Development of a Novel Type Compton Camera: Double-scattering Compton Imager (DOCI)

Abstract

In the present study, a novel type Compton camera, named Double-scattering Compton Imager (DOCI), was developed by using two high-spatial-resolution position-sensitive semiconductor detectors as scatterer detectors and a scintillation detector as absorber detector. The technical rational behind the DOCI is to improve the imaging resolution by accurately determining the trajectory of a photon between two position-sensitive detectors by using two thin, high-spatial-resolution scatterer detectors. To demonstrate this idea, two identical double-sided silicon strip detectors (DSSDs), which are 50 × 50 × 1.5 mm3 in size and have a spatial resolution of 3 × 3 × 1.5 mm3, were placed with a separation of 10 cm to determine the track of the scattered photon between these two detectors. A NaI(Tl) scintillation detector, whose size of 3 in. (D) × 3 in. (H), was right behind the second scatterer detector with 2 cm separation to determine the energy of the double-scattered photon for the energy gating, which is used to reject random-coincidence events. A point-like (1-mm diameter) gamma source, a positron emitter emitting 511 keV annihilation photons, was placed at 3.6 cm in front of the Compton camera. Among a total of 9,616 coincidence events, 354 effective events were selected by applying the energy gate of 511±50 keV. Using the measured effective events, the Compton image was reconstructed by using the list-mode expectation maximization (LM-EM) algorithm. The imaging resolution of the DOCI, which was evaluated in terms of full width at half-maximum (FWHM), was about 4 mm FWHM for the point-like gamma source. In the current version of the DOCI, the energy resolution of the first scatterer detector is the limiting parameter for further improving the imaging resolution of the Compton camera, which issue is under investigation in our group.