Abstract:
A radiation source comprising:
       a source of radiation;   a plate having an aperture formed therein, said aperture facing the source of radiation; and   means for moving the source when no radiation is desired on the side of the plate away from the source.

Description:
RELATED APPLICATIONS 
   The present application is a divisional of U.S. application Ser. No. 09,588,402, filed on Jun. 6, 2000 now U.S. Pat. No. 6,642,523. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the field of medical imaging by PET and by SPECT systems. The present invention relates in particular to PET and SPECT systems with simultaneous, single-photon transmission imaging for attenuation corrections. 
   BACKGROUND OF THE INVENTION 
   Single photon emission computerized tomography (SPECT) and positron emission tomography (PET) are well known nuclear imaging systems in medicine. Generally, in nuclear imaging, a radioactive isotope is injected to, inhaled by or ingested by a patient. The isotope, provided as a radioactive-labeled pharmaceutical (radio-pharmaceutical) is chosen based on bio-kinetic properties that cause preferential uptake by different tissues. The gamma photons emitted by the radio-pharmaceutical are detected by radiation detectors outside the body, giving its spatial and uptake distribution within the body, with little trauma to the patient. 
   SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma-ray pairs that are emitted in coincidence, in opposite directions, due to electron-positron annihilations. In both cases, data from the emitted photons is used to produce spatial images of the “place of birth” of a detected photon and a measure of its energy. In PET, photon detectors also provide an indication of the time when a photon is detected. 
   An Anger gamma camera generally comprises a scintillation crystal, which when struck by a photon emits light, an array of photomultiplier tubes (PMTs) arranged in a conventional hexagonal matrix, for giving the x-y position of the detected photon, various processing circuitry, and a processing unit. Solid state gamma cameras generally include an array of pixel sized detectors and may be based on one of a number of different technologies. Solid state cameras suitable for use in SPECT and/or PET imaging are described in PCT Application No. PCT/IL98/00462 filed Sep. 24, 1998, now WO publication 00/17670 and PCT publication WO 98/23974, the disclosures of which are incorporated herein by reference. 
   Many SPECT and PET systems utilize one or two gamma cameras of either the Anger or solid state type to detect gamma rays. 
   SPECT and PET imaging couple conventional planar nuclear imaging techniques and tomographic reconstruction methods. Gamma cameras, arranged in a specific geometric configuration, are mounted on a gantry that rotates them around a patient, to acquire data from different angular views. Projection (or planar) data acquired from different views are reconstructed, using image reconstruction methods, to generate cross-sectional images of the internally distributed radio-pharmaceuticals. These images provide enhanced contrast and greater detail, when compared with planer images obtained with conventional nuclear imaging methods. 
   In general, it is desirous to have imaging systems that can be used both for PET and for SPECT, depending on the need. 
   A factor that influences the quality of imaging is non-uniform photon attenuation by intervening tissue, that is tissue through the which the gamma rays must pass before being detected by the gamma camera or cameras. Transmission scanning is a technique used to generate an attenuation map for correcting gamma images for non-uniform attenuation. In principle, gamma radiation from a known source, external to the tissue, is transmitted through the tissue to a corresponding scintillation detector. As in the cases of SPECT and PET, the external radiation source and the detector are rotated around the tissue, and transmission data from different angular views, coupled with tomographic reconstruction methods are used generate an attenuation map of the internal structure. 
   Two important points in generating an attenuation map are that transmission scanning must be performed for each patient individually, as patients differ in size and in internal structure and that transmission scanning should be performed on the same spatial registry as the PET or SPECT imaging, else it is difficult to correlate between the attenuation map and the PET or SPECT data. 
   Therefore, transmission scanning is generally performed simultaneously or concurrently with the PET or SPECT imaging, using the same detector system for the PET or SPECT imaging and for the transmission scanning. In many systems, use photons of different energies to differentiate between the transmission scanning photons and those of PET or SPECT imaging. 
   U.S. Pat. No. 5,900,636 to Nellemann, “Dual-Mode Gamma Camera system Utilizing Single-Photon Transmission Scanning for Attenuation Correction of PET Data,” whose disclosure is incorporated herein by reference, describes a system of simultaneous PET of SPECT imaging and transmission scanning.  FIG. 1  illustrates a view in a transverse (x-y) plane, in which two transmission point sources  30 A and  31 A are on the same side as two gamma detectors  10  and  11  in the transaxial (x) direction. Point sources  30 A and  31 A are mounted outside the fields of views (FOVs.) of detectors  10  and  11 . Such mounting avoids blocking the detectors and reduces transmission self-contamination (where radiation from a point source strikes the detector near it). A transmission scan across the entire axial width of detectors  10  and  11  is performed at each angular stop about the z axis. The aggregate effect of these transmission scans with the illustrated placement of point sources  30 A and  31 A is a transmission FOV (of each transverse slice) represented by a circle  70 . The emission field of view (in each transverse slice) is represented by circle  72 . 
   In this device, the allowable width (in the transverse plane) of the fan beam generated by point sources  30 A and  31 A is limited by detectors  10  and  11 . Thus, the transmission FOV  70  is defined by two boundaries, an outside boundary and an inside boundary. The outside boundary is defined by the outer edges of the transmission fan beams  68  and  69  at each of the angular stops about the z axis, while the inside boundary is defined by the circumference of a circle  76 , which represents a blind spot in the attenuation map. In order to prevent the gap from resulting in incomplete data acquisition, the computer system (not shown) causes the examination table (not shown) to move vertically and horizontally relative to the z axis in dependence on the angular positions of the detectors  10  and  11  about the z axis, in order to provide full coverage of the object of interest. 
   SUMMARY OF THE INVENTION 
   An aspect of some preferred embodiments of the present invention relates to providing a PET or SPECT system with simultaneous, single-photon transmission scanning yielding complete coverage of all the tissue that is being examined, so that a complete tissue attenuation map is obtained, with no blind spots. 
   An aspect of some preferred embodiments of the present invention relates to providing a PET or SPECT system with simultaneous or sequential, single-photon transmission scanning wherein transmission scans can be obtained in one pass, rather than in a series of axial stops along the z axis. 
   An aspect of some preferred embodiments of the present invention relates to providing a PET system with simultaneous or sequential, single-photon transmission scanning wherein the transmission scanning can be shut off at will. 
   In one preferred embodiment of the invention, two gamma detectors are positioned facing each other, with a fan beam transmission source at the edge of one detector. The tissue to be examined is placed between them and the system is rotated about an axis. The axis is shifted towards the side of the line source. The tissue is completely within a rectangle bounded by the two detectors and the axis of rotation, is within the field of view of the fan beam line source. The source may be outside or inside the rectangle formed by (or the largest rectangle defined by) the gamma detectors. 
   In other preferred embodiments of the invention, two gamma cameras having slightly different transverse extents are utilized. One edge of the two detectors is so aligned with the other detector such that the other edge of the wider detector extends past a rectangle formed by the other three edges. A transmission source is placed at the edge of detector diagonally across from the extending edge. The extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector. The source may be outside or inside the rectangle formed by (or the largest rectangle defined by) the gamma detectors. 
   In other preferred embodiments of the invention, both effects are utilized and one detector extends past the other and the center of rotation is slightly displaced such that the center of rotation is within the fan beam. 
   An aspect of some preferred embodiments of the present invention relates to providing a transmission line source comprising multiple fan-beams so that scanning of many slices along the z direction can be obtained simultaneously. In preferred embodiments of this aspect, a radio-opaque rod, such as a tungsten rod, is used. Blind holes are drilled along the rod at equal distances, and a radioactive material of the desired properties is inserted into each hole. As such, the rod provides a linear array of point sources. 
   In a preferred embodiment of the invention, a radio-opaque plate is formed with slits spaced the same distance apart as the holes in the rod (and the sources) and having their long direction perpendicular to the axis of the rod. The slits are aligned with the sources such that a series of fan beams are formed. The plate which is thick enough to substantially limit the width of a beam such that the beams are limited in extent in the direction of the rod. 
   An aspect of some preferred embodiments of the present invention relates to providing a transmission source assembly wherein the transmission source can be shut off and turned on at will. 
   In some preferred embodiments of this aspect, a metal rod into which point sources have been inserted, as described above, is used. The metal rod is positioned on a sliding mechanism in a shielded box (for example a lead box or a tungsten box), with the edge of the rod, free of point sources, protruding from the side of the box. The top of the box is formed with open slits, as described above. When it is desired to shut off the point sources, the edge of the rod is pulled, so as to sufficiently misalign the point sources with the slits at the top of the box and to block radiation from exiting from the slits. 
   Alternatively, but less desirably, the rod itself is fixed, but the slits at the top of the box may be shut by putting a shielded cover on them. 
   There is thus provided, in accordance with a preferred embodiment of the invention, a nuclear imaging system comprising: 
   first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent; 
   an radiation source, situated outside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and 
   an axis about which the first and second detectors and the radiation source rotate together; 
   wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation. 
   In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, one edge of the two detectors is aligned with each other such that the other edge of the wider detector extends past the parallelepiped and wherein the radiation source is placed near the edge of the short detector diagonally across from the extending edge. 
   In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, both edges of a short detector being outside said parallelepiped, wherein the radiation source is placed near an edge of the short detector and diagonally opposed edge of the wider detector extends past the parallelepiped by an amount sufficient such that the axis of rotation is within the field of view. 
   Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. 
   Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation. 
   Preferably, the extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector. 
   Preferably, the axis of rotation is substantially at the center of the parallelepiped or of a rectangle defined by edges of the shorter detector and the edge of the longer detector aligned with the edge of the shorter detector. 
   Alternatively or additionally, the axis of rotation is displaced such that the center of rotation is within the field of view of the line source. Preferably,the axis of rotation is displaced in the direction of the aligned edges, from the center of a rectangle defined by edges of the shorter detector and the edge of the longer detector aligned with the edge of the shorter detector. Alternatively or additionally, the axis of rotation is displaced in the direction of the detector farther from the source. 
   In a preferred embodiment of the invention, the first and second detectors have substantially the same extent and are aligned with each other at one side of the parallelepiped and wherein the axis of rotation is displaced from the center of the parallelepiped. 
   In a preferred embodiment of the invention, the first and second detectors have different extents and wherein the edges of the detectors are aligned with each other on one side of the parallelepiped, the source is placed near the opposite edge of the smaller detector and the axis of rotation is displaced from the center of the parallelepiped. 
   Preferably, the axis of rotation is displaced toward the open side of the parallelepiped at which the radiation source is situated. Alternatively, the axis of rotation is displaced in the direction of detector farther from the source. 
   Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation. 
   There is further provided, in accordance with a preferred embodiment of the invention, a nuclear imaging system comprising: 
   first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent; 
   an radiation source, situated inside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and 
   an axis about which the first and second detectors and the radiation source rotate together; 
   wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation and wherein the field of view of the radiation source does not encompass the center of the parallelepiped. 
   In a preferred embodiment of the invention, the first and second radiation detector have different transverse extents, one edge of the two detectors is aligned with each other such that the other edge of the wider detector extends past the parallelepiped and wherein the radiation source is placed near the edge of the short detector diagonally across from the extending edge. Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation. Preferably, the extending edge is made to extend far enough such that the center of rotation of the two detectors is within a fan beam of radiation from the transmission source that is detected by the wider detector. 
   In a preferred embodiment of the invention, the axis of rotation is substantially at the center of the parallelepiped. Alternatively, the axis of rotation is displaced from the center of the parallelepiped, such that the center of rotation is within the field of view of the line source. Preferably, the axis of rotation is displaced in the direction of the aligned edges. Alternatively or additionally, the axis of rotation is displaced in the direction of the detector farther from the source. 
   In a preferred embodiment of the invention, the first and second detectors have substantially the same extent and are aligned with each other at one side of the parallelepiped and wherein the axis of rotation is displaced from the center of the parallelepiped. Preferably, the axis of rotation is displaced toward the open side of the parallelepiped at which the radiation source is situated. Alternatively or additionally, the axis of rotation is displaced in the direction of detector farther from the source. 
   Preferably, the radiation source comprises a line of sources extending along the edge of the shorter detector. Preferably, the irradiation source comprises a plurality of point sources collimated to produce fan beams each defining a plane perpendicular to the axis of rotation. 
   There is further provided, in accordance with a preferred embodiment of the invention a radiation source comprising: 
   a source of radiation; 
   a plate having an aperture formed therein, said aperture facing the source of radiation; and 
   means for moving the source when no radiation is desired on the side of the plate away from the source. 
   In a preferred embodiment of the invention, the plate is a flat plate. 
   Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture. 
   Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam. 
   There is further provided, in accordance with a preferred embodiment of the invention, a radiation source comprising: 
   a plurality of individual sources of radiation; 
   a plate having an apertures formed therein, each said aperture facing a respective individual source of radiation; and 
   means for moving the sources such that they do not face the apertures when no radiation is desired on the side of the plate away from the source. 
   Preferably, the plate is a flat plate. 
   Preferably, the means for moving displaces the sources so that they are situated between the apertures when no radiation is desired. Alternatively or additionally, the means for moving rotates the sources so that they do not face in the direction of the plate. 
   Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture. 
   Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam. 
   There is further provided, in accordance with a preferred embodiment of the invention, a radiation source comprising: 
   a plurality of individual sources of radiation; and 
   a plate having an apertures formed therein, each said aperture facing a respective individual source of radiation, 
   wherein the plate has a thickness that is greater than the smallest dimension of the aperture. 
   Preferably, the plate is a flat plate. 
   Preferably, the means for moving displaces the sources so that they are situated between the apertures when no radiation is desired. Alternatively or additionally, the means for moving rotates the sources so that they do not face in the direction of the plate. Preferably, the plate has a thickness that is greater than the smallest dimension of the aperture. Preferably, the thickness is more than five or ten as large as the smallest dimension of the aperture. 
   Preferably, the aperture has a slit shape, such that the radiation exiting the slit forms a collimated fan beam. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the following detailed description of the preferred embodiments of the invention and from the attached drawings, in which same number designations are maintained throughout the figures for each element and in which: 
       FIG. 1  is a schematic illustration of a gamma camera system utilizing single-photon transmission scanning for attenuation correction of PET data, as known in the art; 
       FIGS. 2A–2F  are schematic illustrations of PET systems with simultaneous, single-photon transmission scanning for attenuation corrections, in accordance with preferred embodiments of the present invention; 
       FIGS. 3A–3C  are schematic illustrations of the fan beam transmission source rod and source assembly, in accordance with preferred embodiments of the present invention; and 
       FIG. 4  is a schematic diagram of a PET system with attenuation correction. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference is now made to  FIG. 2A  which is a schematic illustration of a PET system  100  with simultaneous or sequential, single-photon transmission scanning for attenuation corrections, in accordance with a preferred embodiment of the present invention. PET system  100  comprises two scintillation detectors  102  and  104 , having imaging surfaces  103  and  105  respectively. Detectors  102  and  104  are disposed opposite each other and are mounted on a gantry (not shown). The gantry rotates detectors  102  and  104  around the z axis, at point  0 , perpendicular to the x-y plane. A patient, having a two dimensional projection  110  on the x-y plane is situated between detectors  102  and  104 . 
   A line-source assembly  112  is located near detector  102 , slightly shifted from detector  102  along the x axis and preferably (but not necessarily), flush with imaging surface  103 . The field of view of line source  112  in the x-y plane is described by lines  122  and  123 , connecting line source  112  and the edges of receiving detector  102 . In order for there to be no blind spot in the attenuation map, the field of view of line source  112  must encompass the center of rotation. This situation is achieved by the asymmetry of the setup, in that detector  104 , the receiving detector for the transmission data, extends to the side of detector  102  opposite line source  112 . Note that the amount of asymmetry and the distance of the source from the edge are exaggerated in  FIGS. 2A and 2B . In a practical system in which the detectors have an extent of 54 cm, the source is 3 cm from the edge and the edge is extended by 3 cm or more. In addition, the source may be placed above the plane of detector, for example by 3–5 cm. In this case however, the lower detector must be extended by a greater amount. However, it should be understood that the distances and dimensions are based on the geometry of the system, as described above. 
   By comparison, in  FIG. 1  (from U.S. Pat. No. 5,900,636), a symmetric situation is described. The field of view of source  30 A is defined by lines  68  where internal line  68  is less acute than a line connecting the source with the center of the origin of the x-y plane, and a blind spot, bounded by circle  76 , is generated. 
   Reference is now made to  FIG. 2B  which is a schematic illustration of an alternative SPECT or PET system  200  with simultaneous or sequential, single-photon transmission scanning for attenuation corrections, in accordance with another preferred embodiment of the present invention. In  FIGS. 2A–2F , the numbers n 00 , n 02 –n 05 , n 12 , n 22  and n 23  refer to the same features, where n=1 for  FIG. 2A , n=2 for  FIG. 2B , n=3 for  FIG. 2C , n=4 for  FIG. 2D , n=5 for  FIG. 2E  and n=6 for  FIG. 2E . In system  200 , two scintillation detectors  202  and  204 , of equal dimensions, having imaging surfaces  203  and  205  respectively, are used. Detectors  202  and  204  are rotated about a center O, which is shifted from the center of the rectangle formed by detectors  202  and  204  by an amount large enough so that the center of rotation is within the field of view ( 222 ,  223 ) of a source  212 . 
   In a practical system in which the detectors have an extent of 54 cm, the source is 3 cm from the edge of and the center is offset by 2 cm or more. In addition, the source may be displaced toward the plane of the other detector, for example by 3–5 cm. In this case however, the lower detector must be extended by a greater amount. However, it should be understood that the distances and dimensions are based on the geometry of the system, as described above. 
     FIG. 2C  shows a system  300  in which source  312  is within the boundaries of the rectangle formed by the detectors. Here again, without any changes in the normal center of rotation or the size of the opposing detector, the center of rotation would be outside the fan beam  322 ,  323 . An extension to detector  302  solves the problem. The extent of the parallelepiped formed by the two detectors is indicated by lines  340  and  322  and the center of the parallelepiped is indicated by dot  342  (surrounded by a circle for ease of indication). 
     FIG. 2D  shows a system  400  in which the position of the source is similar to that of  FIG. 3C . However, the solution of the problem is to offset the center of rotation as in  FIG. 2B . The center of the parallelepiped (rectangle) is indicated by dot  442  (surrounded by a circle for ease of indication). 
     FIGS. 2E and 2F  show embodiments of the invention in which the portion of the detector behind the source is removed. This may be possible (although not necessarily desirable) if the source is placed in a housing that blocks the portion of the detector behind the source. 
   While the center of rotation in  FIGS. 2B ,  2 D and  2 F is shown as being offset to the left of the center of the rectangle formed by the detectors, it is also possible to achieve the same effect by moving the center of rotation downward. 
   In the embodiments of  FIGS. 2A–2F , projections through the patient, as the system rotates, cover all parts of the patient cross-section  110 , such that data for an attenuation map of the patient, with no blind spots can be acquired. Additionally or alternatively, if a blind spot does occur, it is possible to interpolate the values from the edge of the blind spot to “fill” the hole with continuous attenuation information. 
   While  FIGS. 2A–2F  show either extension of one of the detectors or movement of the center of rotation, it should be understood that a combination of a smaller extension and a smaller offset may be utilized to overcome the problem of dead space in the center. 
   Reference is now made to  FIG. 3A  which is a schematic illustration of line source rod  140  in accordance with a preferred embodiment of the present invention. The purpose of the line source is to provide multiple fan-beams so that scanning of many slices along the z direction can be obtained at one time.  FIG. 3A  illustrates a radio-opaque (e.g., tungsten) rod into which blind holes  142  are drilled at equal distances. While conical holes are shown, round holes or holes with other shapes may be used. Radioactive material  144  of the desired properties is inserted into each hole  142 . In some preferred embodiments of the invention, glue is used to keep radioactive material  144  in place. Alternatively, the radioactive material  144  is in itself a metal which can be fused in each hole  142 . Alternatively, the radioactive material  144  is molten and poured into each hole  142  where it hardens into a solid. Alternatively, the radioactive material is embedded in a ceramic matrix or embedded in an epoxy material. Conveniently, the radiation source is Cs  137 , having a 662 keV peak. 
   Preferably, the length of rod  140  (which serves as line sources  112  and  212  in  FIGS. 2A and 2B ) in the z direction is substantially the same as the length of detectors  102 ,  104 ,  202  and  204  in the z direction. 
   Reference is now made to  FIGS. 3B and 3C  which schematically illustrate the line-source assembly  150  from top and side views respectively, in accordance with a preferred embodiment of the present invention. Line source assembly  150  comprises a shielded box  152  such as a lead or a tungsten box. Preferably, the length of box  152  is substantially the same as the length of detectors  102 ,  104 ,  202  or  204  in the z direction. Preferably, the width of box  152  is given by a parameter W that will be described shortly. 
   Preferably, box  152  has a shielded top  154 , preferably of the same material as box  152 . top  152  is shown as a flat plate, however, rounded plates can also be used. Preferably slots  156  are formed in top  154 , spaced at the same distances as are sources  144 . In a preferred embodiment of the invention, the distance between the slot centers (and the sources is 21 mm. In a preferred embodiment of the invention, the width of each slot  156  is 2.4 mm and the thickness of the top is 30 mm. 
   Preferably, rod  140  containing multiple point sources  144  is inserted into box  152 , preferably, close to top  154 . Preferably, rod  140  is inserted along a track (not shown) so that it can slide easily in and out. Preferably, rod  140  is positioned so that each source  144  is directly aligned with a slot  156 . Since the radiation being emitted from each slot  156  has a fan-beam shape, assembly  150  is basically a line source of multiple, fan beam sources. 
   A feature of assembly  150  is that the radiation can be shut off, by sliding rod  140  so that point sources  144  are no longer aligned with slots  156 . The slots, which are deep and thin, block the radiation. Alternatively or additionally, the rod may be rotated so that the radio-active material faces away from the slots. Alternatively or additionally, slots  156  may be covered with a shielding material. 
   Preferably, the length of the slots, W is such that the field of view of the radiation passing the slot is the same as that formed by the geometry of the source and detectors, namely the field of view defined in  FIG. 2A  by lines  122  and  123 , and in  FIG. 2B  by lines  222  and  223 . 
   Each of detectors n 02 , n 04  includes a scintillation crystal, an array of photomultiplier tubes (PMTs) arranged in a conventional matrix, various processing circuitry, and a processing unit. Gamma camera detectors such as detectors nO 2 , nO 4  are well known; accordingly, a detailed description of the internal components of the detectors is not necessary to an understanding of the present invention. Detectors n 02 , n 04  may be any gamma detectors or gamma cameras as known in the art, such as solid state detectors. 
   In some preferred embodiments the gantry can rotate detectors n 02  and n 04  individually or in unison, about axis of rotation z. The 180°-detector configuration, shown in  FIGS. 2A–2F , is intended to facilitate coincidence (PET) imaging. For PET imaging a coincidence detector as known in the art is used to determine coincidence of events detected by the opposing detectors. 
   In some preferred embodiments systems n 00  may be used as SPECT systems or as PET systems or as both SPECT and PET systems. 
   For some systems, for example for PET, a collimator is not required. In general, for such systems, if lower energy gamma rays are used for the transmission imaging, a low energy collimator having septa along width of the beam can be used on detectors m 02 . Alternatively, if septa are used in the PET system (as described for example in U.S. application Ser. No. 09,129,078, filed Aug. 5, 1998 and entitled “Gamma Ray Collimator”, now U.S. Pat No. 6,271,524, the disclosure of which is incorporated herein by reference) then a higher energy gamma ray is used for the transmission radiation source, such that the septa are substantially transparent to the transmission radiation. 
   For SPECT systems for which a collimator is generally provided, a high energy transmission source is used, for which the collimator is substantially transparent. 
   It should be understood, that due to the high collimation of the transmission sources and their spacing, collimation for the transmission receiver can be omitted. 
     FIG. 4  shows a functional diagram of a PET system  700  in accordance with a preferred embodiment of the invention. System  700 , includes detectors n 02  and n 04  and associated detector electronics  702  and  704 , which produce position and energy signals x′, y′, E and x″, y″ and E for the positions and energy of the events detected on the detectors. Associated with detector n 04  is an energy filter  706  which passes only events having the correct energy associated with the PET image. An energy filter  708 , associated with detector n 02  passes events associated with the attenuation measurement (transmission radiation), to an attenuation reconstructor  710  which constructs an attenuation map, utilizing methods known in the art. Reconstructor  710  transforms the attenuation map at the transmission energy to attenuation values at the emission energy. 
   Energy filter  708  passes events having energy associated with the PET image to a coincidence detector  710  which also receives events from filter  706  having this same energy. Coincident events are passed to a PET reconstructor  714 , which operates according to any of the algorithms known in the art, for example that shown in PCT application No. PCT/IL97/00128, filed on Apr. 17, 1997, now WO publication WO 98/47103, the disclosure of which is incorporated herein by reference. The PET image is corrected either after or during its reconstruction, based on the attenuation map generated by attenuation reconstructor  710 . The attenuation correction is shown as being performed in a corrector  716 , to produce a corrected PET image  718 . 
   It should be understood that  FIG. 4  is a functional representation of the PET system and does not necessarily represent particular hardware, which may be any suitable hardware as known in the art. Furthermore, some or all of the data processing indicated in  FIG. 4  may be performed by dedicated hardware or by software in a computer or by a combination of the two. Furthermore, while certain steps are shown in a particular order (for example, energy filtering before coincidence detection and attenuation correction after PET reconstruction) the steps can be performed in reverse order or as part of a single procedure. 
   The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. The disclosed embodiments each have a plurality of feature, some of which may be added to other embodiments and some of which may be omitted. Furthermore, the terms “comprise,” “include,” and “have” or their conjugates, mean, when used in the claims, “including but not necessarily limited to.” The scope of the invention is limited only by the following claims.