Sensitivity improvement in 44Sc whole gamma imaging: simulation study

We have proposed a new concept of whole gamma imaging (WGI), which is a combined Compton-PET system by inserting a scattering detector ring inside a PET detector ring. In addition to positron emitters, WGI can visualize single gamma emitters based on the Compton imaging method. Besides, for triple-gamma emitters such as
⁴⁴
Sc that emits a pair of 511 keV photons and a 1157 keV single gamma-ray almost at the same time, a direct imaging method would be possible; the source position will be calculated as an intersection point between a surface of the Compton cone and a line-of-response. In 2017, we succeeded in developing the world's first WGI prototype. However, limited sensitivity for Compton imaging degraded the system sensitivity for triple-gamma emitters. Therefore, in this work, we simulated the WGI system to optimize the scatterer detector for higher sensitivity. Using GEANT4, we modeled the WGI geometry, in which a scatter ring (GAGG crystals, 20 cm inner diameter) was inserted into a PET ring (GSOZ crystals, 66 cm inner diameter). In order to improve the sensitivity of triplegamma imaging and reduce parallax error, we used 4-layer depth-of-interaction (DOI) detectors as the scatterer. Total thickness of the scatterer was increased from 6 mm to 18 mm. However, thick scatterer also increased detection efficiency of 511 keV photons in the scatterer. Therefore, we improved the triple-gamma detection method in order to use the scatterer detection events of 511leV for triple-gamma imaging. Events in the scatterer were identified as positron annihilation and Compton scattering of the 1157 keV gamma-ray based on their energy information. The simulation results showed that (1) the sensitivity of the WGI using the 18-mm thick GAGG scatterer with the new triple-gamma detection method was 2.7 times higher than that of our first prototype (using 6-mm thick GAGG), and (2) the use of 4-layer DOI kept the position resolution in the triple-gamma imaging.