## 1 Introduction  
Peripheral arterial disease (PAD) is characterized by arterial stenosis or occlusion anywhere from the aortoliac axis to pedal arteries [1]. PAD has a high prevalence worldwide, occurring in up to 15% of the population over 70 years, and it is a prevalent cause of repeated hospitalizations and cardiovascular mortality [1; 2; 3; 4]. After physical examination, imaging techniques such as Doppler ultrasound (DUS), computed tomography angiography (CTA), and digital subtraction angiography (DSA) are employed for PAD localization and definition [4]. DSA, historically the gold standard technique for PAD diagnosis, is nowadays limited to cases with a planned endovasculartherapeutic intervention due to its numerous limitations, such as radiation burden, contrast medium load, high costs, and access site complications [5; 6]. For pure diagnostic purposes, DSA has been replaced by other non-invasive imaging techniques. In current clinical practice, CTA, magnetic resonance angiography (MRA) and duplex sonography (DUS) are commonly employed to assess the extent and severity of PAD. In particular, CTA of the lower extremity runoff is nowadays the preferred imaging modality [4; 7] because of its large availability, its high resolution, the fast acquisition, and the suitability of multi-planar reconstructions (MPR) [5; 8; 9; 10]. CTA is also highly accurate in assessing significant luminal lesions and evaluating arterial walls [8; 11]. CTA principal drawbacks include the lack of hemodynamic information, the exposure to ionizing radiation, and the use of iodinated contrast agents. The occurrence of \"blooming\" artifacts caused by highly calcified atheromas resulting in an overestimation of stenosis severity and in false positive findings is also an important limitation [3; 4; 5; 8; 11; 12; 13; 14; 15; 16]. Despite new generation CT scanners and protocols minimizing these issues [17], the increasing use of dual-energy computed tomography angiography (DE-CTA) in the diagnostic assessment of PAD may overcome CTA limitations. Specifically, DE-CTA improves the assessment of the vessel lumen lowering the employed contrast media volume [18]. As \"dual-energy CT\" refers to the use of two-photon spectra, DECT is frequently referred to as \"spectral CT\". In the DECT dataset, since X-ray attenuation depends on the atomic number (Z) of a material, low-Z, and high-Z materials can be differentiated if images are acquired at both high and X-ray spectra (low to high tube voltages) [16]. This process, known as material decomposition, can be obtained by a variety of methods depending on the type of DE-CT scanner (equipped with a double detector panel, two X-ray tubes, or a two-layered single detector panel). The acquisition of two imaging datasets with two different attenuation profiles is thus allowed [5]. In contrast to single-spectrum imaging, DECT imaging is also able to differentiate materials with similar attenuation coefficients based on chemical composition and atomic number [19; 20; 21; 22; 23; 24; 25]. Constituent materials characterized by high atomic numbers, such as iodine and calcium, can be easily differentiated by DECT processes, thus permitting the development of many clinical applications [26]. The DE technique has several applications for clinical imaging and can allow the reduction of the iodine contrast medium dose, as well as the radiation dose. First, material decomposition algorithms may be applied for precise quantification of specific materials such as iodine uptake. They may also generate iodine maps from contrast-enhanced phases, which may be useful in the characterization of many clinical entities [26]. Moreover, post-processing algorithms allow the identification and removal of the calcium content of calcified plaques, obtaining a better evaluation of the contrast-enhanced vessel lumen, similar to DSA images [6; 11; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37]. Imaging of arteries may benefit greatly from these DECT reconstructions.