Experimental Assessment on Data Sufficiency Condition for WGI Compton Imaging

We developed a whole gamma imaging (WGI) prototype composed of a scatterer ring insert and an absorber ring. It was the world’s first realization of a full-ring Compton camera geometry. In this study, we focused on the data sufficiency condition (DSC) for Compton image reconstruction. Contrary to PET which requires a full-ring geometry to satisfy the DSC, Compton imaging does not require a full ring theoretically. Reducing the detector-ring coverages can improve flexibility of the geometry design and reduce manufacturing costs although sensitivity is decreased. However, the DSC for reconstruction is not guaranteed in a practical situation due to actual absorber ring arrangement and detection limits in scattering angle. In this study, we experimentally assessed the DSC for the angular coverages. We evaluated image artifacts on reconstructed Compton images because, in general, images distort and exhibit artifacts when the DSC is not satisfied, degrading quantitative accuracy. We measured a cylindrical phantom filled with 89Zr solution of 10.2 MBq by the WGI prototype for 60 min and extracted 909-keV Compton events. We selected events related to the detector blocks marked as available to change detector angular coverages defined for the phantom bottom point. First, we changed the number of detectors in the scatterer ring, keeping the full-ring absorber. Next, we changed the absorber ring angular coverage by changing the number of detectors, keeping the scatterer ring at the number where the angular coverage just exceeded 180°. As a result, when the angular coverage of the scatterer detector was less than 180°, reconstructed images exhibited artifacts. The absorber ring detector with less than 180° coverage showed artifacts when used with the scatterer detector with more than 180° coverage. We concluded that angular coverages of more than 180° for both the scatterer and absorber detectors against any region on a subject are the DSC for Compton image reconstruction.
IEEE NSS MIC 2021