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Aldawood, Saad (2017): Commissioning of a Compton camera for medical imaging. Dissertation, LMU München: Fakultät für Physik
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Abstract

The interest of using hadron-therapy in cancer treatment, particularly for tumors in the vicinity of critical organs-at-risk, is continuously growing due the ability of this treatment modality to provide high precision dose delivery. In order to fully exploit this beneficial property, it is mandatory to ensure that the well-localized dose deposition (Bragg peak) is located in the tumor volume. This calls for a precise in-vivo monitoring of the particle (proton, ion) beam stopping range. Therefore, the purpose of our project is to develop an in-vivo imaging system based on a Compton camera to verify the particle beam range by detecting (multi-MeV) prompt γ rays, generated as a result of nuclear reactions between the particle beam and biological tissue. In the context of this thesis the prototype of the LMU Compton camera was considerably improved and upgraded, and characterized both in the laboratory as well as under online conditions with particle beams at various accelerator facilities. The Compton camera consists of two main components: a scatterer (tracker), formed by a stack of six double-sided Si-strip detectors (DSSSD), and a monolithic LaBr 3 :Ce scintillation detector (5x5x3 cm 3 ), acting as absorber. The highly segmented DSSSD detectors, each with 128 strips per side (strip pitch: 0.39 mm), is processed by a compact ASIC-based electronics (1536 signal channels), while the scintillation detector is read out by a 256-fold segmented, position-sensitive multi-anode photomultiplier tube, providing energy and time information for each PMT segment. The stacked design of the LMU Compton camera scatter detector allows not only to reconstruct the incident photon origin, but it also allows to track Compton scattered electrons, thus enhancing the reconstruction efficiency compared to the conventional design. The Compton camera absorber (LaBr 3 :Ce scintillator crystal) was characterized in two different side-surface wrapping scenarios, absorptive and reflective. (Position-dependent) energy resolution and time resolution were determined for both coating scenarios, revealing the superior properties of the advanced scintillator material in case of the reflectively coated crystal, providing excellent energy (position independent: ∆E/E =3.8 % at 662 keV) and time resolution (273(6) ps FWHM). In addition, the impact of the crystal wrapping options on the scintillation light distribution was studied by extracting the Light Spread Function (LSF) from the crystal irradiation with a collimated 137 Cs source. Here, as can be expected, the absorptively coated crystal reveals a slightly better FWHM value of the LSF compared to the reflectively coated detector. Nevertheless, the drastic improvement of the other properties with reflective coating motivated this choice for the Compton camera absorber. The capability of the monolithic LaBr 3 :Ce scintillator to provide the γ-ray interaction position, which is a mandatory prerequisite for the targeted photon source reconstruction based on Compton scattering, was determined by applying two specific algorithms (’k-nearest neighbor’(k-NN) and ’Categorical Average Pattern’ (CAP)). These algorithms require a large reference data base of 2D scintillation light amplitude distributions, acquired by perpendicularly irradiating the scintillator front surface with a tightly (1 mm diameter) collimated photon source on a fine grid (0.5 mm step size). Two γ-ray sources, 137 Cs and 60 Co, were used to generate the required reference libraries in order to study the energy-dependent spatial resolution of the LaBr 3 :Ce scintillator. Systematic parameter studies were performed as a function of the photon energy, PMT granularity, irradiation grid size and number of photopeak events acquired in each of the 10 4 irradiation positions. Optimum values for the spatial resolution were achieved with 4.8(1) mm (FWHM) at 662 keV and 3.7(1) mm (FWHM) at 1.3 MeV using the CAP algorithm,thus almost reaching the final design goal of 3 mm envisaged for the prompt-γ energy region of 4-6 MeV. With the observed trend of improving spatial resolution with increasing photon energy, it will be interesting to study this property beyond the realm of γ-ray calibration sources in the higher energy region beyond 4 MeV, provided the availablility of an intense, monoenergetic and collimated photon beam. Furthermore, the Compton camera has been commissioned at different particle beam facilities. The camera components were first calibrated and characterized with monoenergetic 4.44 MeV γ rays generated via the nuclear 15 N(p,αγ) 12 C ∗ reaction at the Helmholtz-Zentrum Dresden Rossendorf (HZDR). The response of both the scatter and absorber detectors was found in good agreement with Monte-Carlo simulations. Moreover, the time-of-flight (TOF) measurement capability of the absorbing scintillator was studied at the Garching Tandem accelerator, using a 20 MeV pulsed (400 ns) deuteron beam hitting a water phantom, showing prompt γ rays well separated from the slower neutron background. The camera was finally commissioned with different clinical proton beams (100 MeV, 160 MeV and 225 MeV) at the research area of the Universitäts Protonen Therapie Dresden, stopping either in a water or a PMMA phantom. Energy spectra were acquired and separated into their prompt and delayed components, extracting the prompt photon contribution via TOF. The Compton electron energy deposit in each DSSSD layer was determined and found in very good agreement with simulation expectations. Hit multiplicities and the correlated electron tracking capability of the scatter/tracker array were investigated and limitations imposed by the present ASIC-based readout electronics, as well as options for further improvements, were identified.