Direct Time-of-Flight for Quantitative, Real-Time In-Beam PET: Concept and Feasibility Study

We extrapolate the impact of recent detector and scintillator developments, enabling sub-nanosecond coincidence timing resolution t, onto in-beam positron emission tomography (in-beam PET) for monitoring charged-hadron radiation therapy. For t <= 200 ps full width at half maximum, the information given by the time-of-flight (TOF) difference between the two opposing gamma-rays enables shift-variant (fixed dual-head), artifact-free tomograms to be utilized for in-beam PET. We present the corresponding fast, TOF-based and backprojection-free, 3D reconstruction algorithm that, coupled with a real-time data acquisition and a fast detector encoding scheme, allows the sampled beta-plus-activity to be visualized in the object during the course of the irradiation. Despite the very low statistics scenario typical of in-beam PET, real-treatment simulations show that in-beam TOF-PET enables high-precision, quantified images to be obtained in real-time, either with closed-ring or with fixed, dual-head in-beam TOF-PET systems. The latter alleviates greatly the installation of in-beam PET at radiotherapeutic sites.