Abstract:
A positron emission detection scanner includes a first plurality of detecting elements arranged in a first two dimensional geometrical array, the detecting elements defining a first detection surface oriented for receiving radiant energy stimulus incident thereto. The detecting elements each have a second surface for communicating light from a scintillation event occurring in response to receiving a radiant energy stimulus. A light transmitting member is provided for receiving light from the scintillation events from each of the detecting elements. A second plurality of light sensing members is arranged in a second two dimensional geometrical array, different from the first geometrical array, the alignment of the light sensing members is independent of the detecting elements. A predetermined group of the light sensing members is responsive to light from said light transmitting member with each one of the light sensing members of the group producing signals proportional to its respective portion of the collected light.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a positron emission tomography (PET) system, and more specifically, to a PET system for detecting positron emissions from a volume through a plurality of detecting elements arranged in a first two dimensional array about the volume, the first array being optically coupled to a plurality of light sensing members in a second two dimensional array so that a scintillation event created in a detecting element by a positron emission emanating from the volume is not necessarily identified as originating from an individual detecting element. 
     As is known, scintillation detectors used in nuclear medicine can be divided into two broad categories. The first category includes detectors which use one or a small number of large sized single scintillation crystals. The second category includes pixilated detectors employing a plurality of smaller sized crystals. In both categories, the detectors determine the position of a positron emission caused by decaying isotopes of a radioactive compound. The position of the positron emission is determined by calculating the locus of two oppositely directed gamma rays (i.e., 180° apart) impinging the detectors and causing scintillation events within the crystals. In this way, “coincident” scintillation events identify a unique position of a positron emission. Typically, the radioactive compound is administered to a subject for rendering a tomographic image whereby the biochemical and physiological condition of the subject can be monitored by way of the detector. 
     Devices which fall into the first category are the PET detectors generally used with nuclear medicine gamma cameras now in general use. These detectors use large sized crystals, typically NaI(TI), coupled to a large number of photomultipliers (PMTs). Identification of the situs of the positron emission is determined by “Anger” logic. Thus, the position of the scintillation event is calculated by complex processing circuitry using the signals from several PMTs (anywhere from 3 to 95) and using a centroid finding technique to determine the situs of the positron emission. 
     The second category, includes detectors with small scintillation crystals known as pixelated detectors. Pixelated scintillation detectors consist of a large number of small sized crystals in which the locus of a gamma ray impinging upon the detector is calculated by identifying the individual crystal in which the event was converted to light (the scintillation process). These detectors typically use small (4 by 8 mm) BismuthGermanate (BGO) crystals. Typically the crystals are grouped such that their outputs are received by a particular group of PMT&#39;s. Often referred to as “block” detectors due to their block like structure, the rectangular array of crystals are coupled to a corresponding rectangular array of PMTS. In general, pixelated scintillation detectors determine a position for every scintillation event and the events which are not coincident are eliminated after the individual crystals are identified, as such the processing circuitry for pixelated detectors is likewise complex and the spatial resolution of the detector is limited to the size of the individual crystals. 
     Presently, a pixelated PET detector is desired which uses many small crystals, but does not require an alignment of the crystals and the PMTs for identifying a scintillation event by a specific crystal. Further a pixelated PET detector is desired in which the processing electronics can be simplified to determine the coincidence of the scintillation event first and then the position of the positron emission causing the coincident event. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprises a positron emission detection scanner. The scanner includes a first plurality of detecting elements arranged in a first two dimensional geometrical array, the detecting elements together defining a first detection surface oriented for receiving radiant energy stimulus incident thereto. The detecting elements each have a second surface for communication light from a scintillation event in response to receiving a radiant energy stimulus. A light transmitting member is provided for receiving light from the scintillation events from each of the detecting elements. A second plurality of light sensing members is arranged in a second two dimensional geometrical array, different from the first geometrical array. The alignment of the light sensing members is independent of the detecting elements, a predetermined group of light sensing members being responsive to light in the light transmitting member. The group of light sensing members collect the light from the light transmitting member and each one of the light sensing members of the group produce signals proportional to its respective portion of the collected light. 
     The present invention further comprises a positron emission scanner which includes a first plurality of detecting elements arranged in a first two dimensional geometrical array. The detecting elements together define a first detection surface oriented for receiving radiant energy stimulus incident thereto. The detecting elements each have a second surface for communicating light from a scintillation event in response to received energy stimulus. A light transmitting member is provided for receiving light from the scintillation events from each of the detecting elements. The light transmitting member has a detection surface and a transmission surface, the light transmitting member channeling light received by the detection surface by photon boundaries formed therein to distribute the light along photon paths such that the light is predictably distributed to exit the transmission surface. A second plurality of light sensing members is arranged in a second two dimensional geometrical array, the alignment of the light sensing members being independent of the detecting elements. In this manner, a predetermined group of light sensing members are responsive to light based on the distribution of the light in the light transmitting member and exiting the transmitting surface. The group of light sensing members collect the light from the light transmitting member transmission surface such that each one of the light sensing members of the group produces electrical signals proportional to a collected portion of the received light. 
     The present invention further comprises a positron emission scanner including a first plurality of detecting elements arranged in a first two dimensional geometrical array. The detecting elements together define a first detection surface oriented for receiving radiant energy stimulus incident thereto and the detecting elements each having a second surface for communicating light from a scintillation event in response to receiving a radiant energy stimulus. A light transmitting member is provided for receiving the light from the scintillation events from each of the detecting elements. The light transmitting member has a detection surface and a transmission surface. The light transmitting member channels light received by the detection surface by photo boundaries formed therein to distribute the light along photo paths such that the light is predictably distributed to exit the transmission surface. A second plurality of light sensing members is arranged in a second two dimensional geometrical array not aligned to the first array, and oriented toward the light transmitting member transmission surface. Each light sensing member collects light through the light transmitting member from one or more of the detecting elements and generates proportional electrical signals. A processor is provided for receiving the electrical signals from each of the light sensing members and for determining the position of the energy stimulus. 
     The present invention further comprises a method of determining the coincidence of a scintillation event. The method comprises the steps of detecting positron emissions from an area with an array of detecting elements in a first two dimensional geometric configuration; transferring light from scintillation events in the array of detecting elements resulting from the detection of the positron emissions to a channeling member; collecting light from the channeling member with an array of light sensing members in a second two dimensional geometric configuration, the alignment of the light sensing members being independent of the detecting elements with each light sensing member producing electrical signals proportional to the portion of light passing thereto; and processing the light sensing member signals to identify coincident scintillation events. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
     FIG. 1 is a side elevational view of a prior art pixelated positron emission scanner design; 
     FIG. 2 is a perspective view of a prior art pixelated “block detector” positron emission detector design; 
     FIG. 3 is a schematic representation of a prior art pixelated quadrant sharing positron emission detector design; 
     FIG. 4 is a schematic representation of a pixelated scintillation detector in accordance with the present invention; and 
     FIG. 5 is a side elevational view of the pixelated scintillation detector of FIG. 4 showing the boundaries of the light sensing member. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right,” “left,” “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the pixelated scintillation detector and designated parts thereof. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import. 
     Referring now to the drawings, FIGS. 1-3 show several prior art pixelated scintillation detectors. In FIG. 1, a pixelated scintillation detector  5  is formed from a one-dimensional, linear ring array of detector elements  10  and a hexagonal array of photomultiplier tubes (PMTs)  15  aligned with the array of detector elements  10 . A light guide  20  provides mechanical support for both of the configurations of the PMTs  15  and the detector elements  10 . The detector elements  10  of pixelated scintillation detector  5  are aligned relative to the PMTs  15 , and are operably linked to processing electronics (not shown). 
     The processing circuitry provides one position calculation circuit for every 3 PMTS. In determining the position of a positron emission, first, the processing electronics identifies an individual element of the detector element array as the position for each detected scintillation event, then the circuitry eliminates all events not found to be in coincidence. In this way, the detector  5  detects an event by identifying a specific detecting element  10  of the linear array rather than by merely calculating the position of detected scintillation events. 
     Referring now to FIG. 2, a prior art pixelated scintillation detector  25  or “block detector” is shown. The block detector  25  includes a rectangular array of detecting elements  30  and a light guide  40  having photon boundaries  45  for predictably distributing photons traveling therethrough. Each group of four PMTs  35  form physically separate blocks (as opposed to the continuous light guide of FIG.  1 ). In this way, the block detector  25  distributes light among the PMTs  35  using the boundaries  45  of the light guide  40 . As in FIG. 1, the detecting elements  30  are aligned relative to the PMTs  35 . Thus, each “block” of 4 PMTs has its own position calculation circuit. Also as in the design of the pixelated scintillation detector of FIG. 1, the detector  25  identifies each individual detecting element receiving a gamma ray and a corresponding detecting element of the array of detecting elements  30  is identified for every event. The events which are not in coincidence are eliminated after the position calculation. 
     Referring to FIG. 3, a schematic diagram showing a modification of the block design of FIG. 2 is shown. A pixelated scintillation detector  50  includes cross-coupled detecting elements  55  and quadrant sharing PMTs  60 . The detecting elements  55  are offset relative to the PMTs  60 . However, the PMTs  60  and detecting elements  55  are still aligned. As with the detectors of FIGS. 1-2, the detector  50  of FIG. 3 similarly determines each of the individual detecting element of the array  55  which receives the gamma ray first and then determines the coincidence of the scintillation events. 
     Referring now to FIGS. 4-5, schematic diagrams of a pixelated scintillation detector  75  in accordance with the present invention are shown. The detector  75  includes a first plurality of detecting elements or crystals  80 , a second plurality of light sensing members or PMTs  85 , and a light transmitting member  90  (shown in FIG.  5 ). 
     The first plurality of detecting elements  80  is arranged in a first two dimensional geometrical array  100 , for receiving radiant energy stimulus. The detecting elements  80  each communicate light from a scintillation event in response to receiving a radiant energy stimulus such as a gamma ray. The array  100  includes many small detecting elements  80 , preferably NaI(TI), GSO, LSO or LGSO crystals. The detecting elements  80  are attached either to a flat or curved light transmitting member  90 . The size of the member  90  can range from 10 cm long by 10 cm wide containing 625 individual detecting elements  80 , to a cylindrical light transmitting member with a 90 cm diameter and 25 cm length containing about 30,000 detecting elements  80 . As can be appreciated, a single light transmitting member or multiple members  90  may be employed by the detector  75 . 
     The light transmitting member  90  receives light resulting from scintillation events from each of the detecting elements  80  in the first geometrical configuration  100 . A second geometrical array  95  of light sensing elements or “PMT&#39;s”  85  are optically coupled to the light transmitting member  90  in an arrangement which is not related to the position or geometry of the detecting elements  80 . For example, as shown in FIG. 4, the detecting elements  80  are arranged in a square array  100  of 5 mm long by 5 mm wide. The array  100  of detecting elements  80  is optically coupled to a hexagonal array  95  of cylindrical light sensing members  85  by way of the light transmitting member  90 , where each light sensing member  85  has a typical diameter of 38 mm. The light transmitting member  90  is preferably manufactured of Lucite™ or some other transparent, lightweight polymeric material, however other materials are known to those skilled in the art. By way of example, the 38 mm diameter PMTs  85  may be arranged in a close-packed hexagonal pattern such that the separation from the center-to-center spacing of rows of PMTs  85  in a first direction (x) is 40 mm. This necessarily results in a separation of 35 mm between rows of PMTs  85  in the other direction (y) perpendicular to first direction (x). Again, by way of example it may be desirable to use detecting elements  80  with dimensions of 4×4 mm square. Thus it is obvious that the detecting elements  80  will be aligned with the PMTs  85  in one direction (i.e. 10 crystals per PMT), but will not be aligned with the PMTs  85  in the other direction (i.e. 8.75 crystals per PMT). 
     Specifically, by not aligning the array  95  of light sensing members or PMTs  85  with the array  100  of detecting elements  80 , the design of the PET scanner can be optimized to achieve a desired performance not possible by requiring an alignment. Furthermore, the design allows as few as two processing circuits (not shown) to determine the position of an event within the detector element array  100 , instead of requiring a large number of processing circuits as is customarily used with prior art block detectors as described above. 
     Referring now more specifically to FIG. 5, the light transmitting member  90  is shown including photon boundaries  110  for channeling photons  115  along predetermined paths for reducing the number of light sensing members  85  necessary to detect a scintillation event. In the prior art block detector  25  shown in FIG. 2, there are physical boundaries, such as grooves of different depths, which are all aligned relative to the detecting elements  30 . Likewise, the present invention employs physical boundaries  110  to redirect the light from the detecting elements  80 . However, the boundaries  110  of the detector  75  of the present invention are employed to be aligned relative to the light sensing members  85 . For example, in the hexagonal array  95  of light sensing members  85  shown in FIG. 4, the pattern of physical boundaries  110  formed in the light sensing member  90  also forms a hexagonal pattern, even though the detecting elements  80  might be arranged in a square array. The boundaries  110  reduce the distance over which the light from the detecting elements  80  spreads within the light transmitting member  90  and redirects the light in such a way that accurate positioning of the event can be achieved with fewer, larger PMTs. For example, it may be possible to replace the PMTs having a diameter of 38 mm with PMTs having a diameter of 45 mm, thereby reducing the total number of PMTs required. 
     The physical boundaries  110  may be formed in patterns wherein the channel walls slope to converge at a vertex for creating a triangular channel. In this way, light is redirected by the sloping channel walls as shown in FIG.  5 . It is recognized by those skilled in the art that a variety of geometrical channel configurations are possible for performing this function. Additionally, boundaries  110  aligned relative to the light sensing members  85  can be formed to have channel widths corresponding to the distance between the light sensing members  85 . In this way, light is redirected away from areas where detection of light is minimal due to a specific geometrical configuration of light sensing members  85 . 
     The second geometrical array  95  of light sensing members  85  is arranged in a geometrical configuration different from the first geometrical array  100  of the detection elements  80 . The alignment of the light sensing members  85  is independent of the detecting elements  80  so that a square array of detecting elements  80  may be combined with a close-packed hexagonal array of round PMTs  85 . The group of light sensing members  85  collect the light from the light transmitting member  90 , and each one of the light sensing members  85  of the second geometrical configuration  95  produces electrical signals proportional to its respective portion of the collected light. A predetermined group of the plurality of light sensing members  85  is responsive to light in the light transmitting member  90 . 
     For example, it may be useful in the center of the array of detecting elements  80  to use seven PMTs  85  to calculate the position of the event, while it may only be necessary to use three PMTs  85  to calculate the position of the event along the edges of the array of detecting elements  80 . 
     The detector  75  is operably linked to processor circuits  105  normally used with continuous scintillation crystals. The processor circuits first determine the coincidence and then determines the position of an event only for those events which are actually in coincidence (typically only 2-3% of all events are in coincidence, therefore the data rate for position calculation is dramatically reduced). The processor circuits do not necessarily identify individual crystals, only general areas of the array. The position of the positron emission event is calculated and repositioned based on a calibration table. This method is well known in standard nuclear medicine gamma cameras and is referred to as either “distortion removal” or “linearity correction”. Thus an individual detecting element  80  is not identified by the detector  75  for determining the position of a positron emission. In this way, the size of the detecting elements  80  does not limit the spatial resolution of the detector  75 . 
     The processor circuits first determine whether two events are in coincidence. If they are found to be in coincidence, the PMT values are transferred to another circuit which next finds the PMT  85  with the largest signal. The processor circuits next use the signals from three to seven of the PMTs in the vicinity of the PMT with the largest signal to calculate the position of the coincident event within the array of detecting elements  80 . Finally, the total amount of signal detected is used to accept those coincident events within a predetermined total signal range, which range may be a function of the position on the array of detecting elements  80 , where the position of the event has been calculated. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the invention as defined by the claims.