LAUSR

Beta-ray imaging systems are widely used for various biological objects to obtain a 2-dimensional distribution of β-ray emitting radioisotopes. However, a conventional β-ray imaging system is unsuitable for multiple-tracer imaging, because the continuous energy distribution of β-rays complicates distinguishing among different tracers by energy information. Therefore, we developed a new type of β-ray imaging system, which is useful for multiple tracers by detecting coincidence γ-rays with β-rays, and evaluated its imaging performance. Our system is composed of position-sensitive β-ray and γ-ray detectors. The former is a 35×35×1-mm3 Ce-Doped((La, Gd)2 Si2 O7 ) (La-GPS) scintillation detector, which has a 300-µm pitch of pixels. The latter is a 43×43×16-mm3 bismuth germanium oxide (BGO) scintillation detector. Both detectors are mounted on a flexible frame and placed in a user-selectable position. We experimentally evaluated the performance of the β-ray detector and the γ-ray efficiencies of the γ-ray detector with different energies, positions and distances. We also conducted point sources and phantom measurements with dual isotopes to evaluate the system performance of multiple-tracer imaging. For the β-ray detector, the β-ray detection efficiencies for 45 Ca (245-keV maximum energy) and 90 Sr/90 Y (545-keV and 2280-keV maximum energy) were 14.3% and 21.9%, respectively. The total γ-ray detection efficiency of the γ-ray detector for all γ-rays from 22 Na (511-keV annihilation γ-rays and a 1275-keV γ-ray) in the center position with a detector distance of 20 mm was 17.5%. From a point-source measurement using 22 Na and 90 Sr/90 Y, we successfully extracted the position of a positron-γ emitter 22 Na. Furthermore, for a phantom experiment using 45 Ca and 18 F or 18 F and 22 Na, we successfully extracted the distribution of the second tracer using the annihilation γ-ray or de-excitation γ-ray coincidence. In all the imaging experiments, the event counts of the extracted images were consistent with the counts estimated by the measured γ-ray efficiencies. We successfully demonstrated the feasibility of our β-ray autoradiography system for imaging multiple isotopes. Since our system can identify not only a β-γ emitter but also a positron emitter using the coincidence detection of annihilation γ-rays, it is useful for PET tracers and various new applications that are otherwise impractical.