Data processing: measuring – calibrating – or testing – Calibration or correction system – Timing
Reexamination Certificate
1998-12-14
2001-12-04
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Timing
C378S207000
Reexamination Certificate
active
06327546
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of instruments that detect events involving small particles. More particularly, the present invention is in the field of calibrating instruments that detect events involving small particles.
BACKGROUND OF THE INVENTION
Many types of equipment are designed to detect physical events such as particle-matter interactions. Event detection is widely used in scientific research and in medicine. An example of useful event detection equipment is a nuclear medicine camera, also referred to as a gamma camera. Such cameras can aid in locating diseased tissue, such as tumors, in the body.
Some conventional nuclear medicine imaging systems have two or more detectors. The detectors are usually planar and include an array of detector devices such as photo multiplier tubes (PMTs). The detectors arrays are positioned above different sides of a patient. Gamma cameras can operate in different modes. For example, some nuclear medicine cameras perform single photon emission computed tomography (SPECT) in which information from a single detector is used to produce information. Other nuclear medicine cameras perform positron emission tomography (PET) in which the detection of two scintillation events, one in each of two detectors that occur 180° apart, are used to compute imaging information. In a PET system, detectors detect scintillation events that result when a photon of a photon pair collides with a crystal.
Before the gamma camera is used in PET mode, the patient is injected with a radiopharmaceutical, such as Flouro Deoxi Glucose (FDG). The radiopharmaceutical emits positrons that interact with electrons in the body. As a result of the interaction, the positrons are annihilated and gamma rays, including photon pairs, result. Photon pairs leave the scene of the interaction in directions of travel that are 180° apart from each other. When a photon comes in contact with a crystal of a detector, a scintillation event occurs. The scintillation event is detected by a photo detector device of the detector creating analog information. The analog information is digitized and processed by electronics and software to produce image information about objects such as tumors in the body.
In SPECT mode, the patient is injected with a radiopharmaceutical and emissions from the patient are then detected independently by detectors of the gamma camera, rather than detecting coincident events. When a trigger signal from a detector is received by processing hardware and software, the outputs of the PMTs are integrated to determine the energy associated with the event. Integration occurs during a constant time window. Therefore, if different trigger signals line up differently with respect to the timing window, some events will be processed to show energy in one area of a spectrum, and another event in another area of the spectrum. All energy spectra are added to generate a global energy spectrum for the system. The narrower the energy distribution, the more accurate the spectrum will be. Ideally, energy resolution should depend primarily on the spectra of the crystal and on the PMT energy. Some inaccuracies are unavoidable, such as variance in process behavior in the crystal by location, and noise in the PMT that prevents signals captured from each gamma ray from being the same. Variations in trigger signal arrival time add to inherent inaccuracies of the SPECT method because they affect energy resolution.
Another example of event detection equipment is a Compton camera. A Compton camera examines coincident events between two detectors to determine the well-known Compton scattering angle associated with an interaction of photons with matter. Known equations are used to find the Compton angle, which in turn is used to calculate the position of the event in the body.
Typical gamma camera, regardless of the mode in which they are operated, include detectors with multiple devices such as PMTs. For various reasons, the propagation time of trigger signals indicating detection of events varies between PMTs. One factor contributing to propagation time variance is the fact that PMTs vary physically in ways that affect their response times. Another factor is the variance in the length of cables used to carry signals associated with different PMTs. Yet another factor is crystal response time variance by area. If the trigger signal is received by processing hardware and software significantly later than the event detected, inaccuracies may result. Inaccuracies may include false detection indications, and images with poor resolution. Therefore, it is critical to calibrate the timing of trigger signals so that they portray, as accurately as possible, what is actually occurring in the tissue of the patient.
Proper calibration of trigger signals can be important in both PET and SPECT systems, as well as systems that include Compton cameras. Currently, calibration is performed on PET systems by positioning a radioactive source between detectors and monitoring rates of coincident events. Such methods of calibration are often time consuming and may be imprecise because the steps performed are not accurately repeatable. Currently, no calibration is known to be performed on detectors of systems, such as SPECT systems, that do not operate by detecting coincident events.
Detector calibration is especially critical in systems, such as PET systems and systems using Compton cameras, that rely on detection of coincident events to produce imaging data. If the collision of one photon of a photon pair with one detector is not reported at the same time as the collision of the other photon of the photon pair with another detector, the coincident event will be missed. Techniques currently exist for calibrating PET systems, but these techniques have several disadvantages. Current techniques are complex and not accurately repeatable. In addition, current calibration operations take a relatively long time to perform.
FIG. 1
is a block diagram of a prior PET coincidence detection system
200
. System
200
includes two detectors
266
and
268
. Detector
266
is divided into four zones
266
(
1
),
266
(
2
),
266
(
3
), and
266
(
4
). Each of zones
266
(
1
-
4
) include multiple PMTs. Zone
266
(
1
) and its connected components operate similarly to other zones in detector
266
and corresponding zones in detector
268
. Zone
266
(
1
) will be described as an example of how a zone of a detector operates. When any PMT in zone
266
(
1
) detects a scintillation event resulting from a collision of a photon with the crystal of detector
266
(not shown), an analog signal is sent to summing circuit
201
. Summing circuit
201
receives signals from all of the PMTs in zone
266
(
1
) and sums their amplitudes in a known manner. Summing circuit
201
outputs a signal to constant fraction discriminator (CFD)
221
. CFD
221
operates as a trigger detector in an amplitude independent manner. CFD
221
outputs a zone trigger signal to programmable delay
241
. Programmable delay
241
is typically controlled by a processor of system
200
and is used to vary the delay of the trigger signal output by CFD
221
during calibration of system
200
. Zones
266
(
2
),
266
(
3
), and
266
(
4
) operate in the same manner as zone
266
(
1
), each outputting a signal from their respective programmable delay circuits indicating that an event has been detected. The outputs of programmable delays
241
,
242
,
243
, and
244
are input to OR gate
256
. Detector trigger signal
270
is active on the output of OR gate
256
when any event is detected in a zone of detector
266
. Detector trigger signal
270
is input to common delay CD
A
260
, which is associated with a detector A. CD
A
260
is a programmable delay circuit that is used to vary the delay of detector trigger signal
270
with respect to detector trigger signal
272
. Adjusted detector trigger signals
274
and
276
are input to coincidence detection circuit
264
. Coincidence detection circuit
264
typically performs an oper
Bertelsen Hugo
Petrillo Michael J.
Scharf Thomas E.
Wellnitz Donald R.
ADAC Laboratories
Barbee Manuel L.
Becker Jordan M.
Wachsman Hal
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