Concrete realization of the whole gamma imaging concept

Abstract

PET is recognized as a successful method to pursue cancer diagnosis and molecular imaging. However, in order to meet emerging demands for widened imaging applications such as textbfin-situ real-time single-cell tracking, we need to break through the principle of PET itself. In this paper, therefore, we propose a new concept of whole gamma imaging (WGI), which is a novel combination of PET and Compton 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. Therefore, in addition to Compton imaging (single-gamma mode), missing pairs of annihilation photons in PET, at least one of which is undetected, can be used for imaging (PET mode). Further large sensitivity gain can be expected for triple gamma emitters such as $^{\mathbf {44}}$textbfSc, that emits a pair of 511 keV photons and a 1157 keV gamma ray almost at the same time (triple-gamma mode). In principle, only a single decay would be enough to localize the source position: (1) the coincidence detection of a pair of 511keV photons locates the source position along a line-of-response (LOR), and (2) the source position is identified as one of two intersection points of the LOR with a Compton cone after measuring the 1157 keV gamma ray. Using GEANT4, we simulated an “insert geometry”, in which a scatter ring (24 x 24 array of 1 x 1 x 6 mm$^{\mathbf {3}}$ GAGG crystals, 20 cm diameter and 5 cm long,) was inserted into a PET ring (16 × 16 × 4-DOI array of 2.9 × 2.9 × 7.5 mm$^{\mathbf {3}}$ GSOZ crystals, 66 cm diameter and 22 cm long). In the single-gamma mode, spatial resolution for the 511keV source obtained by 3D OSEM was 6.2 mm FWHM (center)-3.0 mm FWHM (8 cm off-center). In the triple-gamma mode, the position distribution of a $^{\mathbf {22}}$Na point source projected on a line-of-response was 7.3 mm FWHM at the 5 cm off-center position without applying any image reconstruction. From the simulation results, we were able to develop the first prototype of the WGI system.