Micro-machined retro-reflector for improving light yield in ultra-high resolution gamma cameras

High-resolution imaging with X-rays and gamma rays can be achieved through scintillation crystals that are optically coupled to Charge Coupled Devices (CCDs). The energy and the interaction position of individual gamma quanta can be estimated by real time image analysis of scintillation light flashes ("photon-counting mode"), provided that the images of the CCD can be read out fast enough. The back-illuminated electron-multiplying CCD (BI-EMCCD) is well-suited for fast read-out, since even at high frame rates it has extremely low read-out noise thanks to an internal gain mechanism. BI-EMCCDs can achieve a quantum efficiency of over 90% for the detection of photons in the range of 500 to 650 nm. Here we investigate a gamma camera based on a back-illuminated EMCCD, coupled to a 1.2-millimeter-thick continuous CdWO4 crystal by means of a fiber-optic window. In order to improve the performance of our gamma camera, we have optimized the optical coating of the scintillation crystal. We observe enhancement of the optical output of the crystal by application of reflective optical coatings (i.e. retro-reflective and mirror-reflective) when compared to absorptive coating or no coating on top of the crystal (the side of the crystal that is not read out by the EMCCD). The retro-reflective coating has been designed and micro-machined in-house for this purpose specifically. Applying our set-up to Tc-99 m imaging, we found that use of this retro-reflector improves the intrinsic spatial resolution (Full Width at Half Maximum) by about 17%, allowing us to obtain a resolution of 86 mum, compared to 104 mum when using a non-coated crystal. Therefore, the micro-machined retro-reflector is the optimal coating for our application. We conclude that the use of our enhanced CCD-based gamma cameras offers great potential for applications such as in vivo imaging of gamma emitters.