Spectral Ultrahigh-resolution Photon-counting CT Angiography for Below-knee Peripheral Artery Disease

Objectives: Photon-counting detector (PCD)-CT may improve noninvasive assessment of patients with peripheral artery disease (PAD), yet ideal conditions to unlock its full potential remain unexplored. Hence, this study aims to evaluate the effect of isotropic voxel spacing on image quality and performance in spectral ultrahigh-resolution (UHR) PCD-CT of lower limb CT-angiography (CTA). Materials and Methods: In this IRB-approved post hoc analysis of a prospective study cohort, consecutive patients with PAD underwent lower limb CTA between November 2024 and April 2025 using a dual-source PCD-CT system in spectral-UHR mode (collimation: 120×0.2 mm). Reconstructions included down-sampled images (DS, 0.8 mm section thickness), virtual monoenergetic images at 40 keV (0.4 mm), polychromatic UHR images (0.2 mm), iodine maps (IM, 0.4 mm), and, furthermore, focused per-extremity UHR and IM reconstructions with isotropic voxel spacing (UHR focused and IM focused ). Two readers assessed image quality in consensus using a 4-point Likert scale (4: “excellent”). In the below-knee arteries with calcified stenosis, perpendicular attenuation profiles were used to calculate full width at half maximum for lumen (FWHM lumen ) and plaque (FWHM plaque ). Where available, diagnostic performance was evaluated against digital subtraction angiography for ≥50% stenosis. Results: A total of 59 patients, mean age: 64.6 ± 13.5 years; 40 men (68%), with 111 extremities were included. DS yielded the lowest image quality (median: 2 [2-2]), while UHR focused scored highest (median: 4,[4-4] P <0.001). UHR focused and IM focused achieved the highest lumen visibility (FWHM lumen , UHR: 1.68±0.76; IM: 1.70±0.76) and lowest blooming (FWHM plaque , UHR: 1.01±0.28; IM: 0.98±0.27), indicating superior anatomic resolution, while DS images were prone to blooming artifacts (FWHM lumen : 0.60±0.78; FWHM plaque: 2.11±0.60). UHR focused and IM focused both yielded sensitivity of 93% (95% CI: 77%-99%), while UHR focused demonstrated the highest accuracy of 94% (95% CI: 83%-99%) per segment (n=50). Conclusions: UHR PCD-CT with focused, per-extremity reconstruction using isotropic voxel spacing enhances image quality, improves lumen and plaque delineation, and yields high diagnostic accuracy in below-knee CTA.