In the present study, we compare clinically applied CTA protocols of UHR PCD-CT and third-generation dual-source EID-CT regarding the visualization of peripheral artery high- and low-grade ISRS in a phantom. We demonstrate high image quality of both modalities at a stent lumen of 8 mm, enabling a precise and therefore reliable, noninvasive differentiation between low- and high-grade ISRS.

In femoropopliteal disease, approximately one-third of patients are treated with stent implantation [23]. While postinterventional antiplatelet regimens decreased rates of stent thrombosis, intimal hyperplasia remains a significant complication of the procedure with up to 40% of ISRS within a two-year period [24, 25].

Current guidelines recommend duplex ultrasound (DUS) upon clinical suspicion of ISRS and digital subtraction angiography (DSA) where deemed necessary [10, 11, 26]. Besides, recent data suggest the additional usefulness of intravascular ultrasound and optical coherence tomography regarding ISRS lesion characterization (e.g., stent underexpansion, severely calcified lesions) [27, 28].

However, considering the general methodologic limitations of DUS (i.e., difficulties in anatomical access, reproducibility, documentation, operator’s experience), DSA (i.e., invasiveness, availability, cost), and intravascular imaging such as ultrasound and optical coherence tomography (i.e., invasiveness, availability, operator’s experience), CTA has been used as a diagnostic alternative in clinical practice with a growing body of supportive data [29,30,31]. Yun et al. examined 51 patients with a total of 68 stents and compared DSA to CTA with a dual-source EID-CT and reported an accurate detection of ISRS in EID-CT of 79% compared to DSA [8]. Furthermore, the first clinically approved PCD-CT has enabled improved image quality of coronary and peripheral stents in vitro and in vivo [5,6,7, 14, 17]. Overall, this development emphasizes the importance of further studies to test the accuracy of state-of-the-art CTA scanners in the depiction of previously stented vessels and the identification of ISRS.

In our study, we investigated seven widely used peripheral stents. Across all stent models, observed mean CNR was best in the 0% stenosis and lowest in the 95% stenosis, whereas findings of similar and even higher CNR in UHR PCD-CT compared to EID-CT may result from the improved contrast and lower image noise owing to the possibility of weighing photons differently, depending on their inherent energy [32].

Spatial resolution as measured by FWHM was high for both PCD-CT and EID-CT at any slice thickness, with deviations of the mean no larger than 0.5 mm from true lumen size. Combining the results for non-stenosed, significantly (i.e., 75%), and highly stenosed (i.e., 95%) segments, UHR PCD-CT at a slice thickness of 0.2 mm enabled estimations of lumen width with a precision (= half-width of CI) of 0.34 mm, 0.32 mm, and 0.15 mm, respectively.

We observed underestimation of the true lumen size in PCD-CT at slice thicknesses of 0.5 mm and 1.0 mm such as EID-CT at 1.0 mm. In highly stenosed segments, measurements of residual ISRS lumen diameter were prone to underestimation in both modalities, probably owing to lower CNR.

Qualitative assessment of images regarding sharpness, subjective image noise, blooming, and diagnostic confidence by two raters with basic and advanced experience levels displays good correlation and suggests high diagnostic usability (sum score from 17.79 to 19.43 of 20 possible) of both third-generation dual-source EID-CT and PCD-CT regarding ISRS in 8 mm caliber peripheral stents, whereas images of UHR PCD-CT at a slice thickness of 0.2 mm were rated best. Likely, it is the sum of improvements—lower image noise and higher CNR, enabled by weighing the energy of individual X-ray photons, such as better spatial resolution, achieved by PCD not necessitating reflective septa and respective smaller pixel sizes, that yield an overall improved visual impression of UHR PCD-CT images. The fact that orientation of the stents relative to the z-axis showed no detectable differences is attributable to the isotropic voxel sizes [7, 8].

We conclude that for the investigated stent lumen of 8 mm, both PCD-CT and state-of-the-art EID-CT yield good image quality to detect and accurately quantify ISRS, whereas the smallest slice thicknesses available for both scanners (i.e., 0.2 mm in PCD-CT vs. 0.5 mm in EID-CT) appeared advantageous, especially in high-grade ISRS. Our results stand in line with previous in vitro and in vivo data supportive of the reliably of noninvasive ISRS diagnosis in 8 mm and smaller diameter stents using both scanner types, in PCD-CT more so than EID-CT. The accuracy of both a third-generation dual-source EID-CT and PCD-CT is of clinical importance, especially given the currently limited availability of PCD-CT due to higher associated procurement costs. On another note, in the present study, PCD-CT and EID-CT acquisition parameters were matched using CTDIvol, underlining that the reported improvements in image quality do not come at the cost of excessive radiation. Finally, with prospects of software advancements such as experience in clinical settings, the potential of PCD-CT will certainly expand greatly in the foreseeable future.

The limitations of this study include, first, the in vitro nature, and thus, the absence of potential motion and pulsation artifacts, as well as beam hardening artifacts from bone or calcified tissue. In addition, the ideal contrast concentration inside the stents with the absence of potentially suboptimal filling of the lumen might have created a picture that is only obtainable under optimal clinical conditions and not in every real-world cases. Moreover, due to the comparison of clinically applied PCD-CT and EID-CT protocols with preselect kernels, we cannot exclude the possibility that other kernels might yield equal or even superior images of the presented ISRS-phantom. Last but not least, the study tested two CT scanners from a single manufacturer, which might limit the generalizability of results.