Continuous-spectrum Emission Tomography (CET) for quantifying the absorbed dose of in-vivo bremsstrahlung

Introduction: Personalized dosimetry based on quantitative images of radiopharmaceutical distribution in the body can substantially improve the effectiveness of radionuclide thera- py. Current nuclear medicine imaging systems are optimized for diagnostic radionuclides and suboptimal for bremsstrah- lung X- rays emitted by therapeutic radionuclides.Materials and methods: We present a novel molecular imaging modality, called CET (based on elements from CT, SPECT and PET) and specifically designed for collimating and detecting the high-energy secondary body generated bremsstrahlung X-ray photons. To efficiently detect incoming high-energy X-ray photons, conventional NaI SPECT detectors are replaced with high-density scintillators from PET. A first step in the design of the complete system is the evaluation of monolithic high- resolution detectors for this modality. LYSO, the most com- monly used PET scintillator, is intrinsically radioactive due to the presence of Lu-176. This leads to a continuous background in the energy spectrum, making this scintillator not suitable for CET. Therefore simulations (using the input of the continuous Spectrum of Y-90) and measurements of high-density thick PET BGO crystals are compared to the results of conventional NaI detectors. CET also combines these detectors with pinhole collimators (from small animal SPECT), with small accep- tance angles and minimal penetration.These are specifically designed for a wide energy spectrum and with the primary aim to reduce scatter and collimator penetration. To efficiently construct these complex collimators with high stopping power, additive manufacturing (3D printing) of tungsten powder will be used. Since no scatter windows can be used for X-rays, quantitative reconstructions will be obtained by modeling the remaining contamination in Monte Carlo-based image recon- struction. Finally Monte Carlo or other approaches will pro- vide the information on the absorbed radiation dose.Results. A BGO de