Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2000-05-01
2001-07-03
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S363040
Reexamination Certificate
active
06255655
ABSTRACT:
This invention relates generally to gamma cameras and more particularly to a gamma camera which is capable of performing both Positron Emission Tomography (“PET”) and Single Photon Emission Computed Tomography (“SPECT”) studies.
The invention is particularly applicable to and will be described with specific reference to an Anger type camera equipped with two, two dimensional detector heads containing photomultipliers or solid state detectors and Nal crystals capable of performing PET and SPECT studies. However, those skilled in the art will recognize that the invention has applications outside of gamma cameras and can be used in conventional PET ring scanners to improve the accuracy, spatial response and time scans.
INCORPORATION BY REFERENCE
Our application filed as of this date entitled “Prefilter Collimator for PET Gamma Camera” and assigned to the present assignee, SMV America, is incorporated herein. The invention disclosed and claimed herein is believed separate and distinct from that disclosed and claimed in our application filed concurrently herewith.
In addition, the following patents are incorporated by reference herein (as well as documents incorporated or referred to in the patents) so that details known to those skilled in the art need not be restated herein. The documents incorporated by reference herein do not form part of the present invention.
U.S. Pat. No. 5,532,489 to Yamashita et al., issued Jul. 2, 1996, entitled “Positron Imaging Apparatus”;
U.S. Pat. No. 5,272,343 to Stearns, issued Dec. 21, 1993, entitled “Sorter for Coincidence Timing Calibration in a PET Scanner”;
U.S. Pat. No. 5,241,181 to Mertens et. al., issued Aug. 31, 1993 entitled “Coincidence Detector for a PET Scanner”;
U.S. Pat. No. 5,512, 755 to Vickers et al., issued Apr. 30, 1996, entitled “Gamma Camera Device” and assigned to the assignee of the present invention;
U.S. Pat. No. 5,345,082 to Engdahl et al., issued Sep. 6, 1994 entitled “Scintillation Camera Utilizing Energy Dependent Linearity Correction”;
U.S. Pat. No. 5,023,895 to McCroskey et al., issued Jun. 11, 1991 entitled “Three Dimensional Tomographic System”;
U.S. Pat. No. 4,864,140 to Rogers et al., issued Sep. 5, 1989 entitled “Coincidence Detection System for Positron Emission Tomography”;
U.S. Pat. No. 5,323,006 to Thompson et al., issued Jun. 21, 1994 entitled “Dedicated Apparatus and Method for Emission Mammography”;
U.S. Pat. No. 4,675,526 to Rogers et. al., issued Jun. 23, 1987 entitled “Method and Apparatus for 3-D Encoding”; and
U.S. Pat. No. 4,395,635 to Friauf et. al., issued Jul. 26, 1983 entitled “Gamma Ray Coincidence Analysis System”.
BACKGROUND
Nuclear or scintillation or gamma cameras are conventionally used to perform SPECT studies. A patient ingests or is injected with a radiopharmaceutical, such as Thallium or Technetium, which emits gamma radiation from a body organ which is the subject of a medical study. The gamma camera detects the radiation and generates data indicative of the position and energy of the radiation which is then mathematically processed through a procedure known as reconstruction tomography (performed by computer) to produce pictures or scintigrams (two and three dimensional) of the body organ which is the subject of the medical study.
A typical gamma camera has at least two detecting heads. Each head contains an array of photomultipliers (PMT) which are arranged behind a scintillation crystal, typically Nal, which in turn is positioned behind a lead collimator. Gamma radiation passing through the collimator strikes the crystal which emits bursts of light or scintillations received by the photomultipliers which in turn generate analog signals indicative of the intensity of the light. The PMT analog signals are grouped, digitized, corrected and processed as data indicative of position, x,y, and intensity, z.
Traditionally, PET scanners are different from gamma cameras. In PET, radio nuclides, typically fluorine-18, carbon-11, nitrogen-13 and oxygen-15 are incorporated into substances such as glucose or carbon dioxide to produce radiopharmaceuticals such as FDG (Fluoro-Deoxy-Glucose) which are ingested by the patient. As the radio nuclides decay, positrons are emitted and they collide, in a very short distance, with an electron and become annihilated and converted into two photons, or gamma rays, traveling linearly in opposite directions to one another with each ray having an energy of 511 Kev. PET scanners typically comprise, laterally spaced rings which encircle the patient. Each ring contains detectors extending thereabout. A typical detector within the ring is a BgO crystal in front of a photomultiplier tube. Each ring is thus able to discern an annihilation event occurring in a single plane. The analog PMT signals are analyzed by coincidence detection circuits to detect coincident or simultaneous signals generated by PMT's on opposite sides of the patient, i.e., opposed detectors on the ring. Specifically, when two opposed detectors detect simultaneous 511 Kev events, a line passing through both detectors establishes a line of response (LOR). By processing a number of LORs indicative of annihilation events an image is reconstructed of the organ using computed tomographic techniques.
Although the literature oftentimes refers to one machine for performing PET and SPECT studies, the radiation events are different resulting in commercially different mechanisms for performing the studies. The PET scanner ring, while one dimensional, forms a complete solid angle about the patient and captures all events in a 2-D slice which can be stacked in accordance with normal tomographic techniques to produce a 3-D image. The gamma camera, while having a 2-D detector head giving it a power factor of one over the scanner ring, can not collect all the positron annihilation events about the patient and in fact, experiences an inherently low count rate which is significantly less than that achieved in a scanner ring. If the gamma camera retains its lead septa collimator (necessary for SPECT studies) photons emitted from non-normal positions of the body can not be accounted for and the low count rate further decreases to the point where the gamma camera simply does not acquire sufficient counts to perform any study with any degree of resolution. For this reason, commercial scanner rings are not fitted with collimators. This particular problem is discussed at some length in U.S. Pat. No. 5,323,006 in which a gamma camera is used to perform PET studies during a mammogram study since the breast can be viewed as being essentially compressed to a two-dimensional object which can be fitted between the detector heads.
Another fundamental problem arises from the different energies of the gamma rays sensed in a SPECT study compared with the significantly higher energy of the PET gamma ray. Thallium doped sodium iodide crystals are conventionally used in gamma cameras for SPECT studies while bismuth germanate crystals are conventionally used in PET scanners. For any given crystal having a given density, crystal thickness is sized to the energy of the ray. BgO crystals are not sufficiently sensitive and are generally unacceptable for SPECT studies. Increasing the thickness of the Nal crystal for positron generated 511 Kev photons will increase the sensitivity of the crystal for PET studies but lead to degradation of the gamma camera when used for SPECT studies. See generally U.S. Pat. No. 4,675,526. A more subtle but significant problem occurs with respect to fluoresence. See U.S. Pat. No. 4,864,140. Using a Nal crystal for PET studies significantly increases the fluorescence problem. Of course this problem is present only if the camera must be used to perform both SPECT and PET studies.
Perhaps one of the most serious problems affecting the commercial feasibility of using a gamma camera to perform both SPECT and PET studies stems from the signal processing capabilities of the camera. As noted above, PET detectors must determine first if a coincident event has occurred and then must determine a line of response fo
Heipp Christ H.
Mc Croskey William K.
Vickers David S.
Epps Georgia
Hanig Richard
Nawalanic Frank J.
SMV America, Inc.
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