Color CT, literally

Spectral CT has long been described as "color CT" figuratively, with energy bands of invisible x-rays analogous to the wavelength bands of visible light. Here, we propose to take the idea of "color CT" concept literally: what would happen if the photodiodes used in energy-integrating detectors were color sensitive, just like subpixels in consumer cameras are color sensitive? This idea at first seems to yield no tangible benefit, but we hypothesize that spectral optical imaging can be used to measure spectral X-ray imaging. Using information from publicly available datasheets, we modeled the optical transport of gadolinium oxysulfide (GOS), the most common scintillator used today. GOS emits majority green but minority red light, and the green light preferentially self-absorbed. This explains the characteristic yellow color of raw GOS crystal, and leads us to predict that low energy X-rays, which deposit energy at shallower depths, will produce relatively more red photons that reach the photodiode. This effect is weak but occurs after X-ray quanta have been amplified (roughly 1000-fold) into visible light quanta, so it could be detected with appropriately sensitive digitization electronics. We compare the proposed color detector to conventional dual-layer detectors, and find that the spectral separation is worse primarily because of the GOS K-edge. While its spectral performance would be poor, it could be made widely available because it requires changes in the silicon circuitry only. It is possible that color photodiodes would one day replace monochromatic diodes in energy-integrating detectors, making spectral imaging standard among new scanners.