The Influence of Crystal Configuration and PMT on PET Time-of-Flight Resolution

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1 The Influence of Crystal Configuration and PMT on PET Time-of-Flight Resolution Christopher Thompson Montreal Neurological Institute and Scanwell Systems, Montreal, Canada Jason Hancock Cross Cancer Institute, Edmonton, Canada

2 Introduction: Timing alignment in PET In order to detect positron annihilations the detectors of a PET scanner must detect two coincident gamma rays This requires that gamma rays which are detected at the same time, are reported by the detectors at the same time Different cable lengths, electronic variations, and stability require that the detectors be calibrated by a timing offset.

3 Motivation for Better Timing Alignment A narrower time window improves PET images. Lower random counts. In dynamic PET studies this provides better image quality in early time frames. In whole-body PET scans this permits higher doses to be used providing better image quality with short scan times implying faster patient throughput. Substantial timing improvement would allow the use of time-of-flight information Moses W W: Time of flight in PET revisited (IEEE TNS Oct. 2003)

4 What is Timing Alignment? Timing window shown in blue

5 Why can t the resolving time be made shorter? In order to reduce the resolving time, the detectors must be very well aligned. It is a long, iterative process to perform the timing alignment. The timing response may drift over time. It is better to have noise due to increased random counts than due to poor timing alignment.

6 Positron decay A positron escapes from the parent nucleus The mass difference between the nuclei is given up as kinetic energy randomly distributed between the positron and nucleus The positron must lose its kinetic before it combines with an electron then annihilates. This must precede the detection of the gamma rays by the detectors of the PET scanner. If the energy is lost in a plastic scintillator, the time of the positron decay can be detected!!

Timing Using Positron Detection Central source emits positrons which deposit energy in plastic scintillator. Light flash detected by PMT recorded by timing circuit.

7 Timing Using Positron Detection Central source emits positrons which deposit energy in plastic scintillator. Light flash detected by PMT recorded by timing circuit. Central positron source Gamma ray reaches any PET detector. Time difference measured. Time-difference histogram made for all (12,000!) crystals simultaneously. Time offset for each crystal is the peak of timing histogram. Timing Circuit

8 Central Time Alignment Probe Timing probe used in these studies. The connectors on the left provide V PMT bias and connect the timing anode signal from the PMT

9 Equipment and Properties Canberra 2145 Time-to-Amplitude Converter Ortec CF8000 Oct Constant Fraction Discriminator Walk: <±250 ps Fraction: 40% Internal delay jumpers Canberra 454 Quad Constant Fraction Discriminator Walk: <±100 ps (typically < 30 ps) Fraction: 20% External Delay (with 0.6 ns internal delay)

10 Experiment Setup γ-ray s time-of-flight to PET detector Scanwell PET timing alignment probe PET block detector undergoing tests MCA Σ Σ Start NIM crate Stop CFD Fast/slow Amp X4 Amp Sum Amp TAC CAMAC crate ADC sequencer ADC SCSI DEC Alpha workstation

11 Time arrival delays for crystals in HiRez Detector Arrival time ns ± ns Resolution (FWHM) 1.25 ns ± ns

12 Time arrival delays for crystals in HiRez Detector ~250 ps per frame

13 Time arrival delays for crystals in Focus Detector Arrival time 1.02 ns ± ns Resolution (FWHM) 1.21 ns ± ns

14 Time delays in Focus Detector ~250 ps per frame

Cause of arrival time delays in a PS-PMT The six anodes in the PS-PMT are interconnected by 1kΩ resistors and there is a small but significant (10-20 pf) capacitance between the anodes.

The central anodes provide slower, delayed signal to each end, so the apparent arrival time is later from central crystals when their sum signal is processed by the CFD.

15 Cause of arrival time delays in a PS-PMT The six anodes in the PS-PMT are interconnected by 1kΩ resistors and there is a small but significant (10-20 pf) capacitance between the anodes. As a result, a signal on anode 1 or 6 will have a fast, big signal at one end of the chain, and a slow, small signal at the other end. The central anodes provide slower, delayed signal to each end, so the apparent arrival time is later from central crystals when their sum signal is processed by the CFD. This effect could be eliminated by taking the timing signal from the last dynode instead of the sum of the anode signals Anodes in a PS-PMT connected with 1 kω resistors Shape of the sum signal from both ends of the anode chain when the γ-ray interacts with a crystal near each anode Measured delays for each crystal of Focus detector

time at the anode nearest the crystal of interaction, but appear attenuated and delayed and

16 Signals from anodes in PS-PMT The anode wires are connected with 1kΩ resistors. If they were displayed individually on an oscilloscope the signals would show a fast rise time at the anode nearest the crystal of interaction, but appear attenuated and delayed and integrated by the inter-anode capacitance at the other anodes. Response to input on Anode #3 shown for 100 nsec.

17 Cause of arrival time delays in PET detectors The four PMTs, or one quad-pmt, in a conventional PET detector do not have the same gain. To provide consistent outputs, the anode voltage is reduced on high gain PMTs, however this increases the transit time in that PMT so delays the signal at the anode. This effect is reduced by matching the PMTs gains during assembly of the block detectors, but gain adjustments are still required. t cathode anode = 2D ( n + 1) d d 2eV mn ( d + 1) D d N d = distance between dynodes = number of dynodes V = voltage across PMT e,m = charge and mass of electron The transit time from photo-cathode to anode of electrons in the Photonis Xp-1452 quad PMT used in the GE PET/CT scanner as a function of PMT anode voltage. The apparent arrival times of γ-rays in crystals in a PET detector. The PMT in the near corner has a higher gain so its anode voltage is reduced resulting in the slower electron transit time.

18 Acknowledgements NSERC (PET research grant to Chris Thompson) Sara St. James (Former student) Lissa Zyromski (IPL: Implantation of radio-isotopes) Frank Wilkinson (Alpha Spectra Inc: Scintillator) Steve Kecani (McGill Physics workshop: Machining) US Patent #7,247,844: (July-2007) An instrument and method to improve the timing alignment of a PET scanner C J Thompson and M-L Camborde

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