Abstract:
A single photon emission computed tomography (SPECT) system for cardiac imaging including an open arc-shaped frame. A collimator subsystem is shaped to approximately match the thoracic contour to optimize the geometric efficiency for detecting photons emitted from the heart of patients having different sizes and weights and shaped to surround and position the collimator subsystem closely proximate a heart of a patient of the patients encompassed by at least one predetermined image volume for optimizing collimation of radiation photons emitted from the heart. The collimator subsystem is facilitated by a tracking system that is capable of quickly bringing up the collimator component, which meets a specific set of collimation requirements, into place for imaging. And an open arc-shaped detector system is coupled to the collimator subsystem having a shape closely matching the shape of the collimator subsystem for detecting collimated radiation photons from the collimator subsystem and generating output electrical signals.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/869,274, filed Dec. 8, 2006, which is incorporated by reference herein. 

   FIELD OF THE INVENTION 
   This subject invention relates to an improved single photon emission computed tomography (SPECT) system for cardiac imaging. 
   BACKGROUND OF THE INVENTION 
   SPECT systems are often used to show the distribution of a radioactive substance inside a patient&#39;s body. A source of penetrating radiation is administered to the patient, which typically consists of a pharmaceutical tagged with a radionuclide which emits radiation photons (radiopharmaceutical). The radiopharmaceutical is designed to be absorbed in a target organ, such as the heart muscle, or other organs or body part of interest. The emitted radiation photons are collimated with a collimator subsystem and detected by a detector subsystem which generates output electrical signals which are digitized and processed by a computer system to generate images of the regional distribution of the radioactive sources in and around the target organ. 
   One prior SPECT system proposed by the inventor hereof utilizes a large circular shape design for the frame, or the gantry, collimator subsystem, and the detector subsystem which attempted to accommodate a large patient cross-section while placing the patient&#39;s heart at the geometric center for imaging from multiple directions simultaneously. However, because of the off-center location of heart, the circular geometry had to be fairly large to enclose a large patient&#39;s thorax. As a result, the long-distance collimation offsets the potential gain in geometric efficiency and renders the circular design less than optimum. Furthermore, the design devoted considerable collimator and detector area to the patient&#39;s right-posterior side, where the heart is too distant from the collimator for effective collimation. The typical problem of low photon sensitivity in SPECT is further compounded in cardiac imaging where the desirable radiation photons are scarce: only about 2-4% of the injected dose is absorbed in the myocardium of the heart. This circular design approach results in a limited return of the heavily attenuated and scattered photons and sub-optimal image quality. 
   Conventional and contemporary SPECT systems used and proposed for cardiac imaging suffer from a major weakness: these systems do not provide optimum detection coverage for photons emitted from the heart because they allow a large fraction of high quality photons to escape detection. This is well demonstrated by the requirement of using detector rotation around the patient, e.g., as utilized in conventional dual-head systems. Obviously, as detector rotates in incremental steps to catch photons on the far side of the patient, the photons on the near side escape coverage. Additionally, the detector area of prior systems has not been used efficiently for detection of photons emitted from the heart: most of the time, a large portion of the detector area is directed to the surrounding background area of the thorax. 
   Thus, an optimal system design for cardiac SPECT imaging needs to provide efficient and optimum detector coverage for high quality photons emitted from the heart, while effective collimation and adequate data sampling are achieved at the same time. Therefore, how to obtain an optimal balance between detector coverage, collimation, and sampling is the key to the design of a high-performance SPECT system. 
   The collimator subsystems of conventional SPECT systems are designed with only one predefined set of collimation parameters. For different imaging requirements, or for patient having different sizes, a different set of collimation parameters is often preferred. Therefore a different collimator with different collimation parameters is needed. However, changing the collimator during a conventional SPECT imaging procedure is not realistically feasible because such a procedure takes an inordinate amount of time and effort, and more importantly, it disturbs the patient&#39;s imaging position which is hard to be restored after the collimator change. The result is that conventional SPECT systems are not flexible in accommodating different collimation requirements to suit various clinical situations and patients having different sizes. 
   Additionally, conventional SPECT systems typically incrementally rotate the large, heavy collimator and the detector subsystem about the patient to obtain a plurality of projection images (projections). Each time the collimator and the detector subsystem are rotated step-by-step, the collimator and detector follow the patient&#39;s body contour by successively adjusting their radial and lateral positions. Such a technique is cumbersome, not easily reproducible, prone to both mechanical and electrical errors, slow, inefficient, utilizes expensive hardware to rotate large heavy collimator and detector subsystems, and requires extensive safety measures to protect the patient. As a result, conventional SPECT images have large variations in image quality and reproducibility, which make comparison of images from different facilities or from different times at the same facility difficult. 
   BRIEF SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide an improved SPECT system for cardiac imaging. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging in which the shape and size of the collimator and detector subsystems optimize detection of radiation photons emitted from the heart. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which provides a plurality of predetermined imaging volumes of various sizes and locations for optimizing data acquisition and image quality. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which provides high quality SPECT images for patients having different sizes and shapes in a typical patient population. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which positions a heart to a desired location based on a previous scout image to optimize SPECT imaging. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which provides a plurality of collimation parameters. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which eliminates the problems associated with moving a collimator and a detector subsystem about a patient. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which is easier to use. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which provides images that are reproducible at different facilities. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which provides images that are reproducible at different times at the same facility. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which is cost effective relative to the performance it can achieve. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which improves sensitivity for a given imaging spatial resolution. 
   It is a further object of this invention to provide such a SPECT system for cardiac imaging which improves imaging spatial resolution for a given sensitivity. 
   The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
   This invention features a single photon emission computed tomography (SPECT) system for cardiac imaging including an open arc-shaped frame. A collimator subsystem having an open arc-shape is shaped to approximately match the thoracic contour of patients having different sizes and weights and shaped to surround and position the collimator subsystem closely proximate a heart of a patient of the patients encompassed by at least one predetermined image volume for optimizing collimation of radiation photons emitted from the heart. An open arc-shaped detector system is coupled to the collimator subsystem having a shape closely matching the shape of the collimator subsystem for detecting collimated radiation photons from the collimator subsystem and generating output electrical signals. 
   In another embodiment, the shape of the collimator subsystem and the detector subsystems may optimize collimation and detection of the radiation photons for a majority of the patients of a patient population. The predetermined imaging volume may include a three-dimensional cylindrical imaging volume. The arc-shaped frame, the collimator system, and the detector subsystem may be subtended at an angle in the range of about 180° to 220° with respect to the center of the predetermined imaging volume. The collimator subsystem may include a slit-plate comprising a predetermined number of spaced longitudinal slits each having a predetermined width for transversely collimating the radiation photons. The predetermined width of each of the plurality of spaced longitudinal slits may be configured to adjust spatial resolution of transverse collimation. The system may further include a plurality of slit-guides attached proximate each side of each of the plurality of longitudinal slits. The angle of the slit-guides and the location of the spaced longitudinal slits may be configured to provide a plurality of non-overlapping projections that define the size and location of the at least one predetermined imaging volume. The size and location of the at least one predetermined image volume and the plurality of non-overlapping projections may provide high geometric efficiency in the detection of radiation photons emitted from the heart. The at least one predetermined image volume may be configured for patients having different thoracic contours and/or different sized hearts and/or different locations of the heart relative to a predefined central axis. The at least one predetermined imaging volume may include a large three-dimensional imaging volume for generating a scout image which estimates a three-dimensional center and the general size of the heart. The at least one predetermined imaging volume may include a small three-dimensional imaging volume for generating SPECT images of the heart. The combination of the location of spaced longitudinal slits, the angle of the slit-guides, and the distance between the slit plate and the detector subsystem may be adjusted for minification of a plurality of simultaneous non-overlapping projections such that a maximum number of projections can be cast on the detection system to provide high geometric efficiency for generating one or more SPECT images. The one or more SPECT images may be obtained by using image reconstruction of the plurality of simultaneous non-overlapping projections. The collimator subsystem may include a plurality of transversely spaced slats disposed behind the slit-plate for longitudinally collimating the radiation photons. The location of each of the plurality of transversely spaced slats may be configured to adjust spatial resolution of longitudinal collimation. The transversely spaced slats may be configured to converge on predetermined focal points to accommodate cone-beams of radiation photons emitted from the heart for increasing the number of radiation photons detected by the detector subsystem. The slit-plate may be configured as a flexible loop moveably coupled to the frame having a plurality of sections each configured to provide a unique predetermined imaging volume having a predetermined size and location, and a spatial resolution. A desired section of the flexible loop may be positioned proximate and surrounding the at least one predetermined imaging volume of the patient by driving the flexible loop to a predetermined location on the collimator subsystem. The system may further include a plurality of connected flexible loops moveably coupled to the frame, each loop including a plurality of sections configured to provide a unique predetermined imaging volume of a predetermined size, location, and spatial resolution. The system may further include a patient positioning subsystem for positioning the patient such that the heart is located proximate the center of the predetermined imaging volume based on previous scout images of the heart. The patient positioning subsystem may incrementally rotate the patient about a central longitudinal axis of the at least one predetermined imaging volume for obtaining a plurality of additional projection images. The patient positioning subsystem may intermittently and incrementally rotate the patient about a predefined central longitudinal axis of a small predetermined three-dimensional imaging volume for obtaining a plurality of sequentially acquired sets of simultaneous projections and reconstructing one or more SPECT images. A patient positioning subsystem may position the predefined imaging volume encompassing the heart up and down about a longitudinal axis for acquiring additional cone-beam data set in a longitudinal plane. The open arc-shaped frame may have a shape closely matching the shape of the collimator subsystem. 
   This invention further features a single photon emission computed tomography (SPECT) system for cardiac imaging including an open arc-shaped frame. A collimator subsystem is shaped to approximately match the thoracic contour of patients having different sizes and weights and shaped to surround and position the collimator subsystem closely proximate a heart of a patient of the patients encompassed by at least one predetermined image volume for optimizing collimation of radiation photons emitted from the heart. An open arc-shaped detector system is coupled to the collimator subsystem having a shape closely matching the shape of the collimator subsystem for detecting collimated radiation photons from the collimator subsystem and generating output electrical signals. A patient positioning subsystem positions the patient such that the heart is located proximate the center of the predetermined imaging volume configured to optimize SPECT imaging based on previous scout images of the heart. 
   This invention also features a single photon emission computed tomography (SPECT) system for cardiac-imaging including an open arc-shaped frame. A collimator subsystem is shaped to approximately match the thoracic contour of patients having different sizes and weights and shaped to surround and position the collimator subsystem closely proximate a heart of a patient of the patients encompassed by at least one predetermined image volume for optimizing collimation of radiation photons emitted from the heart. The collimator system further includes a slit-plate configured as a flexible loop moveably coupled to the frame having a plurality of sections each configured to provide a unique predetermined imaging volume having a predetermined size and location, and a spatial resolution. An open arc-shaped detector system is coupled to the collimator subsystem having a shape closely matching the shape of the collimator subsystem for detecting collimated radiation photons from the collimator subsystem and generating output electrical signals. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
       FIG. 1  is a three-dimensional side view of one embodiment of the SPECT system for cardiac imaging of this invention; 
       FIG. 2  is a schematic top view of one embodiment of the SPECT system for cardiac imaging of this invention configured for a predetermined imaging volume (PIV) of a typical sized patient; 
       FIG. 3  is a schematic top view one embodiment of the SPECT system for cardiac imaging of this invention configured for PIV of a large patient; 
       FIG. 4  is a three-dimensional view of a one embodiment of the collimator subsystem and the detector subsystem of this invention shown in  FIGS. 1-3  used for high quality SPECT images of the heart; 
       FIG. 5  is a three-dimensional view of another embodiment of the collimator subsystem and the detector subsystem of this invention shown in  FIG. 1-3  used for obtaining scout images; 
       FIG. 6  is a schematic side view showing a one embodiment of slit-guides disposed on each side of the longitudinal slits shown in  FIGS. 2-5 ; 
       FIG. 7  is a schematic side view showing in further detail the angle of the slit-guide shown in  FIG. 6 ; 
       FIG. 8  is a schematic top view of another embodiment of the SPECT system for cardiac imaging of this invention configured for a large PIV used for scout imaging; 
       FIG. 9  is a schematic top view showing a comparison of the PIVs and patients shown in  FIGS. 2 and 3 ; 
       FIG. 10  is a schematic side view of one embodiment of the SPECT system for cardiac imaging having a collimator subsystem with converging slats; 
       FIG. 11  is a schematic top view of one embodiment of the SPECT system for cardiac imaging of this invention including a movable loop having a plurality of sections for defining multiple PIVs; 
       FIG. 12  is a schematic front view showing one example in detail the two collimator segments in a collimator section shown in  FIG. 11 ; and 
       FIG. 13  is a schematic top view showing in further detail the structure of the segments shown in  FIG. 12 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
   There is shown in  FIG. 1 , one embodiment of SPECT system  10  of this invention. System  10  includes collimator subsystem  18  coupled to frame  16 , which is shaped to an open-arc to approximately match the thoracic contour of patient  26 , as better shown, e.g., in  FIGS. 2 and 3 . Collimator subsystem  18 ,  FIGS. 1-3  is fairly large and has a cross-section, e.g., about 55 cm as shown at d 1 - 21 ,  FIG. 2 , and about 25 cm as shown at d 2 - 23 , or any equivalent dimensions as known by those skilled in the art. Collimator subsystem  18 ,  FIGS. 2-3 , is responsive to radiation photons emitted from heart  40 , e.g., the circle representing the heart of patient  26  as shown in  FIG. 1 . As discussed in the Background section above, radiation photons emit from heart  40  as a result from intravenous injection of a radiopharmaceutical. The shape of collimator subsystem  18 ,  FIGS. 1-3 , is designed to approximately the thoracic contour of patient  26 . This allows collimator subsystem  18  to be closely proximate to heart  40  to optimize collimation of radiation photons from heart  40  encompassed by at least one PIV, e.g., PIV  42 ,  FIG. 2 , of a typical sized patient  26 , or PIV  42   a ,  FIG. 3 , of a larger sized patient. The shape of open-arc collimator subsystem  18 ,  FIGS. 1-3  is also designed to accommodate data sampling for the off-center location of heart  40 . The result is collimator subsystem  18  effectively collimates radiation photons from heart  40  of the majority of the patients in a typical patient population. Collimator subsystem  18  may be oval shaped, elliptical shaped, hyperbolic shaped, or a composite (of any of the aforementioned shapes), or any shape known to those skilled in the art which will result in collimator subsystem  18  approximately matching the thoracic contour of patient  26  and being located closely proximate heart  40  encompassed by PIV  42 ,  FIG. 2 , or PIV  42   a ,  FIG. 3 . 
   Detector subsystem  20 ,  FIGS. 2 and 3 , is located behind collimator subsystem and has a shape which closely matches the shape of collimator subsystem  18 , as discussed above. Detector subsystem  20  is responsive to collimator subsystem  18  and detects collimated radiation photons emitted by heart  40  and generates output electrical signals. Detector subsystem  20  preferably includes a plurality of closely spaced detector modules  72 ,  FIGS. 4 and 5  that maximize packing fraction and detection efficiency while providing high intrinsic spatial resolution (ISR). Detector modules  72  could be made from a variety of radiation detector material, such as scintillation detectors or room-temperature solid-state detectors. Detector modules  72  may be solid-state CZT detectors or advanced pixellated scintillation detectors, as known by those skilled in the art. Computer system  25 ,  FIG. 1 , receives digitized output electrical signals from the plurality of detectors and its associated electronics processing subsystem and generates one or more raw projection images of the heart. 
   Preferably arc-shaped frame  16 ,  FIGS. 1-3 , collimator subsystem  18 , and detector subsystem  20  are subtended at an angle in the range of about 180° to 220° with respect to center  32  of heart  40 . 
   In one design, collimator subsystem  18 ,  FIGS. 2 and 3 , includes slit-plate  30  having a predetermined number of spaced longitudinal slits  52  of a predetermined width. In this example, each slit  52  in slit-plate  30  has a predetermined width of about, e.g., 2 to 5 mm, as shown in  FIG. 4 . Slits  52  transversely collimate the radiation photons emitted from heart  40  encompassed by PIV  42 ,  FIG. 2 , or PIV  42   a ,  FIG. 3 . Each slit  52  functions as a 1D pinhole in the transverse plane casting a plurality of projections  200 ,  FIG. 2 , or a plurality of projections  200   a ,  FIG. 3 , of radiation photons emitted from PIV  42 , or PIV  42   a , respectively, onto detector subsystem  20 . 
   In another design, collimator subsystem  18 ,  FIG. 5 , includes a predetermined number of spaced wider slits  52   a , e.g., about 5 to 8 slits  52   a , having a predetermined wider width of about, e.g., about 8-12 mm. 
   The width of each of longitudinal slits  52 , and slits  52   a ,  FIGS. 2-5 , is configured to adjust spatial resolution and photon sensitivity of transverse collimation of the radiation photons emitted from the heart  40 . Wider slits, e.g., slits  52   a ,  FIG. 5 , provide generally lower spatial resolution but are more sensitive, e.g., for use with scout imaging. Narrower slits, e.g. slits  52 ,  FIG. 2-4  provide generally higher spatial resolution but are less sensitive, e.g., for the construction of more accurate SPECT images of the PIV  42 ,  FIG. 2  or PIV  42   a ,  FIG. 3 , with heart  40  therein. 
   In one preferred embodiment of this invention, collimator subsystem  18 ,  FIGS. 1-5 , includes slit-guides  56 ,  FIG. 6 , attached proximate each side of each of spaced longitudinal slits  52 ,  52   a . In one example, slit-guides  56  are made of lead or similar type radiation-opaque material. The angle θ 1 - 55 ,  FIG. 7 , of slit-guides  56 , and the angle θ 2 - 57 , of slit-guides  56  with respect to axis  59  and the spacing between slits  52 ,  52   a ,  FIGS. 2-5 , are configured to define, inter alia, the size and location of PIV  42 ,  FIG. 2 , for a typical sized patient  26 , PIV  42   a ,  FIG. 3  for a larger patient  26 , and a large PIV  42   b ,  FIG. 8  for scout imaging. The combination of the angle of the slit-guides  56  and the spacing between slits  52 ,  52   a  provides the flexibility to define multiple PIVs at predetermined locations needed for patients having different thoracic contours, weights, and different sized hearts located further from central longitudinal axis  32  than a typical patient  26 . The precisely targeted and optimized design and selection of PIVs  42 ,  42   a , and  42   b , results in high quality SPECT imaging and improved scout images. 
   For example, the angle θ 1 - 55  and θ 2 - 57 ,  FIG. 7 , with respect to axis  59 , of slit-guides  56  and the spacing between slits  52 ,  FIG. 2 , defines a plurality of non-overlapping projections  200  which are cast on detector subsystem  20  to define PIV  42  for typical sized patient  26 . Similarly, the combination of the angle θ 1 - 55  and θ 2 - 57 , of the slit-guides  56 ,  FIG. 7 , and the spacing between slits  52 ,  FIG. 3  defines a plurality of non-overlapping projections  200   a ,  FIG. 3  which are cast on detector subsystem  20  to define PIV  42   a  of a larger sized patient  26   a  having a heart  40  which is larger and located further from central longitudinal axis  32  than a typical patient  26 .  FIG. 9 , where like parts have been given like numbers, shows a comparison of PIV  42  of patient  26  and PIV  42   a  of larger patient  26   a , as shown in  FIGS. 2 and 3 , respectively. Additionally, the combination of the angle θ 1 - 55  and θ 2 - 57  with respect of axis  59 ,  FIG. 7 , and the spacing between longitudinal slits  52   a ,  FIG. 8 , can also be configured to define a plurality of non-overlapping projections  200   b  which are cast on detector subsystem  20  to define a much larger PIV  42   b , in the thoracic cross-section of patient  26  that covers heart  40  for generating a series of scout images to locate the center of heart  40 . In this example, slit-plate  30  of collimator subsystem  18  may include about 5 to 8, e.g., 6 longitudinal slits  52   a , as better shown in  FIG. 5 , each having a width of about 10 mm. Lines  61  and  63 ,  FIG. 8 , show two exemplary peripheral paths of radiation photons emitted from large PIV  42   b  and cast on detector subsystem  20  to create one of the plurality of projections  200   b , indicated at  203 . Center  32  and the longitudinal central axis that passes through center  32  of the heart  40  can be estimated either by inspecting raw projections of scout images or by real-time tomographic reconstruction of the plurality of raw projections  200   b  cast on the detector subsystem  20  to derive three-dimensional scout SPECT images. Thus, the wider width of the slits  52   a  provides quick, low-resolution images of the large PIV  42   b  for scout SPECT imaging. 
   The result is SPECT system  10  for cardiac imaging of this invention provides multiple PIVs at multiple locations needed for high quality SPECT images of heart  40  for both typical and large sized patients having different thoracic contours, weights, and different sized hearts located further from central longitudinal axis  32  than a typical patient  26 . 
   Because the collimator subsystem  18 ,  FIGS. 1-5 ,  8  and  9  is not circular, slits  52 ,  52   a  are not evenly spaced angularly with respect to the center of PIV  42 ,  FIG. 2 , PIV  42   a ,  FIG. 3 , or PIV  42   b ,  FIG. 8 . This unevenness in angular sampling requires computer subsystem  25 ,  FIG. 1  to utilize iterative algorithms, such as Ordered Subset-Expectation Maximization (OS-EM), for tomographic image reconstruction. 
   Collimator subsystem  18 ,  FIGS. 1-5 ,  8  and  9  preferably includes a plurality of transversely spaced slats  54 , as shown in  FIGS. 4 and 5 , disposed behind and physically separate from the slit-plate  30 . Slats  54  longitudinally collimate the radiation photons. In one example, the distance between each of the plurality of transversely spaced slats  54  is configured to adjust spatial resolution of longitudinal collimation. Slats  54  are typically multiple thin parallel lead plates or foils, and may be separated with Styrofoam plates (not shown) of uniform thickness, e.g., about 2-5 mm. Slats  54  basically fill the space (with varied radial length, e.g., 50-100 mm), between the two ends of collimator subsystem  18  and between collimator subsystem  18  and detector subsystem  20 . 
   In one embodiment, SPECT system  10 ,  FIG. 10 , shown in a longitudinal plane, includes collimator subsystem  18   a  having a plurality of transversely-spaced slats  54  which converge on predetermined focal points, shown in two dimensions as point  308 . In this embodiment, collimator subsystem  18   a  of system  10  utilizes a variation of conventional cone-beam geometry (which relies on a collimator design with a plurality of pin holes which are all aligned in three-dimension to a single point beyond the target organ, such as the brain) for each slit of collimator subsystem  18 , e.g., slits  52 ,  FIGS. 2-3 , for PIV  42 ,  42   a , respectively. The advantage of using converging-slats  54  is the increased solid angle for photon detection, and corresponding increased geometric sensitivity. Appropriate cone-beam algorithms are applied in image reconstruction. However, since cone-beam sampling may have limitations in providing artifact-free 3D images, mainly in the upper and lower regions of the cone-beam, a few additional longitudinal sampling could be utilized to reduce these artifacts and provide satisfactory images. In this example, patient positioning subsystem,  FIG. 1 , moves patient  26  up and down along longitudinal axis  311  to acquire more cone-beam data sets from the radiation photons emitted from heart  40  encompassed by PIV  42  in the longitudinal plane. In this example, PIV  42  (or PIV  42   a ,  FIG. 3 ) encompassing heart  40  is sampled three times at positions  317 ,  315 , and  319 . Using converging-slats  54  increases the solid angle of radiation photons received by detector subsystem  20  and increases detection efficiency by detector subsystem  20 . The result is high speed or high quality SPECT imaging of heart  40 . 
   In one preferred design, slit-plate  30 ,  FIGS. 2-5 ,  8  and  9  is configured as movable loop  210 ,  FIG. 11 . Loop  210  is slideably coupled to frame  16 . Movable loop  210  includes a plurality of sections, e.g., section  212 , section  214 , and section  216 . Each of sections  212 ,  214  and  216  include a predefined number of spaced longitudinal slits  52 ,  52   a , each having slit-guides  56  proximate each side thereof at a predetermine angle and each having a predetermined width to define PIV  42 ,  FIG. 2 , PIV  42   a ,  FIG. 3 , and PIV  42   b ,  FIG. 8 , respectively, as discussed above. In this example, section  212  is shown located at the front of collimator subsystem  18 . 
   Each of sections  212 - 216  of each section is preferably coupled to movable cars  132 ,  FIG. 12 . For example, section  212  includes a plurality of segments  130  and  131 , as shown in greater detail in  FIG. 13 , which are coupled on top surface  133 ,  FIG. 12  to cars  132  movably coupled to frame  16  and coupled on bottom surface  135  to movable cars  132  coupled the frame  16 . In one design, biasing devices  134 , e.g., springs, disposed between cars  132  and segments  130  and  131  maintains segments  130  and  131  in an appropriate and reproducible position for imaging. In an exemplary embodiment, stepper-motor  343 ,  FIG. 11 , controlled by computer subsystem  25 ,  FIG. 1  drives cars  132 ,  FIG. 12  on tracks (not shown) in frame  16  so that the desired section of loop  210 ,  FIG. 11 , is located at the front of collimator subsystem  18  for imaging. 
   In one design, collimator subsystem  18 ,  FIG. 11  includes a plurality of connected loops, e.g., loop  210  and loop  211  that each including a plurality of sections and share a common section, e.g., section  210  disposed proximate patient  26 . As discussed above, each of the connected loops  210  and  211  are slideably coupled to frame  16 . A switching system may be used to select the track of choice and allow a specific section to be pulled to the front. The track flexibilities and functionalities are available from well-developed track technology. 
   The result is SPECT system  10  for cardiac imaging,  FIGS. 1-13  of this invention, can select a desired PIV  42 ,  FIG. 2 , PIV  42   a ,  FIG. 3 , or PIV  42   b ,  FIG. 8 , as needed for different sized patients and for scout imaging by simply moving the desired section  212 - 214 ,  FIG. 11 , to the front of collimator subsystem  18  to provide scout images and high quality SPECT images of the heart. 
   In one embodiment of this invention, the distance, d, indicated at  44 ,  FIGS. 2 and 3 , between slit-plate  30  of collimator subsystem  18  and detector subsystem  20  is configured for minification of the plurality of simultaneous non-overlapping projections  200 ,  200   a  on the detector subsystem  20 . To accommodate the large number of plurality of projections  200 ,  200   a , on detector subsystem  20 , each projection needs to be small enough so that there is no overlap of adjacent projections,  200 ,  200   a  and a maximum number of projections can be accommodated on detector subsystem  20 . This is achieved through minification of the plurality of projections  200 ,  200   a  by adjusting distance d- 44  at the appropriately the distance between the slit-plate  30  and detector subsystem  20 , e.g., to a distance of about 5-10 cm and by adjusting the angle of plurality of slit-guides  56 ,  FIGS. 6 and 7 , e.g. θ 1 - 55  and θ 2 - 57  with respect to axis  59 . The angle of slit-guides  56  limits the radiation photons only from PIV  42 ,  FIG. 2 , or PIV  42   a ,  FIG. 3 . This design allows a large number of non-overlapping projections of a finite-sized PIV to be acquired simultaneously and thus provides high geometric efficiency in detection of the radiation photons emitted from PIV  42  or PIV  42   a , used or SPECT imaging of heart  40 . 
   In one design of this invention, patient positioning subsystem  12 ,  FIG. 1 , positions patient  26  to one or more predetermined locations defined by PIV  42 ,  FIG. 2 , PIV  42   a ,  FIG. 3 , or PIV  42   b ,  FIG. 8 , so that patient  26  can be rotated on a central axis of the appropriate PIV which contains the heart  40  throughout the whole series of rotation for SPECT imaging (discussed in detail below). Patient positioning subsystem  12 ,  FIG. 1 , may include chair  13  that is incrementally rotated to obtain a plurality of projection images. The longitudinal axis of the frame  16  may be oriented nearly, but not exactly, vertically such that the patient  26  sits nearly upright. More realistically, patient  26  should sit in a slightly reclined bucket seat, with his back firmly supported so that patient  26  feels comfortable, with low likelihood of torso movement, to go through the imaging procedure. This arrangement facilitates rotation of patient  26  during imaging and allows a small footprint of the system  10 . Upright imaging provides the advantage of lowering the diaphragm of the patient  26 , thus reducing the severity of attenuation and scatter effects caused by sub-diaphragmatic organs and sub-diaphragmatic tracer accumulations. 
   Patient positioning subsystem  12 ,  FIG. 1 , positions patient  26  so that the center of the heart  40  is at center  32 ,  FIGS. 2 and 3 , of the three-dimensional field PIV  42  or PIV  42   a  based on a previous scout imaging of the heart. However, a single set of plurality of 12-20 projections is typically not enough for reconstructing high quality SPECT images of heart  40 . Thus, several (2-5) additional sets of non-redundant projections may be acquired, depending on image quality requirements of the specific clinical application. These additional sets of non-redundant projections can be added by rotating patient  26  on the positioning subsystem  12 ,  FIG. 1 , relative to the other hardware, to sample slightly different projections. Patient positioning subsystem  12 , then incrementally rotates patient  26 , e.g., approximately 3° for a total of 12° to 15° to fill in the angular sampling gaps about a predefined central longitudinal axis, e.g., center  32  of PIV  42 ,  FIG. 2  or center  32 ,  FIG. 3  of PIV  42   a , and then intermittently remains stationary, e.g., for about 30-120 seconds to obtain additional plurality of ECT projections  200 ,  FIG. 2  or a plurality of projections  200   a ,  FIG. 3 . Frame  16 , collimator subsystem  18  and detector subsystem  20  remain stationary at all times. Computer subsystem  25 ,  FIG. 1 , reconstructs SPECT images from the whole sets of the plurality of ECT projections  200 ,  FIG. 2 , or the plurality of projections  200   a ,  FIG. 3 . 
   In one exemplary operation of SPECT system  10 ,  FIGS. 1-13  of this invention, patient set-up and imaging proceeds as follows. After patient  26 ,  FIG. 1  is secured in chair  13  of patient position subsystem  12  with body restraints, patient  26  and chair  13  are first moved to a default position for scout-imaging, e.g., PIV  42   b ,  FIG. 8 , in the frame  16 . Electronic control of the system  10  is provided by computer system  25 ,  FIG. 1 , having a monitor (not shown) for data visualization, as is known to those skilled in the art. Section  216 ,  FIG. 11 , with wider slits  52   a  on loop  210 , e.g., as shown in greater detail in  FIG. 5 , is already in the front of collimator subsystem  18 . Scout SPECT imaging of a large PIV  42   b ,  FIG. 8 , covering the lower thorax is immediately performed with the collimator subsystem  18 . In about 30 seconds, three low-resolution real-time reconstructed SPECT images show up on the monitor for the three standard orthogonal slices across the center of the heart. The location of the heart  40  gradually becomes clear on all three slices as a distinct blurry disk. The operator may then be prompted to verify the computer identified three-dimensional center and the general size of the heart  40 , as indicated on the monitor with a 10 cm or a 14 cm circles as best-matched reference, the outline of a three-dimensional sphere superimposed on each of the three slices. 
   Approximately 1-2 minutes into acquisition, as the displayed three-dimension scout-SPECT images gain more statistics to confirm the match, the operator clicks a software control button to approve the center location of the sphere, the displacement necessary to bring the heart into an appropriate PIV is determined. At the same time, a decision of which PIV, e.g., PIV  42   a ,  FIG. 2 , or PIV  42   b ,  FIG. 3 , will be selected for imaging is also determined and confirmed based on the displayed size of the heart. 
   Following the necessary translations, patient position subsystem  12 ,  FIG. 1 , moves patient  26  in three-dimensions to center the heart  40  at the center  32 ,  FIG. 2  of PIV  42 , or PIV  42   a ,  FIG. 3  and locks in place. At the same time, section  212 ,  FIG. 11 , with PIV  42  on loop  210 , or section  214  with PIV  42   a  on loop  210 , for a larger patient  26 , or for a larger heart, is moved to the front of collimator subsystem  18 . As soon as the patient motion stops and the appropriate collimator subsystem  18 ,  FIGS. 2 and 3 , is properly configured, core SPECT imaging of heart  40  begins with a large number of projections acquired from multiple directions simultaneously for 0.5 to 2 minutes. In an alternative embodiment, scout imaging may be performed using acquired raw projections to determine the 3D center and the size of the heart without SPECT image reconstruction. 
   When acquiring high-resolution core SPECT images of heart  40 , the required rotation of patient is a small angle rotation utilizing only several additional (2-5) steps. Patient positioning subsystem  13  rotates patient  26 , for example, about 3° per step for a total of 15° in five additional steps. Thus, for a collimator system as shown in  FIGS. 2-4 , a total of 72 to 78 projections may be acquired in 3 to 12 minutes. 
   The result is system  10 ,  FIGS. 1-14 , is capable of achieving high performance either in high spatial or high temporal resolution and has significant advantages over the current state-of-the-art SPECT systems. These advantages include, inter alia, high quality SPECT images of the heart, achieved through accurate, optimal and reproducible positioning guided by scout imaging, increased overall detector utilization and detection efficiency, inherent detector stability, mechanical simplicity, the ability to define multiple PIVs to accommodate both typical and larger patients which accommodates the majority of patients of a patient population, compact physical size, predictable and reproducible system and imaging performance, simple, practical, standardized and automated clinical operations. Further, the small footprint of the system meets the need of hospitals and physician offices to reach a large patient population. 
   Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims. 
   In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.