Simulation and Design Optimization of a Dual Layer Plastic Scintillator Intra-operative Probe for Radiolabelled Tumours

The design of intra-operative hand-held imaging devices that assist surgeons with the complete removal of radioactively labelled tumours is an important problem to solve in cancer surgery. A number of different designs for such devices have been proposed previously but they have not been completely successful in providing real-time images due to the difficulty in discriminating the background from the tumour radiation. Recently, a novel approach that uses two layers of ultrathin detector foils to measure the direction of the detected positrons was proposed at Arizona University. This new concept, called a directional charged particle detector, is able to detect the direction of the detected particles by measuring the position of interaction of the positron in each detection layer. Despite having a clear working principle, there are a number of issues to be addressed in the implementation of this concept in an intra-operative probe. In this work, we perform a simulation study to characterize and optimize a probe design that uses two thin plastic scintillators as detector layers and a silicon photomultiplier as a photo-detector. The impact of the scintillator thickness on the spatial resolution and sensitivity of the probe was evaluated using Monte Carlo simulations. Taking into account only the positron physics and a probe-tumour distance of 10 mm, the probe intrinsic spatial resolution was in the range of 3.5-7.5 mm. If a cover foil is added to protect the probe, the resolution is degraded to 7-10 mm. The tumour-gamma background discrimination was studied by simulating a gamma background source coming from the patient's body, and it was found to be negligible due to the thin plastic scintillators and the use of coincidence events. The design was further evaluated including the generation and transport of the optical photons, as well as the photo-detector readout. A scintillator thickness between 25 and 50 μm with a separation between the two layers of 1000 μm proved to be a good compromise regarding spatial resolution and sensitivity. With this parameter choice and without the foil cover, the intra-operative probe could reconstruct a point source with a FWHM of 10 mm, that can be improved to 5 mm when reconstructing the images via iterative methods. According to our study, the reduction of the thickness of the foil cover, the introduction of new and better plastic scintillators and the current advances in silicon photomulpliers are key points to improve the limited spatial resolution achievable with a dual layer intra-operative probe.