Abstract:
A nuclear gamma camera employs a virtual contouring technique in order to maximize the portion of transmission radiation fan beams ( 32   a,    32   b ) which pass through a subject ( 12 ). A plurality of radiation detector heads ( 20   a-   20   c ) having radiation receiving faces and a plurality of radiation sources ( 30   a,    30   b ) are mounted to a gantry ( 16 ). An orbit memory ( 42 ) stores clearance offset orbit ( 45 ) around the subject and a subject support ( 10 ). A tangent calculator ( 46 ) calculates virtual lines ( 48   a,    48   b ) between the radiation sources ( 30   a,    30   b ) and the corresponding radiation detector heads ( 20   a,    20   b ). The virtual lines ( 48   a,    48   b ) correspond to edge rays of the transmission radiation fans ( 32   a,    32   b ). A shift calculator ( 50 ) calculates and sends shift commands to a motor orbit controller ( 52 ) which controls rotational and translational drives attached to the detector heads ( 20   a-   20   c ). The detector heads are translated such that the virtual lines ( 48   a,    48   b ) remain tangent to a predefined contour of the subject throughout rotation of the detector heads about the subject receiving aperture ( 18 ). The detected transmission radiation ( 32   a,    32   b ) is reconstructed ( 64   t ) into an attenuation volumetric image representation and used to correct ( 68 ) detected emission radiation data. The corrected emission data is then reconstructed ( 64   e ) into a volumetric image representation. The virtual contouring minimizes lost rays ( 40 ) of transmission radiation and facilitates an artifact-free attenuation volumetric image representation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the arts of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with gamma or scintillation cameras and will be described with particular reference thereto. It is to be appreciated that the present invention is applicable to single photon emission computed tomography (SPECT), positron emission tomography (PET), whole body nuclear scans, and the detection of radiation for other applications. 
     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, typically including collimators, are placed adjacent to a surface of the subject to monitor and record emitted 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. 
     One of the problems with this imaging technique is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera heads 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 homogeneous 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 an image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission computed tomography image are used to correct or compensate for radiation attenuation in the emission data. 
     Dual and triple head gamma cameras are now equipped for simultaneous collection of transmission and emission data in order to provide enhanced PET and SPECT attenuation correction. Typically, the transmission device consists of a collimated radioactive line source or a point source mounted for movement along a shielded cylinder. The cylinder may be mounted to one or more of the detector heads through a pivoting arm mechanism. In this configuration, the transmission sources are offset from the detector heads, and therefore offset the useful field of view (FOV). 
     With one or more offset transmission sources, the transmission radiation beam is offset from the center of rotation, i.e. the center of the subject, creating unsampled regions. Because information from the central portion of the subject is critical for an artifact-free reconstruction, detector heads have been shifted laterally so that the transmission fan beams cover the center of the subject. While lateral shifting of the detector heads enables transmission radiation to pass through a central region, some regions of the patient are still undersampled, and some radiation passes through the air missing the patient. In order to minimize a patient&#39;s dose of radiation, the transmission radiation source typically generates only a limited number of radiation events per unit time. Wasting a portion of these events or rays reconstructing empty regions next to the patient is inefficient. 
     In order to eliminate these “lost rays” of transmission radiation, prior art techniques concentrate on moving the patient support vertically and horizontally during data acquisition. This technique is disadvantageous because it leads to patient discomfort, especially in rapid acquisition sequences. 
     The present invention contemplates a new and improved contouring technique for use with transmission scans which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a nuclear medicine gamma camera includes a rotating gantry which defines a subject receiving aperture. A plurality of radiation detector heads, which are movably attached to the rotating gantry, rotate about the subject receiving aperture with rotation of the rotating gantry about an axis of rotation. At least one radiation source is mounted to the rotating gantry such that a divergent beam of transmission radiation from the at least one radiation source is directed toward and received by a corresponding detector head positioned across the subject receiving aperture from the radiation source. A rotational drive rotates the plurality of detector heads around the subject receiving aperture and a plurality of translational drives translate independently the plurality of detector heads (i) laterally in directions tangential to the subject receiving aperture and (ii) radially in directions orthogonal to the axis of rotation. An orbit memory stores a predefined orbit which clears a subject disposed in the subject receiving aperture. A tangent calculator calculates the position of a virtual line between the at least one radiation source and an edge of a radiation receiving face of the corresponding detector head which receives transmission radiation from the at least one radiation source. A shift calculator calculates lateral and radial shifts for the plurality of detector heads such that the detector head positions are dynamically adjusted in order to maintain the virtual line tangent to an outer boundary of the subject throughout rotation of the gantry around the subject receiving aperture. A motor orbit controller controls the plurality of translational drives and the rotational drive in accordance with the orbit from the orbit memory and shift inputs from the shift calculator. 
     In accordance with another aspect of the present invention, a method of diagnostic imaging using a nuclear medicine gamma camera includes placing a subject in a subject receiving aperture and injecting the subject with a radiopharmaceutical. A plurality of radiation sources and corresponding radiation detector heads are positioned about the subject receiving aperture such that the radiation sources are across the subject receiving aperture from their corresponding radiation detector heads. A contour of the subject is obtained and radiation emitted by the injected radiopharmaceutical is detected using the plurality of radiation detector heads. The positions of virtual lines extending from each radiation source to an edge of a radiation receiving face disposed on each corresponding radiation detector head are calculated. The detector heads are shifted laterally such that the virtual lines are tangent to the contour of the subject. Radiation from the radiation sources is transmitted toward the corresponding radiation detector heads positioned across the subject receiving aperture and detected using one of the plurality of radiation detectors. The detected tranmsission and emission radiation is reconstructed into a volumetric image representation. 
     In accordance with another aspect of the present invention, a nuclear camera system includes a rotating gantry which defines a subject receiving aperture and a plurality of real radiation detector heads movably attached to the rotating gantry. The real detector heads rotate about the subject receiving aperture with rotation of the rotating gantry. A plurality of radiation sources are mounted to the plurality of real detector heads such that transmission radiation from the radiation sources is directed toward and received by the corresponding real detector heads positioned across the subject receiving aperture from the plurality of radiation sources. A plurality of virtual detector heads impose shift restrictions on the real detector heads during rotation about the subject receiving aperture. A rotational drive rotates the real detector heads about the subject receiving aperture and a pair of translational drives translate independently the real detector heads at least one of laterally and radially with respect to the subject receiving aperture. An orbit memory stores a predefined contour of a subject disposed in the subject receiving aperture. A shift calculator calculates shifts in the real detector heads according to the predefined contour of the subject and the shift restrictions imposed by the virtual detector heads. A motor orbit controller controls the translational and rotational drives in response to commands from the shift calculator. 
     In accordance with another aspect of the present invention, a nuclear camera includes a rotating gantry on which at least first and second detector heads are mounted. The first detector head carries an offset transmission radiation source that projects a fan bean of transmission radiation to the second detector head, where the fan beam extends between edge rays. A rotating drive rotates the rotating gantry continuously or in steps and a radial drive moves the detector heads in a radially inward direction toward a center of rotation of the rotating gantry and a radially outward direction away from the center of rotation. A lateral drive moves the detector heads with a component of motion orthogonal to the radially inward and outward directions. The nuclear camera is controlled by positioning a subject on a subject support with a region of interest at the center of rotation. A clearance offset orbit around and displaced from the subject and subject support is calculated. A subject orbit around the region of interest is calculated and the subject is injected with a radiopharmaceutical. The rotating drive and radial drive are controlled such that the detector heads are maintained tangent to the clearance offset orbit as the detector heads are rotated around the subject. The lateral drive is controlled such that one of the fan beam edge rays is maintained tangent to the subject orbit as the detector heads rotate. 
     One advantage of the present invention is that it maximizes the fraction of the transmission radiation beam which interacts with the subject. 
     Another advantage of the present invention is that it provides a full set of transmission correction data. 
     Another advantage of the present invention resides in that it facilitates reduction of the radiation dose. 
     Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiment. 
    
    
     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. 2A is a side view of a preferred orientation of detector heads in a two head nuclear medicine gamma camera illustrating an unsampled region at the center of the subject; 
     FIG. 2B is a perspective view of a preferred orientation of detector heads in a two head nuclear medicine gamma camera; 
     FIG. 3 is a diagrammatic illustration of a preferred orientation of detector heads in a three head nuclear medicine gamma camera in accordance with the present invention; 
     FIG. 4A is a side view of a preferred orientation of detector heads in a two head nuclear medicine gamma camera illustrating a minimal lateral shift in accordance with the present invention; and, 
     FIG. 4B is a side view of a preferred orientation of detector heads in a two head nuclear medicine gamma camera illustrating an optimal lateral shift in accordance with 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 supports a subject  12  being examined and/or imaged. The subject  12  is injected with one or more radiopharmaceuticals or radioisotopes such that emission radiation is emitted therefrom. Optionally, the subject support  10  is selectively height adjustable so as to center the subject  12  at a desired height, e.g., the volume of interest is centered. A first or stationary gantry  14  rotatably supports a rotating gantry  16 . The rotating gantry  16  defines a subject receiving aperture  18 . In a preferred embodiment, the first gantry  14  is moved longitudinally along the subject support  10  so as to selectively position regions of interest of the subject  12  within the subject receiving aperture  18 . Alternately, the subject support  10  is advanced and retracted to achieve the desired positioning of the subject  12  within the subject receiving aperture  18 . 
     Detector heads  20   a,    20   b,    20   c  are individually positionable on the rotating gantry  16 . The detector heads  20   a - 20   c  also rotate as a group about the subject receiving aperture  18  (and the subject  12  when received) with the rotation of the rotating gantry  16 . The detector heads  20   a - 20   c  are radially and circumferentially adjustable to vary their distance from the subject and spacing on the rotating gantry  16 , as for example, in the manner disclosed in U.S. Pat. No. 5,717,212. Separate translation devices  22   a,    22   b,    22   c,  such as motors and drive assemblies, independently translate the detector heads radially and laterally in directions tangential to the subject receiving aperture  18  along linear tracks or other appropriate guides. 
     Each of the detector heads  20   a - 20   c  has a radiation receiving face facing the subject receiving aperture  18 . Each head includes a scintillation crystal, such as a large doped sodium iodide crystal, that emits a flash of light or photons in response to incident radiation. An array of photomultiplier tubes receive the light flashes and convert them into electrical signals. A resolver circuit resolves the x, y-coordinates of each flash of light and the energy 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 photons are received by the photomultiplier tubes and the relative outputs of the photomultiplier tubes are processed and corrected 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 PET imaging, the detector head outputs are monitored for coincident radiation events on two or more heads. From the position and orientation of the heads and the location on the faces at which the coincident radiation was received, a ray between the peak detection points is calculated. This ray defines a line along which the radiation event occurred. 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. 
     For SPECT imaging, the detector heads  20   a - 20   c  include mechanical collimators  24   a,    24   b,    24   c,  respectively, removably mounted on the radiation receiving faces of the detector heads  20   a - 20   c.  The collimators include an array or grid of lead vanes which restrict the detector heads  20   a - 20   c  from receiving radiation not traveling along selected rays in accordance with the selected imaging procedure. For PET imaging, a SPECT camera without collimators on the detector heads may be employed. Alternately, PET imaging is performed using collimators to minimize stray radiation. 
     FIG. 2A illustrates a two-head embodiment, including a first detector head  20   a  and a second detector head  20   b  arranged on the rotating gantry  16  on opposite sides of the subject receiving aperture  18  such that the radiation receiving faces of the first and second detector heads face one another. A first radiation source  30   a  is mounted to the first detector head  20   a  and is collimated such that transmission radiation (represented by the arrows  32   a ) from the radiation source  30   a  is directed toward and received by the second detector head  20   b  positioned across the subject receiving aperture from the radiation source  30   a.  A second radiation source  30   b  is mounted to the second detector head  20   b  and collimated such that transmission radiation  32   b  therefrom is directed toward and received by the first detector head  20   a.  The first and second radiation sources  30   a,    30   b  are mounted at opposite ends of the radiation receiving faces of the first and second detector heads  20   a,    20   b  as shown. The preferred collimators  24   a,    24   b  are configured such that the detector heads  20   a,    20   b  receive both the emission radiation and the transmission radiation  32   a,    32   b.  That is to say, the collimators  24   a,    24   b  restrict the detector heads  20   a,    20   b,  (in the embodiment of FIG. 2A) from receiving those portions of transmission radiation not traveling along direct rays from the source to the radiation receiving faces of the detector heads. Alternately, other collimation geometries are employed for different applications and radiation sources, such as a line source. Additional collimation may take place at the source. 
     FIG. 3 illustrates a three-head embodiment, including a first detector head  20   a,  a second detector head  20   b,  and a third detector head  20   c  arranged on the rotating gantry  16  spaced from one another around the subject receiving aperture  18 . A first radiation source  30   a  is mounted to the first detector head  20   a  such that transmission radiation  32   a  therefrom is directed toward and received by the second detector head  20   b.  A second radiation source  30   b  is optionally mounted to the second detector head  20   b  such that transmission radiation therefrom can be directed toward and received by the first detector head  20   a.  It is to be appreciated that radiation sources may be mounted to all three detector heads. 
     In one embodiment, the radiation source  30   a  contains a radioactive point source  36   a  adjustably mounted inside a shielded steel cylinder which is sealed at the ends. In this configuration, the radioactive point source generates a radiation fan beam which passes through the subject receiving aperture  18 . As shown diagrammatically in FIG. 2B, as the radiation source  30   a  rasters longitudinally, the fan beam moves across the field of view. In a step and shoot mode, the transmission source undergoes a full raster (or integer number of rasters) at each step. In a continuous rotate mode, the fan beam spirals through the examination volume. The steel cylinder is adjustably mounted onto the corresponding detector head through a pivoting arm mechanism for retraction when the transmission source is not used. Alternately, the radiation source  30   a  is a bar source, flat rectangular source, disk source, flood source, tube or vessel filled with radionuclides, or active radiation generators such as x-ray tubes. 
     FIG. 2A illustrates the two-head embodiment in which the radiation sources  30   a,    30   b  are mounted outside the field of view (FOV) of the first and second radiation detector heads  20   a,    20   b.  Those skilled in the art will appreciate that having the radiation sources outside the FOV of the detector heads results in a “hole” or blind spot  38  in the transmission FOV. In other words, the transmission radiation from the first and second radiation sources does not pass through a region  38  surrounding a center C of the orbit. In order to receive that valuable transmission information from this central region  38 , the detector heads  20   a,    20   b  are shifted laterally, as shown in FIG. 4A, such that the transmission radiation fans  32   a,    32   b  pass through the center C of the orbit. Shifting the detector heads  20   a,    20   b  laterally just enough for the transmission radiation to pass through the center region of the FOV results in “lost rays”  40  which pass through air, rather than through the subject. In order to minimize or eliminate these lost rays  40  of transmission radiation, the detector heads are shifted further as shown in FIG.  4 B. This optimal shift maximizes the portion of the transmission radiation fans  32   a,    32   b  which pass through the subject being examined. The optimal shift is determined based on the location of virtual lines, which are described more fully below. 
     With reference again to FIG. 1, prior to running an imaging operation, the outer boundaries or contour of the subject  12  are defined and stored in an orbit memory  42 . In one embodiment, the outer boundaries are entered manually into the orbit memory  42  based on the size of the subject. In another embodiment, the outer boundaries of the subject are determined during an initial contouring scan of the subject. During the contouring operation, the translation drives  22   a - 22   c  translate the detector heads laterally in directions tangential to the subject receiving aperture  18  and a contouring processor  44  calculates outer boundaries of the subject  12  based on the transmission radiation received by the detector heads. The edges of the subject are registered when the subject interferes with the transmission radiation emitted from the radiation sources as detected by the corresponding detector heads. That is to say, as the relative positions of the radiation sources and the corresponding detector heads are varied, the outer boundary of the subject interferes and/or crosses the path of the transmission radiation as it is transmitted across the subject receiving aperture. The rotating gantry is incrementally rotated with the contouring device  44  measuring the outer boundaries of the subject at a number of angular orientations to obtain a complete outer contour of the subject. A clearance offset calculator calculates a clearance offset  45 , i.e., a minimum distance of approach between the head and the subject including the support. 
     Once the outer boundaries of the subject, including the clearance offset, are determined and stored in the orbit memory  42 , a tangent calculator  46  calculates a first virtual line  48   a  between the first radiation source  30   a  and an edge of the second detector head  20   b.  Conversely, the tangent calculator calculates a second virtual line  48   b  between the second radiation source  30   b  and an edge of the first detector head  20   a.  These virtual lines  48   a,    48   b  correspond to the end rays of the radiation fans generated by the first and second radiation sources  30   a,    30   b,  respectively. It is to be appreciated that the virtual lines may be calculated based on the known geometry of the scanner. Once the virtual lines  48   a,    48   b  are calculated by the tangent calculator  46 , a shift calculator  50  calculates initial lateral shifts for each of the detector heads  20   a,    20   b  as a function of angular position of the heads. The initial lateral shifts are determined such that each virtual line  48   a,    48   b,  corresponding the end rays of each radiation fan  32   a,    32   b,  is tangent to the predetermined orbit, corresponding to the outer boundaries or contour of the subject, as shown in FIG.  4 B. As the imaging operation commences, a motor orbit controller  52  controls the rotational and translational drives  22   a - 22   c  moving the heads in and out with angular rotation to maintain the heads tangential to the clearance offset orbit and shifting the heads in response to shift inputs from the shift calculator  50 . During the imaging operation, the shift calculator  50  determines lateral and radial shifts for each of the detector heads  20   a - 20   c  such that the positions of the detector heads are dynamically adjusted in order to maintain the virtual lines  48   a,    48   b  tangent to the contour of the subject  12  throughout rotation of the gantry  16  around the subject receiving aperture  18 . In other words, the mathematical relationship between the virtual lines and the predefined orbit around the patient is used to control lateral shifting of the detector heads throughout the acquisition of transmission radiation data. Artisans will appreciate that this technique maximizes the amount of transmission radiation which passes through the region of interest during a transmission scan by minimizing lost rays  40 . Further, this technique is applicable to eliminate transmission data truncation caused by the edge of the predefined orbit being outside the end rays of the transmission radiation fan beam. 
     Maintaining the virtual lines tangent to the predefined orbit throughout the transmission scan adds a constraint on the detector heads in addition to keeping them moving along the oval orbit that defines the region of interest. From the perspective of the control software, namely the shift calculator  50  and motor orbit controller  52 , the additional virtual line constraint is analogous to having a scanner with two additional “virtual detector heads”  48   a,    48   b.  During an imaging operation the real detector heads  20   a,    20   b  are dynamically adjusted according to constraints placed upon them and the positions of adjacent detector heads. For example, adjustment of a third virtual detector head  48   a,  corresponding to a virtual line from the first radiation source  30   a  to the second detector head  20   b,  results in a responsive adjustment of the real detector heads  20   a,    20   b  based on the additional constraint. 
     Running an imaging operation includes a reconstruction process for emission and transmission data. The reconstruction process changes according to the type of radiation collected and the types of collimators used (i.e., fan, cone, parallel beam, and/or other modes). Emission radiation from the subject  12  is received by detector heads  20   a - 20   c  and transmission radiation  32   a,    32   b  from the radiation sources  30   a,    30   b  is received by the detector heads  20   a,    20   b  to generate emission projection data and transmission projection data. The emission data normally contains inaccuracies caused by varying absorption characteristics of the subject&#39;s anatomy. A sorter  60  sorts the emission projection data and transmission projection data, such as on the basis of their relative energies or the detector head which originated the data. The data is stored in a projection view memory  62 , more specifically in a corresponding emission data memory  62   e  and transmission data memory  62   t.  A reconstruction processor  64   t  uses a fan beam reconstruction algorithm to reconstruct the transmission data into a transmission image representation or volume of attenuation factors stored in a memory  66 . Each voxel value stored in the memory  66  is indicative of attenuation of tissue in a corresponding location within the subject  12 . 
     An emission data correction processor  68  corrects each emission data in accordance with the attenuation factors determined from the transmission data. More specifically, for each ray along which emission data is received, the emission correction processor  68  calculates a corresponding ray through the transmission attenuation factors stored in the memory  66 . Each ray of the emission data is then weighted or corrected by the emission data correction processor  68  in accordance with the attenuation factors. The corrected emission data are reconstructed by an emission radiation reconstruction processor  70  to generate a three-dimensional emission image representation that is stored in a volumetric emission image memory  72 . A video processor  74  withdraws selected portions of the data from the image memory  72  to generate corresponding human-readable displays on a video monitor  76 . Typical displays include reprojections, selected slices or planes, surface renderings, and the like. 
     It is to be appreciated that the emission and transmission acquisition portions of the imaging operation need not be performed in a set order. In addition, emission and transmission radiation data may be acquired simultaneously. 
     Although positioning edge rays  48   a,    48   b  tangent to the subject  12  is optimal for most applications, in some applications it may be desirable to over or undershift the heads. In the overshifted example, the heads are shifted such that the edge rays  48   a,    48   b  are displaced a selected distance into the subject. The degree of such overshifting can be angularly dependent, e.g., deepest into the subject when the point of tangency is on the major axis and tangent or even displaced from the subject when the point of tangency lies on the subject&#39;s minor axis. In terms of constraints, the vertical heads  48   a,    48   b  are constrained to be tangent to a different preselected orbit than the physical heads  20   a,    20   b.  As yet another alternative, the edge rays can be monitored in real time by the detector heads or a separate edge ray detector and the shifting performed dynamically in real time based on monitored deviations from tangent. 
     The invention has been described with reference to the preferred embodiment. 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.