Abstract:
A subject ( 10 ) is disposed adjacent a detector array ( 18 ) for the purposes of nuclear imaging. The subject ( 10 ) is injected with a radioactive isotope ( 14 ) and γ-ray emissions indicative of nuclear decay are detected at the detector array ( 18 ) these signals are processed and reconstructed into an image representation of the anatomy of the subject ( 10 ). A dual level arbitration system orders detected signals for ease of processing and efficiency of reconstruction. The first level of the arbiter monitors a group of individual detectors ( 22 ). It locks out any signal that arrives from its group of detectors if a previous signal is still being analyzed. This avoids paralyzation of the system. The second level of the arbiter consists of a plurality of memories, one for each group of individual detectors ( 22 ) that store an address and energy of each processed signal. These memories pass a token around among themselves, and if they possess the token, as well as data, they load that data onto a data collection bus, which transfers it to an event archive ( 44 ) for storage until enough data is collected to be reconstructed into an image of the subject ( 10 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention deals with the nuclear arts. It finds particular application in conjunction with electronics used in SPECT cameras and will be described with particular reference thereto. However, it is to be appreciated that the present invention may find application in PET and other nuclear cameras and the like and is not limited to the aforementioned application. 
     Nuclear imaging employs a source of radioactivity to image the anatomy of a subject. Typically, a radiopharmaceutical is injected into the patient. This radiopharmaceutical contains atoms that decay at a predictable rate. Each time an atom decays, it releases a γ-ray. These γ-rays are detected, and from information such as their detected position and energy, a representation of the interior of the subject is reconstructed. 
     Typically, a nuclear camera has one, two, or more detector heads. Each head has a large scintillator sheet, such as doped sodium iodide, which converts incident radiation into scintillations, i.e., flashes of light. An array of photomultiplier tubes is disposed in back of the scintillator to monitor for light flashes. The output of the photomultiplier tubes and associated circuitry indicates the coordinates of each scintillation on the sodium iodide crystal and its energy. Unfortunately, there are numerous non-uniformities and inaccuracies when using a large scintillator crystal and an array of photomultiplier tubes. 
     This type of detector is only capable of processing one nuclear event at a time. Γ-rays incident upon the detector temporally too close together are typically discarded, i.e., ignored. The reset time is determined in part by the afterglow of the crystals, and to a lesser extent, the processing time of the electronics. 
     Rather than using a single, large scintillator and photomultiplier tubes, others have proposed using an array of small scintillators, each optically coupled to a corresponding photodiode or other photoelectrical device which senses a scintillation in each individual scintillation crystal. Other types of individual solid-state detectors have also been suggested. These radiation-light-photodetection systems are again limited in speed by crystal afterglow. Moreover, energy-to-light conversion efficiency of the scintillators limits detector size, hence resolution. 
     The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a nuclear imaging apparatus is given. A source of radioactivity emits γ-rays that strike an array of solid state detector arrays. An array of electronics process nuclear decay event information from the detector arrays, and a reconstruction processor converts them into a human readable format. 
     In accordance with another aspect of the present invention, a nuclear camera is given. An array of groups of detectors detect incident radiation and send output signals in response. Signal processing circuits process the signals. First level arbiters decide which signals gain priority, and a second level arbiter orders the signals from the first level arbiters. A reconstruction processor the converts the signals to an image format. 
     In accordance with another aspect of the present invention, a method of nuclear imaging is given. Γ-rays are detected that produce signals and are arbitrated upon. The signals are then reconstructed into an image representation. 
     In accordance with another aspect of the present invention, a logic circuit arbitrator is given. Signal generators generate signals that are selectively processed by signal processors, labeled, and sent to temporary memories where they wait for a data collection bus. A token allows the signals to be passed from the temporary memories to the data collection bus. 
     One of the advantages of the present invention is that it decreases the effective event processing time of the system. 
     Another advantage resides in a direct detection of incident radiation. 
     Another advantage resides in its compact size. 
     Another advantage is that it achieves concurrent detection of radiation events. 
     Yet another advantage is that excellent spatial resolution is achieved. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a nuclear imaging device in accordance with the present invention; 
     FIG. 2 is a perspective view of the detector array; 
     FIG. 3 is a perspective view of the detector array, circuit boards, and cooling fans; 
     FIG. 4 is a circuit that performs of a first level of arbitration; 
     FIG. 5 is a diagram of the circuit boards associated with each line of detector arrays; 
     FIG. 6 is a block diagram of digital readout circuitry that performs a second level of arbitration; 
     FIG. 7 is a timing diagram for the second arbitration level. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a region of interest of a subject  10  defines an imaging region  12 . In an emission tomography embodiment, a radioactive isotope  14 , such as a radiopharmaceutical is injected into the or subject, near a region to be imaged. For example, if a physician wants to view a blockage in the aorta, the isotope is injected into the bloodstream upstream from the blockage. The radioisotope is monitored as it is carried by the blood to image the circulatory system. As another example, the radiopharmaceutical which is preferentially absorbed by tumors or other tissue of interest is injected into the circulatory system. The radiopharmaceutical is monitored after absorption by tissue of interest to image the tissue of interest. 
     As quantum physics predicts, atomic nuclei of the radioactive isotope decay over time. Energy is released at the time of decay in the form of a photon, more specifically, a γ-ray of characteristic energy. 
     Many of the γ-rays produced during an emission imaging process are lost, propagating in useless directions. However, some of the γ-rays pass through collimators  16 , thin tungsten vanes in the preferred embodiment, and strike a detector array  18 . In the preferred embodiment and with reference to FIG. 2, the detector array  18  includes a 4×24 array of cadmium zinc telluride (CZT) crystal arrays  20 , each having 4×8 individual detectors  22 . 
     For transmission tomography, a radiation source  24  is disposed across the subject from the detector head. Preferably, the transmission source is a radioisotope with a different characteristic energy from the injected radiopharmaceutical. 
     In the preferred embodiments, the detector array  18  and collimators  16  are mounted on a mechanized drive  30  go that moves the detector array. Preferably, the array moves with lateral rotational components of motion, although various trajectories within the detector head are contemplated. Alternately, the detector array is stationarily mounted within a detector head. In tomographic applications, the detector head is indexed around the region of interest. 
     In the preferred embodiment, the detector head is mounted on a rotatable gantry  32  which extends fully around the subject  10 . A motor control  34  selects a range of motion of the detector array  18 , if any, within the rotatable gantry and rotation of the gantry  32  stepwise or continuously around the image region. 
     In SPECT imaging, the collimator  16  limits access to the detector array  18  to radiation following prescribed paths or trajectories, e.g., parallel vane collimators define trajectories in a slice perpendicular to the plane of the detector array  18 . In this manner, each radiation event defines a trajectory along which a radioisotope decayed. If the movable gantry  32  remains stationary, the detectors define a projection image of the radioisotope distribution in the region of interest. 
     An array of event analyzers  42  determines the location which each events strikes each group of the detector arrays, i.e., which detector within a group receives it and the amount of energy of the radiation event. The radiation events collected at each stationary position of the detector array are stored in an archive  44 . When the rotatable gantry  32  is rotated to different angular positions around the subject, a plurality of projection images from different angular orientations are collected. A reconstruction processor  46  backprojects or otherwise reconstructs the data from the archive memory  44  into a volumetric image representation for storage in a volumetric image memory  48 . A video processor  50  under operator control selectively withdraws portions of the volumetric image representation and converts them into appropriate form for display on a video or other human readable monitor  52 . 
     With reference to FIG.  3  and continuing reference to FIGS. 1 and 2, received γ-rays are detected and their energy measured by electronics attached to the detector array  18 . In the preferred CZT embodiment, a potential difference of −600 V is applied across the detector arrays. P-ASICS or other low noise pre-amplifier circuits process and shape the current pulses from the detector array that result from detected γ-rays. Four P-ASICs are mounted on each side of a circuit board or pair of back-to-back circuit boards  62  that support four of the detector arrays. 
     The CZT crystals rely on propagating electrons rather than scintillations to indicate detection of events. The high voltage potential sets up a substantially uniform electric field in the detector array. This electric field loosens the bonds of electrons that are naturally found in the CZT. When a γ-ray strikes the detector, it liberates many of these electrons from their bonds to local nuclei, and they get caught in the electric field and are propagated to the other side of the detector. It is important to note that the detectors of the preferred embodiment are solid state detectors that rely on the propagation of charge to identify a radiation event. 
     Each time a γ-ray strikes one of the detectors, it releases electrons producing an output electrical pulse. Associated electronic components  64  which are powered by a voltage regulator  66  amplify each pulse, spread the pulse, measure the area under the pulse, convert the area to an output, otherwise processes the received radiation data. Multiplexers  68  multiplex the resulting signal into a series of outputs to portions of the event analyzer  42  mounted remotely. 
     The electrical signals detected by the array  18  are ordered and processed. The data acquisition architecture of the preferred embodiment uses two levels of arbitration and data reduction to pass signals from the 3072 individual detectors  22  to a single output port at very high event rates. First, analog signals from the 3072 individual detectors are collected into more manageable groups, in the preferred embodiment,  48  independent groups of  64  signals. Within each group of  64  signals, a first level of arbitration selects a signal of interest, and provides an output channel for that signal, along with a digital representation of that signal&#39;s address. It is to be understood that the number of independent signals may be smaller or larger, depending on the size of the detector array  18 . 
     The first level arbiter consists of  64  independent signal comparators, each of which provides a digital output trigger signal when the corresponding analog signal input exceeds an adjustable threshold voltage. The first digital signal to reach the arbitration logic, when the logic is in a ready state, wins the arbitration contest. The logic then encodes a 6-bit address representing the channel number of the digital signal which triggered it, enables the analog multiplexer  68  to send the selected analog signal to an output port, provides a composite trigger signal indicating to subsequent circuitry that an analog signal has been selected for processing, and locks out any further triggering events until subsequent circuitry is finished extracting the energy information from the signal. This property of the logic design renders it immune from (and thus not paralyzable by) additional radiation events from other detectors in the arbitration group while an individual pulse is being processed. 
     The first level arbitration logic returns to a ready state (reset) by a signal from subsequent circuitry indicating that the energy information (peak value of the pulse) has been determined. The remaining portion of the original analog signal is then ignored, and the arbitration logic returns to the ready state, indicating accept new triggering inputs. 
     The first level arbitration logic operates fast enough to provide the analog signal through an analog multiplexer prior to the signal reaching its peak (100 s of nanoseconds). The logic also operates fast enough to inhibit any possible additional triggering inputs from producing an erroneous arbitration address (10 s of nanoseconds). 
     With reference to FIG. 4, the multiplicity of digital trigger signals  70 ,  64  in the preferred embodiment, each drive a clock input  72  of a flip-flop  74 . If the logic is currently ready, the flip-flop  74  sensing a transition on its clock input  72  sets an output bit  76  true. Logic implementing a Boolean “OR” function produces a composite trigger signal  80  when any of the input flip-flops have been set. This composite trigger signal  80  remains true until all of the input flip-flops  74  have been reset by a done signal  82  from subsequent external circuitry indicating that the energy information (peak value of the pulse) has been determined. The composite trigger  80  thus also indicates that the logic is not in a ready state to accept further inputs, and therefore may be fed back into an inverted condition to all of the flip-flops to inhibit any of them from being set true by additional individual triggering inputs. 
     With reference to FIG.  5  and continuing reference to FIG. 4, when any of the output bit signals  76  are set true, a unique address is generated to represent which of the multiple trigger inputs  70  won the arbitration. This operation is preferably done with a simple binary decoder  84  in which output bit is set depending on its state in the binary representation of the channel number. Any decoding which provides a unique address for any single input  76  may be used. This decoder logic also provides the enabling signals to analog multiplexers  68  to pass the selected analog signal to a first level output port  90 . 
     In the event that more than one trigger input  70  becomes active before the composite trigger signal  80  can be fed back into the input flip-flops  74 , all are ignored. Optionally a more complex implementation of the arbitration logic examines the  64  inputs  70  to find out their trigger times and selectively chooses one among them and vetoes any others. 
     The first level arbitration logic has additional circuitry to “close the loop” and provide the reset signal  82 . This is circuitry which determines and reports the peak value of the pulse following a successful arbitration. The circuitry to perform this task might be implemented in several different ways, such as analog peak detectors employing diodes or sample/hold amplifiers. The method of the preferred embodiment uses a free running ADC of relatively high sampling speed (20 MHZ) with additional control logic to detect the occurrence of the signal peak and average (sum) samples of the signal around the peak to improve the resolution of the reported peak value. Following the determination and saving of the peak value, The control logic releases the original arbitration logic with the reset signal  82 . 
     With reference to FIG. 6, the signal processing logic for the peak detection allows a highly accurate result (12 bits resolution) from relatively low cost, low resolution (10 bit) analog to digital converters by over sampling the analog pulse (sampling period pulse duration/20), and summing (averaging) four samples near the maximum value observed in an ADC output data stream  92 . A simple magnitude comparitor  94  indicates when the signal has stopped rising, and by means of a digital storage pipeline, two samples preceding and two samples following the detected peak sample are added in a digital accumulator. The sum of the four 10 bit signals provides the 12 bit output signal. 
     The operation of the peak detector pipeline and accumulator is controlled by a logical state machine, which responds to a natural progression of events following a trigger, such as observing a monotonically-increasing leading edge, peak value, and monotonically decreasing values following the peak. This same state machine controller is programmed to trap and exit from states which do not follow the natural progression of signal behavior, such as electronic “glitches” or radiation induced “pile-up” events, which would render the reported energy value inaccurate. 
     The first level arbitration logic requires a number of individual sensors  22  to gain access to a common output port. This naturally reduces the possible bandwidth of the data acquisition system below what is achievable with an output port per sensor. However, a tradeoff in cost vs. performance/complexity is advantageous in a system with thousands of sensors. In a non-paralyzable system such as just described, the point at which 20% of events are lost due to the unavailability of the processing channel is a common figure of merit for system throughput. 
       R   out   =R   in /(1 +R   in   *DT )=0.8 *R   in   
     where DT is the dead time associated with an event to be processed. If the DT is 2 μs then the 20% loss rate occurs with an input rate R in =125,000 events/sec, and R out =100,000 events/sec. On the other hand, if the detection system processes each sensor individually, the achievable throughput rate is multiplied by the number of sensors. Without the 64:1 arbitration, this would equate to 6.4×10 6  events/second. 
     The total throughput of the system is maintained by having a large number of these first level arbiters. In the case of a system with  3072  sensors,  48  first level arbiters each channel  64  sensors to one peak detector. Thus, for the system, the 20% loss point occurs with an event rate of R in(system) =48×125,000 events/sec=6.0×10 6  events/second, and an output rate of R out(system) =48×100,000 events/second=4.8×10 6  events/sec. 
     The data acquisition process of the described embodiment has  48  independent arbiters each choosing  64  possible data sources, and producing a digital measurement of each of those sources. The second level arbitration provides for access of any of these measurements to a single output port for eventual storage in the event archive  44 . 
     In the second level arbitration, the data from individual peak detectors is stored in temporary memory to await access to a second level output port  102 . Each peak detector provides an energy measurement, as well as the address of the sensor pixel which won the first level arbitration to that peak detector. At the temporary memory in the second arbitration level, additional address bits are also stored indicating which peak detector provided the data. 
     The peak detectors and temporary memory are preferably organized such that a number of peak detectors arbitrate for access to a common memory. The number of peak detectors per independent temporary memory block may vary based on system cost and complexity constraints. Each temporary memory is designed to assert a “data ready” flag signal indicating to an external data collection bus that it has received data from at least one of its contributing peak detectors. Before data from one of the temporary memories can be placed on the external data collection bus, additional address bits are stored indicating which temporary memory provided the data. 
     The arbitration for the output port from the individual temporary memories is controlled by a tokenring scheme  104 . In this scenario, a “token” signal is passed from one memory to the next in a loop fashion. If the receiving memory has its “data ready” flag set, that memory is permitted to place the data from its next available address on the output data collection bus, assert a “data valid” strobe on the bus, and then pass the token (supplies a signal) to the next memory in the loop. If the receiving memory does not have its “data ready” flag set, it simply passes the token to the next memory without placing any data or “data valid” strobe on the 
     It is to be understood that alternate embodiments of the present invention are easily scalable to include more or fewer individual arbiters in the loop. 
     With reference to FIG. 7, a sample token ring scheme is given. In this example, one token is being passed around between eight magnitude comparitors  94 . Comparitors  2 ,  3  and  6  simultaneously have data ready, and have their data ready flags set. When the token reaches one of the comparitors  94  that has data ready, the data is transferred to a motherboard data bus  110 , and on its way to the event archive  44 . For comparitors  94  that do not have a data ready flag set, the token is passed on in the next clock cycle. The first line  112  is a motherboard clock, mediating any data transfers. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.