Patent Application
20080075342
Referring to
A scintillating crystal is one that, when illuminated by a gamma ray, briefly generates visible light. This visible light is detected by the photo-multiplier tubes, which in turn generate an electrical signal indicative of detection of an incident gamma ray photon, hereafter referred to as detection of an “event.”
To image a portion of a patient with a PET scanner 10, one introduces a radioactive material into the patient. As it decays, the radioactive material emits positrons. A positron, after traveling a short distance through the patient, eventually encounters an electron. The resulting annihilation of the positron and the electron generates two gamma ray photons traveling in opposite directions. To the extent that neither of these photons is deflected or absorbed within the patient, they emerge from the patient and strike two of the detector modules 16A-K.
In particular, when one of these photons strikes a first detector module 16A, the other photon strikes a second detector module 16E, F, G, or H that is opposed to the first detector module. This results in two events: one at the first detector module 16A and the other at the opposed second detector module 16E, F, G, or H. Each of these events indicates the detection of a gamma ray photon. If these two events are detected at the first detector module 16A and the second detector module 16E, F, G, or H nearly at the same time, it is likely that they indicate an annihilation occurring somewhere along a line connecting first detector module 16A and the second detector module 16E, F, G, or H. If these two events are detected at the first detector module 16A and the second detector module 16E, F, G, or H at almost the same time, it is likely that they indicate an annihilation occurring somewhere along a line connecting first detector module 16A and the second detector module 16E, F, G, or H.
It is apparent that what is of interest in a PET scanner 10 are pairs of events detected by opposed detector modules 16A, 16E-F at, or almost at, the same time. A pair of events having these properties is referred to as a “coincidence.” In the course of a PET scan, each detector module 16A-K detects a large number of events. However, only a limited number of these events represent coincidences.
Associated with each detector module 16A-K is a module processor 18A-K that responds to events detected by its associated detector module 16A-K. A module processor 18A-K includes a processing element and a memory element in data communication with each other. The processing element includes an arithmetic logic unit (“ALU”) containing combinatorial logic elements for performing various logical operations, an instruction register, associated data registers, and a clock. During each clock interval, the processor fetches an instruction from the memory element and loads it into the instruction register. Data upon which the instruction is to operate is likewise loaded into the associated data registers. The structure and operation of the module processors 18A-K and their interaction with a coincidence processor is described in detail in Worstell, et al., U.S. Pat. No. 6,828,564, the contents of which are herein incorporated by reference.
A module processor 18 is intended to respond only to events detected by detector blocks 17. However, in many cases, detector blocks 17 generate spurious signals that arise from causes other than events. A trigger protects the module processor 18 from being overwhelmed by such signals. The function of a trigger is to reject those signals that are unlikely to represent an event, and to provide the remaining signals to the module processor 18.
Similarly, the candidate signal 24 is also provided to a low band-pass filter 32. The low band-pass filter has a pass-band that encompasses lower frequencies than the pass-band of the high band-pass filter. The resulting low band-pass filtered candidate signal 34 is then sampled at a second sampling frequency that is lower than the first sampling frequency. These samples, which will be referred to as the “low-frequency samples,” are stored in a second memory 36, such as a FIFO memory.
The two distinct memories 30, 36 shown in
In general, the passbands of the high and low band-pass filter 32, 26 will overlap to some extent. The passband of the high band-pass filter 26 should have an upper cut-off frequency below the point at which high frequency noise begins to impair the ability to identify the occurrence of the leading edge of the candidate signal 24. The passband of the low band-pass filter 32 is selected to provide sufficient bandwidth for obtaining information concerning the amplitude of the candidate signal, but without accepting high frequency components tainted by excessive noise.
The characteristics of the candidate signals 24 differ from time to time. In some cases, these characteristics are consistent with a detected gamma ray. In other cases, they are not. For example, some candidate signals 24 may indicate a voltage that is unusually low. Or, the time-evolution of a candidate signal 24 may be atypical of a gamma ray interaction.
A digital trigger 38 protects the module processor 18 from being overwhelmed by specious candidate signals, such as those that are likely to have arisen from something other than a gamma ray interaction. The digital trigger 38 is programmed to examine data derived from a candidate signal 24 and to then execute certain rules that classify that candidate signal 24 as likely to have arisen from a gamma ray interaction, or as being a stray signal.
The particular rules for classifying a candidate signal 24 are, in general, programmable. Because the rules are programmable, the digital trigger 38 can implement a variety of classification algorithms for determining whether a candidate signal is likely to indicate an event. The programmability of the digital trigger 38 enables the classification algorithm to be changed to suit changing circumstances, either by an operator or adaptively. For example, a qualifying condition might cause the classification algorithm to change by branching, considering additional inputs, or by changing parameters considered by the classification algorithm. In some implementations, the digital trigger 38 can be programmed or changed even while data processing continues, without interrupting the processing.
The digital trigger 38 need not rely on analog information in making a decision concerning a candidate signal 24. As a result, the rules executed by the digital trigger 38 are unconstrained by difficulties associated with manipulation and storage of analog information. Such difficulties include the degradation of analog information that often occurs during its manipulation and storage.
The digital trigger 38 is in data communication with one or both memories 30, 36. As a result, the programmable rules can readily exploit information from candidate signals other than a current candidate signal 24, or information from different parts of the waveform that makes up the current candidate signal 24. The digital trigger 38 can thus implement rules that classify a current candidate signal 24 on the basis of previous signals. Or, the digital trigger 38 can postpone classifying a candidate signal 24 until additional candidate signals have been acquired. This enables the digital trigger 38 to implement classification rules that depend on information not yet available as of the time a candidate signal 24 to be classified is received.
In the embodiment shown in
Since the output of the timing qualifier 40 and the output of the energy qualifier 42 are available at different times, the comparator 44 is implemented by placing the earlier of the two outputs into a delay line. The delay line delays the earlier of the two outputs until the later one becomes available. At that point, the outputs of the energy qualifier 42 and the timing qualifier 40 are both available, and can therefore be compared.
One rule implemented by either or both the timing qualifier 40 and the energy qualifier 42 is a single-pair slope test. In implementing the single-pair slope test, the qualifier 40, 42 examines the slope associated with a pair of samples stored in the memory 30, 36. In some embodiments, these samples are consecutive. However, in other embodiments, these samples are separated by one or more other samples. If the resulting slope is in excess of a threshold, the qualifier 40, 42 classifies the candidate signal as being likely to represent an event. The resulting classification is represented by a logical output signal 46, 48 provided to the comparator 44.
Another rule implemented by either or both the timing qualifier 40 and the energy qualifier 42 is a multi-pair slope test. When executing a multi-pair slope test, the qualifier 40, 42 examines the slopes associated with two or more pairs of samples stored in the memory 30, 36. If the number of pairs having associated slopes in excess of a slope threshold exceeds a count threshold, then the qualifier 40, 42 classifies the candidate signal has being likely to represent an event. The resulting classification is represented by a logical output signal 46, 48 provided to the comparator 44.
The multi-pair and single-pair slope tests are useful for detecting the edge of the candidate signal waveform. By measuring the difference in slope at two different times, the qualifier 40, 42 decreases sensitivity to baseline shift. By appropriately re-programming the qualifier 40, 42, for example by defining different thresholds for the slope difference and by changing the times at which the slope differences are measured, one can tune the qualifier 40, 42 to accept or reject candidate signals on the basis of different ranges of slopes.
Other rules executed by a qualifier 40, 42 include those that correct for signal pile-up caused by receiving two candidate signals in rapid succession. In such cases, the tail of the earlier candidate signal may not have decayed sufficiently to avoid being added to the beginning of the later candidate signal. A qualifier 40, 42 corrects this by storing a tail amplitude for a candidate signal and causing a decaying fraction of this amplitude to be subtracted from a subsequent candidate signal. The rate of decay and the parameters required to trigger the use of tail cancellation are both programmable.
Other rules executed by a qualifier 40, 42 include those that correct for the reduced sensitivity of a photo-detector 22 at the periphery of its field of view. This reduction in sensitivity as one approaches the periphery of the field of view is referred to as “crowning.”
The digital trigger 38 thus uses samples in memory 30, 36 to make decisions concerning whether or not a candidate signal 24 represents a gamma ray interaction. The availability of such stored samples permits extensive and programmable processing of information before making a commitment to a particular decision. This renders practicable decision-making about a current sample on the basis of information embodied in preceding samples. Because samples are stored in memory 30, 36 until a decision is made, samples representing different times can be read from different portions of memory 30, 36 at any time, thereby enabling the rules implemented by the digital trigger 38 to be varied in complexity without altering the data storage requirements.
The foregoing features enable the digital trigger 38 to execute multiple independent processes in parallel, as shown in
Although
Referring now to
The architecture shown in
At each stage, information concerning the outcome of a test on a candidate signal can be forwarded to the following stage for use by that stage. Those candidate signals that pass through all the stages, and hence “graduate” from the digital trigger 38, are optionally tagged with a weighting coefficient to be used by the coincidence processor in determining whether that event is likely to be part of a coincidence.
A multi-stage digital trigger 45 as shown in
The parallel, serial or cascaded, and recursive architectures described above can be combined into a hybrid architecture. For example, a multi-stage digital trigger can incorporate stages in which processes execute independently as shown in
The multi-stage architecture for a digital trigger 45, as shown in
It is evident that those skilled in the art may now make numerous modifications of and departures from the apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature, and novel combination of features, present in or possessed by the apparatus and techniques herein disclosed and limited only by the spirit and scope of the appended claims.
