Whole gamma imaging (WGI) concept: simulation study of triple-gamma imaging

Objectives: PET is recognized as a successful method to pursue cancer diagnosis and molecular imaging. However, in order to meet emerging demands for super high-sensitive imaging such as in-vivo single-cell tracking, we need to break through the principle of PET itself. In this work, we propose a new concept of whole gamma imaging (WGI), which is a novel combination of PET and Compton imaging.

Methods: WGI is a concept of utilizing every detectable gamma rays for imaging. An additional detector ring, which is used as the scatterer, is inserted in a conventional PET ring so that single gamma rays can be detected by the Compton imaging method. For positron emitters, missing pairs of annihilation photons, at least one of which is undetected, can be used for imaging. In addition, further large sensitivity gain can be expected for triple gamma emitters such as ⁴⁴Sc, that emits a pair of 511 keV photons and a 1157 keV gamma almost at the same time. In theory, localization from a single decay is possible by identifying the intersection point between a coincidence line and a Compton cone. Using GEANT4, we simulated a WGI geometry, where a scatter ring (24 x 24 array of 1 x 1 x 6 mm³ GAGG, 20cm diameter and 5cm long,) was inserted into a PET ring (16 x 16 x 4-DOI array of 2.9 x 2.9 x 7.5 mm³ GSOZ, 66cm diameter and 22cm long). We simulated a ²²Na point source, which is also a triple gamma emitter (e⁺ and 1274 keV gamma).

Results: At the 5cm off-center position, the source position distribution projected on a line-of-response was 7.3mm FWHM without applying any image reconstruction.

Conclusion: Simulation results showed the feasibility of the triple-gamma WGI concept. Research Support: This work was supported by the Grant-in-Aid for Scientists Research (A) of Kakenhi (16H02641).