Photon-counting computed tomography

Photon-counting computed tomography (PCCT) is an emerging technology (c.2021) in CT that could represent the next major technological milestone in the field. Briefly, photon-counting CT uses energy-resolving detectors, thereby enabling scanning at multiple energies.

Physics

Current clinical CT systems rely on energy-integrating detectors (EID), which measure the total x-ray energy reaching the detector during the measurement period. The photon-counting detectors (PCD) in a photon-counting CT system count the exact number of incoming x-ray photons and also measure their energy individually. As a consequence, photon-counting detectors always obtain spectral information and can effectively filter out electronic noise unlike energy-integrating detectors, resulting in significantly improved signal-to-noise ratio ^1,2. 

Practical points

Photon-counting CT readily differentiates between tissue types and contrast agents much like spectral CT. In the future, photon-counting CT could offer higher signal-to-noise ratio, better spatial resolution, superior virtual non-contrast imaging, and spectral imaging data much like dual-energy CT currently does. It could reduce radiation exposure, reduce the amount of contrast agent needed, and lower the amount of CT artifacts. It could also make simultaneous imaging with multiple contrast agents (e.g. iodine, gadolinium, or gold nanoparticles) feasible. Unlike conventional CT, photon-counting CT can readily measure the exact concentration of materials within the voxel (e.g. calcium, iodine), resulting in improved accuracy of studies such as perfusion imaging, kidney stone characterization, or bone density measurements ². 

Initial technical challenges were primarily posed by cross-talk between the detector elements and the extremely fast detector readout required to separately count each incident x-ray photon ^1,2. The first commercial photon-counting CT device was given US Food & Drug Administration (FDA) approval in 2021 ³. Early clinical results demonstrate a substantial improvement in spatial resolution and reduction of noise compared to the existing state-of-the-art CT systems ⁴.