WHOLE GAMMA IMAGING: A NOVEL COMBINATION OF PET AND COMPTON IMAGING

Introduction: PET is recognized as a successful method to pursue cancer diagnosis and molecular imaging as well as range verification in particle therapy. However, in order to meet emerging demands for widened imaging applications, we may need to break through the principle of PET itself. Therefore, we proposed a new concept of whole gamma imaging (WGI), which is a novel combination of PET and Compton imaging. Methods: An additional detector ring, which is used as the scatterer, was inserted in a conventional PET ring so that single gamma rays can be detected by the Compton imaging method. In addition to a conventional PET mode, Compton imaging (single-gamma mode) is possible. Further large impact can be expected for triple gamma emitters such as 44Sc (about 4 h half-life), 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, 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. Carbon-10, which is a positron emitter to be produced through fragmentation reaction in particle therapy, is also a triple gamma emitter to be visualized by the β+-γ coincidence.Results: We developed the first prototype of the WGI system (figure 1). Table 1 summarizes the major parameters of the prototype. All interaction events were recorded as list-mode data, and event selection such as coincidence detection was done in software. We measured a 137Cs point source in the single-gamma mode and a 22Na point source with conventional coincidence detection. The 22Na point source was also used to demonstrate the triple gamma mode as it emits a 1275 keV gamma ray after a positron decay.In the single-gamma mode, spatial resolution for the 137Cs point source obtained by 3D list-mode OSEM was 13.1 mm FWHM (center) - 4.4 mm FWHM (8 cm off-center). Spatial resolution values for the 22Na point source, obtained by the absorber-absorber coincidence and the scatterer-scatterer coincidence, were almost the same (be-low 2 mm). In the triple gamma mode, where only simple backprojection was applied and no image reconstruction algorithm was applied, spatial resolution for the 22Na point source was 5.7 mm FWHM (center) - 4.8 mm FWHM (8 cm off-center).MMND-ITRO 202