Spectrally Encoded Schlieren Imaging for Ionizing Radiation Detection via Modulation of Optical Properties

Dramatically improving the coincidence time resolution (CTR) in time-of-flight positron emission tomography (ToF-PET) down to 10-picosecond can bring benefits in the clinical setting, such as improved lesion detection and quantification, lower dose or scan time, or a mixture of these. As detection of individual 511 keV photons is required, however, an extremely sensitive and fast detection mechanism is necessary. To achieve such a CTR, the mechanism of optical property modulation was proposed as a possible alternative to scintillation to detect the fast transient charge tracks induced by ionizing radiation. Here, we continue to explore more sensitive setups, this time based on the ‘Schlieren imaging’ technique, which measures the optical contrast of a signal against background noise. We chirped a pulsed probe laser beam to accommodate asynchronous arrival times of the ionizing radiation while maintaining high timing precision. A single-photon avalanche diode (SPAD) array provided both spatial localization of the laser probe beam over the ionization-induced charge tracks, and spectral encoding of the time resolution for ultrafast timing. In this paper, we report a 1.43% shift of the mean value in the distribution of the transmitted optical signal with and without Tl-204 beta source irradiation, indicating small but measurable ionization-induced optical property modulations. We selected a time frame when a radiation source was present that had the maximum counts among the 1.1e6 frames collected, which we induce is from single ionizing beta particle detection with this method. In summary, this novel research direction of optical property modulation detection seems promising as we head towards our goal of 10 ps CTR for ToF-PET, and we will continue to improve the sensitivity our setup with lower background, and spectral encoding of timing information.