Dual-radioisotope PET data acquisition and analysis

This study experimentally evaluates a novel approach for dual radioisotope PET imaging. The method relies on one radioisotope being a pure positron emitter and the other radioisotope emitting a prompt high energy gamma along with the positron. Using the differential count rates of dual and triple coincidences allows for quantitative reconstruction of the individual radioisotope activities. The objective of the present study was to perform an experimental proof-of-principle test of the method. We used two cMiCE detector modules (with 400 keV lower energy threshold) mounted directly across from one another and a third module (850 keV threshold) at 90 degrees to the first two modules. Coincidence logic was set to either monitor a coincidence between modules 1 and 2 and NOT 3, or a coincidence between all three modules. These were used to measure dual and triple coincidence count rates from ²²Na and ⁶⁸Ge point sources, scanned either separately or together. While ⁶⁸Ge was considered to be a pure positron emitter, for ²²Na, 90.4% of decays produce a positron and a 1.275 MeV prompt gamma. In addition to the count rate test, full tomographic data were collected and images reconstructed for both dual and triple coincidence data sets. Reconstructed images demonstrate the ability of the method to clearly distinguish between ²²Na and ⁶⁸Ge sources based on triple-coincidence criterium. For both dual and triple coincidence event modes, the coincidence rates for simultaneous acquisition of the ²²Na and ⁶⁸Ge sources were 10-19% higher than the sum of the coincident rates for the radioisotopes acquired individually. We speculate this is due to energy pulse-pile up and are continuing evaluations of this effect. We were able to demonstrate the basic validity of differentiating the individual activity levels of ²²Na and ⁶⁸Ge sources even when acquired at the same time. Quantitative accuracy can likely be improved using normalization methods, and evaluations of this approach are ongoing.