Simulation, Image Reconstruction and SiPM Characterisation for a Novel Endoscopic Positron Emission Tomography Detector (2017)

In the scope of the EndoTOFPET-US project, a novel multimodal device for ultrasound(US) endoscopy and positron emission tomography (PET) is being developed. The projectaims at detecting and quantifying morphologic and functional biomarkers and developingnew biomarkers for pancreas and prostate oncology. The detector system comprisesa small detector probe mounted on an ultrasound endoscope and an external detectorplate. The detection of the gamma rays is realised by scintillator crystals with SiliconPhotomultiplier (SiPM) read-out.For the characterisation of over 4000 SiPMs for the external plate, an automatisedmeasurement and data analysis procedure is established. The key properties of the SiPMslike breakdown voltage and dark count rate (DCR) are extracted. This knowledge isneeded both as a quality assurance as well as for the calibration of the detector. Thespread between minimum and maximum breakdown voltage within a SiPM array of 44is at maximum 0.43 V with a mean of 0.15 V and an RMS of 0.06 V. This assures theoptimal biasing of each SiPM at its individual operating voltage. The mean DCR amountsto 1.49 MHz with an RMS of 0.54 MHz and is thus well below the acceptable thresholdof 3 MHz. Two spare modules from the external plate are re-measured and analysedseveral years after the module assembly, revealing a potential alteration of the SiPMnoise properties over time. For the characterisation of SiPMs from dierent vendors, asoftware framework for the automatic extraction of performance parameters from pulseheightspectra, including a t of the entire spectrum, is developed and tested.In order to facilitate the modelling of the response of the EndoTOFPET-US detector,a framework is developed which is built around the Geant4-based simulation toolkitGAMOS, to simulate and reconstruct realistic imaging scenarios with this asymmetricPET detector. The simulation studies are used to compare dierent possible detectordesigns, guide the development of the custom image reconstruction software, and assessthe image quality qualitatively and quantitatively. With the help of simple simulatedphantoms, the inuence of the amount of probe rotation and acquisition time on the resultingimage quality and image resolution is investigated. It is shown that a scan time ofapproximately 10 minutes and small rotation of around 10 yields a saturation in imagequality. Furthermore, studies using full-body patient data are presented. The resultssuggest that the endoscopic approach is able to separate the prostatic lesion well from the background radiation from prostate and bladder.After the assembly of the detector system, the rst pre-clinical commissioning is presented,including data acquisition using simple phantoms as well as in-vivo examinationsof pigs. It is shown that data can be acquired and reconstructed in the freehand mode,i.e. using a hand-held and moving probe. It is demonstrated that the entire data takingand image reconstruction chain is functional, including slow control, tracking and ultrasoundacquisition and that in-vivo acquisitions with the device are possible. However,the acquired pig images do not show the desired results, and there are spatial distortionsin the reconstructed images of the phantoms, necessitating further pre-clinical trials oncethe technical commissioning phase is completed.