Sharper, Smarter, Safer: Unpacking the Potential of Photon-Counting CT in Urolithiasis Imaging

See also the article by Huflage et al in this issue.

Dr Nariman Nezami is an associate professor of Vascular and Interventional Radiology (Georgetown University Medical Center and the Lombardi Comprehensive Cancer Center). His focus is interventional oncology and locoregional immune-oncologic therapies for primary and metastatic tumors in the liver and lung.

Dr Ashkan Malayeri is a staff clinician and chief of the body imaging section in the Department of Radiology and Imaging Sciences (National Institutes of Health; Bethesda, Md). He is the recipient of grant funding from the RSNA Research and Education Foundation and the NIH Research Award for Staff Clinicians Program. His research interests focus on optimizing MRI and CT techniques for clinical problem solving.

Photon-counting detector (PCD) CT is a groundbreaking advancement in medical imaging and may soon eclipse the energy-integrating detector (EID) CT across clinical applications (1). This optimistic outlook stems from the superior performance of PCD CT compared with conventional EID CT. However, there is a pressing need for extensive clinical trials to explore the efficacy of PCD CT in specific tasks.

In this issue of Radiology, Huflage et al (2) analyze the performance of PCD CT with tin prefiltration compared with a top-tier dose-optimized EID CT model (Somatom Force; Siemens Healthineers) in evaluating renal stones. This comprehensive study of 507 patients (190 women; 37.5%) focused on identification of urinary stones in two cohorts: a group who underwent contrast-unenhanced PCD CT (229 patients; mean age, 51.6 years ± 17.7 [SD]; 90 women) and a group who underwent EID CT (278 patients; mean age, 51.7 years ± 17.2; 100 women). Three radiologists assessed the presence or absence of urinary calculi in kidneys and urinary tracts in both the EID and the PCD CT groups. Their findings showed that PCD CT not only enhanced signal-to-noise ratio and reduced image noise but also had robust diagnostic accuracy and consensus among reviewers. Importantly, radiation dose was cut by an impressive 43.6% relative to EID CT (2).

Unlike EID CT, PCD in PCD CT directly converts the energy from x-ray photons into electronic signals. This is achieved as photons strike the semiconductor material within the PCD, generating electron-hole pairs through the application of voltage. PCD not only detects individual photons with high efficiency but also provides an energy spectrum for each photon, a stark contrast to EID technology (3).

EID measures the total energy deposited in a pixel based on the visible light emitted from converting the x-ray. Relying on emitted light requires the use of reflective septa within EIDs to direct this light toward the optical photon sensor (4). However, PCD eliminates the need for such septa, allowing for a reduction in detector pixel size without compromising geometric detection efficiency (fill factor). This spectral or energy-resolved CT technique, coupled with the absence of septa, enhances image quality in PCD, boosts spatial resolution, and opens the door to lowering radiation exposure for patients (5). Beyond its intrinsic dose efficiency, the ability of PCD to minimize radiation dose is augmented by innovations such as adding tin filters. The tin filters effectively block low-energy photons that do not contribute to image quality, thereby reducing unnecessary radiation without compromising diagnostic performance of CT.

The superior resolution and enhanced dose efficiency distinguish PCD CT from other CT platforms, but these are not its only benefits. The capability of PCD CT to precisely depict the photon energy allows for the categorization of photon energies into multiple levels beyond the conventional binary energy levels. This advancement makes possible nuanced differentiation of various materials (6). PCD CT features superior spectral separation, which assists specific diagnostic tasks such as characterization of renal stones. Research into the ability of PCD CT to help identify the mineral composition of renal stones has shown it to be similar or even superior to dual-energy platforms, including in the assessment of small renal stones (7). However, the use of ultra-low-dose imaging, as it was used in this study (2), particularly with tin filters, poses challenges for optimal material decomposition from restricting the detected photon energy spectrum. Nevertheless, the tradeoff for achieving submillisievert imaging levels—minimizing radiation exposure while maintaining diagnostic accuracy for renal stone detection—represents an important advancement (2). This balance between dose reduction and diagnostic efficacy underscores the transformative potential of PCD CT in refining patient care.

PCD CT technology, despite being in its early stages, has continued to demonstrate promising advancements since the U.S. Food and Drug Administration approved the first clinically ready PCD CT system in September 2021. Innovations have continued to emerge, notably the development of silicon-based detector materials, which enhance both sensitivity and dose efficiency. However, the widespread adoption of PCD CT faces hurdles, primarily because of the high costs of manufacturing these advanced detectors (3).

One of the capabilities of PCD CT is its ultra-high-resolution scanning ability, achieving spatial resolutions of approximately 0.3 mm. This precision is beneficial for applications requiring fine detail such as coronary plaques or small renal stones. However, this increased resolution comes with challenges, notably a heightened susceptibility to motion artifacts, which can be caused by patient movements, aortic pulsation, or bowel loops peristalsis (8). These artifacts can compromise image quality, detracting from the diagnostic utility of the scans. To mitigate the impact of motion artifacts, researchers are actively pursuing various strategies of refining PCD CT technology. Among these are the development and implementation of advanced reconstruction algorithms specifically designed to manage or compensate for movement during image acquisition. Additionally, the adoption of faster detectors in PCD CT systems may reduce motion artifacts. By enabling rapid image acquisition, the faster PCDs can diminish the effects of patient motion, thereby improving image quality and reliability (9). Together, these advancements and ongoing research efforts are pivotal in overcoming current limitations, paving the way for broader clinical application and maximization of potential benefits of PCD CT.

A critical focus in the evolution of PCD CT is the development of imaging protocols optimized to minimize electronic noise, which has an important role in image degradation. This challenge becomes more critical when the PCD CT scanner is used in an ultra-high-resolution mode. By improving the signal-to-noise ratio, PCD CT systems can offer superior resolution at lower doses of radiation compared with EID CT, positioning PCD CT to be an advantageous modality for a broad spectrum of clinical applications ranging from oncological imaging, where high precision is paramount, to vessel wall imaging, which requires detailed visualization.

The integration of artificial intelligence in image processing, especially the rapid development of artificial intelligence–based denoising algorithms, is a promising area of innovation in PCD CT systems. These advanced algorithms help to counteract noise levels in ultra-high-resolution PCD CT, thereby enhancing image quality without compromising detail or increasing radiation exposure (10). Using artificial intelligence in this context represents a step forward in diagnostic imaging technology and underscores the potential of PCD CT to redefine standards of care across various medical specialties by providing clearer, more accurate images at lower doses.

Although the study by Huflage et al (2) highlights the technical and clinical benefits of using PCD CT in depicting renal stones while reducing radiation exposure, several critical questions remain unanswered. We wonder about the potential drawbacks of depicting stones at ultra-low doses without characterizing their mineral composition. We also question how the optimal balance is found between minimizing radiation exposure and ensuring the diagnostic quality of CT scans, especially in identifying possible extrarenal pathology.

As radiologists, we strive to enhance the quality of patient care and minimize risk. The study by Huflage et al (2) shows the potential of PCD CT in increasing diagnostic confidence in renal stone depiction, improving image quality, and decreasing radiation dose, marking an important advancement toward this goal.