Patent Publication Number: US-6337481-B1

Title: Data binning method and apparatus for pet tomography including remote services over a network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 09/200,289, entitled “Data Binning Method And Apparatus For Pet Tomography” by Charles W. Stearns et al. filed on Nov. 25, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of medical diagnostic systems, such as imaging systems. More particularly, the invention relates to a technique for storing collected imaging data to a stand alone memory device during data acquisition so as to reduce the amount of memory required by a processor to acquire an entire set of imaging data. 
     Positrons are positively charged electrons which are emitted by radionuclides which have been prepared using a cyclotron or other device. The radionuclides most often employed in diagnostic imaging are fluorine-18 ( 18 F), carbon-11 ( 11 C), nitrogen-13 ( 13 N), and oxygen-15 ( 15 O). Radionuclides are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances such as glucose or carbon dioxide. One common use for radiopharmaceuticals is in the medical imaging field. 
     To use a radiopharmaceutical in imaging, the radiopharmaceutical is injected into a patient and accumulates in an organ, vessel or the like, which is to be imaged. It is known that specific radiopharmaceuticals become concentrated within certain organs or, in the case of a vessel, that specific radiopharmeceuticals will not be absorbed by a vessel wall. The process of concentrating often involves processes such as glucose metabolism, fatty acid metabolism and protein synthesis. Hereinafter, in the interest of simplifying this explanation, an organ to be imaged will be referred to generally as an “organ of interest” and prior art and the invention will be described with respect to a hypothetical organ of interest. 
     After the radiopharmaceutical becomes concentrated within an organ of interest and while the radionuclides decay, the radionuclides emit positrons. The positrons travel a very short distance before they encounter an electron and, when the positron encounters an electron, the positron is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to medical imaging and particularly to medical imaging using photon emission tomography (PET). First, each gamma ray has an energy of essentially 511 keV upon annihilation. Second, the two gamma rays are directed in substantially opposite directions. 
     In PET imaging, if the general locations of annihilations can be identified in three dimensions, the shape of an organ of interest can be reconstructed for observation. To detect annihilation locations, a PET camera is employed. An exemplary PET camera includes a plurality of detectors and a processor which, among other things, includes coincidence detection circuitry. Each time a 511 keV photon impacts a detector, the detector generates an electronic signal or pulse which is provided to the processor coincidence circuitry. 
     The coincidence circuitry identifies essentially simultaneous pulse pairs which correspond to detectors which are generally on opposite sides of the imaging area. Thus, a simultaneous pulse pair indicates that an annihilation has occurred on a straight line between an associated pair of detectors. Over an acquisition period of a few minutes millions of annihilations are recorded, each annihilation associated with a particular detector pair. After an acquisition period, recorded annihilation data is used via any of several different well known procedures to construct a three dimensional image of the organ of interest. 
     PET cameras have been configured in many geometries. Because annihilation data has to be collected from essentially 360 degrees about an organ which is to be imaged, one popular PET camera configuration includes small detectors arranged to form an annular gantry about the imaging arc. In this case data from all required degrees can be collected at the same time, separated into data from different angles about the imaging area and then back projected as different profile type views to form the tomographic image. Unfortunately annular cameras require large numbers of detectors and therefore are extremely expensive which renders annular cameras unsuitable for many applications. 
     Referring to FIG. 1, another common PET camera configuration  10  includes first and second cameras  12 ,  14 , respectively, each camera  12 ,  14  including an impact surface  13 ,  15 , respectively, for detecting impacting gamma rays. Each camera  12  and  14  is characterized by a width W across which hardware which can distinguish M different impact locations is arranged. To detect coincident gamma ray pairs, first and second cameras  12  and  14  are positioned a distance D apart and such that surfaces  13  and  15  oppose each other on opposite sides of an imaging area  16  and define a field of view (FOV). With the opposing camera configuration, instead of collecting tomographic data from all angels about imaging area  16  simultaneously as with an annular configuration, during an acquisition session, first and second cameras  12  and  14  are rotated (see arrows  18 ,  20 ) about imaging area  16  through approximately 180 degrees, the cameras maintained at different stop angles for short acquisition periods which together comprise the acquisition session. 
     For the purposes of this explanation the term “profile view” or simply “view” will be used to describe all annihilation data collected during a data acquisition period which emanates from the imaging area along parallel paths. At each camera position, cameras  12  and  14  collect annihilation data corresponding to several different profile views. A more detailed analysis of FIG. 1 can be used to better understand profile views and how data corresponding to several views is collected at each camera position. 
     Referring to FIG. 1, an initial camera position angle θo is defined by a line between and perpendicular to impact surfaces  13  and  15 . During rotation, a stop angle θs is defined by the angle between the initial position angle θo and the instantaneous line between and perpendicular to impact surfaces  13  and  15 . While some systems operate with a continuously changing stop angle θs during data acquisition, unless indicated otherwise and in the interest of simplifying this explanation, it will be assumed that an exemplary system actually stops at different stop angles and only acquires data while stationary. 
     Referring still to FIG. 1, assuming cameras  12  and  14  are in the initial position illustrated so that stop angle θs is zero degrees, if an annihilation event occurs at the center of imaging area  16  as indicated by point  22 , the annihilation event may generate a corresponding gamma ray pair which emanates along virtually any path. However, with cameras  12  and  14  positioned as illustrated, cameras  12  and  14  can only collect generated gamma rays if the rays are directed within an angle range between a maximum negative flight path angle −θm and a maximum positive flight path angle +θm (and within a z-axis plane which is perpendicular to the illustration). In the interest of simplifying this explanation it will be assumed that cameras  12  and  14  are single dimensional (i.e. z=1) and, although range −θm through +θm may span several different ranges, it will be assumed that range −θm through +θm spans 30° (i.e. 15° on either side of an instantaneous stop angle θs). 
     Referring also to FIG. 2, assume the annihilation event at point  22  (i.e. a center of imaging area  16 ) which is being studied generates gamma rays which are directed along the a flight path  50  which is parallel to initial position angle θo. In addition, assume that other annihilation events occur at other positions indicated at points  24 ,  26 ,  28 ,  30  and  32  and that each of those events, like the event at point  22 , generates a pair of gamma rays which emanate along flight paths parallel to path  50 . Because all of the ray pairs in FIG. 2 are parallel, the pairs together form a profile view of data, in essence indicating what the collected data “appears” like from a view which is perpendicular to the gamma ray flight paths. For the purposes of this explanation the profile view corresponding to the flight path illustrated in FIG. 2 will be referred to as a first profile view. 
     While some gamma rays are traveling along the paths indicated in FIG. 2 during a data acquisition period, other gamma rays travel along other flight paths. For example, referring also to FIG. 3, annihilation events are indicated at points  34 ,  36 ,  38  and  40 , each of which causes gamma rays having flight paths which cause rays to impact surfaces  13  and  15  at a projection ray angle θf. Because all of the ray pairs in FIG. 3 are parallel, the pairs together form a profile view of data, in essence indicating what the collected data “appears” like from a view which is perpendicular to the gamma ray flight paths. For the purposes of this explanation the profile view corresponding to the flight path illustrated in FIG. 3 will be referred to as a second profile view. 
     Thus, annihilation data corresponding to two different profile views is collected simultaneously while cameras  12  and  14  are at the initial position illustrated in FIGS. 2 and 3. In fact, for every angle within range −θm to +θm, data is collected for a separate profile view corresponding to the angle, the number of angles limited only by the ability of camera  12  and  14  hardware to distinguish between angles. For example, when collecting data at the initial position illustrated, data corresponding to one thousand different profile views might be collected. 
     To distinguish between data from different profile views, collected annihilation data is stored as a function of two coordinates, a first coordinate, a projection ray angle θf, indicating the projection ray path associated with a specific profile view and a second coordinate, a distance R, indicating the location of the projection ray path within the profile view. For example, referring still to FIG. 3, the annihilation event which occurs at point  38  is associated with projection ray angle θf 1  which is a distance R 1  from imaging area central point  22 . Other events illustrated in FIG. 3 are characterized by the same projection ray angle θf 1  but different distances R. Similarly, referring to FIG. 2, the annihilation event which occurs at point  30  is characterized by a projection ray angle coordinate θf 2  (not illustrated) which is zero and a distance R 2 . 
     During data acquisition coincident counts are organized in a processor memory as a set of two-dimensional arrays, one array for each profile view, each array having as one of its dimensions projection ray angle θf and the other dimension distance R. 
     While annihilation data is collected which corresponds to many different profile views at each stop angle, there is no stop angle at which all data corresponding to a single profile view is collected. For example, referring again to FIG. 2, when cameras  12  and  14  are positioned at the initial stop angle (i.e. θs=0°), the event at point  30  is detected and data associated therewith is stored in a corresponding coincident count associated with angle θf 2  and distance R 2 . When cameras  12  and  14  are rotated to a different stop angle indicated in phantom and by numerals  12 ′ and  14 ′, an annihilation event at point  30  which generates gamma rays having the projection ray path illustrated still impacts both cameras  12 ′ and  14 ′ causing an annihilation detection which corresponds to the first profile view discussed above. This annihilation detection or event is added to the coincident count corresponding to angle θf 2  and distance R 2  as illustrated in FIG.  2 . 
     Referring again to FIG. 1, after data is collected at the initial camera position illustrated, cameras  12  and  14  are rotated through a small angle in a clockwise direction to a second stop angle. For the purposes of this explanation it will be assumed that the stop angle increment in between consecutive stop angles is 2 degrees. Thus, while the initial stop angle corresponds to a zero degree position, the second stop angle corresponds to a 2° position, the third stop angle corresponds to a 4 degree position and so on. 
     It should be apparent that, after cameras  12  and  14  are rotated to the second stop angle, range −θm to θm changes such that, upon commencing data acquisition at the new stop angle, no data is collected at original angle −θm. Similarly, each time the stop angle is changed by clockwise rotation to a new stop angle, angle −θm changes and data is not collected at the previous angle −θm during the next acquisition period. For example, where the range −θm through θm is 30°, the range −θm through θm is −15° through 15° during data acquisition at the initial stop angle. At the second stop angle where θs=2°, range −θm through θm is between −13° and 17° and so on. During acquisition at the second stop angle data is not collected which corresponds to initial angle −θm (i.e. −15°) while data is collected which corresponds to angle −θm=−13°. Hence, more data is collected which corresponds to angle −13° than corresponds to angle −15°. 
     It should be appreciated that only incomplete data corresponding to projection ray angles between −15° and +15° is acquired during the first half of an acquisition session. The dearth of data for initial projection ray angles θf between −15° and +15°, if not supplemented, reduces resulting image quality. To complete data between projection ray angles −15° and +15°, additional data is collected at the end of a data acquisition session when the stop angle exceeds 180−2θm degrees. In the present case, where range −θm through θm is 30°, data collection to complete projection ray coincident counts for projection rays having angles between −15° and 15° begins when the stop angle is 150°. When the stop angle exceeds 150°, annihilation events along original angle −θm (i.e. −15°) are again detected and corresponding coincident counts associated with projection ray angle −θm are increased. This process of supplementing the −15° to 15° projection ray coincident counts continues through a stop angle equal to 180−θin degrees. In the present case, because stop angle increment θin is 2°, data acquisition continues through 178°. 
     Thus, data corresponding to initial angle −θm and distance R is not completed until after essentially 180° of data acquisition. In conventional data acquisition systems the processor which collects coincident count data effectively maintains coincident count data for each possible projection ray (R, θf) during data acquisition and modifies counts for each ray (R, θf)) as data is collected. 
     Over the course of 180° of data acquisition, in histogram space, the effect of the acquisition protocol described above is schematically illustrated in FIG. 4 which shows an exemplary histogram  70  having flight angle θf and stop angle θs along the vertical axis and distance R along the horizontal axis. During a data acquisition, cameras  12  and  14  rotate through stop angles ranging from 0 to 178°. At each stop angle θs, a diamond shaped region of coincident count data is acquired, each point within a diamond shaped region representing a separate coincident count coordinate (R, θf). For example, in FIG. 4 the diamond shaped region corresponding to data acquisition at the initial stop angle θs (as illustrated in FIG. 1) where θs is 0° is identified by numeral  44 . As indicated above, at the initial stop angle θs coincident count data is collected which corresponds to gamma ray flight paths having projection ray angles between−θm and θm degrees. 
     For projection ray angles θf equal to angles −θm or θm, while cameras  12  and  14  are at the initial stop angle the only possible value for distance R is zero. Thus, as illustrated in FIG. 4, at angle −θm which is at the top of diamond shaped region  44 , region  44  is restricted indicating that data corresponding to a single coincident count is stored for angle −θm at the first stop angle. Similarly, at angle θm which is at the bottom of diamond shaped region  44 , region  44  is restricted indicating that data corresponding to a single coincident count is stored for angle θm at the initial stop angle. 
     Referring to FIGS. 1 and 2, while the cameras are at the initial stop angle θs=0°, for projection rays having angles θf equal to 0° so that associated rays are perpendicular to impact surfaces  13  and  15 , data is collected for all distance R values between −W/2 and W/2. Thus, referring also to FIG. 4, at the initial stop angle θs, data corresponding to coincident counts associated with projection rays having angles Øf equal to 0° are represented by a horizontal line which bisects region  44  and extends between distance R=−W/2 and distance R=W/2. 
     For every projection ray angle θf between initial angle −θm and 0° there are a number of distances R, the number of distances R depending on how close a specific flight angle θf is to 0°. The number of R coordinates for a specific projection ray angle θf increases as the projection ray angle θf gets closer to 0°. Similar comments can be made with respect to the histogram space between 0° and θm. Thus, region  44  is diamond shaped because there are a large number of R values at 0° angles θf, a single R value at each of angles θf equal to −θm and θm and a linearly decreasing number of R values between the 0° angle θf and angles −θm and θm. 
     Referring still to FIG. 4, at the second stop angle (i.e. a 2 degree stop angle in the present example), a second diamond shaped region  44 ′ of data is acquired. The shape of region  44 ′ is identical to the shape of region  44 , the only difference being that region  44 ′ is shifted 2° along the vertical axis. At the second stop angle, while many of the projection ray angles for which data is collected are the same as during acquisition at the first stop angle, some of the projection ray angles are different. Specifically, the range of projection ray angle acquisition is (−θm+2°) through (θm+2°). 
     As stop angles are changed to acquire data from different perspectives about imaging area  16 , the diamond shaped region is shifted down the vertical axis until the stop angle is equal to 178°. Because range −θm through θm is 30°, the range during data acquisition at the final stop angle is between 163° and 193° and the entire data set illustrated in FIG. 4 extends from −15° to 193° (i.e. approximately −θm through 180+θm degrees). 
     One problem with PET imaging systems is that the amount of data which must be acquired during data acquisition is extremely large and therefore a huge memory is required. One solution for reducing memory size is to generate data in a compact histogram form. To this end, referring again to FIG. 4, the symmetrical relationship P(R, θf)=P(−R, θf±180) between polar coordinate data can be used to store coincident counts corresponding to projection ray angles θf which are less than 0° or greater than 180°, to projection ray coincident counts which are within the 0° to 180° range. This compacting is represented by arrows  60  and  62  and results in the compact histogram form  72  illustrated in FIG.  4 . For example, referring again to FIG. 1, rays along projection ray paths having projection ray angles θf of −15° (i.e. the initial −θm angle) are outside the compact histogram form  72 . This data can be directed to a histogram address within compact histogram form  72  by changing the sign of distance R and adding 180° to angle −θm. The new angle is 165° which is within compact form  72 . 
     Unfortunately the compact histogram solution has several shortcomings. First, while the mathematics to convert data which is outside the compact form into data which is inside the compact form is relatively simple to derive, solving the mathematics in real time to form the compact histogram during the acquisition process is impractical for a number of reasons. As an initial matter, the arithmetic required for the conversion is relatively involved due at least in part to the fact that data from outside the compact form must be reflected about the distance R=0 axis which requires a plurality of conditional steps. In addition, although possible, it would be extremely difficult to provide a lookup table for conversion of all coincident detection pair possibilities and stop angles to distance R and projection ray angle θf possibilities. Moreover, as some systems change stop angle θs continuously during data acquisition, even if it where possible to provide a suitable lookup table, it may be impossible to update using a lookup table whenever stop angle θs changes. 
     A second and perhaps more vexing problem with forming a compact histogram form is that such a form makes it extremely difficult to save any collected data to an inexpensive stand alone storage device which could alleviate some of the burden on the processor&#39;s memory. Storage outside a processor memory during acquisition is difficult because coincident counts for some coordinates (R, θf) must be maintained during an entire acquisition session. For example, as described above, data acquired at the initial stop angle θs is not completed until acquisition at the final stop angle is completed at the end of an acquisition session. Because coincident counts for some coordinates have to be maintained throughout an acquisition session, it is extremely difficult to define an acquisition regime which saves raw data to a stand alone storage device during acquisition. 
     Solutions to the problems described above have not heretofore included significant remote capabilities. In particular, communication networks, such as, the Internet or private networks, have not been used to provide remote services to such medical diagnostic systems. The advantages of remote services, such as, remote monitoring, remote system control, immediate file access from remote locations, remote file storage and archiving, remote resource pooling, remote recording, remote diagnostics, and remote high speed computations have not heretofore been employed to solve the problems discussed above. 
     Thus, there is a need for a medical diagnostic system which provides for the advantages of remote services and addresses the problems discussed above. In particular, there is a need for remote storage which provides large memory capacity for the large amount of data acquired with PET imaging systems. Further, there is a need for remote calculation of complicated computations of PET imaging systems. Even further, there is a need for providing PET imaging systems with remote services over a network. 
     SUMMARY OF THE INVENTION 
     One embodiment relates to a method to be used with an imaging system which includes two opposed cameras mounted for rotation among a plurality of acquisition angles about an imaging axis for acquiring imaging data throughout an arc about the axis, the cameras collecting data corresponding to an acquisition angle range including a plurality of flight path angles at each acquisition angle. The system also includes a processor having a processor memory. The method reduces the amount of processor memory required to acquire imaging data. The method includes the steps of (a) after gathering imaging information at an acquisition angle, for each flight path angle within the acquisition angle range, determining if additional data will be collected during acquisition at the next consecutive Q acquisition angles; (b) where imaging data for a flight path angle will be collected during data acquisition at at least one of the next Q acquisition angles, maintaining imaging data for the flight path angle in the processor memory; and (c) where imaging data for a flight path angle will not be collected during data acquisition at at least one of the next Q acquisition angles, communicating the imaging data to a remote facility to store the imaging data for the flight path angle in a remote storage device via a network and to provide remote services to the imaging system. 
     Another embodiment relates to an apparatus for use with an imaging system including two opposed cameras mounted for rotation among a plurality of acquisition angles about an imaging axis for acquiring imaging data throughout an arc about the axis, the cameras collecting data corresponding to an acquisition angle range including a plurality of flight path angles at each acquisition angle. The system also includes a processor having a processor memory. The apparatus reduces the amount of processor memory required to acquire imaging data. The apparatus includes a programmed data processor for: (a) after gathering imaging information at an acquisition angle, for each flight path angle within the acquisition angle range, determining if additional data will be collected during acquisition at the next consecutive Q acquisition angles; (b) where imaging data for a flight path angle will be collected during data acquisition at at least one of the next Q acquisition angles, maintaining imaging data for the flight path angle in the processor memory; and (c) where imaging data for a flight path angle will not be collected during data acquisition at at least one of the next Q acquisition angles, communicating the imaging data via a network to a remote facility to store the imaging data for the flight path angle in a remote storage device and to provide remote services to the imaging system. 
     Another embodiment relates to an imaging system including two opposed cameras mounted for rotation among a plurality of acquisition angles about an imaging axis for acquiring imaging data throughout an arc about the axis, the cameras collecting data corresponding to an acquisition angle range including a plurality of flight path angles at each acquisition angle. The system also includes a processor having a processor memory. The system reduces the amount of processor memory required to acquire imaging data. The system includes a communications module which transmits and receives data for remote services, a remote facility coupled to the communications module via a network, and a programmed data processor coupled to the communications module. The remote facility includes a processing system coupled to a system of databases and communication components. The programmed data processor (a) after gathering imaging information at an acquisition angle, for each flight path angle within the acquisition angle range, determines if additional data will be collected during acquisition at the next consecutive Q acquisition angles; (b) where imaging data for a flight path angle will be collected during data acquisition at at least one of the next Q acquisition angles, maintains imaging data for the flight path angle in the processor memory; and (c) where imaging data for a flight path angle will not be collected during data acquisition at at least one of the next Q acquisition angles, communicates the imaging data via a network to the remote facility, stores the imaging data for the flight path angle in the system of databases, and provides remote services to the imaging system. 
     Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred exemplary embodiments are described below with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
     FIG. 1 is a schematic representation of a dual head PET camera configuration; 
     FIG. 2 is a schematic similar to FIG. 1, albeit illustrating slightly different information; 
     FIG. 3 is similar to FIG. 1, albeit illustrating slightly different information; 
     FIG. 4 is a schematic representation of a prior art histogram before and after compacting; 
     FIG. 5 is similar to FIG. 4, albeit illustrating a non-compact histogram according to the present invention; 
     FIG. 6 is a schematic diagram illustrating the components of the inventive PET system; 
     FIG. 7 is a flow chart illustrating a preferred embodiment of the inventive method; 
     FIG. 8 is a diagrammatical representation of a series of medical diagnostic systems coupled to a service facility via a network connection for providing remote services and data interchange between the diagnostic systems and the service facility; 
     FIG. 9 is a block diagram of the systems shown in FIG. 8 illustrating certain functional components of the diagnostic systems and the service facility; 
     FIG. 10 is a block diagram of certain functional components within a diagnostic system of the type shown in FIG.  8  and FIG. 9 for facilitating interactive remote servicing of the diagnostic system; 
     FIG. 11 is a block diagram of certain of the functional components of the service facility illustrated in FIG.  8  and FIG. 9 for rendering interactive remote service to a plurality of medical diagnostic systems; and 
     FIG. 12 is a flow chart illustrating steps performed in an exemplary remote services process between the diagnostic system and the remote service facility. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, wherein like reference characters and symbols represent corresponding elements and signals throughout the several views, and more specifically referring to FIG. 6, the present invention will be described in the context of an exemplary PET scanner system  80 . System  80  includes an acquisition system  82 , an operator work station  90 , a data acquisition processor  84 , a secondary storage device  86  and an array processor  88 . 
     System  82  includes a gantry  92  which supports first and second gamma cameras  12  and  14  about a central bore which defines an imaging area  98 . Each camera  12 ,  14  has a width W and cameras  12  and  14  are separated by a distance D. A patient table (not illustrated) is positioned in front of gantry  92  and is aligned with imaging area  98 . A patient table controller (also not illustrated) moves a table bed into imaging area  98  in response to commands received from work station  90 . 
     A gantry controller (not illustrated) is mounted within gantry  92  and is responsive to commands received from operator work station  90  through a serial communication link  100  to operate gantry  92 . For example, gantry  92  can be tilted away from vertical on command from an operator, can perform a “transmission scan” with a calibrated radio nuclide source to acquire attenuation measurements, can perform a “coincidence timing calibration scan” to acquire corrective data, or can perform a normal “emission scan” in which positron annihilation events are counted. 
     Construction and operation of an exemplary PET camera  12  and  14  is described in detail in U.S. Pat. No. 5,272,343 which issued on Dec. 21, 1993 and is entitled “Sorter for Coincidence Timing Calibration in PET Scanner” and which is incorporated herein by reference. Each of cameras  12  and  14  generates analog signals when a gamma ray is detected. The analog signals are provided to processor  84  via data buses  102  and  104 . 
     Processor  84  includes acquisition, event locator and coincident (ALC) circuitry  106 , a sorter  108  and a processor memory  110 . ALC circuitry  106  receive the analog signals via buses  102  and  104  and perform several different functions. First, ALC circuitry  106  identifies the total energy associated with a perceived event and compares the total energy to an expected range of energies corresponding to a likely true annihilation event. To this end, the expected energy range of a true annihilation event is typically 511 keV±20%. ALC circuitry  106  discards perceived events which are outside the expected range. 
     Second, ALC circuitry  106  determines the location on a camera impact surface  13 ,  15  at which a gamma ray was detected and the exact time at which a gamma ray was detected. Methods to determine impact location are well known in the PET imaging art. 
     Third, ALC circuitry  106  determines if any two events (i.e. detected gamma rays) are in coincidence. Coincidence is determined by a number of factors. First, the time markers in each event data packet must be within a specified time interval (e.g. 12.5 nanoseconds) of each other, and second, the locations indicated by the two event data packets must lie on a straight line which passes through the field of view (FOV) in imaging area  98 . Events which cannot be paired as coincidence events are discarded, but coincidence event pairs are located and recorded as a coincidence data packet that is conveyed through a serial link  112  to sorter  108 . Each coincidence data packet includes a pair of digital numbers which precisely identify the locations of coincident events on each of first and second cameras  12 ,  14 , respectively. 
     The primary purpose of sorter  34  is to efficiently store coincidence data packets thereby reducing required processor memory while maintaining data integrity. The set of all projection rays which have identical flight path angles θf and pass through the camera FOV is a “profile view”. A distance R between a particular projection ray and a center  114  of the FOV locates that projection ray within the FOV. As shown in FIG. 7, for example, a positron annihilation  116  occurs along a projection ray  118  which is located in a view at the projection ray angle θf and the distance R. Sorter  108  counts all events which occur on this projection ray (R,θf) during an acquisition period by sorting out the coincidence data packets which occur between camera locations along projection ray  118 . 
     In the interest of simplifying this explanation, it will be assumed that, in order to collect coincidence count data for all possible projection angles θf about imaging area  98 , cameras  12  and  14  are rotated in a clockwise direction and parked at discrete stop angles. The entire acquisition time is referred to as an acquisition session and the time required to acquire coincident count data at each stop angle is referred to as an acquisition period. Consecutive stop angles are separated by 2°. Rotation about area  98  begins at the initial position illustrated in FIG. 7 which is arbitrarily referred to as the initial or 0° angle position. Rotation ends at 178° after an essentially half gantry rotation about central point  114  and after data has been acquired from 90 different stop angles (i.e. 180°/2°=90 stop angles). 
     During a data acquisition, the coincidence counts (e.g. there is a separate coincidence count for each projection ray (R, θf)) are organized in memory  43  as a set of two-dimensional arrays, one for each axial image (in the z direction), and each having as one of its dimensions the projection ray angle θf and the other dimension the distance R. This θf by R map of detected events is called a histogram. 
     Coincidence events occur at random and sorter  108  quickly determines the projection ray angle θf and R values from the two locations in each coincidence data packet and increments the coincident count of the corresponding histogram array element. In the case of flat surface detector or camera  12  and  14 , the values of θf and R may be calculated as follows: 
     
       
           θf=arctan ([ W/DM][M −1−( x   1   +x   2 )])  (1) 
       
     
     
       
           R =( W/ 2 M )( x   1   −x   2 )cos θ f   (2) 
       
     
     where 
     M=number of separate detector locations identifiable by each detector along width W; and 
     x 1 , x 2 =head indexes on first and second detectors, respectively; and 
     D=Detector separation distance 
     During data acquisition, sorter  108  maintains some coincident count data in processor memory  110  and transfers some coincident count data to secondary storage device  86 . The method by which sorter  108  determines which data to maintain and which to store to device  86  is described in more detail below. After an acquisition session has been completed all coincident count data is stored on secondary device  86 . 
     Array processor  88  reconstructs an image from the coincident count data in device  86 . First, however, a number of corrections are made to the acquired data to correct for measurement errors such as those caused by attenuation of the gamma rays by the patient, detector gain nonuniformities, randoms and integrator deadtime. In one embodiment, each row of the corrected histogram array is filtered and the filtered data is then back projected to form an image array which is also stored in device  86 . The filtering an backprojecting processes are also described in more detail below. The image array can then be accessed for viewing via work station  90 . 
     Station  90  includes a CPU  120 , a CRT display  122  and a keyboard  124 . CPU  120  connects to network  100  and scans key board  124  for input information. Through keyboard  124  and associated control panel switches, an operator can control calibration of the system  82 , its configuration, and the positioning of the patient table during an acquisition period. 
     Referring to FIG. 5, according to the present invention coincident count data is acquired in a non-compact histogram  128  over a range from −θm to π+θm. Where θm is 15°, histogram  128  ranges between −15° and 195°. To this end, at the initial position illustrated in FIG. 6 which corresponds to a first stop angle θs 1 , data is collected into the diamond-shaped region  130  at the top of histogram array  128 . As cameras  12  and  14  are rotated to different stop angles in the clockwise direction, the data collecting “active” diamond shaped region  130  progresses down array  128  until region  130  is reached at the bottom of array  128  at the final stop angle as illustrated by diamond shaped region  130 ′. The resulting non-compact histogram array  128  consists of several regions which can generally be grouped into the following types: 
     1. Projection rays where all count data for the projection ray (R, θf) has been acquired and stored into a corresponding data element, these rays collectively referred to by numeral  136 ; 
     2. Projection rays at angles outside the range from 0 to 180° for which some but not all count data has been acquired and stored into a corresponding data element, these rays collectively referred to by numeral  138 ; 
     3. Projection rays within the range from 0 to 180° for which some but not all count data has been acquired and stored into a corresponding data element, these rays collectively referred to by numeral  140 ; and 
     4. Projection rays for which no coincident count data is collected and for which corresponding data elements contain exactly zero counts, these rays collectively referred to by the numeral  142 . 
     Referring to FIGS. 4 and 5, clearly array  128  is larger than compact array  70 . Array  130  is larger than array  70  by a factor of 1+(2θm/π). In the present case where θm is 15°, array  130  is 1.1667 times as large as array  70 . However, according to the present invention, as data acquisition at each stop angle is completed, several rows of array  130  are moved from processor memory  110  to secondary storage device  86 . 
     Specifically, according to the present invention, after data is acquired at a specific stop angle which corresponds to a set of projection rays, sorter  108  determines, for each projection ray (R, θf) in the set, if data will be acquired during data acquisition at the next stop angle. If data for a specific projection ray (R, θf) will be acquired during data acquisition at the next stop angle, sorter  108  maintains the projection ray coincident count data in memory  110 . However, if data for a specific projection ray (R, θf) will not be acquired during data acquisition at the next stop angle, sorter  108  stores the projection ray coincident count data in for the specific projection ray in storage device  86  and purges the projection ray coincident count data in for the specific projection ray from memory  110 . The inventive process is illustrated in FIG.  7 . 
     Referring to FIG. 7, initially it is assumed that data acquisition has already occurred at at least a current stop angle θs. Starting at process block  150  sorter  108  identifies the current stop angle θs which is the stop angle θs at which the cameras are currently parked. In addition, at block  150 , sorter  108  identifies projection rays (R, θf) corresponding to the current stop angle and at which data has been acquired during the most recent acquisition period. Moreover, at block  150  sorter  108  identifies the next stop angle which is the stop angle at which the cameras will be parked during the next data acquisition period and also identifies projection rays (R′,θf′) corresponding to the next stop angle and at which data will be acquired during the next acquisition period. 
     At process block  152  sorter  108  compares each projection ray (R,θf) which is associated with the current stop angle to the projection rays (R′,θf′) in the next projection ray set to determine if each projection ray (R,θf) is in the next projection ray set. At decision block  154 , if a specific ray (R,θf) is in the next projection ray set meaning that data will be acquired during the next acquisition period which corresponds to the specific ray (R,θf), sorter control passes to process block  156 . If a specific ray (R,θf) is not in the next projection ray set meaning that data will not be acquired during the next acquisition period which corresponds to the specific ray (R,θf), sorter control passes to process block  158 . 
     At block  156  sorter  108  maintains specific ray (R, θf) coincident count data in processor memory  110 . At block  158  sorter  108  stores specific ray (R, θf) coincident count data in secondary storage device  86  and purges the specific ray (R, θf) coincident count data from memory  110  thereby freeing memory  110  for other purposes. After each of blocks  156  and  158  control passes to block  160 . 
     At block  160  cameras  12  and  14  are rotated to the next stop angle and sorter  108  resets the current stop angle and associated projection array set to equal the next stop angle and associated projection ray set. In addition, at block  160  cameras  12  and  14  cooperate with ALC circuitry  106  to acquire data for each projection ray (R,θf) in the new current projection ray set. Sorter  108  uses the new data to update the coincident counts associated with each ray (R,θf). 
     At decision block  162  sorter  108  determines if the current stop angle is the last stop angle. In the present case, the last stop angle is 178°. If the current stop angle is not the last stop angle control passes back up to process block  150  and the process begins again. If the current stop angle is the last stop angle so that all data required to generate an image has been acquired, control passes to block  164 . At block  164  the current projection ray set is stored to secondary storage device  86 . 
     Referring again to FIG. 5, at the initial stop angle θs 1  memory  110  must only provide sufficient storage space to accommodate counters for all possible projection rays within the range between −θm and θm. In the present case that means that storage space for rays throughout only a 30 degree range is required. Similarly, as cameras  12  and  14  are rotated to different stop angles to acquire data, the total range of data which must be supported always remains 30°. For example, when the stop angle is changed from the fist angle (i.e. 0°) to the second angle by 2° of rotation, range −θm through θm changes to −13 to 17° and projection rays corresponding to the first 2° (i.e. between −15 and −13°) are stored to secondary device  86 . 
     Clearly, using the inventive method processor memory  110  is substantially reduced. Generally, using the present invention memory  110  size will be reduced by a factor of 180/2θm. Thus, in the present case where θm is 15°, memory  110  is reduced by a factor of 6. 
     After an entire acquisition period when all imaging data has been collected and is stored on secondary device  86 , one of two general procedures may be performed which are consistent with the present invention. First, it would be possible to transform the acquired data array  128  into a compact array over the range from 0 to 180° as described above with respect to the prior art. Methods to achieve such a transformation are known in the art and therefore will not be explained again here in detail. 
     Second, a suitably configured filtered back projection process can backproject each profile view to construct an image, the back projection process accomplishing the addition of projection rays which are outside the 180 degree histogram to associated rays within the histogram. This back projection process has the same effect on a reconstructed image as combining the non-compact histogram data into a compact form prior to image reconstruction. This is because filtered back projection is a linear operation, so the filtered back projection operation can be distributed through the projection combining step. 
     Mathematically, filtered back projection can be expressed as follows. A true histogram row P θ (r) includes both the θf and θf±π rows of histogram  128 , given by the expression Dθ(r)+Dθ±π(−r), where the plus or minus is chosen as appropriate for the target row of the histogram. Assuming the filtering and back projection operations are denoted by the letters F and B respectively, the effect of the true histogram row on an image can be expressed by the relationship: 
     
       
           BFP   θ ( r )= BF{D   θ ( r )+ D   θ ±π(− r )}= BFD   θ ( r )+ BFD   θ ±π(− r )  (3) 
       
     
     Thus, by filtering and backprojecting two separate coincident counts sets which corresponds to a single projection row, the resulting data merge. Note that Equation 3 relies on good linearity of the filtered back projection operation. In this method filtering the incomplete projections generates artifacts which must cancel out exactly in the back projection process. 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, while the invention is described as one wherein, when data will not be acquired for a projection ray during a next consecutive acquisition period at a next stop angle, the coincident count data for the projection ray is stored in secondary storage and purged from memory  110 , the invention also contemplates another method wherein, only if no more data will be collected for a projection ray during the remainder of an acquisition session, will the sorter store the projection ray data in the secondary device. Thus, referring again to FIG. 5, data corresponding to the range between −15° and 15° would remain in memory  110  until the end of the acquisition session. During the last 30° of data acquisition the data corresponding to the first 30° would be supplemented. While this solution may not be as advantageous as the preferred solution it does still reduce memory size. For example, in the present example, memory  110  would only have to support 60° of data instead of the entire 180° as in the prior compact histogram example. 
     Referring now to FIG. 8, a service system  1010  is illustrated for providing remote service to a plurality of medical diagnostic systems  1012 , including systems such as exemplary PET scanner system  80  described with reference to FIG.  6 . In the embodiment illustrated in FIG. 8, the medical diagnostic systems include a magnetic resonance imaging (MRI) system  1014 , a computed tomography (CT) system  1016 , and an ultrasound imaging system  1018 . The diagnostic systems may be positioned in a single location or facility, such as a medical facility  1020 , or may be remote from one another as shown in the case of ultrasound system  1018 . The diagnostic systems are serviced from a centralized service facility  1022 . Moreover, a plurality of field service units  1024  may be coupled in the service system for transmitting service requests, verifying service status, transmitting service data and so forth as described more fully below. 
     In the exemplary embodiment of FIG. 8, several different system modalities are provided with remote service by the service facility. Remote services include but are not limited to services, such as, remote monitoring, remote system control, immediate file access from remote locations, remote file storage and archiving, remote resource pooling, remote recording, and remote high speed computations. Remote services are provided to a particular modality depending upon the capabilities of the service facility, the types of diagnostic systems subscribing to service contracts with the facility, as well as other factors. In general, however, the present technique is particularly well suited to providing remote service to a wide variety of medical diagnostic system modalities, including MRI systems, CT systems, ultrasound systems, positron emission tomography (PET) systems, nuclear medicine systems, and so forth. Moreover, the various modality systems serviced in accordance with the present techniques may be of different type, manufacture, and model. 
     Depending upon the modality of the systems, various subcomponents or subsystems will be included. In the case of MRI system  1014 , such systems will generally include a scanner, a control and signal detection circuit, a system controller, and an operator station. MRI system  1014  includes a uniform platform for interactively exchanging service requests, messages and data with service facility  1022  as described more fully below. MRI system  1014  is linked to a communications module  1032 , which may be included in a single or separate physical package from MRI system  1014 . In a typical system, additional components may be included in system  1014 , such as a printer or photographic system for producing reconstructed images based upon data collected from the scanner. 
     Similarly, CT system  1016  will typically include a scanner, a signal acquisition unit, and a system controller. The scanner detects portions of x-ray radiation directed through a subject of interest. The controller includes circuitry for commanding operation of the scanner and for processing and reconstructing image data based upon the acquired signals. CT system  1016  is similar to PET system  80  described with reference to FIG.  6 . As such, PET system  80  may be included in service system  1010  and would be included in system  1010  as CT system  1016  is. CT system  1016  is linked to a communications module  1048  for transmitting and receiving data for remote services. Moreover, like MRI system  1014 , CT system  1016  will generally include a printer or similar device for outputting reconstructed images based upon data collected by the scanner. 
     In the case of ultrasound system  1018 , such systems will generally include a scanner and data processing unit and a system controller. Ultrasound system  1018  is coupled to a communications module  1062  for transmitting service requests, messages and data between ultrasound system  1018  and service facility  1022 . 
     Although reference is made herein generally to “scanners” in diagnostic systems, that term should be understood to include medical diagnostic data acquisition equipment generally, not limited to image data acquisition, as well as to picture archiving communications and retrieval systems, image management systems, facility or institution management systems, viewing systems and the like, in the field of medical diagnostics. 
     Where more than one medical diagnostic system is provided in a single facility or location  1020 , as indicated in the case of MRI and CT systems  1014  and  1016  in FIG. 8, these may be coupled to a management station  1070 , such as in a radiology department of a hospital or clinic. The management station may be linked directly to controllers for the various diagnostic systems. The management system may include a computer workstation or personal computer  1072  coupled to the system controllers in an intranet configuration, in a file sharing configuration, a client/server arrangement, or in any other suitable manner. Moreover, management station  1070  will typically include a monitor  1074  for viewing system operational parameters, analyzing system utilization, and exchanging service requests and data between the facility  1020  and the service facility  1022 . Input devices, such as a standard computer keyboard  1076  and mouse  1078 , may also be provided to facilitate the user interface. 
     It should be noted that, alternatively, the management system, or other diagnostic system components, may be “stand-alone” or not coupled directly to a diagnostic system. In such cases, the service platform described herein, and some or all of the service functionality nevertheless be provided on the management system. Similarly, in certain applications, a diagnostic system may consist of a stand-alone or networked picture archiving communications and retrieval system or a viewing station provided with some or all of the functionality described herein. 
     The communication modules mentioned above, as well as workstation  1072  and field service units  1024  may be linked to service facility  1022  via a remote access network  1080 . For this purpose, any suitable network connection may be employed. Presently preferred network configurations include both proprietary or dedicated networks, as well as open networks, such as the Internet. Data may be exchanged between the diagnostic systems, field service units, and remote service facility  1022  in any suitable format, such as in accordance with the Internet Protocol (IP), the Transmission Control Protocol (TCP), or other known protocols. Moreover, certain of the data may be transmitted or formatted via markup languages such as the HyperText Markup Language (HTML), or other standard languages. The presently preferred interface structures and communications components are described in greater detail below. 
     Within service facility  1022 , messages, service requests and data are received by communication components as indicated generally at reference numeral  1082 . Components  1082  transmit the service data to a service center processing system, represented generally at reference numeral  1084  in FIG.  8 . The processing system manages the receipt, handling and transmission of service data to and from the service facility. In general, processing system  1084  may include one or a plurality of computers, as well as dedicated hardware or software servers for processing the various service requests and for receiving and transmitting the service data as described more fully below. 
     Service facility  1022  also includes a bank of operator workstations  1086  which may be staffed by personnel who address the service requests and provide off and on-line service to the diagnostic systems in response to the service requests. Also, processing system  1084  may be linked to a system of databases or other processing systems  1088  at or remote from the service facility  1022 . Such databases and processing systems may include extensive database information on operating parameters, service histories, and so forth, both for particular subscribing scanners, as well as for extended populations of diagnostic equipment. 
     FIG. 9 is a block diagram illustrating the foregoing system components in a functional view. As shown in FIG. 9, the field service units  1024  and the diagnostic systems  1012  can be linked to the service facility  1022  via a network connection as illustrated generally at reference numeral  1080 . Within each diagnostic system  1012 , a uniform service platform  1090  is provided. 
     Platform  1090 , which is described in greater detail below with particular reference to FIG. 10, includes hardware, firmware, and software components adapted for composing service requests, transmitting and receiving service data, establishing network connections and managing financial or subscriber arrangements between diagnostic systems and the service facility. Moreover, the platforms provide a uniform graphical user interface at each diagnostic system, which can be adapted to various system modalities to facilitate interaction of clinicians and radiologists with the various diagnostic systems for service functions. The platforms enable the scanner designer to interface directly with the control circuitry of the individual scanners, as well as with memory devices at the scanners, to access image, log and similar files needed for rendering requested or subscribed services. Where a management station  1070  is provided, a similar uniform platform is preferably loaded on the management station to facilitate direct interfacing between the management station and the service facility. In addition to the uniform service platform  1090 , each diagnostic system is preferably provided with an alternative communications module  1092 , such as a facsimile transmission module for sending and receiving facsimile messages between the scanner and remote service facilities. 
     Messages and data transmitted between the diagnostic systems and the service facility traverse a security barrier or “firewall” contained within processing system  1084  as discussed below, which prevents unauthorized access to the service facility in a manner generally known in the art. A modem rack  1096 , including a series of modems  1098 , receives the incoming data, and transmits outgoing data through a router  1100  which manages data traffic between the modems and the service center processing system  1084 . 
     In the diagram of FIG. 9, operator workstations  1086  are coupled to the processing system, as are remote databases or computers  1088 . In addition, at least one local service database  1102  is provided for verifying license and contract arrangements, storing service record files, log files, and so forth. Moreover, one or more communication modules  1104  are linked to processing system  1084  to send and receive facsimile transmissions between the service facility and the diagnostic systems or field service units. 
     FIG. 10 illustrates diagrammatically the various functional components comprising the uniform service platform  1090  within each diagnostic system  1012 . As shown in FIG. 10, the uniform platform includes a device connectivity module  1106 , as well as a network connectivity module  1108 . Network connectivity module  1108  accesses a main web page  1110  which, as mentioned above, is preferably a markup language page, such as an HTML page displayed for the system user on a monitor at the diagnostic system. Main web page  1110  is preferably accessible from a normal operating page in which the user will configure examination requests, view the results of examinations, and so forth such as via an on-screen icon. Through main web page  1110 , a series of additional web pages  1112  are accessible. Such web pages permit remote service requests to be composed and transmitted to the remote service facility, and facilitate the exchange of other messages, reports, software, protocols, and so forth as described more fully below. 
     It should be noted that as used herein the term “page” includes a user interface screen or similar arrangement which can be viewed by a user of the diagnostic system, such as screens providing graphical or textual representations of data, messages, reports and so forth. Moreover, such pages may be defined by a markup language or a programming language such as Java, perl, java script, or any other suitable language. 
     Network connectivity module  1108  is coupled to a license module  1114  for verifying the status of license, fee or contractual subscriptions between the diagnostic system and the service facility. As used herein, the term “subscription” should be understood to include various arrangements, contractual, commercial or otherwise for the provision of services, information, software, and the like, both accompanies with or without payment of a fee. Moreover, the particular arrangements manages by systems as described below may include several different types of subscriptions, including time-expiring arrangements, one-time fee arrangements, and so-called “pay per use” arrangements, to mention but a few. 
     License module  1114  is, in turn, coupled to one or more adapter utilities  1116  for interfacing the browser, server, and communications components with modality interface tools  1118 . In a presently preferred configuration, several such interface tools are provided for exchanging data between the system scanner and the service platform. For example, modality interface tools  1118  may include applets or servlets for building modality-specific applications, as well as configuration templates, graphical user interface customization code, and so forth. Adapters  1116  may interact with such components, or directly with a modality controller  1120  which is coupled to modality-specific subcomponents  1122 . 
     The modality controller  1120  and modality-specific subcomponents  1122  will typically include a preconfigured processor or computer for executing examinations, and memory circuitry for storing image data files, log files, error files, and so forth. Adapter  1116  may interface with such circuitry to convert the stored data to and from desired protocols, such as between the HyperText Transfer Protocol (HTTP) and DICOM, a medical imaging standard for data presentation. Moreover, transfer of files and data as described below may be performed via any suitable protocol, such as a file transfer protocol (FTP) or other network protocol. 
     In the illustrated embodiment, device connectivity module  1106  includes several components for providing data exchange between the diagnostic system and the remote service facility. In particular, a connectivity service module  1124  provides for interfacing with network connectivity module  1108 . A Point-to-Point Protocol (PPP) module  1126  is also provided for transmitting Internet Protocol (IP) packets over remote communication connections. Finally, a modem  1128  is provided for receiving and transmitting data between the diagnostic system and the remote service facility. As will be appreciated by those skilled in the art, various other network protocols and components may be employed within device connectivity module  1106  for facilitating such data exchange. 
     Network connectivity module  1108  preferably includes a server  1130  and a browser  1132 . Server  1130  facilitates data exchange between the diagnostic system and the service facility, and permits a series of web pages  1110  and  1112  to be viewed via browser  1132 . In a presently preferred embodiment, server  1130  and browser  1132  support HTTP applications and the browser supports java applications. Other servers and browsers, or similar software packages may, of course, be employed for exchanging data, service requests, messages, and software between the diagnostic system, the operator and the remote service facility. Finally, a direct network connection  1134  may be provided between server  1130  and an operator workstation, such as management station  1070  within the medical facility (see FIGS.  8  and  9 ). 
     In a present embodiment, the components comprising network connectivity module may be configured via an application stored as part of the uniform platform. In particular, a Java application licensed to a service engineer enables the engineer to configure the device connectivity at the diagnostic system to permit it to connect with the service facility. 
     FIG. 11 illustrates exemplary functional components for service facility  1022 . As indicated above, service facility  1022  includes a modem rack  1096  comprising a plurality of modems  1098  coupled to a router  1100  for coordinating data communications with the service facility. An HTTP service server  1094  receives and directs incoming and outgoing transactions with the facility. Server  1094  is coupled to the other components of the facility through a firewall  1138  for system security. Operator workstations  1086  are coupled to the port manager for handling service requests and transmitting messages and reports in response to such requests. 
     An automated service unit  1136  may also be included in the service facility for automatically responding to certain service requests, sweeping subscribing diagnostic systems for operational parameter data, and so forth, as described below. In a presently preferred embodiment, the automated service unit may operate independently of or in conjunction with the interactive service components comprising processing system  1084 . It should be noted that other network or communications schemes may be provided for enabling the service facility to communicate and exchange data and messages with diagnostic systems and remote service units, such as systems including outside Internet service providers (ISP&#39;s), virtual private networks (VPN&#39;s) and so forth. 
     Behind firewall  1138 , an HTTP application server  1140  coordinates handling of service requests, messaging, reporting, software transfers and so forth. Other servers may be coupled to HTTP server  1140 , such as service analysis servers  1142  configured to address specific types of service requests, as described more fully below. In the illustrated embodiment, processing system  1084  also includes a license server  1144  which is coupled to a license database  1146  for storing, updating and verifying the status of diagnostic system service subscriptions. Alternatively, where desired, license server  1144  may be placed outside of fire wall  1138  to verify subscription status prior to admission to the service facility. 
     Handling of service requests, messaging, and reporting is further coordinated by a scheduler module  1148  coupled to HTTP server  1140 . Scheduler module  1148  coordinates activities of other servers comprising the processing system, such as a report server  1150 , a message server  1152 , and a software download server  1154 . As will be appreciated by those skilled in the art, servers  1150 ,  1152  and  1154  are coupled to memory devices (not shown) for storing data such as addresses, log files, message and report files, applications software, and so forth. In particular, as illustrated in FIG. 11, software server  1154  is coupled via one or more data channels to a storage device  1156  for containing transmittable software packages which may be sent directly to the diagnostic systems, accessed by the diagnostic systems, or supplied on pay-per-use or purchase basis. Message and report servers  1152  and  1150  are further coupled, along with communications module  1104 , to a delivery handling module  1158 , which is configured to receive outgoing messages, insure proper connectivity with diagnostic systems, and coordinate transmission of the messages. 
     In a presently preferred embodiment, the foregoing functional circuitry may be configured as hardware, firmware, or software on any appropriate computer platform. For example, the functional circuitry of the diagnostic systems may be programmed as appropriate code in a personnel computer or workstation either incorporated entirely in or added to the system scanner. The functional circuitry of the service facility may include additional personal computers or workstations, in addition to a main frame computer in which one or more of the servers, the scheduler, and so forth, are configured. Finally, the field service units may comprise personal computers or laptop computers of any suitable processor platform. It should also be noted that the foregoing functional circuitry may be adapted in a variety of manners for executing the functions described herein. In general, the functional circuitry facilitates the exchange of remote service data between the diagnostic systems and a remote service facility, which is preferably implemented in an interactive manner to provide regular updates to the diagnostic systems of service activities. 
     As described above, both the diagnostic systems and the field service units preferably facilitate interfacing between a variety of diagnostic system modalities and the remote service facility via a series of interactive user-viewable pages. Exemplary pages include capabilities of providing interactive information, composing service requests, selecting and transferring messages, reports and diagnostic system software, and so forth. Pages facilitate the interaction and use of remote services, such as, remote monitoring, remote system control, immediate file access from remote locations, remote file storage and archiving, remote resource pooling, remote recording, and remote high speed computations. 
     The user can access specific documents described in text areas of the pages by selection of all or a portion of the text describing the documents. In the presently preferred embodiment, the accessed documents may be stored in local memory devices within the diagnostic system, or selection of the text may result in loading of a uniform resource locator (URL) for accessing a remote computer or server via a network link. 
     Advantageously, service system  1010  (FIG. 8) provides remote services, such as, remote file storage, remote computation, remote servicing, remote control, remote diagnostics, remote monitoring, and remote sales. As such, PET scanner system  80  (FIG. 6) incorporated into service system  1010  includes the capability of providing large memory capacity at remote locations for the large amount of data aquired with PET scanner system  80 . Multiple systems may then cooperate to share memory capacity or storage equipment located at remote locations. 
     FIG. 12 illustrates exemplary process involving remote services provided by service system  1010 , in particular the remote service of remote image data storage is illustrated. Initially it is assumed that data acquisition has already occurred at at least a current stop angle θs. Starting at process block  2150  sorter  108  identifies the current stop angle θs which is the stop angle θs at which the cameras are currently parked. In addition, at block  2150 , sorter  108  identifies projection rays ( R, θf) corresponding to the current stop angle and at which data has been acquired during the most recent acquisition period. Moreover, at block  2150  sorter  108  identifies the next stop angle which is the stop angle at which the cameras will be parked during the next data acquisition period and also identifies projection rays (R′,θf′) corresponding to the next stop angle and at which data will be acquired during the next acquisition period. 
     At process block  2152  sorter  108  compares each projection ray (R,θf) which is associated with the current stop angle to the projection rays (R′,θf′) in the next projection ray set to determine if each projection ray (R,θf) is in the next projection ray set. At decision block  2154 , if a specific ray (R,θf) is in the next projection ray set meaning that data will be acquired during the next acquisition period which corresponds to the specific ray (R,θf), sorter control passes to process block  2156 . If a specific ray (R,θf) is not in the next projection ray set meaning that data will not be acquired during the next acquisition period which corresponds to the specific ray (R,θf), sorter control passes to process block  2158 . 
     At block  2156  sorter  108  maintains specific ray (R,θf) coincident count data in processor memory  110 . At block  2158  sorter  108  communicates specific ray (R,θf) coincident count data to remote facility (FIG. 7) and purges the specific ray (R,θf) coincident count data from memory  110  thereby freeing memory  110  for other purposes. After each of blocks  2156  and  2158  control passes to block  2160 . 
     At block  2160  cameras  12  and  14  are rotated to the next stop angle and sorter  108  resets the current stop angle and associated projection array set to equal the next stop angle and associated projection ray set. In addition, at block  2160  cameras  12  and  14  cooperate with ALC circuitry  106  to acquire data for each projection ray (R,θf) in the new current projection ray set. Sorter  108  uses the new data to update the coincident counts associated with each ray (R,θf). 
     At decision block  2162  sorter  108  determines if the current stop angle is the last stop angle. In the present case, the last stop angle is 178°. If the current stop angle is not the last stop angle control passes back up to process block  2150  and the process begins again. If the current stop angle is the last stop angle so that all data required to generate an image has been acquired, control passes to block  2164 . At block  2164  the current projection ray set is stored by processing system  1084  to, for example, databases  1088 . 
     While the embodiments illustrated in the Figures and described above are presently preferred, it should be understood that the embodiments are offered by way of example only. Other embodiments may include remote control of the imaging system whereby imaging cameras are controlled from a remote location. The invention is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations that nevertheless fall within the scope and spirit of the appended claims.