# Synchrotron X-ray Radiation (SXR) in Medical Imaging: Current Status and Future Prospects
## 4 Future Direction
### Photon-Counting Detector (PCD)  
The potential of SXR imaging can further be realized by utilizing photon-counting detectors (PCD) in place of energy integrating detectors (EID). EIDs integrate the whole electrical current generated by the radiation sensor and ignore the charge amplitude of each individual photon detection events. The charge amplitude of each photon is proportional to the energy of the detected photon. During this integration, the summation and measurement of detector leakage current and charges produced by X-ray detection led to no information about the energy of each photon [43].  
The chemical structure, mass density of the object, and the energy of the X-ray photons are factors that affect the X-ray linear attenuation coefficients. As a result, the transmitted X-ray spectra transfer information about different tissue. However, the EIDs measure the energy-integrated signals of X-ray photons, consequently losing all of the energy-dependent information. Moreover, EIDs add electronic, swank noise, and weight lower energy photons less, which leads to larger contrast between tissues than higher energy photons, thus increasing noise and decrease contrast [44].  
On the other hand, when X-ray photons hit the PCD which is composed of semiconductor material with two electrodes (cathode and anode) and application-specific integrated circuits (ASICs), the photons will convert to the electrical charges directly. PCD can count the number of photons above a specified energy level. The charge produced is proportional to the incident energy of the X-ray photons. To remove electronic noise (including Fano noise), which can be interpreted by a PCD as a photon with energy located at the lower end of a typical X-ray spectrum, several energy thresholds can be set. Each detected photon is assigned to one of the energy windows determined by the threshold [45].  
The energy counting capabilities of PCDs are widely used in nuclear medicine. However, in X-ray-based imaging, the use is limited due to its incapability to record a higher number of photons [46; 47]. With the potential clinical benefits, higher atomic number detector materials and high-spatial-resolution fast ASICs are being developed. Cadmium telluride (CTd), or cadmium zinc telluride (CZT), is the most commonly used semiconductor for PCD along with silicon (Si) and gallium arsenide (GaAs) [48; 49; 50].  
Since, SXR can provide monochromatic X-rays, using suitable PCDs, the scatter information within the object could be estimated, as in the case of nuclear medicine and can be utilized to increase the soft-tissue contrast and spatial resolution of the image [51; 52]. However, this will require the development of ultrafast ASICs (nanosecond peaking time) along with the detector materials as the incoming SXR flux is several magnitudes higher than that of a conventional X-ray source. This is also to be noted that because of the small pixel pitches of the PCDs, inter-crystal scattering during photon detection process reduces spatial resolution. For SXR-based application using PCD, the actual impact on image blurring arising from this phenomenon is yet to be investigated.