Prefilter collimator for PET gamma camera

• US 5,751,000 A
• Filed: 01/08/1997
• Issued: 05/12/1998
• Est. Priority Date: 01/08/1997
• Status: Expired due to Term

• Current Assignee: SMV America Incorporated
• Original Assignee: SMV America Incorporated
• Inventors: Vickers, David S., McCroskey, William K.
• Primary Examiner(s): Tokar, Michael J.
• Assistant Examiner(s): HANIG, RICHARD E
• Application Number:
  US08/780,642
• Time in Patent Office: 489 Days
• Field of Search: 250/363.03, 250/363.04, 250/369, 250/363.02, 378/147, 378/149
• US Class Current: 250/363.03
• CPC Class Codes: A61B 6/037 Emission tomography
• CPC Class Codes: A61B 6/4258 for detecting non x-ray radiation, e.g. gamma radiation A61B6/037 takes precedence
• CPC Class Codes: G01T 1/1642 using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
• CPC Class Codes: G01T 1/1648 Ancillary equipment for scintillation cameras, e.g. reference markers, devices for removing motion artifacts, calibration devices adapted for flow studies G01T1/1647

Abstract

A gamma camera is equipped with a graded filter plate in lieu of a collimator so that the gamma camera can perform PET studies. The filter plate prevents detection of Compton scattered, positron annihilation photons produced in the patient while permitting detection of photons which have experienced Compton scattering within the camera'"'"'s scintillation crystal to significantly increase the camera'"'"'s usable count rate. Additionally, the filter is fitted with a protruding baffle shielding the camera from stray 511 Kev radiation.

Claims

1. A gamma camera for performing PET studies in which a patient has ingested or been injected with a radiopharmaceutical emits positrons, each positron undergoing an annihilation which produces a pair of photons, each photon traveling in a linear opposite direction to the other and each having a photopeak energy of about 511 Kev, some photon pairs having a photon striking a first detector head of said camera while the opposite traveling photon strikes a second detector head of said camera, said patient positioned between said first and second detector heads, said camera comprising:

a) each detector head having a scintillation crystal and a plurality of photomultipliers adjacent one another on one side of said crystal producing pulse signals indicative of the intensity of scintillations produced by said crystal when struck by said photons;

b) signal triggering means for determining when said pulse signal of any given photomultiplier becomes a triggering signal having at least a set photopeak energy;

c) coincidence detecting means for determining when any given triggering signal from one detector head occurs within a preset time of any given triggering signal from the other detector head to establish a matched pair of triggering signals indicative of a positron annihilation event;

d) processing means for correcting and refining each matched pair of triggering signals so that tomographic images can be produced therefrom; and

e) each crystal having on its side opposite that adjacent said photomultipliers, a filter extending over the area of said crystal, said filter having an atomic number Z and thickness sufficient to reduce the photopeak energy of photons resulting from positron annihilations which have experienced attenuation attributed to Compton scattering to a value not detected by said signal triggering means whereby photons having photopeak energies of about 511 Kev which experience attenuation attributed to the Compton effect within said crystal actuate said signal triggering means to increase the count of the positron annihilation events.

2. The camera of claim 1 wherein said filter substantially absorbs gamma radiation attributed to positron annihilation which has experienced attenuation attributed to the Compton effect.

3. The camera of claim 1 wherein said filter substantially absorbs about 250 Kev of photopeak energy of gamma radiation striking said filter.

4. The camera of claim 1 wherein said signal triggering means is set to detect pulse signals having photopeak energies in excess of about 250 Kev.

5. The camera of claim 1 wherein said crystal is Nal.

6. The camera of claim 1 wherein each detector head includes a body housing containing said crystal and said photomultipliers and a frame removably secured to said body housing, said frame containing said filter and having a continuous, shielding baffle circumscribing said filter adjacent the peripheral edge of said filter and protruding away from said body housing whereby said baffle is adapted to be positioned in close proximity to the imaged patient'"'"'s body organ for shielding said detector head from stray radiation.

7. The camera of claim 6 wherein said baffle is substantially lead.

8. The camera of claim 1 wherein each detector head includes a body housing containing said crystal and said photomultipliers and a frame removably secured to said body housing, said filter includes a thin copper sheet facing said crystal and a thin lead sheet glued to said copper sheet facing the patient, said copper sheet and said lead sheet anchored within a lead/antimony mounting, said mounting secured to said frame.

9. In a gamma camera having first and second detector heads between which a patient is placed, each detector head having a scintillation crystal and a plurality of photomultiplier tubes adjacent the crystal side away from said patient;

triggering means for sensing and determining which photopeak pulse signals generated by each photomultiplier are to be processed as triggering signals;

coincident detecting means for sensing when photons produced by a positron annihilation event simultaneously strike each detector head and matching their respective triggering signals into matched pairs and signal processing means for forming tomographic images from a plurality of matched triggering signals when detected by said coincident means, the improvement comprising;

each crystal having on its side facing said patient a filter adjacent thereto and extending over the area of said crystal, said filter having an effective atomic number Z and a thickness sufficient to reduce the photopeak energy of photons resulting from positron annihilations which have experienced attenuation attributed to the Compton effect to a photopeak energy not sensed by said triggering means whereby photons resulting from positron annihilation events which experience attenuation attributed to the Compton effect within the crystal are detected as positron annihilation events by said triggering signal means.

10. The improvement of claim 9 wherein said filter substantially absorbs about 250 Kev of photopeak energy of gamma radiation striking said filter.

11. The improvement of claim 9 wherein each detector head includes a body housing containing said crystal and said photomultipliers and a frame removably secured to said body housing, said frame containing said filter and having a continuous, shielding baffle circumscribing said filter adjacent the peripheral edge of said filter and protruding away from said body housing whereby said baffle is adapted to be positioned in close proximity to the imaged patient'"'"'s body organ shielding said detector head from stray radiation.

12. The improvement of claim 11 wherein said baffle is lead.

13. The improvement of claim 9 wherein each detector head includes a body housing containing said crystal and said photomultipliers and a frame removably secured to said body housing, said filter includes a thin copper sheet facing said crystal and a thin lead sheet glued to said copper sheet facing the patient, said copper sheet and said lead sheet anchored within a lead/antimony mounting, said mounting secured to said frame.

14. The improvement of claim 9 wherein said crystal is a thallium doped sodium iodide crystal about 1/2" thick.