An introduction to photon-counting detector CT (PCD CT) for radiologists

Abstract

The basic performance of photon-counting detector computed tomography (PCD CT) is superior to conventional CT (energy-integrating detector CT: EID CT) because its spatial- and contrast resolution of soft tissues is higher, and artifacts are reduced. Because the X-ray photon energy separation is better with PCD CT than conventional EID-based dual-energy CT, it has the potential to improve virtual monochromatic- and virtual non-contrast images, material decomposition including quantification of the iodine distribution, and K-edge imaging. Therefore, its clinical applicability may be increased. Although the image quality of PCD CT scans is superior to that of EID CT currently, further improvement may be possible. The introduction of iterative image reconstruction and reconstruction with deep convolutional neural networks will be useful.

Pros and Cons of Dual-Energy CT Systems: “One Does Not Fit All”

“…Photon-counting detector CT scanners are promising for imaging large patients, reducing electronic noise, and improving CT number stability at low photon flux [ 44 , 45 ]. For now, noise-reduction iterative reconstruction techniques may be used to improve image quality in large patients [ 15 ].…”

“…Photon-counting scanners are advanced spectral CT systems that have been under investigation and development for more than ten years. The first clinical CT system using this technology was launched in 2021 by Siemens Healthineers [ 45 ].…”

“…They can reduce the electronic circuit noise, beam hardening, and metal artifacts; increase iodine CNR, spectral separation, and spatial resolution; and generate greater quality VMIs. Additionally, dose efficiency is improved [ 1 , 45 , 72 , 73 , 74 ]. Labeling nanoparticles with elements that may be detected by K-edge imaging allows for molecular CT imaging, and the injection of multiple contrast agents (such as iodine and gadolinium) allow multiple-phase images at the same time, reducing the number of acquisitions and radiation doses, although further research is needed before human scanning approval [ 7 , 45 , 75 ].…”

“…Some physical challenges remain: For instance, there is substantial overlap between the different energy bins (possibly corrected) and the system must be capable of ultrafast photon processing (possible due to new developments). If the signal process is slow, an overlap of electric pulses occurs (“pulse pile-up”) and two or more photons are detected as one higher-energy photon, underestimating the count (possibly lessened by several strategies) [ 6 , 7 , 45 ].…”

Imaging in translational cancer research

“…Developments in CT detectors have included the introduction of smaller detector elements as well as so-called photon counting detectors ( Figure 1 ). In a conventional, energy-integrating detector, X-rays enter a solid-state scintillator detector, thus generating scintillation light through interaction, which hits a photodiode that converts the light into an electrical signal proportional to the sum of all detected X-ray photon energies, regardless of the individual photon energies 25 . In photon counting detectors (PCDs), however, the interaction of X-ray photons with the single layer semiconductor detector material (typically cadmium zinc telluride) produces positive and negative charges, wherein the negative charges are drawn to pixelated anodes and record individual X-ray photons, thus enabling direct conversion to an electronic signal proportional to the photons’ individual energies 26 .…”