Counting Photons: The Next Era for CT Imaging?

HomeRadiologyVol. 303, No. 1 PreviousNext Reviews and CommentaryFree AccessEditorialCounting Photons: The Next Era for CT Imaging?Martin J. Willemink, Thomas M. Grist Martin J. Willemink, Thomas M. Grist Author AffiliationsFrom the Department of Radiology, Stanford University School of Medicine, Palo Alto, Calif (M.J.W.); and Department of Radiology, University of Wisconsin–Madison, School of Medicine and Public Health, 600 Highland Ave, Madison, WI 53705 (T.M.G.).Address correspondence to T.M.G. (e-mail: [email protected]).Martin J. WilleminkThomas M. Grist Published Online:Feb 15 2022https://doi.org/10.1148/radiol.213203MoreSectionsPDF ToolsImage ViewerAdd to favoritesCiteTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinked In See also the article by Rajendran et al in this issue.Martin J. Willemink, MD, PhD, is an instructor in the Department of Radiology at Stanford University School of Medicine. His research focuses on CT, with a special interest in cardiovascular imaging. Dr Willemink is a fellow of the Society of Cardiovascular Computed Tomography, Fulbright laureate, and junior deputy editor of European Radiology.Download as PowerPointOpen in Image Viewer Thomas M. Grist, MD, is the John H. Juhl Professor of Radiology and Medical Physics and chair of the Department of Radiology at the University of Wisconsin–Madison. He is a cardiovascular radiologist and past president of the International Society for Magnetic Resonance in Medicine and past chair of the Radiological Society of North America R&E Foundation.Download as PowerPointOpen in Image Viewer The first clinical CT scanner was introduced in 1972. The system was slow; image acquisition took approximately 5 minutes, and the spatial resolution of 80 × 80 pixels was very low (1). Soon after the introduction of CT imaging, technical developments advanced rapidly. The introduction of multisection CT systems in 1998 allowed for larger detector coverage along the patient. Dual-energy CT was introduced in the mid-2000s, which allowed for the separation of low- and high-energy x-ray photons. Most recently, CT scanners adapted for cardiac CT have used fast rotation times of approximately 250 msec while covering the whole heart in a single heartbeat.As CT developed and use increased, CT radiation doses also increased. A primary limitation in this regard is the manner by which x-rays are detected. Up to this point, all clinical CT scanners have used so-called energy-integrating detectors (EIDS) to detect an x-ray photon. As pixel sizes become smaller, the CT scanners using EIDS tend to become very inefficient in capturing incident x-ray photons. As a result, the spatial resolution of most CT systems (on the order of 0.5 mm) has not changed significantly over the past decade (2).Another important limitation of current CT imaging is the beam-hardening artifact. Beam-hardening artifact is well known to the clinical radiologist in association with severe artifacts located around metal implants. A less noticeable but equally important aspect of beam-hardening artifact is noted beneath the skull, around the kidneys, or in the spinal canal where CT attenuation numbers deviate markedly from their correct values. Finally, current CT scanners with EIDs have limited contrast resolution. A common example of this is limited identification of gray versus white matter in the brain.In the past 15 years, photon-counting detector (PCD)–based CT has been introduced (3,4). In contrast to EIDs, PCDs count individual x-ray photons and measure their associated energy (5). There are several benefits of PCDs: (a) PCDs are more efficient than EIDs, meaning that they can detect lower radiation doses. This allows lower patient radiation doses. (b) PCD elements are smaller (by a factor of two or more) than those of conventional detectors. Thus, PCDs can achieve at least twofold greater spatial resolution at a lower radiation dose than conventional detectors. (c) PCDs measure the energy of each individual x-ray photon. This allows for more accurate energy binning, resulting in improved spectral separation compared with dual-energy CT without the need for complex workflow adjustments. Better spectral resolution could allow for lowering the dose of iodine contrast agents or even for the use of alternative contrast agents, such as gadolinium, bismuth, or other materials (2). (d) PCDs allow better contrast resolution than conventional detectors. This advantage is expected to increase low contrast resolution for soft tissues (such as the brain) for PCD CT scanners compared with conventional CT.In this issue of Radiology, Rajendran and colleagues (6) present a technical systematic evaluation of the first clinical PCD CT system (Naeotom Alpha, Siemens Healthineers). Multiple phantoms as well as four participants were scanned with a dual-source PCD CT system. The PCD CT system has a gantry rotation time of 250 msec. An American College of Radiology CT accreditation phantom, a tungsten wire, a gold foil, a water phantom, and an iodine phantom allowed for systematic assessment of image quality (including noise and noise power spectra), spatial resolution, and quantitative multienergy CT performance. The authors include the results for CT scanning of four participants for proof of principle, with all participants having both a PCD CT scan as well as a conventional CT scan with EIDs. This allows the reader an opportunity for a head-to-head comparison of the PCD CT scanner and a conventional, yet still state-of-the-art CT scanner.The phantom studies showed that the sharpest kernel that was evaluated (Br96, or body-regular, sharpness level 96) resulted in 125-μm limiting in-plane spatial resolution. While Rajendran et al did not acquire phantom images with an EID CT system for comparison purposes, their data can be compared with previously published results. The 10% modulation transfer function of current clinical EID CT systems is generally lower than 25 line pairs per centimeter, even with the smallest detector elements that are currently available (0.25 × 0.25 mm, Canon Medical Systems) (7). The PCD CT system showed an increased 10% modulation transfer function of up to 36.1 line pairs per centimeter. One should note that direct comparison of the modulation transfer function should be done with kernels used for similar clinical tasks. Measurements of iodine concentrations had a mean error of 5.7% and a root mean squared error of 0.5 mg/mL, indicating that the quantitative multienergy performance of PCD CT to quantify iodine concentrations is accurate.The participant study showed the potential for radiation dose reduction of 30% for temporal bone PCD CT compared with EID CT while using a thinner section thickness (0.2 mm vs 0.4 mm, respectively). In dose-matched examinations, PCD CT showed a lower image noise (eg, 47% lower in a participant with multiple myeloma examined for skeletal survey). Overall, the spatial resolution of PCD CT improved compared with that of conventional EID CT in the participant examinations.These results are important because after the decades-long promise of PCD CT, Rajendran and colleagues have now systematically evaluated the technical performance of the first clinical PCD CT system that is approved by the U.S. Food and Drug Administration. Despite the lack of a direct comparison of the phantom results with EID CT and despite the low number of participant examinations (n = 4), this study clearly indicates the promise and potential of this new technology.There are challenges for PCD CT that need to be addressed. First, a relatively high tube voltage (120 kV or more) is needed for optimal spectral separation, which is associated with an increased radiation dose if other parameters are kept the same. A potential solution is to compensate this by using lower tube currents, which may be possible due to the more optimal noise behavior of PCDs compared with conventional EIDs. Second, as with most new technologies, the initial costs of PCD CT systems of more than $2 million are relatively high. This may decrease over time once the technology becomes more widespread. Last, the increased spatial resolution and spectral data sets may not only result in longer reconstruction times, but there will also be a need for larger storage space. Further work will need to address human factors necessary to handle the large amount of information provided by these systems.We are at the beginning of an exciting new era in CT imaging, where PCDs are likely to completely replace conventional EIDs over time. The coming years will prove whether PCD CT will allow for further reduction of the x-ray radiation dose, increased spatial resolution, enhanced low-contrast resolution, lower iodine contrast dose, and whether iodine may be replaced by other contrast agents. Besides technical assessments such as the study by Rajendran et al, it will be essential to conduct larger patient studies that assess the incremental diagnostic value of PCD CT in the clinical setting.Disclosures of Conflicts of Interest: M.J.W. Grants to institution from Siemens Healthineers; consulting fees from Segmed; patent pending for amorphous chalcogenide alloy for dual-layer x-ray detector; unpaid leadership role in the Society of Cardiovascular Computed Tomography and member of the Radiology Fellowship Awards Peer Review Committee of the American Heart Association; stockholder in Segmed. T.M.G. Advisory board member for Bracco Diagnostics; equity stockholder in Elucent, HistoSonics, and Shine Medical; institutional research support from GE Healthcare, Bracco Diagnostics, Siemens, Hologic, and Change Healthcare.References1. Hounsfield GN. Computerized transverse axial scanning (tomography). 1. Description of system. Br J Radiol 1973;46(552):1016–1022. Google Scholar2. Willemink MJ, Persson M, Pourmorteza A, Pelc NJ, Fleischmann D. Photon-counting CT: technical principles and clinical prospects. Radiology 2018;289(2):293–312. Link, Google Scholar3. Schlomka JP, Roessl E, Dorscheid R, et al. Experimental feasibility of multi-energy photon-counting K-edge imaging in pre-clinical computed tomography. Phys Med Biol 2008;53(15):4031–4047. Crossref, Medline, Google Scholar4. Pourmorteza A, Symons R, Sandfort V, et al. Abdominal imaging with contrast-enhanced photon-counting CT: first human experience. Radiology 2016;279(1):239–245. Link, Google Scholar5. Leng S, Zhou W, Yu Z, et al. Spectral performance of a whole-body research photon counting detector CT: quantitative accuracy in derived image sets. Phys Med Biol 2017;62(17):7216–7232. Crossref, Medline, Google Scholar6. Rajendran K, Petersilka M, Henning A, et al. First clinical photon-counting detector CT system: technical evaluation. Radiology 2022;303(1):130–138. Link, Google Scholar7. Onishi H, Hori M, Ota T, et al. Phantom study of in-stent restenosis at high-spatial-resolution CT. Radiology 2018;289(1):255–260. Link, Google ScholarArticle HistoryReceived: Dec 17 2021Revision requested: Dec 28 2021Revision received: Dec 28 2021Accepted: Jan 2 2022Published online: Feb 15 2022Published in print: Apr 2022 FiguresReferencesRelatedDetailsCited ByCharacterizing the Heart and the Myocardium With Photon-Counting CTEmeseZsarnóczay, AkosVarga-Szemes, TilmanEmrich, BálintSzilveszter, Niels R.van der Werf, DomenicoMastrodicasa, PálMaurovich-Horvat, Martin J.Willemink2023 | Investigative Radiology, Vol. 58, No. 7Accompanying This ArticleFirst Clinical Photon-counting Detector CT System: Technical EvaluationDec 14 2021RadiologyRecommended Articles Photon-counting CT: Technical Principles and Clinical ProspectsRadiology2018Volume: 289Issue: 2pp. 293-312First Clinical Photon-counting Detector CT System: Technical EvaluationRadiology2021Volume: 303Issue: 1pp. 130-138Seeing More with Less: Clinical Benefits of Photon-counting Detector CTRadioGraphics2023Volume: 43Issue: 5Abdominal Imaging with Contrast-enhanced Photon-counting CT: First Human ExperienceRadiology2016Volume: 279Issue: 1pp. 239-245Next-Generation Hardware Advances in CT: Cardiac ApplicationsRadiology2020Volume: 298Issue: 1pp. 3-17See More RSNA Education Exhibits Photon Counting Detector CT A New Powerful Addon Tool To Forensic Autopsy.Digital Posters2021Clinical Photon Counting Abdominopelvic CT: A Crash Course!Digital Posters2022Pediatric Applications of Photon Counting Detector Computed Tomography (PCD CT)Digital Posters2022 RSNA Case Collection Osteochondral injuryRSNA Case Collection2022COVID-19 PneumoniaRSNA Case Collection2020Gout-KneeRSNA Case Collection2021 Vol. 303, No. 1 Metrics Altmetric Score PDF download