FIELD OF INVENTION 
     The present invention relates generally to gamma ray detectors and specifically to gamma ray collimation in nuclear medicine. 
     BACKGROUND OF THE INVENTION 
     Gamma ray imaging is currently used in medicine to obtain 3D images of patients&#39; internal organs. Positron Emission Tomography (PET) is a medical gamma ray imaging technique frequently used for this purpose. FIGS. 2A,  2 B and  3  show representative prior art systems. Prior to an imaging procedure, a patient is given a radiopharmaceutical, which contains a positron emitting substance and which is selectively accumulated in a region of interest. When a positron emitted by the radiopharmaceutical encounters an electron, the electron-positron pair annihilates, emitting two gamma photons of 511 keV each, flying in opposite directions. The simultaneous detection of these two 511 keV gamma photons by two gamma detectors  40  positioned opposite to each other (as shown in FIG.  1 ), indicates that a positron has been emitted and annihilated inside an organ of a patient  500 . The simultaneous attribution of 2D coordinates to each one of the photons allows for the determination of the photon&#39;s line of flight. The position of the annihilation is along this line. When a multitude of gamma photon pairs are detected and the information processed using appropriate algorithms, electronic circuitry, software, etc., a 3D image of the organ under examination is reconstructed. 
     Further and more detailed descriptions and analysis of PET will be found in “Performance Parameters of a Positron Imaging Camera”, by Gerd Muehllehner et al., IEEE Transactions of Nuclear Science, Volume NS-23, No. 1, February 1976 and in “Performance Parameters of a Longitudinal Tomographic Positron Imaging System,” by Paans et al., Nuclear Instruments and Methods, Volume 192, Nos. 2, 3, pages 491 -500, Feb. 1, 1982, the disclosures of which are incorporated herein by reference. 
     Low energy, stray, gamma photons, resulting from 511 keV gamma photons scattered within patient&#39;s body, are also present during coincidence measurements and may also reach one or both detectors. These scattered low energy gamma photons do not contain any usable and/or valid information. If these stray gamma photons are allowed to reach the detectors, they increase the count rate at the detector while not adding any usable information. These additional counts, while they may be rejected later, reduce the ability of the detector to detect “real” events, at a high rate. 
     Another problem encountered in coincidence gamma imaging concerns attenuation artifacts caused by absorption by the patient body and scattering. In order to correct for these effects, a 3D distribution of patient&#39;s absorption is preferably previously measured. 
     Attenuation may be measured (see FIG. 2A) by scanning patient  500  using a collimated line source  98  situated opposite a collimated detector  40  or two collimated line sources  98 , opposite two collimated perpendicular detectors  40  and  40 ′. When attenuation and coincidence measurements are to be performed consecutively, the configuration of the apparatus has to be changed. This procedure is very time consuming and cumbersome for the following reasons: 
     a) During coincidence measurements, detectors  40  are positioned parallel to each other (see FIG.  2 B). 
     b) In order to improve the resolution of the attenuation measurements, a collimator  54  is used on the detector side (see FIG.  2 A). Coincidence measurements use no collimators on detectors (see FIG.  2 B); 
     c) A “Filter”  56  (see FIGS.  2 B), used in coincidence measurements contains a graded absorber  58  that selectively absorbs, and thus, protects the detectors from large flux of low energy, scattered, stray gamma photons. A line source  98  used in attenuation measurements is, for practical reasons, a source emitting low energy gamma photons (e.g., 100 keV Gd 153). These gamma photons cannot penetrate graded absorber  58 . 
     Another class of problems concerns the conditioning (collimation) of the radiation from a line source in transmission attenuation measurements. Reference is now made to FIGS. 4A-4C. Collimation of line source radiation is performed in one of the following ways: 
     if a collimation width  62  is larger than source  98  diameter  66 , no substantial collimation exists (see FIG.  4 A); 
     if collimation width  62  is substantially the same as line source diameter  66 , sensitivity related to manufacturing tolerances is maximal and non-uniform radiation is generated (see FIG.  4 B); and 
     if line source diameter  66  is larger than collimation width  62 , and a loss of potential radioactivity  68  results. The smaller the ratio of the width of the slit to its length in the direction of the rays, the better the collimation and the greater the loss. 
     Yet another problem present in coincidence measurements concerns the lack of depth discrimination due to the finite thickness of the scintillation crystal. 
     Reference is now made to FIGS. 5A and 5B. In coincidence measurements, a true  68  or a calculated  68 ′ line of flight of gamma photons  70  is determined by the location of a pair of interaction points  72 , of both photons in a pair  76 , with detectors scintillation crystals  42 . The resolution of a detector in coincidence measurements depends on: 
     a) The detector intrinsic resolution, i.e., the ability of the detector to accurately determine location  72  of interaction of a gamma photon  70 , with scintillation crystal  42 . The thicker the crystal, the higher the probability that the photon interacts with the crystal. However, as is evident from FIGS. 5A and 5B, the detector intrinsic resolution is reduced with increasing thickness. In the absence of accuracy in depth discrimination, gamma photons  70  are assumed to interact with the scintillation crystal at its median  84 ; 
     b) The accuracy with which the gantry position is determined; and 
     c) Loss of resolution due to reconstruction algorithms. 
     Of these three causes, the loss of resolution in depth discrimination, (shown as X on FIG.  5 B), which strongly depends on incident angle  86  and is a function of crystal&#39;s thickness  80 , is most important. In order to increase depth discrimination in coincidence measurements, either the crystal thickness is reduced, or only those photons that have an angle of incidence  86 , under a certain limit are counted, for example, by reducing the flux of photons with large angle of incidence. 
     “Septa” or “Filter” shields  56  (see FIGS. 2B,  3  and  6 ) have no substantial localization function per se. They only remove scattered gamma photons  60 , with large axial incidence angle  86 , most of which are not useful for PET and are rejected by the software. To provide this function, the septa are generally about 1 cm apart and have an acceptance angle of about 10 degrees. Prior art septa are placed parallel to the slices of the reconstructed 3D image. The limited collimation of the Septa indirectly improves resolution by reducing the effect of the lack of depth discrimination on location accuracy 
     It is desirable, in PET, to improve gamma detectors efficiency by reducing the number of stray photons detected relative to the number of non-stray photons detected and to improve the depth discrimination in coincidence measurements without having to reduce the scintillation crystals thickness. It is also desirable to perform attenuation and coincidence measurements in sequence without moving or replacing parts of the imaging system and, in attenuation measurements, to reduce radioactivity losses due to line source diameter while using a large diameter source to improve statistics by increasing the total radiation while keeping the source strictly collimated. 
     SUMMARY OF THE INVENTION 
     It is an object of some preferred embodiments of the invention to improve Nuclear Medicine (NM) image quality in coincidence measurements such as in PET imaging. 
     It is an object of some preferred embodiments of the invention to improve NM detector efficiency in coincidence measurements by increasing the ratio of useful radiation to stray radiation reaching a detector. 
     It is an object of some preferred embodiments of the invention to provide an NM imaging apparatus, with a single collimator assembly, which allows for high resolution measurements with both higher energy and lower energy gamma photons. Preferably the higher energy photons are used for PET imaging and the lower energy photons are used for emission imaging. Preferably, at least one region of the collimator assembly has a plurality of apertures and preferably it allows for collimation in two different directions. 
     It is an object of some preferred embodiments of the invention to provide an NM imaging method that increases depth discrimination, preferably without reducing crystal thickness, in coincidence measurements. Preferably increase in depth discrimination is obtained by introducing transaxial collimation. 
     In accordance with a preferred embodiment of the present invention, the collimator assembly comprises at least two different regions preferably with different collimation capabilities. 
     In accordance with a preferred embodiment of the present invention, each of the regions of the collimator comprises a multitude of apertures. 
     In accordance with a preferred embodiment of the present invention, preferably one of the regions is a strip collimator. Preferably a second region of the collimator is a “Septa collimator.” 
     In accordance with some preferred embodiments of the present invention, the Septa collimator region preferably also has a graded absorber that reduces the flux of low energy scattered gamma photons. 
     In accordance with some preferred embodiments of the present invention, preferably one of the regions is a strip collimator while the other region contains a graded absorber with no septa. 
     In accordance with some preferred embodiment of the present invention, the strip collimator is an axial fan beam collimator. 
     It is an object of some preferred embodiments of the invention to provide a method of source collimation and a source collimator, which allows for the use of line source with a diameter larger than the collimation width, without excessive loss of radiation. 
     In accordance with some of preferred embodiments of the present invention, the source collimator has many apertures distributed in a plurality of rows. Preferably the apertures are of rectangular shape. 
     There is thus provided, in accordance with a preferred embodiment of the invention, a gamma ray collimator assembly comprising a first collimator portion and a second collimator portion, said first and second portions having different gamma ray acceptance angles. 
     In a preferred embodiment of the invention, the collimator portions are formed by spaced septa and wherein the septa spacing is different for the two collimator portions. Preferably, the collimator portions pass radiation received from different regions. Preferably, the first and second collimator portions are designed for operation with gamma rays of different energies. Preferably, the first and second collimator portions are secured in a single frame. Preferably, the first and second collimator portions are positioned side by side, having openings in the same direction. 
     Preferably, the first collimator portion lies adjacent one edge of the collimator assembly. Preferably, the first collimator portion lies along substantially said entire edge of the assembly. 
     In a preferred embodiment of the invention, the assembly comprises an acceptance area within which gamma rays are passed and the first collimator portion covers a smaller portion of the acceptance area than does the second collimator portion. 
     Preferably, the first collimator portion has a substantially lower acceptance angle than does the second collimator portion. Preferably, the acceptance angle of the first collimator portion is between 0.2 and 5 degrees, more preferably between 0.5 and 3 degrees. 
     Preferably, the first collimator portion is designed to collimate gamma rays having an energy of between about 70 and 150 keV. 
     In a preferred embodiment of the invention, the second collimator portion is comprised of strips which block radiation, disposed parallel to each other. 
     Preferably, the second collimator portion has a relatively high acceptance angle as compared to that of the first portion. Preferably, the acceptance angle for the second collimator portion is between about 5 and 30 degrees, more preferably, between about 7 and 20 degrees. 
     In a preferred embodiment of the invention, the second collimator portion is designed to operate with gamma rays having an energy of between 400 and 600 keV more preferably, about 511 keV. 
     In a preferred embodiment of the invention, the energy at which the second collimator portion is designed to operate is at least twice, more preferably at least three times, most preferably at least five times, that at which the first collimator portion is designated to operate. 
     In a preferred embodiment of the invention, the first collimator portion is substantially transparent to gamma rays having an energy at which the second collimator portion is designed to operate. 
     In a preferred embodiment of the invention the assembly includes a gamma ray absorber underlying the second collimator portion, said absorber being designed to absorb gamma rays having an energy lower than that at which the second collimator portion is designed to operate. Preferably, the absorber is graded. 
     In a preferred embodiment of the invention the second collimator portion is a two dimensional collimator. Preferably, the two dimensional collimator comprises apertures formed of a material at their peripheries that does not transmit gamma rays having an energy at which the second collimator portion is designed to operate. In a preferred embodiment of the invention, the apertures are substantially rectangular apertures. Alternatively, the apertures have a non-rectangular shape. 
     There is further provided, in accordance with a preferred embodiment of the invention, a gamma ray collimator assembly comprising a collimator portion, and an absorber portion, said absorber portion covering a substantially greater portion of the collimator assembly than the collimator portion. Preferably, the collimator portion is designed to collimate gamma rays having a first, relatively low energy and wherein the absorber is designed to block gamma rays of said first energy and to pass gamma rays having a second, relatively higher energy. Preferably, the collimator portion and the absorber are secured in a single frame. Preferably, the collimator portion and the absorber are positioned side by side, facing in the same direction. Preferably, the collimator portion lies adjacent one edge of the collimator assembly, more preferably along substantially an entire edge of the assembly. 
     Preferably, the acceptance angle of the collimator portion is between 0.2 and 5 degrees, more preferably the acceptance angle of the collimator portion is between 0.5 and 3 degrees. Preferably, the collimator portion is designed to collimate gamma rays having an energy of between about 70 and 110 keV. Preferably, the absorber is designed to pass gamma rays used for PET imaging. Preferably, the PET imaging energy is about 511 keV. Preferably, the collimator is substantially transparent to gamma rays having the PET imaging energy. Most preferably, the absorber is graded. 
     There is further provided, in accordance with a preferred embodiment of the invention, a gamma ray collimator assembly for PET imaging comprising a two dimensional array of acceptance apertures said apertures being formed at their edges of a material through which gamma rays used for PET imaging do not pass. Preferably, the collimator has a relatively high acceptance angle. Preferably, the acceptance angle for the collimator between about 5 and 30 degrees, most preferably, the acceptance angle degrees. 
     In a preferred embodiment of the invention the collimator assembly includes a gamma ray absorber underlying the collimator, the absorber being designed to absorb gamma rays having an energy lower than that at which the collimator assembly is designed to operate. Preferably, the absorber is graded. Preferably, the apertures of the collimator are substantially rectangular apertures. Alternatively, the apertures have a non-rectangular shape. 
     There is further provided, in accordance with a preferred embodiment of the invention, a gamma ray imaging system comprising: 
     a. a gamma ray collimator assembly having a first collimator portion and a second collimator portion, said first and second portions having different acceptance angles and wherein the collimator portions are formed by spaced openings and wherein the septa openings are different for the two collimator portions; and 
     b. a gamma ray detector wherein said gamma ray collimator assembly is positioned adjacent a gamma ray acceptance surface of the detector. 
     Preferably, the collimator assembly covers substantially the entire detector surface. 
     There is further provided, in accordance with a preferred embodiment of the invention, a gamma ray imaging apparatus comprising more than one gamma ray imaging system wherein the first and second collimator portions are designed for operation with gamma rays of different energies. Preferably, the gamma ray imaging apparatus further comprises a line source positionable opposite detector. Preferably, the first and second collimator portions are designed for operation with gamma rays of different energies. Preferably, the first and second collimator portions cover substantially the entire detector surface. Preferably, the first and second collimator portions are designed for operation with gamma rays of different energies. 
     There is further provided, in accordance with a preferred embodiment of the invention, an apparatus comprising: 
     a. a line source having a given width and length; and 
     b. a plurality of apertures opposite to the line source. 
     Preferably, the apertures are narrower than the line source width and are distributed in the direction of the width of the source, more preferably, the apertures are distributed in a plurality of rows running along the length of the length of the source. 
     There is further provided, in accordance with a preferred embodiment of the invention, a method of improving depth discrimination in PET measurements comprising: 
     a. providing an area detector; and 
     b. providing a collimator at the detector that blocks gamma photons having an incident transaxial angle larger than a predetermined value. 
     Preferably, the method improves detector efficiency in PET measurements and more preferably, the collimator also blocks gamma photons with an axial incident angle larger than a predetermined value. 
     There is further provided, in accordance with a preferred embodiment of the invention, a method of performing attenuation and coincidence measurements sequentially comprising: 
     a. providing at least one area detector; 
     b. providing at least one collimator, covering part of at least one detector; 
     c. irradiating a patient with gamma radiation from a source positioned opposite the detector; 
     d. collimating a flux of the gamma radiation passing through the patient from the source; 
     e. detecting the collimated flux utilizing the portion of the area detector covered by the collimator; 
     f. determining a two dimensional attenuation map of at least a portion of the patient from the detected flux; and 
     g. performing a PET imaging sequence without removing the collimator. 
     There is further provided, in accordance with a preferred embodiment of the invention, dual energy imaging apparatus for simultaneous imaging of relatively low energy and relatively high energy photons emitted by sources of radiation, comprising: 
     at least one detector which produces signals responsive to high and low energy events throughout a given time period; 
     a collimator situated between a detector of the at least one detectors and the source, wherein the collimator collimates the low energy photons and is relatively transparent to the high energy photons; 
     a dual energy detector, which receives the signals and determines therefrom whether the signal was generated by a relatively low energy photon or a relatively high energy photon; 
     an image processing system that separately processes the high energy signals and the low energy signals to produce images based on the detected high energy and low energy photon. 
     Preferably, the collimator is substantially transparent to the high energy photons. 
     In a preferred embodiment of the invention, the high energy image is a PET image. Alternatively, the high energy image is a SPECT image. In a preferred embodiment of the invention the high energy image is a planar image. In a preferred embodiment of the high energy image is a transmission image. 
     In a preferred embodiment of the invention, the low energy image is a SPECT image. Alternatively, the low energy image is a planar image. In a preferred embodiment of the invention, the low energy image is a transmission image. 
     Preferably, the at least one detector comprises a pair of planar detectors. 
     In a preferred embodiment of the invention the apparatus includes a high energy collimator situated between the detector and the sources of radiation, one of said high energy and low energy detectors overlying the other. 
     In a preferred embodiment of the invention, the detectors have photon acceptance faces that are parallel to each other. Alternatively, the detectors have photon acceptance faces that are perpendicular to each other. Alternatively, the detectors have photon acceptance faces that are oriented at an angle different from 0 and 90 degrees with respect to each other. 
     There is further provided, in accordance with a preferred embodiment of the invention, a hybrid collimator for collimating high energy photons and low energy photons in gamma cameras, comprising: 
     a first collimator which collimates low energy photons and is relatively transparent to high energy photons; and 
     a second collimator that collimates high energy photons wherein one of the first and second collimators underlies the other of the first and second collimators overlies the other. 
     Preferably, the second collimator is a PET collimator. 
     Preferably, the second collimator is suitable for use in SPECT and planar nuclear medicine images. The invention will be more clearly understood from the following detailed description of non-limiting preferred embodiments thereof, read in conjuction with the drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a general block diagram of an embodiment used in accordance with medical coincidence measurements of prior art and the present invention; 
     FIG. 2A schematically shows a prior art configuration for attenuation measurements with two gamma detectors positioned at 90 degrees; 
     FIG. 2B schematically shows a prior art configuration for coincidence measurements with two gamma detectors positioned at 180 degrees; 
     FIG. 3 schematically shows a “Scintillation Crystal-Septa” assembly used in prior art coincidence measurements, positioned relative to a patient; 
     FIGS. 4A-4C schematically show different prior art geometries for line source collimation; 
     FIGS. 5A-5B schematically illustrates the events geometry of gamma photon pair interaction in scintillation crystals; 
     FIG. 6 illustrates a prior art “Septa”; 
     FIG. 7A and 7B represent, respectively, a top view and a side view of a preferred embodiment of a collimator assembly in accordance with a preferred embodiment of the present invention; 
     FIG. 8A and 8B represent respectively a top view and a sectional view of a preferred embodiment of a collimator assembly having no collimation function in its wide area region, in accordance with a preferred embodiment of the present invention; 
     FIGS. 9A and 9B schematically show side views of preferred embodiments of a collimator as assembly used in attenuation and coincidence measurements respectively; 
     FIG. 10A and 10B respectively show a top view and a cross section view of a preferred embodiment of a collimator assembly comprising a strip collimator and a septa grid in accordance with a preferred embodiment of the present invention; 
     FIG. 11 schematically shows a septa grid with apertures of a different geometrical shape in accordance with preferred embodiments of the present invention; 
     FIGS. 12A and 12B respectively represent a side view and a top view of a source collimator which comprises a plurality of apertures distributed in a plurality of rows, in accordance with preferred embodiments of the present invention; 
     FIG. 13 schematically shows an axial fan beam collimator and a septa assembly opposite to a collimated line in accordance with preferred embodiments the present invention; 
     FIG. 14 is a schematic representation of a system for dual energy imaging in accordance with a preferred embodiment of the invention; and 
     FIG. 15 is a hybrid collimator in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a block diagram of a system used in accordance with medical coincidence measurements of the prior art and in the present invention. A pair of gamma detectors  40 , each optically coupled to a scintillation crystal  42 , are disposed parallel to each other. Detector pair  40  is preferably mounted on a gantry that can rotate about a patient  500  resting on a table  502 . Additionally, either detector pair  40  or patient  500  can be transversely displaced in the direction perpendicular to the plane of the figure. This configuration allows for total body scanning and/or static imaging, both well-known techniques in NM coincidence measurements. 
     System hardware and software, schematically described in FIG. 1 by blocks  44 - 50 , allows for coincidence measurements in accordance with technology well known in the art. Thus, no further details on system operation will be given in the description of preferred embodiments in accordance with the present invention, except for distinctive features of the invention. This hardware generally includes an energy discriminator that rejects events having a low energy. Such events are presumed to be caused by scatter. 
     FIG. 7A and 7B show respectively a top and a sectional view of a shield  52  in accordance with a preferred embodiment of the present invention. Shield  52  preferably has two distinct regions, namely strip collimator  54  and wide area region  92 , which have a common frame  88  preferably made of aluminum. Strip collimator  54  preferably comprises a plurality of apertures having an acceptance angle of between 0.2 and 5 degrees, more preferably about 2 degrees. Preferably, wide area region  92  comprises a plurality of septa  56  that preferably block stray radiation having a large axial incident angle. As stray radiation includes many gamma photons with large angle of incidence, the septa may also be regarded as collimating and blocking of such radiation in the direction parallel to septa&#39;s long side  96 . Therefore, the term “septa collimator” is interchangeably used herein with “septa”. Exemplary dimensions for collimator assembly  52  are: P =5 cm, Q =35 cm, R =54 cm, S=1 cm (see FIG.  7 A). The acceptance angle of the septa collimator is preferably between 5 and 30 degrees, more preferably about 10 degrees. The slice spacing for reconstruction varies, but in general is only a fraction of S. 
     In accordance with a preferred embodiment of the present invention, wide area region  92  is preferably covered by a graded absorber  58  that prevents low energy gamma photons from reaching detectors  40 . Additionally or alternatively, part of base  88  situated immediately under wide area region  92  is not covered by graded absorber  58 . 
     FIGS. 8A and 8B respectively show a top and a side view of shield  52  in accordance with a preferred embodiment of the present invention, in which part of frame  88  corresponding to wide area region  92 , is covered by graded absorber  58  and does not comprise septa. 
     In accordance with a preferred embodiment of the present invention, shield  52  is preferably conceived, designed and manufactured as to comprise in one single mechanical and functional structure (see FIGS. 7A,  7 B,  8 A and  8 B), collimator  54  and “Septa”  56 . FIGS. 9A and 9B, schematically show shield  52  mounted on gamma detectors in accordance with a preferred embodiment of the present invention. FIGS. 9A and 9B are a side view, in the direction of arrow A, of embodiments illustrated in FIGS. 2A and 2B wherein collimator  54  and “Septa”  56  of FIGS. 2A and 2B have been replaced by shield  52 . For reasons of clarity, head  40 ′ and source  98 ′ of FIG. 2A are not shown if FIG.  9 A. All functional and structural descriptions given hereafter which refer to head  40  and/or source  98  are to be considered as equally applicable to both heads  40  and  40 ′ as well as sources  98  and  98 ′. 
     FIG. 9A schematically depicts a set up for transmission attenuation measurements in accordance with a preferred embodiment of the present invention. Collimated line source  98  is positioned opposite strip collimator  54  of shield  52 , which covers detector  40 . Strip collimator  54  and line source  98  may also have an axial fan beam shape as depicted in FIG.  13 . During attenuation measurements, line source  98  and strip collimator  54  do not move relative to each other. Detector  40 , shield  52  and line source  98  are mounted on a gantry  100  that can be rotated around patient  500 . 
     To scan the patient in attenuation measurements, both heads  40  and  40 ′ and sources  98  and  98 ′ are rotated 90 degrees around the patient to complete imaging of one single slice. Then, bed  502  or patient  500  is laterally translated, preferably a distance equal to FWHM of transmitted radiation, to image the next slice. Axial rotation of gantry  100  and linear displacement of bed (with patient) are repeated until the entire region of interest is scanned. During this imaging session, only data from scintillations occurring behind strip collimator  54  are used. 
     At the end of the transmission attenuation measurements, head  40 ′ and line source  98  in FIG. 2A are rotated 90 degrees clockwise to go from an “L” configuration (as in FIG.  2 A), in which heads and line sources are positioned at 90 degrees relative to each other, to an “H” configuration of FIG. 2B, in which heads  40  and  40 ′ are positioned opposite to each other while line sources  98  and  98 ′ are parked one behind the other and closed so that no radiation emanates from them. Heads  40  and  40 ′ are positioned so that “Septa” (or “Filter”)  56 , region of collimator assembly  52  is positioned opposite to the region of interest in patient&#39;s body as schematically shown in FIG.  9 B. Preferably at this stage, the patient is injected with a radiopharmaceutical that contains a positron emitting substance that preferably selectively accumulates in an organ of interest in body  500 . 
     Pairs of gamma photons emitted in opposite directions (see FIG. 5A) and indicative of annihilation of positrons emitted by the pharmaceutical, are collected by detectors  40  and  40 ′ (see FIGS. 2A,  2 B and  9 B) for coincidence measurements. Heads  40  and  40 ′ (FIG.  9 B), are positioned with respect to the patient so that strip collimator  54  of collimator assembly  52  remains outside the border which delimits the region be imaged. During coincidence measurements, graded absorber  58  in “Septa” (or “Filter”) preferably selectively removes low energy and generally large angle of incidence, patient body scattered, gamma photons. Heads  40  and  40 ′ are preferably rotated at least 180 degrees during coincidence measurements. The images obtained for each projection angle at which heads  40  and  40 ′ were positioned during the entire scanning process, are then used by system&#39;s software to reconstruct a 3D image of the region of interest. 
     Alternatively, strip collimator  54  is substantially transparent to the high energy (511 keV) gamma rays. In this case, the entire detector is utilized for the high energy measurements. 
     It is understood that neither the attenuation measurement sequences nor the coincidence measurements sequences, performed in accordance with a preferred embodiment of the present invention, are limited to what has been described above. Both measurements may be performed in static or dynamic configuration with or without rotational or transversal displacement of the gantry (heads  1  and  2 ), the line source or the bed on which the patient lies. Furthermore, helical scanning, in which the bed is laterally translated while heads  40  and/or  40 ′, and line sources  98  and/or  98 ′, are rotated around the patient may be used. 
     Further, in some preferred embodiments of the present invention, “Septa”  56  as shown in FIG. 7 are modified in order to increase system efficiency in coincidence measurements. Rejecting both gamma photons pairs and stray radiation with large angle of incidence, (for example larger than ±20 degrees in both axial and transaxial directions, increases the efficiency of the imaging system in coincidence measurements by increasing its resolution in depth discrimination in both directions. Thus, in accordance with a preferred embodiment of the present invention, crossed septa act as an axial and transaxial collimator. This modification, which is schematically showed in FIGS. 10A and 10B, is preferably made by replacing the one dimension septa structure of FIG. 7 by a crossed septa grid  102 . The crossed septa grid consists of a plurality of substantially square or hexagonal apertures  116 , in the wide area region  92  of collimator assembly  52  sketched in FIGS. 7A,  7 B,  9 A and  9 B. Preferably, the dimensions of the apertures are variable and adjustable. Exemplary dimensions of the crossed septa grid is similar to that of the single direction septa of FIG.  7 A. 
     Reference is again made to FIGS. 5A,  5 B,  10 A and  10 B. Septa grid  102 , comprising graded absorber  58 , is in fact a partial collimator in both axial and transaxial directions. As such, it rejects scattered or coincidence gamma photons preferably with incidence angle larger than a certain value including many low energy gamma photons. By rejecting the scattered gamma photons and thus reducing useless gamma photons flux, septa grid  102 , increases detector&#39;s efficiency through improvement of signal to noise ratio. By limiting the counted coincidence gamma photons, preferably, only to those with an angle of incidence below a certain value for example ±10 degrees, septa grid  102  also increases detector&#39;s efficiency in depth discrimination relative to annihilation events that take place inside an organ of interest. 
     It will be appreciated by a person skilled in the art that, descriptions and/or preferred embodiments detailed hereinbefore are only representative of their functionality. Any other combination of collimator assembly  52  comprising strip collimator and “Septa” or septa grid with or without graded absorber may be used in some of preferred embodiments of the present invention and should be regarded as pertaining to the present invention. In accordance with some preferred embodiments of the invention the (two-dimensional) septa grid may be used with or without strip collimator. The collimators described herein may equally be used in PET and/or PET-SPECT devices. 
     Furthermore while square shaped openings are shown for the septa of FIG. 10, non-square openings may be used such as rectangular shaped openings (regular or with offset rows) and hexagonal shaped openings. An asymmetric hexagonal septa system is shown in FIG.  11 . 
     Furthermore while regular circular openings as shown for strip collimator  54 , other shaped openings, as known in the art, may be substituted. 
     In some of preferred embodiments of the present invention, line source  98  in FIGS. 2A or  9 A is collimated by a line collimator. In order to overcome the above mentioned problems related to radiation because of geometry, an improved collimator, as shown in FIG. 12 is preferably used. In some preferred embodiments of the present invention, line source  98  is collimated by an improved collimator  106  having a plurality of apertures preferably distributed in a plurality of rows. Preferred embodiments designed and manufactured in accordance with the configuration shown in FIG. 12B allow for simultaneous collimation in planes parallel to both long and short dimensions of line source  98  (whose axis runs from right to left in FIG.  12 B). This collimation may (but need not) be the same in both directions. Thus, line source  98 , which preferably has diameter larger than the width of a single aperture, and would otherwise be collimated by line collimator in accordance to description of FIG. 4C with large radioactivity losses, is collimated by collimator  106  (see FIGS. 12A and 12B) with minimal radioactivity losses. Preferably, septa material is such that penetration of gamma photons through walls  110  between apertures  118  is minimal. Additionally or alternatively, in some preferred embodiments of the present invention, walls  110  are as thin as possible in order to avoid nonuniformity of radiation  112  in regions of the irradiated area. While apertures  108  of collimator  106  are preferably substantially rectangular, they may be of any geometrical shapes. 
     Alternatively, the same principle of multi-hole collimators may be used to generate a fan beam running along the length of collimator  54  from a point source. In this configuration, collimator  54  would be a fan beam collimator focused at the point source (also the focus of the point collimator. Similarly, the same principle may be used to generate an efficient, well collimated point source of radiation from a relatively large “point” source. 
     In many cases one can obtain complementary information from simultaneous imaging of two isotopes, which emit radiation. A first important example is the simultaneous imaging of myocardium with a Tc99m (radiating at 140 keV) labeled radiopharmaceutical and an F 18  (positron emitter radiating at 511 keV) labeled radiopharmaceutical. Two difficulties exist in such simultaneous imaging. One difficulty is that a collimator used must be suited to the high energy and will thus give poorer resolution of the low energy isotope than could be obtained if only it were used. Another difficulty is the low sensitivity of crystals suitable for low energy imaging when used to image the high energy gamma rays. 
     A second situation in which high and low energy imaging may be performed simultaneously is in the simultaneous acquisition of emission and transmission images. In such cases the emission image, which must be of high quality, is generally at a lower energy than the transmission image. Such imaging is performed for both PET images (in which, for example, the lower energy is 511 keV and the upper energy is 662 keV derived from a transmission source made of Cs 137) and for planar or SPECT images (in which, for example, the lower energy is 140 keV and the upper energy may be any suitable energy from 180 to 800 keV). Alternatively, in a third example, as described above, a transmission image for correcting PET images may be acquired using Tc99m at 140 keV, simultaneously with the PET image. 
     The efficiency and resolution of positron emitter imaging can be considerably improved by performing coincidence without a collimator or with a collimator with a large acceptance angle. Unfortunately, other isotopes and other imaging schemes require a collimator. 
     In accordance with a preferred embodiment of the invention, a camera is equipped with dual energy discrimination in the front end electronics. The camera utilizing a collimator optimized for low energy. Such collimators provide optimal low energy images and are, to a large extent, transparent to the high energy radiation. For the first example described above, the camera would image the Tc99m image utilizing the low energy collimator and the high energy PET image as though the collimator was not there. An example of such a system is shown in FIG. 14, which is similar to FIG.  1  and in which the same numerals are used for the same elements. FIG. 14 differs from FIG. 1 in a low energy collimator  120  mounted on each of crystals  42  and in having a separate low energy discriminator  122 , which receives energy signals from detectors  40  and determines whether a detected event was generated by the low energy isotope used. If a low energy photo is detected, an acquisition processor  124  records the x and y positions of the event on the crystal as determined by circuitry  126 . When sufficient data is acquired, image processor  128  produces either a planar or SPECT image from recorded events. This circuitry, which is preferably purely conventional in nature, is shown for only one of the crystals. However, such circuitry can be provided for both crystals. This is especially useful for SPECT imaging using the low energy, since it doubles the acquisition rate. In this case, a common acquisition computer may be used to acquire the data and/or a common image processor may be used to form the SPECT image. It should also be understood that, while a particular configuration of circuitry is shown in FIG.  14  and in others of the drawings, conventional circuitry having other configurations may be used in preferred embodiments of the invention, to perform various acquisition and image processing functions. 
     Considering again the first example described above, in cardiac imaging, usually only 180 degrees from the left posterior to the right anterior view are used in SPECT imaging, since they are the closest to the heart. In a preferred embodiment of the invention, one of the detectors in a dual head camera utilizes a low energy collimator and the other utilizes a high energy collimator. The low energy detector is used to image the heart utilizing the optimal angles, since the low energy radiation has the potential of rendering a high resolution image. The high energy image may be acquired at less optimal angles, if simultaneous imaging is desired, or may be acquired in the optimal position, by further rotation of the detectors. Note that the angle between the detectors may be 180 degrees or some other lesser angle. 
     In a further preferred embodiment of the invention, yet another collimation scheme similar to that shown in FIG. 14 is used. However, instead of low energy collimators  120  being used on both detectors a dual layer collimator  130 , as shown in FIG. 15, is used. One layer  132  of this collimator is a low energy collimator as is known in the art and a second layer  134  is a high energy collimator as known in the art, or as described herein. Either layer can be above the other. 
     The above detailed descriptions and drawings of non-limiting preferred embodiments of the present invention, are only illustrative. Various combinations of features of collimators and scanning regimes described above may be used separately and in various combinations. The invention is not meant to be limited by the specific embodiments disclosed, but only by the claims in which:
A gamma ray collimator assembly comprising a first portion and a second collimator portion, the first and second portions having different gamma ray acceptance angles.