Photon-Counting CT in Cardiovascular Imaging: Clinical Applications

Abstract

Photon-counting CT (PCCT) uses semiconductor detectors to directly convert X-ray photons to electrical signals, the intensity of which is directly proportional to the energy of the individual photons. PCCT offers several advantages in cardiovascular imaging, including ultra-high-resolution (UHR) imaging, improved multi-energy capabilities, reduced noise and artifacts, and improved iodine signal and radiation dose efficiencies. UHR imaging enhances the assessment of small vessels, dense calcifications, and stents. Multienergy mode enhances the iodine signal, reduces artifacts, and allows for material separation and lesion characterization. In this article, we review PCCT technology, highlight the benefits of PCCT in cardiovascular imaging using case examples, and discuss its challenges.

Keywords: CT; Cardiac imaging; Photon-counting CT; Plaque; Stent.

Fig. 1. PCCT scan technology. A: Schematic of a conventional energy-integrating detector CT scanner. An X-ray photon strikes a scintillator (e.g., gadolinium oxysulfide), which emits visible light (illustrated as a polygon). The light is then detected by a photodiode and converted into an electrical signal, resulting in a loss of information about the original photon during the intermediate light conversion. B: Schematic of a PCCT scanner. An X-ray photon strikes a semiconductor detector (e.g., cadmium telluride) and is directly converted into an electrical signal, the energy of which is proportional to that of the incoming photon. PCCT = photon-counting CT

Fig. 2. Improved assessment of dense calcified arteries. A: Axial oblique coronary CTA from an EID-CT scanner in a 71-year-old male shows dense calcified plaque in the proximal and mid left anterior descending coronary artery, resulting in severe stenosis. Calcium margins are poorly defined. B: Coronary CTA in the same patient from a PCCT scanner shows sharper plaque definition with reduced blooming, revealing moderate stenosis. By reducing blooming, PCCT enables more accurate estimation of luminal stenosis, which correlates more closely with invasive coronary angiography. C: Axial CTA in a 75-year-old male shows a heavily calcified popliteal artery with severe stenosis and marked calcium blooming on EID-CT (red circle). D: Axial CTA in the same patient from PCCT shows sharper plaque definition, reduced blooming, and improved assessment of stenosis (red circle). CTA = CT angiography, EID = energy-integrating detector, PCCT = photon-counting CT

Fig. 3. Material-based calcium separation. A: Coronary CT angiography from photon-counting CT in a 75-year-old female shows dense calcified plaque in the left main coronary artery (arrow) with moderate stenosis. B: Virtual non-calcium reconstruction (PureLumen) using a three-material decomposition algorithm replaces calcium pixels with gray color (arrow), mitigating the effect of calcium blooming and improving stenosis assessment.

Fig. 4. Improved evaluation of stents. A: Axial oblique (left) and short-axis (right) coronary CTA in a 64-year-old male from an EID-CT scanner shows a proximal left anterior descending coronary artery stent (arrows) with poor strut definition due to blooming artifact. B: Coronary CTA in the same patient from PCCT (left = oblique axial; right = short-axis) shows sharper stent definition, reduced blooming, and improved visualization of stent struts and lumen. C: Coronal oblique CTA in a 55-year-old female from EID-CT shows poor definition of a renal artery stent. D: Coronal oblique CTA in the same patient from PCCT shows sharper stent margins, reduced blooming, and improved luminal assessment. CTA = CT angiography, EID = energy-integrating detector, PCCT = photon-counting CT

Fig. 5. Improved visualization of small vessels. A-C: Axial CTA (A), coronal MIP image (B) and 3D-volume rendered construction (C) from PCCT in a 16-year-old male with osteosarcoma of the first rib demonstrates the mass (arrows) which abuts and severely narrows the left subclavian artery (curved arrows). The tumor was resected, and a graft restored patency. D: Sagittal oblique CTA from PCCT in a 63-year-old male with metastatic hepatocellular carcinoma in the RV shows a mass (arrow) narrowing the RV outflow tract, infiltrating the RV free wall (arrowhead), and supplied by small marginal RCA branches (curved arrows). E: Coronal oblique CTA in the same patient shows the mass (arrow), RV wall infiltration (arrowhead), and feeding marginal branches (curved arrows). CTA = CT angiography, MIP = maximal intensity projection, PCCT = photon-counting CT, RV = right ventricle, RCA = right coronary artery

Fig. 6. Salvage of suboptimally enhanced CTA using multienergy technique. A: Coronal oblique CTA from photon-counting CT in a 75-year-old male shows suboptimal aortic opacification (mean, 191 HU) due to contrast extravasation during the scan. B: Coronal oblique 50-keV image in the same patient shows improved iodine signal (mean, 450 HU). Low-keV virtual monoenergetic image enhances iodine conspicuity, allowing for a reduced contrast dose and salvaging suboptimally enhanced studies. CTA = CT angiography, HU = Hounsfield unit

Fig. 7. Vessel wall and plaque characterization. A: Short-axis and axial oblique CTA from an energy-integrating detector-CT in a 61-year-old female shows a non-calcified plaque in the mid right coronary artery with positive remodeling (arrows), causing mild stenosis. B: CTA from photon-counting CT in the same patient shows improved plaque definition with clear visualization of a central low-attenuation lipid core (arrows), indicating a high-risk plaque prone to rupture. CTA = CT angiography

Fig. 8. Multienergy characterization of incidental lesion. A: Coronal oblique CT angiography in a 42-year-old male post-Bentall procedure, subaortic resection, Konno root enlargement, and tricuspid annuloplasty shows a complex hyperattenuating mediastinal hematoma (arrow). B, C: Axial (B) and coronal (C) iodine maps in the same patient show absence of iodine in the lesion (arrows), confirming the lesion as a postoperative seroma.

Fig. 9. Calcium scoring from virtual non-iodine image. A: Coronary CT angiography from photon-counting CT in a 75-year-old female shows dense calcified plaque in the left main coronary artery (arrow) with moderate stenosis. B: Virtual non-iodine reconstruction (PURE Calcium) subtracts iodine but retains calcium (arrow), enabling calcium scoring without acquiring a separate non-contrast CT scan, thus reducing radiation exposure.

Fig. 10. Myocardial assessment. Short-axis 50-keV virtual monoenergetic image from photon-counting CT in a 51-year-old male with hypertrophic cardiomyopathy shows patchy mid-myocardial late iodine enhancement in the basal anteroseptum (arrow).

Fig. 11. Combined PCCT modes. A: Coronal oblique CTA from PCCT in UHR-ME mode in an 81-year-old male post-fenestrated endovascular aortic repair shows sharp definition of the stent structure, including renal artery stents. B: ME reconstruction in the same patient (50 keV) shows enhanced contrast visualization. C: Sagittal oblique UHR image shows calcified descending thoracic aortic plaque. D: PureLumen reconstruction shows calcium separation with reduced blooming. E: Axial CTA from PCCT in Flash-ME mode in 43-year-old male shows motion-free pulmonary arteries with embolus in the posterior segmental right lower lobe branch (arrow). F: Iodine map in the same patient shows a corresponding perfusion defect (arrow). Flash-ME mode enables simultaneous motion-free imaging of the coronary, aortic, and pulmonary arteries, along with ME maps (iodine, virtual monoenergetic image). PCCT = photon-counting CT, CTA = CT angiography, UHR = ultra-high-resolution, ME = multienergy

Fig. 12. Challenges of PCCT. A: Axial coronary CTA from PCCT in a 61-year-old male using Bv48 filter shows good image quality. B: Axial coronary CTA in the same patient using sharper Bv60 filter shows increased noise. Noise is a key limitation of ultra-high-resolution images, especially with thin slices, sharp kernels, and high matrix size. C: Coronal CTA from PCCT in a 45-year-old male using Bv44 filter shows artificially “painted” margins. D: Coronal CTA in the same patient using sharper Bv56 filter shows improved, more natural image appearance. PCCT = photon-counting CT, CTA = CT angiography