Gamma-Ray Imaging Using Coincident Detection of Cherenkov Photons for Medical Applications

A novel detection concept for high-energy gamma rays for medical applications has been tested using the coincident detection of the Cherenkov photons from Compton-electrons in an optically transparent radiator material. Such detection capability would facilitate imaging of the distribution of certain targeted alpha-emitter therapies and also could be applicable in range verification for proton therapy, which are both connected to prompt gamma emission. To prove the concept, a demonstrator experiment at energies below the MeV scale was conducted: measurements with a
²²
Na gamma source emitting two 511-keV photons were performed and the Cherenkov light from Compton and photo electrons in UV transparent polymethyl metacrylate (PMMA) was detected in coincidence. Using a
$8\times 8$
silicon photomultiplier (SiPM) array and fast FPGA-based read-out electronics, efficiency in the order of 0.1 % was achieved. A strong increase up to about 3 % for higher gamma energies up to 1.5 MeV is indicated by the Monte-Carlo simulations. The number of detected Cherenkov photons per event and per channel was counted using the time over threshold (TOT) of the SiPM signal. The measured number of photons and their distribution from accumulated events are in good agreement with simulations, and the measured patterns of photon hits indicate spatial sensitivity. The ability to detect 511-keV photons with the Cherenkov light was demonstrated successfully.