Optimization of the WLS design for positron emission mammography and Total-Body J-PET systems

Total body positron emission tomography (PET) instruments are medical imaging devices that detect and visualize metabolic activity in the entire body. The PET scanner has a ring-shaped detector that surrounds the patient, which detects the gamma rays emitted by the tracer as it decays. Usually, these detectors are made up of scintillation crystals coupled to photodetectors that convert the light produced by the scintillation crystal into electrical signals. The Jagiellonian Positron Emission Mammograph (J-PEM) is the first J-PET prototype module based on a novel idea with a plastic scintillator and wavelength shifter (WLS). At the same time, it is a prototype module for the total-body J-PET system. J-PEM can be an effective system for the detection and diagnosis of breast cancer in its early stage by improving sensitivity. This can be achieved using superior timing properties of plastic scintillators and combined with the WLS sheets readout. In this paper, we present an application of the Geant4 program for simulating optical photon transport in the J-PEM module. We aim to study the light transport within scintillator bars and WLS sheets to optimize gamma-ray hit position resolution. We simulated a pencil beam of 511 keV photons impinging the scintillator bar at different locations. For each condition, we calculated the value of the pulse height centroid and the spread of the photon distribution. Some free parameters of the simulation, like the reflectivity and the effective attenuation length in the sheet, were determined from a comparison to experimental data. Finally, we estimate the influence of the application of WLS layer in the total-body J-PET on the scatter fraction. To optimize the performance of the J-PEM module, we compared geometry with the number of WLS strips 50 and 83. It was found, that spatial resolution was 2.7 mm and 3.5 mm FWHM for 50 and 83 WLS strips respectively. Despite the better granularity, the 83-strip WLS geometry exhibits poorer resolution due to fewer photons being transmitted to the strip, resulting in large fluctuations of signal.