## 4 Discussion  
This prospective study based on 40 consecutive extremities DE-CTA scans was carried out to compare the image quality of TNC to VNC images, including attenuation and noise, as well as estimating the potential reduction in radiation dose. Based on the study results, firstly, VNC showed a good reliability in the detection of PAD in comparison to standard TNC phase; secondly, VNC reconstructions reported a high diagnostic quality at subjective analysis; and, thirdly, DE-CTA scans with the single VNC reconstructed images could result in a reduction of the administered mean effective dose. The role of CTA in the assessment of the extent and severity of symptomatic PAD has been widely recognized by both the latest Guidelines from the American College of Cardiology/American Heart Association and the European Society of Cardiology [4,13]. Due to the recent advances in protocols and post-processing reconstructions and in lowering the amounts of radiation dose and contrast, CTA has therefore effectively replaced DSA as a first-line examination for diagnosing PAD. As reported by previous studies, the advancement of third generation DECT and material decomposition algorithms may help in limiting blooming artifacts caused by dense calcifications and high iodine concentrations. DECT also enables a better assessment of the arterial lumen, avoiding stenosis overestimations [16]. With these assumptions, DECT imaging may be considered a valid alternative to conventional maximum intensity projection (MIP) standard reconstructions [16]. After a careful literature review, several studies were found to have attempted to account for VNC reconstructions as an alternative to standard non-contrast phases, all with controversial results [40, 41, 42, 43, 44, 45]. Most of these studies were limited to the aorto-iliac attenuation values, except for one series, which has also been extended to the femoral axis [40]. Our study is one of the few to evaluate arterial lower limb vessels from the aortic axis to popliteal segments. Since the anatomy and pathophysiology of femoropopliteal axis leads to specific challenges for both percutaneous and endovascular treatment, its non-invasive diagnosis is crucial to define the appropriate treatment strategy [2, 3, 4]. A great part of clinically significative PAD arises from femoropoliteal calcified lesions. Furthermore, endovascular treatments of this district are often challenging due to the important reactions to injuries of the thick arterial walls, which are prone to scar, proliferate, and thrombose. For these reasons, VNC reconstructions may significantly improve the evaluation of highly calcified steno-occlusive lesions of these segments [5]. Some authors reported a substantial agreement of intravascular attenuation values measured in the abdominal aorta between VNC reconstructions and TNC acquisitions [40,41]. Other studies have instead demonstrated a statistically significant difference in attenuation values at the same level, as confirmed by our series [40,43]. A greater difference in attenuation between the two CT protocols was detected in proximal vessels (aorto-iliac axis) and less pronounced in distal vessels (popiliteal artery), except for the external iliac artery. This phenomenon could be explained by the different behavior of the homogeneous iodine distribution in parenchomatous tissues compared to laminar intravascular flow. The latter may lead to inaccuracies during VNC reconstructions due to blood motion artifacts. As proof of this, the greater accuracy in attenuation measurements at the level of the external iliac artery may be due to its anatomical characteristics and its lower flow [40, 41, 42, 43, 44, 45]. The significatively lower attenuation values of VNC Images could be due to the very high concentration of iodine during the arterial phase. This phenomenon determinates, against the expectations, an excessive iodine removal during reconstructions, and consequent lower HU-values in VNC images [45]. As previously suggested by [PERSON] et al.