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Examination and optimization of high resolution PET detector modules PhD Theses CECÍLIA OCSOVAINÉ STEINBACH Supervisor: Emőke Lőrincz, PhD Budapest University of Technology and Economics Department of Atomic Physics 2016. 1 Research history Positron Emission Tomography (PET) is one of the today’s most modern functional medical imaging techniques. PET applications provide information about the dispersion of the applied radiotracer within the patient’s body by detecting coincidence of the annihilation γ -photon pairs. Multicrystal scintillator matrices are used for the detection that produce UV/visible photons after absorbing the γ-photons. These photons are then typically detected by photomultiplier tubes (PMT-s) that are coupled to the scintillator matrix via a light guide layer. Spatial resolution and image quality is basically defined by the accuracy of determining the pair of crystal pins that absorbed the γ-photons. Since optical photons play the key role in this phase of detection, image resolution can be improved by using optical solutions that influence positively the propagation and detection of photons. To reduce cost and time requirements of the research, there is an increased demand for reliable simulation models that are capable to model correctly the optical parameters and processes of the system. Department Atomic Physics and Mediso Ltd. conducted joint research over many years to model and optimize light propagation within the detector module. Present doctoral thesis is created within the framework of this cooperation. Objectives Aim of my PhD work was the development and experimental validation of the optical model of the PetDetSim simulation tool used for modeling PET detector modules. I looked for the optical key parameters and their correct incorporation to get a reliable model within the limits of the given simulation environment. My aim was to measure the necessary optical parameters, within this to develop a new direct method to measure the optical scattering length of the scintillator crystals. In the literature the scattering length is determined indirectly with an error of several orders of magnitude. The quantum efficiency of PMTs given by the manufacturer needs complex recalculation before its incorporation into the simulation. I applied a new method considering the reflectance of the PMT window glass and the photocathode. Important goal of my work was the experimental validation of the model using a set of easily reproducible measurements that previously missed from the literature. The easiest way to identify the crystal pin that absorbed the γ-photon is the so-called ‘Anger-logic’ based on the calculation of the center of gravity of the PMTs’ signals. Practical goal of my work was to apply the model to work out an optical solution that is capable to linearize the position of crystal pins (the so-called ‘Anger-position’) in the Anger-image and thus improve the spatial resolution of the PET. My results are summarized in 4 thesis statements. 2 Theses 1. I improved the optical model of the Detect2000 based PetDetSim simulation tool used for modeling PET detector modules by applying chamfered crystal geometry that prevents the photons from closing inside the crystal, and considering angle dependent reflectance of the PMT’s photocathode, and I experimentally validated the PetDetSim by a set of measurements consisting of various crystal configurations. [1]-[6] 2. I worked out a new method for the recalculation of the PMTs’ quantum efficiency given by the manufacturer and for its incorporation into the model considering the reflectance of the PMT window glass and the photocathode. [1, 7] 3. For the determination of the optical scattering length in scintillator crystals, I worked out a new direct method that is less sensitive to the accuracy of the measured parameters and considers the nature of the scattering at the given wavelength. I ascertained that light scattering is negligible in LYSO crystals at the emission wavelengths of the scintillator. [8, 9] 4. I designed and built a new structured light guide that is capable to resolve peripheral, often indistinguishable crystal pins. Applying double prisms matched to the crystals nearly linearized the previously non-linear Anger positions, and reduced the uncertainty of calculated positions. [10, 11] Application of the results The model I built is used both of my colleagues and the staff of Mediso Ltd. in their research and development work. Aspects recognized in the modeling have given an important support for the research running in the Department of Atomic Physics within the framework of an FP7 project. The approach of modeling the PET detector modules from the point of view of optics is a niche. 3 Publications related to the theses [1] C. O. Steinbach, Á. Szlávecz, B. Benyó, T. Bükki, and E. Lőrincz, “Validation of Detect2000 Based PetDetSim by Simulated and Measured Light Output of Scintillator Crystal Pins for PET Detectors”, IEEE Transactions on Nuclear Science, Vol. 57, No. 5, pp. 2460–2467, October 2010. [2] Á. Szlávecz, T. Bükki, C. Steinbach and B. Benyó, “A novel model-based PET detector block simulation approach”, Biomedical Signal Processing and Control, Vol. 6, No. 1, pp. 27–33, 2011. [3] E. Lőrincz, G. Erdei, I. Péczeli, C. Steinbach, F. Ujhelyi, T. Bükki, I. Müller, “Modeling and Optimization of Scintillator Array for PET Detectors”, IEEE Transactions on Nuclear Science, Vol. 57, No. 1, pp. 48–54, February 2010. [4] Á. Szlávecz, B. Benyó, C. Steinbach, T. Bükki, “A novel model and an environment for PET detector block simulation”, Proceedings of the 7th IFAC Symposium on Modelling and Control in Biomedical Systems, Aalborg, Denmark, August 12–14, pp. 304–308, 2009. [5] E. Lőrincz, G. Erdei, I. Péczeli, C. Steinbach, F. Ujhelyi, T. Bükki, I. Müller, “Light Output Analyzes of Scintillator Crystal Pins and Array for PET Detector Modules”, IEEE Nuclear Science Symposium and Medical Imaging Conference, pp. 4868–4871, Dresden, 2008. [6] Steinbach C., Erdei G., Péczeli I., Ujhelyi F., Lőrincz E., Bükki T., “Szcintillátor kristálytűk fényhasznosításának modellezése PET detektormodulhoz”, Kvantumelektronika, P-5, 2008. [7] C. O. Steinbach, F. Ujhelyi, E. Lőrincz, “Enhanced model of quantum efficiency for the optical simulation of photodetectors”, IEEE Nuclear Science Symposium and Medical Imaging Conference, pp. 2555–2558, Anaheim, 2012. [8] C. O. Steinbach, F. Ujhelyi, E. Lőrincz, “Measuring the Optical Scattering Length of Scintillator Crystals”, IEEE Transactions on Nuclear Science, Vol. 61, No. 5, pp. 2456– 2463, 2014. [9] C. O. Steinbach, F. Ujhelyi and E. Lőrincz, “Optical Scattering Length of LYSO Scintillator Crystals”, IEEE Nuclear Science Symposium and Medical Imaging Conference, Conference Record, pp. 2353–2356, 2012. [10] C. O. Steinbach, G. Erdei, F. Ujhelyi, P. Major, E. Lőrincz, “Optimized Light Sharing Module for High Resolution PET Block Detectors”, IEEE Transactions on Nuclear Science, IEEE Transactions on Nuclear Science, Vol. 59, No. 3, pp. 507–512, 2012. [11] C. O. Steinbach, G. Erdei, I. Péczeli, F. Ujhelyi, T. Bükki and E. Lőrincz “Optimized Light Sharing Module for PET Block Detectors”, IEEE Nuclear Science Symposium and Medical Imaging Conference, Conference Record, pp. 2817–2820, 2009. 4