Photon-counting technology, or photon-counting computed tomography (PCCT), is a form of x-ray computed tomography (CT) in which x-rays are detected using a photon-counting detector (PCD) where it can directly transform x-ray photons into electrical signals. The PCDs are better than the currently used CT detectors as they provide much higher spatial resolution with improved contrast-to-noise ratio at lower radiation dose, and with intrinsic spectral information.
PCDs do not require a separate layer to convert x-rays into light but consist of a single layer of semiconductor diodes. If an incident x-ray is absorbed in the semiconductor, it generates a cloud of positive and negative charges that get pulled away from each other rapidly. The moving charges generate an electrical pulse in the wires attached to the electrodes, which is registered with an electronic readout circuit.
PCDs thus convert individual x-ray photons directly into an electric signal, unlike energy-integrating detectors (EIDs) used in a conventional CT, which requires the additional step of converting photons to visible light. Fig. 1 illustrates the principles of EID and PCD technologies.
Basically, a conventional CT requires two conversion steps to turn photons into a medical image. Here, x-rays are collected by a scintillator, which uses many photons to generate an optical signal. A photodiode is then used to convert this optical signal to an electrical signal.
This new approach of image acquisition requires an entirely new detector material, a high-purity cadmium telluride (CdTe) crystal for generating a high spatial resolution.