CHRISTIAN LOEWE
Division of Cardiovascular and Interventional Radiology
Dept. of Bioimaging and Image-Guided Therapy
Medical University Vienna
All you need to know about PCCT
Photon-counting computed tomography (PCCT) is a new CT technology that utilises a direct conversion X-ray detector, in which the energy of the incident X-ray photon is directly recorded as electronical signal.
There are several advantages to PCCT over the traditional CT scanners incorporating energy-integrating detectors (EIDs). These include improvements in spatial resolution (via smaller detector pixel design) and iodine signal (via count weighting), still permitting multi-energy imaging, electronic noise elimination, and artefact reduction (via energy thresholds).
EID versus PCCT
A. EIDs use a scintillator to generate visible light when an incident X-ray photon hits them, then the light is recorded by a photodiode with reflective septa in between detector elements to reduce crosstalk.
B. PCCT uses a semiconductor to directly generate positive and negative charges, with negative charges going to pixelated anodes to record each individual photon and its energy.
Adapted from Esquivel A, et al. Photon-Counting Detector CT: Key Points Radiologists Should Know. Korean J Radiol. 2022 Sep;23(9):854-865. doi: 10.3348/kjr.2022.0377.
Unique technical properties and their impact on CT images
| Property | Impact | |
| Direct conversion of X-ray to electronic signal proportional to photon energy | · Routine multi-energy information with a single X-ray tube voltage: virtual mono-energy images, virtual non-contrast, virtual non-calcium, iodine maps
· Increased iodine signal (no down-weighing of low energy signals) |
|
| Smaller detector pixel size | · Improved spatial resolution | · No attenuating filters that increase radiation dose in ultra-high spatial resolution imaging
· No radiation dose penalty, therefore, ultra-high can be performed on larger body regions |
| No reflective septa | · Radiation dose reduction
· Improved spatial resolution |
|
| Elimination of electronic noise | · Only quantum noise present | |
| X-ray beam shaping with tin filters, energy thresholds and tube potential selection | · Radiation dose reduction
· Reduction of metal and blooming artefacts |
|
Adapted from Esquivel A, et al. Photon-Counting Detector CT: Key Points Radiologists Should Know. Korean J Radiol. 2022 Sep;23(9):854-865. doi: 10.3348/kjr.2022.0377.
Clinical applications under investigation
| Anatomical part | Potential advantage over EID-CT |
| Head and neck imaging | · Improvement of the quality of carotid and intracranial angiography
· Staging of laryngeal and hypolaryngeal cancer |
| Temporal bone imaging | · High-resolution study of small structures such as ossicles |
| Chest imaging | · Precise assessment of nodules and the smallest pulmonary structures (e.g., terminal divisions of the bronchial tree) and the interstitium |
| Breast imaging | · Detection of small lesions and soft tissue differentiation |
| Cardiovascular imaging | · Determination of myocardial damage during infarction
· Estimation of coronary artery calcium · Coronary stent imaging · Detection of aortic stentgraft and endoleaks |
| Abdominal imaging | · Streamlined hepatic lesion detection
· Peritoneal metastasis detection |
| Musculoskeletal imaging | · Assessment of articular cartilage health and monitoring the evolution of osteoarthritis
· Evaluation of bone oedema · Detection of small metastatic lesions |
ADVANTAGES AT A GLANCE
Higher spatial resolution
Many diagnostic tasks in lung and musculoskeletal imaging require scanning large body regions and simultaneously displaying small structures. The improved spatial resolution of PCCT may consequently assist with diagnostic tasks in pulmonary and musculoskeletal imaging. Moreover, the inherently higher spatial resolution of PCCTs than of EIDs allows for low dose musculoskeletal CT imaging. Also, the detection, delineation, and characterisation of renal stone with high-resolution PCCT can be advantageous. Last, but not least, spatial resolution is important when imaging small bones, specifically the temporal bone.
Improved iodine signal
PCCT enables improved iodine contrast at the same tube potential as EID CT, with the additional benefits of multi-energy display and material decomposition. PCCT improves iodine signal thanks to the lack of the down-weighting of low-energy photons. The image contrast optimisation attainabled with PCCT has multiple potential applications in the abdomen, including improved detection and characterisation of neoplasms within parenchyma, especially so-called “low contrast” lesions, for which the CT number of the target lesion is similar to the background.
Multi-energy Imaging
Reconstructions of multi-energy CT scans are pertinent to musculoskeletal imaging and enable visualisation of gout and virtual non-calcium imaging of bone oedema. Unlike dual-source dual-energy CT systems, PCCTs do not restrict the scan field of view in multi-energy applications and extend the benefits of multi-energy CT imaging to large patients.
Radiation dose reduction
PCCT implementation is particularly useful to paediatric patients. The high spatial resolution and excellent contrast-to-noise ratio improve the visibility of anatomic structures in smaller patients together with increased dose efficiency, which facilitates a further dose reduction. Using high-resolution PCCT, radiation dose can be decreased by 20%–30% without sacrificing image quality. In particular, ultra-low dose chest CT is an ideal application for patients that require repeated imaging studies from a young age, e.g., individuals with cystic fibrosis.
Artefact reduction
The reduction of common image artefacts, including but not limited to streaks, beam hardening, metal, and calcium blooming can be obtained with PCCT.
New contrast media
The implementation of PCCT is creating new opportunities for the development of novel contrast media to be used alone or in combination with the existing media. The new contrast media will have improved spectral differentiation in X-ray attenuation allowing for more flexibility in imaging protocols, lower radiation doses, and further opportunities for material decomposition.
LIMITATIONS
The limitations of PCCT include limited availability, high cost, and challenges in handling high photon flux and spectral distortions. Moreover, more real-world evidence is needed to understand fully the benefits of the PCCT technology.
References
- Esquivel A, et al. Photon-Counting Detector CT: Key Points Radiologists Should Know. Korean J Radiol. 2022 Sep;23(9):854-865. doi: 10.3348/kjr.2022.0377.
- Jost G, et al. New Contrast Media for K-Edge Imaging with Photon-Counting Detector CT. Invest Radiol. 2023 Jul 1;58(7):515-522. doi: 10.1097/RLI.0000000000000978.