TIME OF FLIGHT MEASUREMENTS IN POSITRON EMISSION TOMOGRAPHY

• US 20090236532A1
• Filed: 07/18/2007
• Published: 09/24/2009
• Est. Priority Date: 07/28/2006
• Status: Active Grant

• Current Assignee: Koninklijke Philips N.V.
• Original Assignee: Koninklijke Philips N.V.
• Inventors: Solf, Torsten, Frach, Thomas, Thon, Andreas
• Granted Patent: US 8,164,063 B2
• Time in Patent Office: Days
• US Class Current: 250/363.40
• CPC Class Codes: G01R 33/481 MR combined with positron emission tomography [PET] or single photon emission computed tomography [SPECT]
• CPC Class Codes: G01T 1/1611 using both transmission and emission sources sequentially SPECT imaging G01T1/1642; PET imaging G01T1/2985; detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects G01V5/20
• CPC Class Codes: G01T 1/2985 In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis); using external radiation sources A61B6/02

Abstract

A positron emission tomography apparatus (100) includes a plurality of radiation sensitive detector systems (106) and selective trigger systems (120). The selective trigger systems identify detector signals resulting from detected gamma radiation (310) while disregarding spurious detector signals (310). In one implementation, the apparatus (100) includes a time to digital converter which decomposes a measurement time interval (T_max) according to a binary hierarchical decomposition of level H, where H is an integer greater than equal to one.

Claims

1. A radiation detection apparatus comprising:

a scintillator which generates scintillation photons in response to received radiation photons;

a photosensor in optical communication with the scintillator and which generates a photosensor signal in response to the scintillation photons;

a first signal detector which generates a first output signal if the photosensor signal satisfies a first signal criteria;

a second signal detector which generates a second output signal if the photosensor signal satisfies a second signal criteria;

a radiation signal detector which evaluates the first and second output signals to identify photosensor signals indicative of radiation received by the scintillator.

2. The apparatus of claim 1 wherein the first signal criteria identifies a photosensor signal indicative of temporally initial scintillation photons generated by the scintillator in response to a received radiation photon.

3. The apparatus of claim 2 wherein the second signal criteria identifies a photosensor signal indicative of temporally subsequent scintillation photons generated by the scintillator in response to the received radiation photon.

4. The apparatus of claim 1 wherein the first signal criteria includes a first level threshold and the second signal criteria includes a second level threshold, and wherein the magnitude of the second threshold is greater than the magnitude of the first threshold.

5. The apparatus of claim 1 wherein the photosensor generates dark counts and wherein the radiation signal detector rejects photosensor signals resulting from dark counts generated by the photosensor.

6. The apparatus of claim 1 wherein the radiation signal detector determines whether a second output signal is generated within an acceptance time window following the generation of a first output signal.

7. The apparatus of claim 1 further including a time measurement system operatively connected to the radiation signal detector, wherein the time measurement system generates time stamps indicative of the identified photosensor signals.

8. The apparatus of claim 7 wherein the time stamp is the time at which the photosensor output signal satisfies the first signal criteria.

9. The apparatus of claim 7 wherein the time measurement system includes pulse width reduction means for shortening a width of a measured pulse.

10. The apparatus of claim 9 further including a pulse shrinking time to digital converter which measures a width of the shortened pulse.

11. The apparatus of claim 1 further including an energy measurement system operatively connected to the radiation signal detector, wherein the energy measurement system measures the preferentially identified photosensor output signals so as to generate a signal indicative of received radiation photons.

12. The apparatus of claim 1 further including a TOF PET system.

13. A method comprising:

evaluating signals from a gamma radiation detector to identify candidate detector signals, the candidate detector signals including detected gamma radiation signals and spurious signals;

evaluating the candidate detector signals to identify detected gamma radiation signals and reject spurious signals;

generating an output indicative of at least one of a measured gamma radiation energy and a measured detection time of the identified detector signals.

14. The method of claim 13 wherein the radiation detector includes an avalanche photodiode operating in the Geiger mode and wherein the spurious signals include dark counts generated by the avalanche photodiode.

15. The method of claim 13 wherein the detected gamma radiation includes gamma photons indicative of positron annihilations.

16. The method of claim 13 wherein evaluating received signals from a gamma radiation detector includes comparing the received detector signals to a first threshold and evaluating the candidate detector signals includes comparing the received detector signals to a second threshold.

17. The method of claim 16 including:

measuring a characteristic of a candidate signal;

using the measured characteristic to adjust the second threshold.

18. The method of claim 16 wherein the first threshold has a value which corresponds to an output of the detector immediately following detection of a gamma radiation photon, wherein the first threshold value also corresponds to a detector noise output.

19. The method of claim 18 wherein the first threshold corresponds to the output of the detector in response to at least one of temporally first and second scintillation photons generated by a scintillator in response to the detected gamma radiation photon.

20. The method of claim 13 wherein evaluating the candidate detector signals includes

establishing an acceptance time window following identification of the candidate signal;

evaluating a characteristic of the detector signal during the acceptance time window.

21. The method of claim 20 wherein the acceptance time window is about 5 ns.

22. A positron emission tomography apparatus comprising:

a plurality of detector channels, wherein at least a portion of the detector channels include;

a radiation sensitive detector;

means for evaluating an output of the detector to identify candidate detector signals, the candidate detector signals including detected gamma radiation signals and spurious signals;

means for qualifying the candidate detector signals so as to identify the gamma radiation signals;

a coincidence detector operatively connected to the detector channels, wherein the coincidence detector identifies qualified detector signals indicative of positron annihilations.

23. An apparatus comprising:

a gamma radiation sensitive detector;

first means operatively connected to the radiation sensitive detector for selectively identifying and qualifying candidate detector signals indicative of gamma radiation received by the radiation sensitive detector;

pulse shrinking time to digital converter means operatively connected to the first means for generating time stamp data which identifies a detection time of the qualified detector signals.

24. A time to digital conversion method comprising:

receiving a signal to be measured;

dividing a measurement time period into a plurality of sub-periods, wherein each sub-period includes a width;

shortening a width of the signal by an amount which corresponds to the width of at least one of the sub-periods;

measuring a width of the shortened signal; and

using the amount and the measured width to determine the width of the signal.

25. The method of claim 24 wherein the measuring a width includes using a pulse shrinking time to digital converter to measure the width of the shortened signal.

26. The method of claim 24 wherein dividing includes dividing the measurement time period into 2^x sub-periods, wherein x is an integer greater than equal to 1.

27. The method of claim 24 wherein each sub-period has an equal width.

28. The method of claim 24 wherein the measurement time period is divided according to a binary hierarchical decomposition of level H, wherein H is an integer greater than or equal to one.

29. The method of claim 24 including using a delay line to shorten the width of the signal.

30. An apparatus for measuring the width of a signal, the apparatus comprising:

means for dividing a measurement time period into a plurality of sub-periods, wherein each sub-period includes a width;

means for shortening a width of the signal by an amount which corresponds to the width of at least one of the sub-periods;

means for measuring a width of the shortened signal; and

means for using the amount and the measured width to calculate the width of the signal.

31. The apparatus of claim 30 wherein the means for measuring includes a pulse shrinking time to digital converter, and wherein the pulse shrinking time to digital converter includes an asymmetric inverter.