Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-06-30
2002-04-23
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
Reexamination Certificate
active
06376841
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to generating images using a coincidence event detector. More particularly, the present invention relates to normalizing the triggering events in a coincidence event detector, thus reducing image artifacts in the generated image.
BACKGROUND OF THE INVENTION
Many types of equipment are designed to detect physical events such as particle-matter interactions. In the prior art, event detection has been widely used in scientific research and medicine to generate images of a patient's internal structures. For example, nuclear medicine cameras (also referred to as a gamma cameras) may perform positron emission tomography (“PET”) on emitted photon pairs to map the originating location and energy of the emitted photon pairs, thus allowing the nuclear medicine cameras to generate images of tumors or other cancerous developments.
Typically, to generate the image, a patient is injected with a radio pharmaceutical (such as Flouro Deoxi Glucose) that interacts with electrons in the body of the patient. As a result of the interaction, positrons are annihilated and gamma rays, including photon pairs, are emitted from the body of the patient in directions of travel that are 180° apart from each other. An example of a prior art PET may be found in the dual single-photon completed emission tomography (“SPECT”)/PET system of U.S. Pat. No. 5,608,221 of Bertelsen et. al (issued Mar. 4, 1997 and assigned to ADAC Laboratories of Milpitas, Calif.).
Certain nuclear medicine camera systems include two planar detector assemblies that are positioned on opposite sides of the patient. Each detector assembly includes a crystal covered by an array of photo multiplier tubes (“PMTs”). To perform the PET, nuclear medicine cameras detect the collision of photons with the detector assemblies. Specifically, when a photon comes in contact with the crystal of a detector assembly, a scintillation occurs. In response to the scintillation, the PMTs exposed to the scintillation event generate electrical outputs. Accordingly, the electrical output of the PMTs may be used to detect the emission of a coincidence photon pair—hereinafter referred to as a coincidence event.
As previously described, certain nuclear medicine camera systems include two planar detector assemblies. Accordingly, to detect a coincidence event nuclear medicine cameras use a PET coincidence detection system. In particular, in a PET coincidence detection system, the PMTs of a first detector assembly are divided into spatial quadrants (or zones) and the electrical outputs of the zones are summed. The summated outputs are digitized and logically combined with the digitized outputs of all the zones in the first detector assembly, thus generating a first coincidence trigger. Similarly, the PMT outputs of the second detector are summed and digitized to generate a second coincidence trigger. Accordingly, if the two coincidence triggers transition within a predetermined time window (typically 10-20 nanoseconds), the scintillations causing the transition of the coincidence triggers are deemed to be from a coincidence event. Thus, the location and energy of the scintillations on the PMTs may be used to
5
determine the origin of the event.
FIG. 1
is a block diagram of a prior PET coincidence detection system
100
. System
100
includes two detectors
166
and
168
. Detector
166
is divided into four zones
166
(1),
166
(2),
166
(3), and
166
(4). Each of zones
166
(1-4) include multiple PMTs. Zone
166
(1) operates similarly to other zones in detector
166
and corresponding zones in detector
168
. Accordingly, zone
166
(1) will be described as an example of how a zone of a detector operates. When any PMT in zone
166
(1) detects a scintillation event resulting from a collision of a photon with the crystal of detector
166
(not shown), an analog signal is sent to summing circuit
101
. Summing circuit
101
receives signals from all of the PMTs in zone
166
(1) and sums their amplitudes in a known manner for example via a series weighted resistors. Summing circuit
101
outputs a signal to constant fraction discriminator (“CFD”)
131
. CFD
131
operates as a trigger detector in an amplitude independent manner. CFD
131
outputs a zone trigger signal to programmable delay
141
.
Programmable delay
141
is typically controlled by a processor of system
100
and is used to vary the delay of the trigger signal output by CFD
131
during calibration of system
100
. Zones
166
(2),
166
(3), and
166
(4) operate in the same manner as zone
166
(1), each outputting a signal from their respective programmable delay circuits indicating that a coincidence event has been detected. The outputs of programmable delays
141
,
142
,
143
, and
144
are input to OR gate
156
. Detector trigger signal
170
is active on the output of OR gate
156
when any event is detected in a zone of detector
166
. Detector trigger signal
170
is input to common delay CD
A
160
, which is associated with a detector
166
. CD
A
160
is a programmable delay circuit that is used to vary the delay of detector trigger signal
170
with respect to detector trigger signal
171
. Adjusted detector trigger signals
174
and
176
are input to coincidence detection circuit
164
. Coincidence detection circuit
164
typically performs an operation such as an AND operation for determining when scintillation events have been detected simultaneously in detectors
166
and
168
.
As illustrated in
FIG. 1
, system
100
includes multiple delay elements to synchronize the detection of coincidence events. In particular, programmable delays
141
-
144
and
151
-
154
are adjusted to compensate for variances such as response time of different PMTs, different cable lengths between zones, and different component tolerances between zones. Additionally, CD
A
160
and CD
B
161
are adjusted for the purpose of synchronizing the global, or common, delays of signals
170
and
171
due to cable lengths between OR gates
156
and
158
and coincidence detection circuit
164
.
Although a PET system using system
100
may generate accurate images based on the detection of events, the coincidence detection system has numerous disadvantages. One disadvantage, results from the non-uniformity of the images generated by the PET. In particular, the low number of zones in system
100
create “dead areas” during the detection of multiple events. For example, if an event causes a scintillation in the PMTs shared by zone
166
(1) and zone
166
(2), neither zone is available to detect subsequent scintillations until the PMT signals correlating to the event have been processed. Thus, during processing of the event, zone
166
(1) and zone
166
(2) may not detect a coincidence event.
Another disadvantage results from the prior art techniques used to overcome the non-uniform images generated by PET systems. In particular, prior art PET systems use calibration techniques to overcome the generation of non-uniform images. Typically, the calibration technique involves the characterization of the sensitivity profiles of the coincidence detection system. Subsequently, the characterization profiles are used to reduce the non-uniformity of the PET generated image. The use of calibration techniques, however, tends to result in the loss of image data.
SUMMARY OF THE INVENTION
An apparatus and method for an event detector system are described. The event detector system comprises a detector having a plurality of zones, each zone comprising a plurality of detector devices. For one embodiment, each detector device is operable to generate an event signal in response to an energy source. The event detector system also comprises a threshold circuit coupled to a detector device of the detector. For another embodiment, the threshold circuit is operable to selectively transfer the event signal of the detector device. Additionally, the event detector system comprises a summation circuit coupled to the threshold circuit. For yet another embodiment, the summation circu
Adam Robert A.
Petrillo Michael J.
Hannaher Constantine
Mak Theodorus N.
Philips Electronics N.A. Corp.
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