Ph.D. degree thesis
accepted by National Centre for Nuclear Research, Poland in 2012

Tomasz Szczesniak

Supervisor: prof. Marek Moszynski

Optimization of Detectors for Time-of-Flight Positron Emission Tomography

 Keywords:

• Photon detectors for UV, visible and IR photons (vacuum) (photomultipliers, HPDs, others)
• Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators)
• Photon detectors for UV, visible and IR photons (solid-state) (PIN diodes, APDs, Si-PMTs, G-APDs, CCDs, EBCCDs, EMCCDs etc)
• Gamma camera, SPECT, PET PET/CT, coronary CT angiography (CTA)

 Abstract:
The aim of this thesis is to understand the time resolution limitations of scintillation detectors proposed for Time of Flight Positron Emission Tomography (TOF PET). This goal is achieved by an analysis of the fundamental properties of the scintillation detector, such as the photomultiplier's quantum efficiency and time jitter or the scintillator's light output and decay time constant. The first part of the thesis presents the most important aspects of positron emission tomography (PET) as well as possible improvements due to incorporation of the time of flight (TOF) technique. Scintillation detectors are also described in detail, including their general properties and time resolution capabilities, along with a theoretical description of timing with photomultiplier (PMT) systems. In the second part, the experimental data obtained by the author from a wide set of scintillation detectors are presented. These results, mostly dealing with timing resolution, are discussed in terms of the properties and dependencies introduced in the first part of the thesis.
At the beginning of the experimental part, the limits of achievable time resolution with an LSO scintillator and fast, 2-inch diameter XP20D0 photomultiplier are presented. These data prove that the idea of a TOF PET system with detector modules based on the LSO crystal is highly realistic. Next, time resolution optimization of the LSO-based detector is shown and discussed with regard to the Hyman theory of timing. An analysis of the most important parameters affecting the time resolution of a detector consisting of a XP20D0 phototube and LSO scintillator is further extended to a wide set of photomultipliers, including not only fast, timing devices but also slow, general purpose tubes and multi-channel PMTs. A summary of all the gathered data allows for a conclusion about the general dependency of time resolution on the time jitter of the photomultiplier used. This linear dependency is shown for many types of photomultipliers in a plot of normalized time resolution (normalized to a photoelectron number and excess noise factor) as a function of the time jitter in the center of a photocathode. The plot is later extended to other types of photodetectors, such as silicon photomultipliers (SiPMs).
The thesis also deals with alternative concepts of PET block detector configuration. One of the studied ideas is a detector where one monolithic scintillator, read by multi-anode PMT, is used instead of many pixelated crystals. The time resolution of such a design is presented and compared with data obtained with the use of the classic XP20D0 PMT and small LSO samples. Another tested improvement of the PET block detector is the application of LSO crystals co-doped with calcium. The timing characterization of such LSO samples with different co-doping percentages, ranging from 0 to 0.4 atomic % of Ca added to the starting raw material, is presented and discussed with respect to a high quality LSO sample without co-doping. The results show that calcium added to the LSO composition considerably improves its scintillating and timing properties.
Finally, the timing properties of a scintillation detector with a light readout by means of a silicon photomultiplier are presented and discussed. The results of the timing measurements made with LSO and LFS scintillators and 3x3 mm2 SiPM are compared with the data obtained with the use of the PMTs, presented in the previous chapters.