HomeTechnological cornerPhoton-Counting Detector CT: from theory to practice

Photon-Counting Detector CT: from theory to practice

    Loewe

    CHRISTIAN LOEWE
    Division of Cardiovascular and Interventional Radiology
    Dept. of Bioimaging and Image-Guided Therapy
    Medical University Vienna


    All you need to know about PCD-CT

    Photon-counting detector (PCD) computed tomography (CT) 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 PCD-CT 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 PCD

    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. PCD-CT 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 PCD-CT may consequently assist with diagnostic tasks in pulmonary and musculoskeletal imaging. Moreover, the inherently higher spatial resolution of PCDs than of EIDs allows for low dose musculoskeletal CT imaging. Also, the detection, delineation, and characterisation of renal stone with high-resolution PCD-CT can be advantageous. Last, but not least, spatial resolution is important when imaging small bones, specifically the temporal bone.

    Improved iodine signal

    PCD-CT enables improved iodine contrast at the same tube potential as EID CT, with the additional benefits of multi-energy display and material decomposition. PCD-CT improves iodine signal thanks to the lack of the down-weighting of low-energy photons. The image contrast optimisation attainabled with PCD-CT 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, PCDs 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

    PCD-CT 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 PCD-CT, 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 PCD-CT.

    New contrast media

    The implementation of PCD-CT 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 PCD-CT 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 PCD-CT 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.