Methods for increasing the sensitivity of simultaneous multi-isotope positron emission tomography

Multi-isotope PET (MIP) can distinguish thebiodistributions of two simultaneously injected PET tracers, where one tracer has an isotope that is a pure positron emitter that generates two-photon coincidences, and the other emits a positron and a gamma ray in cascade yielding a triple coincidence.1 However triple coincidence detection suffers from very low sensitivity. This work presents a method to significantly enhance the sensitivity of triple coincidences for multi-isotope PET by adding an extra detector dedicated for the detection of the third prompt gamma in coincidence with the annihilation photons. We performed Monte Carlo simulations with I124, Sc-44, and F-18 isotopes, and measurements with Na-22 and Ge-68. F-18 and Ge-68 are pure positron emitters; on the other hand I124, Sc-44, and Na-22 emit 603 keV, 1156 keV and 1275 keV prompt gamma rays respectively. For the simulations, a phantom was acquired in a simulated Siemens Inveon system with 8 cm diameter, 5 cm thick detector slab of BGO placed at one end of the system to increase detection efficiency of the third gamma ray. The simulations indicate about a twofold increase in sensitivity with the extra detector added. For the measurements, we arranged two LYSO crystals coupled to Hamamatsu MMPC silicon photomultipliers (SiPM)s for the detection of 511 keV photons in coincidences and one large 8cm diameter, 2cm thick detector slab of LYSO coupled to a PMT dedicated for the detection of the 1275 keV gamma ray. The measured ratio of the triple-coincidence counts divided by the double-coincidence detection counts is 0.037±0.003, which compares well with the analytically calculated ratio of 0.042 estimated from intrinsic and geometry efficiency considerations. Furthermore, the 511 keV scintillation detectors were mounted on a linear stage that translated the detectors while acquiring double and triple coincidences counts simultaneously in order to generate one-dimensional profiles of the Na-22 and Ge-68 - oint sources. The triple-coincidence allows the distinction of Na-22 point source profile from the standard 511 keV double-coincidence profile of the Ge-68 source.