervals around the subject. Typically, the heads are movable radially 
toward and away from the patient and the three heads are rotatable, as a 
unit, around the patient. In each case, the detector face is placed as 
close as possible to the patient during a diagnostic scan for collimated 
imaging. The close proximity is necessary to minimize the loss in spatial 
resolution due to collimator blur. 
Each of the foregoing systems has various advantages and disadvantages. The 
cost of a gamma camera system increases as additional detector heads are 
added. Systems having two opposed detector heads are particularly useful 
for whole-body imaging. Wide field of view detectors, which permit 
scanning of the entire width of the body, are preferably used in this 
application. Systems having two orthogonal detectors are commonly used for 
cardiac imaging. Because a wide field of view is not required in cardiac 
applications, smaller detectors are preferably used to allow the detectors 
to be placed as close as possible to the patient. Three detector head 
systems are often used in connection with high sensitivity brain and 
cardiac imaging. Although wide field of view detectors are desirable for 
body imaging, their physical size again limits performance in head 
imaging. Because the large detectors cannot be placed as close as possible 
to the patient's head, the system spatial resolution is compromised. The 
placement of the three detector heads also limits the utility of three 
detector systems in whole body and brain applications. Furthermore, as the 
detectors are moved, their associated field-of-view and resolution may 
change resulting in inaccurate collection of data and inaccurate 
reconstruction of images. 
The present invention provides a new and improved diagnostic imaging system 
and method which overcomes the above-referenced problems and others. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a diagnostic imaging method 
utilizes a gamma camera having first and second detectors disposed in a 
relative angular orientation with respect to an examination region. The 
first and second detectors have respective first and second radiation 
sensitive faces which face the examination region and first and second 
transverse fields of view, and the region of interest extends beyond the 
fields of view. The method includes the steps of using the first and 
second detectors to detect radiation indicative of radionuclide decays 
occurring within the examination region, and rotating the first and second 
detectors about the examination region. The steps of using and rotating 
are repeated a plurality of times so as to detect radiation with the first 
and second detectors located at a plurality of projection angles. In 
coordination with the rotation of the first and second detectors about the 
examination region, the first and second detectors are moved in a 
transverse direction so that, at each of the projection angles, the 
effective transverse field of view of the first and second detectors 
includes the region of interest. An image indicative of the detected 
radiation, for example a conventional SPECT image which includes one or 
more image slices orthogonal to the longitudinal axis of a patient, is 
generated. 
According to a more limited aspect of the present invention, the method 
includes positioning the first and second detectors at first and second 
transverse positions at a plurality of the projection angles. 
According to a still more limited aspect of the present invention, at a 
plurality of the projection angles, the first detector is translated 
between a first position wherein the first detector's field of view 
extends to at least a perimeter of the region of interest and a second 
position wherein the first detector's transverse field of view extends to 
at least an opposite perimeter of the region of interest. 
According to another still more limited aspect, the detectors are disposed 
in a ninety degree configuration. 
According to another more limited aspect of the invention, the first and 
second detectors each generate a signal indicative of an axial and 
transverse position on the face of the detector at which radiation is 
detected. According to another more limited aspect, the method includes 
reconstructing a plurality of parallel image slices indicative of the 
detected radiation. 
According to another more limited aspect of the present invention, the 
method includes, rotating the first and second detectors about the 
examination region so that the first detector is located at a first 
projection angle and the second detector is located at a second projection 
angle, utilizing the first detector to detect radiation indicative of 
radionuclide decays occurring within the examination region, a first 
portion of the region of interest being located outside the first 
detector's field of view, rotating the first and second detectors about 
the examination region so that the second detector is located at the first 
projection angle, positioning the second detector so that the first 
portion of the field of interest is within the second detector's field of 
view, and utilizing the second detector to detect radiation indicative of 
radionuclide decays occurring within the examination region. 
According to a still more limited aspect of the invention, the first and 
second detectors are disposed in a 180 degree opposed configuration. 
According to yet another still more limited aspect of the present 
invention, the method includes the steps of, with the second detector 
located at the second projection angle, utilizing the second detector to 
detect radiation indicative of radionuclide decays occurring within the 
examination region, a second portion of the region of interest being 
located outside the second detector's field of view, rotating the first 
and second detectors about the examination region so that the first 
detector is located at the second projection angle, positioning the first 
detector so that the second portion of the region of interest is within 
the first detector's field of view, and utilizing the second detector to 
detect radiation indicative of radionuclide decays occurring within the 
examination region. 
According to another aspect of the present invention, a diagnostic imaging 
method utilizes a gamma camera having a detector which includes a 
radiation sensitive face which faces an examination region. The detector 
has a transverse field of view. The method includes using the detector to 
detect radiation indicative of radionuclide decays occurring within the 
examination region, rotating the detector about the examination region, 
repeating the steps of utilizing and rotating a plurality of times so as 
to detect radiation with the detector located at a plurality of projection 
angles, and generating an image indicative of the detected radiation. At a 
plurality of the projection angles, the detectors are placed in first and 
second transverse positions whereby the effective transverse field of view 
of the detector is greater than the actual transverse field of view of the 
detector. 
According to a more limited aspect, a parallel hole collimator is disposed 
between the radiation sensitive face and the examination region. 
One advantage of an embodiment of the first invention is that radiation 
data is accurately collected in accordance with current field-of-views of 
each detector. 
Another advantage is that the effective field of view of the detectors may 
be advantageously increased. 
Another advantage is that the size of the radiation sensitive detectors may 
be decreased. Another advantage is that a larger region of interest may be 
examined than was heretofore possible. 
Yet another advantage is that greater flexibility in obtaining images is 
available. 
Still further advantages of the present invention will become apparent to 
those of ordinary skill in the art upon reading and understanding the 
following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, a diagnostic imaging system includes a subject 
support or table 10 which is mounted to stationary, vertical supports 12 
at opposite ends. The subject table is selectively positionable up and 
down to center a subject 16 in the center of a circle along a longitudinal 
axis 14. 
An outer gantry structure 20 is movably mounted on tracks 22 which extend 
parallel to the longitudinal axis. This enables the outer gantry structure 
to be moved parallel to the longitudinal axis 14. An outer gantry 
structure moving assembly 24 is provided for selectively moving the outer 
gantry structure 20 along the tracks 22 in a path parallel to the 
longitudinal axis. In the illustrated embodiment, the longitudinal moving 
assembly includes drive wheels 26 for supporting the outer gantry 
structure on the tracks. A motive power source, such as a motor 28, 
selectively drives one of the wheels which frictionally engages the track 
and drives the outer gantry structure and supported inner gantry structure 
and detector heads therealong. Alternately, the outer gantry can be 
stationary and the subject support configured to move the subject along 
the longitudinal axis. 
An inner gantry structure 30 is rotatably mounted on the outer gantry 
structure 20. A first camera or radiation detector head 32a is mounted to 
the inner gantry structure. Second and third radiation detector heads 32b, 
32c are mounted to the inner gantry structure orthogonal to the first 
camera head. Of course, the detectors may be positioned to oppose each 
other at any angle suitable for detecting radiation. It is to be further 
appreciated that a greater or lessor number of detectors can be provided 
and detectors having non-planar radiation receiving surfaces can be used. 
The detectors 32a, 32b, 32c detect radiation, the type of which depends on 
the type of imaging performed. The inner gantry structure defines a 
central, subject receiving examination region 36 for receiving the subject 
table and, particularly along the longitudinal axis. The examination 
region 36 is enlarged to receive the detector heads in any of a variety of 
displacements from a central axis and angular orientations. 
The detectors each include a scintillation crystal disposed behind a 
radiation receiving face 38a, 38b, 38c, respectively, that is viewed by an 
array of photomultiplier tubes. In SPECT imaging, a collimator (such as a 
parallel hole collimator) is disposed between the radiation receiving face 
and the examination region so as to limit the acceptance angle of 
radiation received by the face. The scintillation crystal emits a flash of 
light in response to incident radiation. The array of photomultiplier 
tubes convert the light into electrical signals. A resolver circuit 
resolves the x,y-coordinates of each light flash and the energy of the 
incident radiation. The relative outputs of the photomultiplier tubes are 
processed and corrected, as is conventional in the art, to generate an 
output signal indicative of a position coordinate on the detector head at 
which each radiation event is received, and an energy of each event. A 
data collection processor collects and processes the radiation data in 
accordance with the type of radiation received. It is to be appreciated 
that a collimator may be mounted on the detectors in accordance with the 
type data desired to be collected. 
To increase the image quality obtained during a diagnostic scan, the 
radiation detectors are positioned as close as possible to a region of 
interest of the subject. To position the radiation detectors in desired 
orientations and distances from the subject, a motor and drive assembly 
50a, 50b, 50c is connected to each detector, respectively, which includes 
means for canting, shifting, and zooming the detectors in and out from the 
subject, for example, as described in U.S. Pat. No. 5,838,009 which issued 
on Nov. 17, 1998 and is assigned to the present assignee, expressly 
incorporated by reference herein. Alternately, a single motor and drive 
assembly controls movement of all detector heads individually or as a 
unit. 
With reference to FIG. 2, an example of the three detector camera is shown 
where the three detectors 32a, 32b, 32c have been zoomed-in and shifted in 
an iris-type motion from their original positions 60a, 60b, 60c, 
respectively, represented by the dotted fines. The new orientation of the 
three detectors results in portions of each detector to be overlapped by 
another detector such as area 62 of detector 32c. In other words, a 
field-of-view of each detector is reduced such that areas of a detector 
within the field-of-view are active areas and areas outside the 
field-of-view are inactive areas. In their original positions 60a, 60b, 
60c, the detectors are configured to receive radiation over their entire 
radiation receiving surface which has predefined field-of-view and a 
predefined resolution. However, once the detectors are zoomed and shifted, 
the field-of-view, the resolution and associated correction such as for 
center-of-rotation are correspondingly changed and the radiation received 
can no longer be processed correctly under the original field-of-view, 
resolution and correction parameters. 
With reference again to FIG. 1, to compensate for the new orientation of 
the detectors, a data collection processor 70 is linked with the motor and 
drive assemblies 50a, 50b, 50c of each detector so that position and 
orientation data of each detector is communicated to the data collection 
processor. The data collection processor collects radiation data 72 and 
includes a means 74 for determining the field of-view or active area of 
each detector and a suitable resolution based on the position and 
orientation data. Areas of each detector which are overlapped by another 
detector or are simply outside the field-of-view are referred to as 
inactive areas and are also determined. It is assumed that radiation 
received, if any, by inactive areas is unsuitable for image reconstruction 
and is therefore filtered out using any known filtering algorithm. In 
other words, radiation data collected at coordinate positions on a 
detector falling within an inactive area of a detector is disregarded. 
The remaining radiation data which is collected by active areas is adjusted 
76 based on the adjusted field-of-view of each detector. Similarly, the 
resolution of each radiation detector is adjusted based on the current 
active area and field-of-view of each detector. A reconstruction processor 
78 then reconstructs the adjusted radiation data into an image 
representation 80 into a human readable form in accordance with any known 
reconstruction or tomographic algorithm. 
With reference to FIG. 3A, an alternative diagnostic system is shown 
including two radiation detectors 90, 92 positioned at a 90.degree. angle 
to one another which are rotated around the subject 16 during a scan. In 
FIG. 3A, the detectors 90, 92 are positioned in a conventional arrangement 
where they are aligned with their respective mechanical center-of-rotation 
axes 90a and 92a. The detectors are not translated with respect to each 
other. Each detector receives radiation over an active area 94 and 96 
which does not cover the entire surface of the respective radiation 
receiving faces of each detector due to mechanical structural limitations. 
Typically, a region of interest 98 within the subject 16 does not 
completely fall within the field-of-views of each detector where the 
field-of-views are represented by 100 and 102 for detectors 90 and 92, 
respectively. The shaded area of the region of interest 98 is shown to be 
outside the field-of-views of the detectors. 
With reference to FIG. 3B, the detectors 90, 92 are translated with respect 
to each other thereby creating an overlapped region 110 on detector 90. 
The translation movement shifts the field-of-views 100, 102 of the 
detectors so that the region of interest 98 completely falls within the 
field-of-views. Projection data is generated based on the radiation data 
collected during the scan. 
Turning now to FIG. 4, a system having two detectors 200, 202 positioned at 
a 90 degree relative angular orientation is shown. With the detectors 200, 
202 in the positions shown by the solid lines, the centers of the fields 
of view coincide with the center of rotation. Each detector 200, 202 
receives radiation over an active area 204, 206 which is smaller than the 
width of the front face of the detector. Again, a region of interest 208 
within the subject extends beyond the transverse field of views 210, 212 
of the respective detectors 200, 202. 
It is desirable that the detectors 200, 202 receive radiation from the 
entire region of interest 208, even though its dimension is greater than 
the fields of view 210, 212 of the respective detectors. To accomplish 
this, the detectors are translated in a direction tangential to the 
examination region 36 as indicated by the arrows 214, 216 so that each 
detector receives data corresponding to the entire region of interest. In 
a first extreme position 200a, the detector 200 is translated to a first 
transverse position wherein the edge of its field of view 210 extends at 
least to the perimeter of the region of interest 208. In a second extreme 
position 200b, the detector 200 is translated to a second transverse 
position wherein the edge of its field of view 210 extends at least to the 
opposite perimeter of the region of interest 208. Radiation may be 
collected continuously as the detector 200 is being translated. The 
translation velocity may be constant, or the transverse velocity of the 
detector 200 may be varied as a function of transverse position so as to 
provide a desired transverse sensitivity profile. Sensitivity in the 
regions near the first and second extreme positions 200a, 200b may be 
enhanced by reducing the transverse velocity relative to more median 
positions. Similarly, sensitivity in the more median positions may be 
increased by reducing the transverse velocity in those areas. In 
particular, the transverse velocity may be selected to increase the 
relative transverse sensitivity in regions where the object exhibits 
relatively high attenuation characteristics or to reduce the relative 
sensitivity where the object exhibits relatively lower attenuation. Thus, 
the attenuation provided by the object in the transverse direction may be 
estimated and the transverse velocity profile adjusted based on known 
characteristics of the object (e.g., thickness and/or composition or by 
direct measurement) so that the relative transverse sensitivity profile is 
complementary thereto. Alternately, however, the detector 200 may be moved 
to one or more discrete positions with radiation data collected at each. 
The relative sensitivity profile may be adjusted by varying the time 
during which data is collected at each of the positions. While the 
foregoing discussion has focused on the detector 200, it is equally 
applicable to the detector 202. 
Translation of the detectors 200, 202 is preferably conducted in 
coordination with rotation of the detectors about the examination region 
36 to a plurality of projection angles, for example in a conventional 
circular or elliptical orbit. The detectors are translated in an amount 
sufficient to obtain a complete data set covering the region of interest 
for each projection angle. Depending on the shape of the region of 
interest and the size of the field of view, the magnitude of the requisite 
translation may vary as a function of the projection angle. In fact, 
translation may not be required at one or more of the projection angles. 
As data is collected, the tangential position of the detectors is 
determined. The data collection processor 70 uses this information to 
determine the transverse coordinate on the face of the detector 200, 202 
at which radiation was received. In this way, a data set representative of 
a field of view larger than the transverse field of view 210, 212 is 
generated. The data is used by the reconstruction processor 78 to 
reconstruct a human readable image as is conventional in the art. While 
the foregoing discussion focuses on a gamma camera having two detectors in 
a 90 degree configuration, it is equally applicable to gamma cameras 
having two detectors disposed in other relative angular orientations or 
having three or more detectors. The technique may also be implemented 
using a gamma camera having a single detector. 
Turning now to FIG. 5A, 180 degree opposed first 300 and second 302 
detectors are disposed at respective first and second projection angles. 
As shown in FIG. 5A, the detectors are offset in a transverse direction so 
that the centers of their respective fields of view do not coincide with 
the center of rotation. A portion 304 of the region of interest 308 
extends beyond the field of view of the first detector 300. Likewise, a 
portion 306 of the region of interest 308 extends beyond the field of view 
of the second detector 302. 
Turning to FIG. 5B, translation of the detectors 300, 302 is coordinated 
with rotation about the examination region. As shown in FIG. 5B, the first 
300 and second detectors have been rotated about the examination region by 
180 degrees and translated tangentially with respect to the imaging 
region. Again, a portion 312 of the region of interest 308 extends beyond 
the field of view of the first detector 300. A portion 310 of the region 
of interest likewise falls outside the field of view of the second 
detector 302. However, it will be appreciated that, at the second 
projection angle, the first detector is positioned so that it receives 
data from the portion 306 of the region of interest 308 outside the field 
of view of the second detector 302 prior to rotation. Likewise, at the 
first projection angle, the second detector 302 is positioned so that it 
receives data from the portion 304 of the region of interest 308 outside 
the field of view of the first detector 300 prior to rotation. 
Corresponding data from the first and second projection angles can then be 
combined to form a complete data set. Thus, a complete data set may be 
collected even though, for a given projection angle, data is collected 
with the detectors located at only a single tangential position. It will, 
of course, be appreciated that FIGS. 5A and 5B depict only two of a 
multiplicity of projection angles at which data is collected. While the 
foregoing discussion focuses on a gamma camera having two detectors 
disposed in a 180 degree configuration, it is equally applicable to gamma 
cameras having two detectors disposed in other relative angular 
orientation or having three or more detectors. 
Typical image reconstruction reconstructs the projection data based on the 
center-of-rotation. However, the translation causes the detectors to be 
offset from the center of rotation. Thus, to reconstruct an accurate 
image, the projection data is adjusted in accordance with the offset 
positions at which the projection data was collected. Once adjusted, the 
reconstruction processor 78 reconstructs the adjusted radiation data and 
generates an image representation 80 in a human readable form. 
The invention has been described with reference to the preferred 
embodiments. Obviously, modifications and alterations will occur to others 
upon reading and understanding the preceding detailed description. It is 
intended that the invention be construed as including all such 
modifications and alterations insofar as they come within the scope of the 
appended claims or the equivalents thereof.