LAUSR

When I started my education in neuroradiology almost 30 years ago, there was conventional x-ray, angiography, computed tomography (CT), and magnetic resonance imaging (MRI). Truth be told, I did not expect to see a new imaging modality come into routine use before I would retire. I may have been wrong. After 50 years of clinical use, CT is about to change in a way that feels to me like a completely new imaging modality has emerged. The first generation of CT scanners equipped with photon-counting detectors is currently being installed in hospitals for (high-end) routine use. This technology results in lower radiation dose, increased spatial resolution, and less artifacts. After many years without real progress in the field of CT imaging, this improvement is very much appreciated. But behold, I think there is much more to it. Imagine you are “looking” at a flower garden but can detect intensity of light only. What you get, of course, is a black and white photography. Does that black and white image depict the reality accurately? In some way, yes. But everyone will readily admit that an important dimension has been missed completely: the spectral information of light, namely the various colors of the flowers! What has this got to do with photon-counting CT? Throughout most of my professional life, I believed that CT scanners “detect” X-ray radiation; however, technically speaking, detectors in conventional CT scanners were never able to directly react to X-rays. As a workaround X-rays are received by a glass layer (i.e., scintillation crystal) which converts the X-rays to flashes of visible light. Behind the glass layer is a photodiode (comparable to what is inside digital cameras) which detects the light flashes and sums them up over a given period of time, e.g. within a few milliseconds. Thereby, the information of thousands of X-rays