Abstract:
A gamma camera includes a plurality of detectors ( 32, 34 ) for detecting emission radiation emitted from within a subject and transmission radiation which has traversed a subject to be imaged, the subject attenuating the radiation. Each detector generates position and energy data. At least one transmission radiation source ( 54, 54 ′) transmits transmission radiation through an examination region ( 36 ) to a first segment ( 74, 74 ′) of the opposite detector. In one embodiment, segment selector circuitry ( 88 ) connected with the detectors selectively disables a portion of each detector head during collection of emission data, transmission data, or both. In another embodiment, transmission radiation is received by the first segment ( 74, 74 ′) simultaneously with emission radiation being received by a second segment ( 72, 72 ′) of each detector. The first segment is uncollimated or collimated for the transmission radiation source. In SPECT imaging, the second segment carries a collimator for defining trajectories of received emission radiation. A first electronic storage medium ( 90 ) connected with the segment selector circuitry stores transmission data and a second electronic storage medium ( 86 ) connected with the segment selector circuitry stores emission data. A first reconstruction processor ( 92 ) connected with the first electronic storage medium generates a transmission image representation ( 94 ). A second reconstruction processor ( 100, 101 ) connected with the second electronic storage medium generates an emission image representation ( 102, 103 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the art of nuclear medicine. It finds particular application to nuclear imaging techniques and apparatuses employing emission and transmission tomography. Although the present invention is illustrated and described herein primarily in reference to positron emission tomography (PET) and single photon emission computed tomography (SPECT), it will be appreciated that the present invention is also amenable to other noninvasive investigation techniques and other diagnostic modes in which a subject or patient is examined with transmitted radiation. 
     Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject&#39;s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads are placed adjacent to a surface of the subject to monitor and record emitted radiation. For SPECT imaging, collimators are typically placed on the detector heads. For PET imaging, a coincidence detector detects concurrent receipt of a radiation event on two oppositely disposed heads. PET imaging can be performed using a thin collimator or axial filter to minimize stray radiation. 
     Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject. Such images typically provide functional and metabolic information. 
     Positron emission tomography (PET) is a branch of nuclear medicine in which a position-emitting radiopharmaceutical, such as  18 F-fluorodeoxyglucose (FDG), is introduced into the body of a subject. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays for FDG. The gamma rays are emitted in directions 180° apart, i.e., in opposite directions. 
     A pair of detectors registers the position and energy of the respective gamma rays. Two concurrently received events define a ray which provides information as to the position of the annihilation event and hence the positron source. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct a clinically useful image. 
     Single photon emission computed tomography (SPECT) is another nuclear imaging technique used to study the radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals are injected into a subject. The radiopharmaceuticals are commonly injected into the subject&#39;s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads are placed closely adjacent to a surface of the subject to monitor and record emitted radiation. Collimators mounted on the heads define the trajectory of radiation that is recorded by the head. In SPECT imaging, the detector head or heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation emission data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject. 
     One of the problems in nuclear imaging techniques such as PET and SPECT is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head(s) distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogenous in terms of radiation attenuation, with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest. 
     In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Radiation that is not attenuated is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into a transmission image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission image representation are used to correct for radiation attenuation in the emission data. 
     Since transmission image data is structural or anatomical in nature; whereas, emission image data is functional or metabolic in nature, it would be desirable to use transmission image data for image localization and/or image registration with a structural image of the same region from another imaging modality. The combination of a functional emission image with a structural transmission image or an image from another imaging modality can provide the diagnostician with insights that could not be obtained with either image alone, thus improving diagnostic accuracy. For example, in the area of oncology, precise positioning of localization of functional images enables a clinician to assess lesion progression and/or treatment effectiveness. Also, such diagnostic studies are used in surgical and/or radiotherapeutic planning, where precise positioning is necessary to minimize the effect on healthy cells surrounding the target cells. 
     While transmission data has heretofore been largely successful in determining attenuation correction factors for correction of the emission image data, the transmission image data itself has generally been of less than ideal resolution. The coarseness of the images could create uncertainties when localizing the emission image with respect to anatomical features. Also, the quality, and thus diagnostic value, of image registration could be improved with a transmission map of increased quality. 
     Imaging devices which combine a CT-like device with a gamma camera are known in the art. Typically, the patient is registered with only one of the nuclear and CT devices at a time. Such a combined device is a less than optimal solution to the problem of nuclear medicine image localization due to cost, temporal and spatial registration difficulties, and for logistical reasons. Also, although different modalities are combined in a single system, this type of device retains the conventional approach of addressing separately attenuation correction and precise nuclear medicine image localization. 
     Transmission image quality can also be increased through increasing the number of counts, i.e., by increasing the source activity, increasing the imaging time, or both. Increasing the source activity, however, has the disadvantage of increasing cost and shielding requirements. Increasing the imaging time is generally undesirable for patient handling reasons. Also, both increasing source activity and increasing imaging time undesirably increase the dose of radiation received by the subject. 
     It is also known to optimize a collimator for either transmission or emission imaging, however, optimizing for one or the other suffers from the drawback that the two sets of requirements can result in conflicting design parameters. For example, a collimator having a geometry that closely matches the position of the transmission source can increase transmission image quality. However, doing so can impose constraints in terms of geometry and potentially cause truncation of the emission data, known to cause severe artifact in the reconstruction. 
     Accordingly, the present invention contemplates a new and improved nuclear medicine imaging method and apparatus which overcome the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, a method of diagnostic imaging with a nuclear camera which includes a rotating gantry on which at least first and second detector heads are mounted, each of the detector heads carrying an offset transmission radiation source transmitting transmission radiation through an examination region to the other detector head is provided. The method comprises injecting a subject to be imaged with a radiopharmaceutical composition generating emission radiation and during an emission imaging phase, detecting emission radiation events from the radiopharmaceutical composition and generating emission data based on the emission radiation events detected. During a transmission imaging phase, transmission radiation is transmitted through a subject to be imaged, the subject attenuating the transmission radiation. Also during the transmission imaging phase, transmission radiation events are detected and transmission data based on the transmission radiation events are generated, wherein each detector head is configured to collect one or both of emission data and transmission data using only a portion of each detector head. 
     In a further aspect, a gamma camera includes a plurality of detectors for detecting emission radiation emitted from within a subject and transmission radiation which has traversed a subject to be imaged, the subject attenuating the radiation, each detector generating position and energy data. A plurality of transmission radiation sources each transmit transmission radiation through an examination region to a detector. Segment selector circuitry connected with the detectors selectively disables a portion of each detector head during collection of emission data, transmission data, or both. A first electronic storage medium connected with the segment selector circuitry stores transmission data and a second electronic storage medium connected with the segment selector circuitry stores emission data. A first reconstruction processor connected with the first electronic storage medium generates a transmission image representation. A second reconstruction processor connected with the second electronic storage medium generates an emission image representation. 
     One advantage of the present invention is that it allows optimization for emission imaging while allowing transmission imaging to be performed at a high count rate. 
     Another advantage of the present invention is that it provides a highly detailed attenuation map for anatomical localization and image registration. 
     Another advantage is that nonimaging segments of the detector head can be turned off to reduce the count rate load on the detector during transmission imaging. 
     Another advantage is that emission and transmission data can be acquired concurrently with optimized acquisition strategies for each. 
     Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a nuclear medicine gamma camera in accordance with aspects of the present invention. 
     FIG. 2 illustrates a nuclear camera detector head segmented in the longitudinal direction in accordance with this teaching. 
     FIG. 3 is a side view of a first preferred orientation of detector heads in a two head nuclear camera of the present invention employing segmented detector heads. 
     FIGS. 4A and 4B are partial sectional views taken along line  4 — 4  in FIG. 3, showing segmentation of the detector heads in accordance with two embodiments of the present invention. 
     FIG. 4C illustrates a preferred configuration of the embodiment shown in FIG.  4 A. 
     FIG. 5 is a side view of a second preferred orientation of detector heads in a two head nuclear camera of the present invention employing segmented detector heads. 
     FIGS. 6A and 6B are partial sectional views taken along line  6 — 6  in FIG. 5, showing segmentation of the detector heads in accordance with two embodiments of the present invention. 
     FIG. 7 is an illustration of a coincidence imaging system in accordance with aspects of the present invention. 
     FIG. 8 is an illustration of a single photon emission imaging system in accordance with aspects of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a diagnostic imaging apparatus includes a subject support  10 , such as a table or couch, which is mounted to stationary supports  12  at opposite ends. The table  10  is selectively positionable up and down to position a subject  16  being imaged or examined at a desired location, e.g., so that regions of interest are centered about a longitudinal axis  14 . 
     An outer gantry structure  20  is movably mounted on tracks  22  which extend parallel to the longitudinal axis  14 . An outer gantry structure moving assembly  24  is provided for selectively moving the outer gantry structure  20  along the tracks  22  on a path parallel to the longitudinal axis  14 . In the illustrated embodiment, the longitudinal moving assembly includes drive wheels  26  for supporting the outer gantry structure  20  on the tracks  22 . A motive power source  28 , such as a motor, selectively drives one of the wheels which frictionally engages the track  22  and drives the outer gantry structure  20  and supported inner gantry  30  and the detector heads  32  and  34  therealong. Alternatively, the outer gantry structure  20  is stationary and the subject support  10  is configured to move the subject  16  along the longitudinal axis  14  to achieve the desired positioning of the subject  16 . 
     An inner gantry structure  30  is rotatably mounted on the outer gantry structure  20  for stepped or continuous rotation. The rotating inner gantry structure  30  defines a subject receiving aperture  36 . One or more detector heads, preferably two or three, are individually positionable on the rotatable inner gantry  30 . The illustrated embodiment includes detector heads  32 ,  34 , and optionally a third detector head  35 . The detector heads also rotate as a group about the subject receiving aperture  36  and the subject  16 , when received, with the rotation of the rotating gantry structure  30 . The detector heads are radially, circumferentially, and laterally adjustable to vary their distance from the subject and spacing on the rotating gantry  16  to position the detector heads in any of a variety of angular orientations about, and displacements from, the central axis. For example, separate translation devices, such as motors and drive assemblies, are provided to independently translate the detector heads radially, circumferentially, and laterally in directions tangential to the subject receiving aperture  36  along linear tracks or other appropriate guides. The embodiments described herein employing two detector heads can be implemented on a two detector system or a three detector system. Likewise, the use of three-fold symmetry to adapt the illustrated embodiments to a three detector system is also contemplated. 
     The detector heads  32 ,  34 , and  35  each include a scintillation crystal, such as a large doped sodium iodide crystal, disposed behind a radiation receiving face  38 ,  38 ′ that faces the subject receiving aperture  36 . The scintillation crystal emits a flash of light or photons in response to incident radiation. The scintillation crystal is viewed by an array of photomultiplier tubes that receive the light flashes and converts them into electrical signals. A resolver circuit resolves the x, y-coordinates of each flash of light and the energy (z) of the incident radiation. That is to say, radiation strikes the scintillation crystal causing the scintillation crystal to scintillate, i.e., emit light photons in response to the radiation. The relative outputs of the photomultiplier tubes are processed and corrected in conventional fashion to generate an output signal indicative of (i) a position coordinate on the detector head at which each radiation event is received, and (ii) an energy of each event. The energy is used to differentiate between various types of radiation such as multiple emission radiation sources, stray and secondary emission radiation, scattered radiation, transmission radiation, and to eliminate noise. 
     In SPECT imaging, a projection image representation is defined by the radiation data received at each coordinate on the detector head. In SPECT imaging, a collimator defines the rays along which radiation is received. 
     In PET imaging, the detector head outputs are monitored for coincident radiation events on two heads. From the position and orientation of the heads and the location on the faces at which the coincident radiation is received, a ray between the coincident event detection points is calculated. This ray defines a line along which the radiation event occurred. In both PET and SPECT, the radiation data from a multiplicity of angular orientations of the heads is then reconstructed into a volumetric image representation of the region of interest. 
     Each of the detector heads  32  and  34  is segmented into a plurality of regions such that only a portion of the field of view (FOV) is imaged during transmission scanning, emission scanning, or both. Optional detector head  35 , when utilized, is configured in like manner. The detector heads are segmented by (1) selectively enabling and disabling regions of the detector heads during successive emission and transmission scans and/or (2) employing in an emission imaging region of the detector a collimator suitable for emission imaging (i.e., restricting radiation received by the emission imaging region to radiation traveling along a desired projection path, such as a parallel beam, a cone beam, fan beam collimator, etc.) and employing in a transmission imaging region an open frame crystal or a collimator or axial filter suitable for transmission imaging, such as a collimator or axial filter which matches the geometry of the transmission radiation source or for which the transmission source is substantially penetrating. In the preferred embodiments, the collimator is omitted for the transmission segments of the detectors and an open frame crystal is used. For example, a collimator is not required for a transmission radiation point or line source, or where the transmission radiation is collimated at the source. 
     FIG. 2 illustrates a detector head, such as the detector head  32 , segmented along the axial direction  14 . A collimator  52  includes a first region  72  which is configured for single photon emission or positron emission imaging and a second region  74 , configured for transmission imaging. In the SPECT embodiment, the region  72  includes a collimator, e.g., parallel beam, cone beam, fan beam, etc., and the region  74  is uncollimated (open frame crystal) or includes a collimator having a geometry matching the transmission beam or for which the transmission radiation is substantially penetrating. The collimator typically absorbs a high percentage of the transmission radiation. The use of an open frame crystal for transmission imaging greatly increases the sensitivity of the device. For a typical configuration, a 10 mCi source in a collimated holder generates about 40-50 Kcps on a 19 mm crystal through a low energy collimator. The same source and arrangement generates between 350-400 Kcps when the collimator is removed. The level of counts available through a collimator is generally sufficient for a general attenuation map for attenuation correction. However, when a greater level of detail is desired from the transmission image, e.g., for anatomical localization or image registration, transmission imaging without a collimator is preferred. Having access to approximately an order of magnitude more counts for transmission imaging can compensate, at least in part, for the fact that only a part of the detector is now available for imaging. Thus, the segmented approach of the present invention is particularly advantageous if a high statistics transmission map or image is required and/or if half of the detector size is sufficient. 
     FIG. 3 illustrates a two-head embodiment, including a first detector head  32  and a second detector head  34  arranged on the inner gantry structure  30 . The configuration of FIG. 3 is suitable for SPECT imaging. A radiation source  54 ′ is mounted on the detector head  34  such that transmission radiation therefrom is directed toward and received by the collimator  52  and the detector head  32 . Likewise, radiation source  54  is mounted on the detector head  32  such that transmission radiation therefrom is directed toward and received by the detector head  34  and its collimator  52 ′. It is to be recognized that a third detector head may optionally be employed, with or without a transmission radiation source mounted thereon in like manner. 
     In one embodiment, each of the radiation sources  54  and  54 ′ includes a radioactive point source adjustably mounted inside shielded steel cylinders  60  and  60 ′, respectively, which are sealed at the ends. In this configuration, the radioactive point source generates a radiation cone beam which passes through the subject receiving aperture  36 . The radiation sources can be rastered along the longitudinal axis  14  longitudinally, thus moving the fan beam across the field of view. The steel cylinder  60  and  60 ′ are adjustably mounted onto the corresponding detector head through pivoting arm mechanisms  62  and  62 ′, respectively, for retraction when the transmission source is not used. Alternately, the radiation source is a line source, flat rectangular source, disk source, flood source, or an x-ray tube. 
     Referring now to FIG. 4A, there is shown a partial sectional view taken along line  4 — 4  of the embodiment depicted in FIG.  3 . The detector head  32  is segmented, in the direction along longitudinal axis  14 , into a first region  72  and a second region  74 . A region  76  is preferably provided to isolate the regions  72  and  74 . The detector head  34  is segmented into imaging regions  72 ′ and  74 ′ in like manner. 
     Referring now to FIG. 4C, there is shown a partial sectional view of a preferred configuration of the FIG. 4A embodiment. The detector  32  is segmented, in the direction along longitudinal axis  14 , into a first emission imaging region  72  and a second transmission imaging region  74 . A region  76  is provided to isolate the regions  72  and  74 . The region  72  uses a parallel beam collimation for single photon emission imaging and the region  74  of collimator provides fan beam collimation, e.g., asymmetric fan beam collimation, for transmission imaging. The regions  72 ′ and  74 ′ of the detector  34  are configured in like manner. Alternately, no collimation is used in the regions  74  and  74 ′. 
     In operation, the embodiments of FIGS. 4A and 4C employ sequential emission and transmission phases, wherein the gantry is translated longitudinally between the two phases so that the field of view during each phase is aligned with the same region of the subject. The regions  74  and  74 ′ are turned off during the emission imaging phase. 
     In FIG. 4B, there is shown a configuration which differs from the embodiment of FIG. 4A in that the regions  74 ′ and  72 ′ are reversed on the head  34  relative to the head  32 . In operation, single photon emission and transmission imaging are performed simultaneously or sequentially with no longitudinal translation of the gantry between the phases. By alternating the regions in the manner shown in FIG. 4B, a full detector field of view is accommodated without shifting along the longitudinal axis. Together, the detectors provide a full “reconstructible” SPECT data set and a full transmission data set after 360° of rotation. Although the embodiments of FIGS. 4A and 4C are only rotated about 180° before shifting, whereas the FIG. 4B embodiment is rotated 360°, the imaging times for the embodiments are the same since, for the FIG. 4B embodiment, only one subject position is scanned, and the time for longitudinal translation is eliminated. 
     Referring now to FIG. 5, a second detector head configuration, wherein the detector heads are in generally opposing and facing relation and employing a segmented field of view is shown. The arrangement of FIG. 5 is suitable for PET or coincidence imaging. 
     Referring now to FIGS. 6A and 6B, there are shown partial sectional views of two embodiments taken along line  6 — 6  of the embodiment depicted in FIG.  5 . The detector head  32  is segmented, in the direction along longitudinal axis  14 , into a first imaging region  72  and a second imaging region  74 . A region  76  is preferably provided to isolate the regions  72  and  74 . Likewise, the detector head  34  is segmented, in the direction of the longitudinal axis  14 , into a first emission imaging region  72 ′ and a second transmission imaging region  74 ′. In FIG. 6A, the region  72  opposes the region  72 ′ and the region  74  opposes the region  74 ′. Preferably, the transmission image data is collected in about a half rotation. After a longitudinal shift, emission data is collected from the same region over a half rotation. In FIG. 6B, the region  72  opposes the region  74 ′ and the region  74  opposes the region  72 ′. 
     In FIG. 6B, wherein the regions are alternating, PET imaging is performed using a full field of view followed by full field of view transmission imaging on the regions  74  and  74 ′. The regions  72  and  72 ′ can be turned off during transmission imaging or, alternatively, the regions  72  and  72 ′ can be used for mock scan acquisition during transmission imaging. 
     The configuration of the detector heads  32  and  34  for various imaging protocols are listed in TABLE 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Detector 32 
                 Detector 34 
                   
               
             
          
           
               
                 Example 
                 Region 72 
                 Region 74 
                 Region 72′ 
                 Region 74′ 
                 Protocol 
               
               
                   
               
               
                 1 
                 Any collimator 
                 Open frame 
                 Any collimator 
                 Open frame 
                 SPECT 
               
               
                 2 
                 Any collimator 
                 Open frame 
                 Open frame 
                 Any collimator 
                 SPECT 
               
               
                 3 
                 Any collimator 
                 Axial filter 
                 Any collimator 
                 Axial filter 
                 SPECT 
               
               
                 4 
                 Any collimator 
                 Axial filter 
                 Axial filter 
                 Any collimator 
                 SPECT 
               
               
                 5 
                 Axial filter 
                 Axial filter 
                 Axial filter 
                 Axial filter 
                 PET 
               
               
                 6 
                 Axial filter 
                 Axial filter 
                 Axial filter 
                 Axial filter 
                 PET 
               
               
                   
               
             
          
         
       
     
     In Example 1, low-, medium-, or high-energy, sequential emission and transmission imaging is employed with translation of the FOV between the two phases. The regions  74  and  74 ′ (FIG. 4A) of the detectors  32  and  34  are turned off during the emission imaging phase. The regions  72  and  72 ′ are turned off during the transmission imaging phase. 
     In Example 2, low-, medium-, or high-energy, sequential emission and transmission imaging is employed with no translation of the FOV between the two phases. The region  74  of the detector  32  and the region  72 ′ of the detector  34  (FIG. 4A) are turned off during the emission phase. The region  72  of the detector  32  and the region  74 ′ of the detector  34  are turned off during the transmission phase. 
     In Example 3, low-, medium-, or high-energy, sequential emission and transmission imaging is employed with translation of the FOV between the two phases. The regions  74  and  74 ′ (FIG. 4A) of the detectors  32  and  34  are turned off during the emission imaging phase. The regions  72  and  72 ′ are turned off during the transmission imaging phase. 
     In Example 4, low-, medium-, or high-energy, sequential emission and transmission imaging is employed with no translation of the FOV between the two phases. The region  74  of the detector  32  and the region  72 ′ of the detector  34  (FIG. 4A) are turned off during the emission phase. The region  72  of the detector  32  and the region  74 ′ of the detector  34  are turned off during the transmission phase. 
     In Example 5, full FOV emission (coincidence) imaging is followed by transmission imaging on the region  72  of the detector  32  and the region  74  of the detector  34 , with the region  74  of the detector  32  and the region  72 ′ of the detector  34  (FIG. 6A) turned off. 
     In Example 6, full FOV emission imaging is followed by transmission imaging on the region  72  of the detector  32  and the region  74 ′ of the detector  34  (FIG.  6 A). The region  74  of the detector  32  and the region  72 ′ of the detector  34  are used during transmission imaging for acquisition of mock scan data. 
     In reference to FIG. 7, a nuclear medicine imaging apparatus is shown in a configuration suitable for PET imaging in accordance with the present invention. In the embodiment shown, the two detector heads,  32  and  34 , are arranged on the rotating gantry  30  on opposite sides of the receiving aperture  36  in facing relation. The receiving faces of the detectors are advantageously aligned in generally parallel planes for receiving the coincidence emission events. A transmission radiation source  54  is mounted on the detector head  32  or the rotating gantry  30  and is collimated such that when its shutter is opened, transmission radiation is directed toward and received by the detector head  34  positioned across the subject receiving aperture from the radiation source. Likewise, a radiation source  54 ′ is mounted on the detector head  34  or the rotating gantry  30  and is collimated such that transmission radiation is directed toward and received by the detector head  32  positioned across the subject receiving aperture from the radiation source. 
     As the gantry and thus the detectors  32  and  34  are rotated about the subject, the x-y coordinates of radiation events on each head are collected  80 , as well as the position and angular orientation of each head from a position sensor or encoder  82 . The head position sensor  82  may be, for example, optical, mechanical, or optomechanical. Annihilation radiation events are identified by coincidence circuitry  84  which identifies simultaneous scintillations in both heads and discards noncoincident and piled-up events. The coincidence circuitry  84  also includes a coincidence data processor, e.g., a ray processor, which uses the x, y coordinates and head position information to generate coincidence data which is stored in a coincidence data memory  86 . The coincidence events are detected using the entire field of view of heads  32  and  34 . 
     Following collection of the coincidence data  86 , transmission data is collected using a segmented field of view in accordance with this teaching. In a first embodiment, the transmission data is collected using aligned facing regions of the detectors  32  and  34 , e.g., the regions  74  and  74 ′ of the configuration shown in FIG.  6 A. Alternatively, the transmission data is collected using nonfacing regions of the detectors  32  and  34 , e.g., the regions  74  and  74 ′ of the configuration shown in FIG.  6 B. In another embodiment, one segment of one or both of the detector heads receives the transmission radiation concurrently while the other segments receive emission radiation. Due to the high transmission count rates, the transmission radiation source can be active intermittently, with one segment time sharing between PET and transmission modes. 
     A segment selector  88  is provided to selectively collect data from the desired regions, e.g., sorting the emission and transmission data based on the x, y coordinates of each head. In some embodiments, the data from the excluded regions is discarded, or more preferably, the unused detector segment is electronically disabled. The regions  72  and  72 ′ can be turned off during transmission scanning, or alternatively, the regions  72  and  72 ′ can acquire emission data concurrently. The segment selector  88  can further include energy discrimination or sorting circuitry to sort emission and transmission events based on detected energies. 
     Transmission radiation data, including x,y position and head position, are collected  80  from the selected regions of the segmented detectors  32  and  34 . Energy discrimination circuitry is optionally included to discard detected events which do not correspond in energy to the transmission radiation sources  54  and  54 ′. The transmission data are stored in a transmission data memory  90 . 
     A transmission reconstruction processor  92  reconstructs the transmission data stored in the transmission data memory  90  to generate a transmission image representation which is stored in the transmission image memory  94 . The reconstruction process can change according to the mode of collection, the nature of the study, and the types of collimators used (i.e., fan, cone, parallel beam, and/or other modes). 
     The transmission image representation  94  is used to determine attenuation correction factors which are stored in an attenuation factor memory  96 . A coincidence data correction processor  98  corrects the emission data in accordance with the attenuation factors  96 . For example, for each ray along which coincidence data is received, the coincidence correction processor  98  calculates a corresponding ray through the transmission attenuation factors stored in the memory  96 . Each ray of the coincidence data is then weighted or corrected by the coincidence data correction processor  98  inversely with the attenuation factors. The corrected coincidence data are reconstructed by a coincidence reconstruction processor  100  to generate a three-dimensional coincidence image representation that is stored in a volumetric coincidence image memory  102 . Alternately, the attenuation correction is performed as a part of the reconstruction process. A combiner circuit  104  is advantageously provided to combine the coincidence and transmission image. A display format selector  106  is also advantageously provided to allow a user to select for viewing the transmission image  94 , the coincidence image  102 , or a fused coincidence and transmission image. A video processor  108  withdraws selected portions of the data from the image selected for viewing and generates a corresponding human-readable display on a video monitor  110 . Typical displays include reprojections, selected slices or planes, surface renderings, and the like. Other human readable output formats, such as printed output, are also contemplated. 
     In reference to FIG. 8, a nuclear medicine imaging apparatus is shown in a configuration for SPECT imaging in accordance with the present invention. In the embodiment shown, the two detector heads,  32  and  34 , are arranged on the rotating gantry  30  about the receiving aperture  36 . A transmission radiation source  54  is mounted on the detector head  32  or the rotating gantry  30  and is collimated such that transmission radiation is directed toward and received by one segment of the detector head  34  positioned across the subject receiving aperture from the radiation source. Likewise, another radiation source  54 ′ is mounted on the detector head  34  or the rotating gantry  30  and is collimated such that transmission radiation is directed toward and received by one segment of the detector head  32  positioned across the subject receiving aperture from the radiation source. 
     As the gantry and thus the detectors  32  and  34  are rotated about the subject, the x-y coordinates of emission and transmission radiation events are collected  80 , as well as the position and angular orientation of each head from a position sensor or encoder  82 . 
     In one embodiment, the emission and transmission data are collected sequentially, using the segmented fields of view discussed above. The transmission data is collected using aligned facing regions of the detectors  32  and  34 , i.e., the regions  74  and  74 ′ of the detectors  32  and  34 , respectively, as illustrated in FIGS. 4A and 4C. In a second embodiment, the transmission data is collected using nonfacing regions of the detectors  32  and  34 , i.e., the regions  74  and  74 ′ of the detectors  32  and  34 , respectively, as illustrated in FIG. 4B. A segment selector  88  is provided to sort emission and transmission data from the desired regions, e.g., sorting the data based on the x, y coordinates, electronically disabling the undesired regions, and so forth. In a third embodiment, the emission and transmission data are collected simultaneously. 
     The transmission radiation data, including x, y position and detector head position, are collected  80  from the selected regions of the segmented detectors  32  and  34 . Energy data may also be included to discard detected events which do not correspond in energy to the transmission radiation source  54 . The transmission data are stored in a transmission memory  90 . 
     A transmission reconstruction processors  92  reconstructs the transmission data stored in the transmission data memory  90  to generate a transmission image representation which is stored in the transmission image memory  94 . The reconstruction process can change according to the mode of collection, the nature of the study, and the types of collimators used (i.e., fan, cone, parallel beam, and/or other modes). 
     The transmission image representation  94  is used to determine attenuation correction factors which are stored in an attenuation factor memory  96 . An emission data correction processor  98  corrects the emission data from an emission data memory  85  in accordance with the attenuation factors  96 . For example, for each ray along which emission data is received, the emission correction processor  98  calculates a corresponding ray through the transmission attenuation factors stored in the memory  96 . Each ray of the emission data is then weighted or corrected by the emission data correction processor  98  inversely with the attenuation factors. The corrected emission data are reconstructed by an emission reconstruction processor  101  to generate a three-dimensional emission image representation that is stored in a volumetric emission image memory  103 . Alternately, the attenuation correction is performed as a part of the reconstruction process. A combiner circuit  104  is advantageously provided to combine the emission and transmission image. A display format selector  106  is also advantageously provided to allow a user to select for viewing the transmission image  94 , the emission image  102 , or a fused emission and transmission image. A video processor  108  withdraws selected portions of the data from the image selected for viewing and generates a corresponding human-readable display on a video monitor  110 . Typical displays include reprojections, selected slices or planes, surface renderings, and the like. Other human readable output formats, such as printed output, are also contemplated. 
     The transmission image representation obtained in connection with the embodiments of FIGS. 7 and 8, can be used for anatomical localization of the emission and coincidence images, and for registering such images with an image from another modality, such as a CT, MR, or ultrasound image representation. Accordingly, an image registration processor may advantageously be provided to register a coincidence or emission image representation with a digital image representation acquired from another imaging modality. Because the segmented detector can obtain transmission information in at high count rate, e.g., without a collimator, a more accurate representation of anatomical features is provided. Thus, the emission or coincidence image, which is machine registered to the transmission image is more accurately registered with the other modality image representation. The registered images may then be combined to form a fused or superimposed image representation. 
     As another alternative, the detector head(s) are segmented into two halves with a dividing line parallel with the longitudinal axis  14 . A radiation source is mounted opposite one segment. The other segment is collimated for SPECT imaging. In any given position of the head, one segment of the detector receives emission radiation from one half of the subject and the other segment received transmission radiation, through the other half of the subject. When the head is rotated 180°, emission radiation is received from the opposite half of the subject, and the same for the transmission radiation. 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.