A new method has been invented to directly image the distribution of electron emitting isotopes at very high resolution and sensitivity, which will benefit microdosimetry studies of radiopharmaceuticals, especially beta-emitting therapeutic drugs. The method uses an ultra-thin phosphor, a monolayer of 3-μm P47 phosphor powder deposited on 3-μm clear Mylar foil, layered on radioactive objects to convert the kinetic energy of emitted electrons into light, which is then coupled to a digital light sensor by optical coupling means. The images of the fluorescent light patterns represent the isotope distributions under study. Our group at the Center for Gamma-Ray Imaging has completed a proof-of-concept prototype system using a large-area low-noise CCD detector and various large-aperture imaging lenses. When using optics at unit magnification, the system has resolved the actual annular shape of a 3-mm 100-nCi Y-90/Sr-90 β source, ostensibly a point source. From the images, the spatial resolution is measured at 60 μm, and the detection limit of this source is about 185 disintegrations.
Besides β emissions, the system is also very sensitive to other charged particles such as positrons and conversion electrons from γ emitters. When imaging samples in vitro, our system at unit magnification can provide similar spatial resolution, sensitivity, and linear response range to state-of-art digital autoradiography systems using storage phosphors. The resolution is improved further with optics at higher magnifications. Unlike autoradiography, our system is capable of imaging small animals in vivo. Our group has used the system to image in vivo the 18-F-FDG uptakes in 3 types of tumors implanted in dorsal skin chamber on 3 mice respectively. The images also showed the heterogeneity of FDG distribution inside a tumor about 5 mm in spatial dimension. Due to the high sensitivity of the system, we also demonstrated the dynamic imaging of the 18-F-FDG utake in tumor during a 1-hour period.