The use of preoperative IVUS during lower-extremity PAD has shown improved accuracy in assessing vessel diameter and hemodynamic significance, surpassing that of angiography alone [3]. Typically, angiography is regarded as the gold standard for estimating vessel size and for EVT. However, it has several inherent limitations, including providing only a two-dimensional image of a three-dimensional luminal structure, difficulty in recognizing intravascular structures, and susceptibility to confounding artifacts caused by arterial wall motion [12,22]. In contrast, IVUS offers a real-time cross-sectional image of the vessel, clearly depicting the thickness of the vessel wall as well as the size and shape of the lumen, which allows for accurate measurements of vessel lumen diameter and CSA. A comparative analysis of angiography versus IVUS in evaluating peripheral atherosclerosis revealed that although angiography is highly reliable for quantitative intraluminal assessment, its measurements of true vessel diameter, actual stenosis area, calcification, and plaque concentricity were significantly inconsistent with those obtained from IVUS [12]. Similarly, a prospective study by Shammas et al. [19] showed that the mean vessel diameter of the infrapopliteal artery measured by IVUS was approximately 25% larger than that measured by angiography. In the assessment of femoral and iliac arteries, IVUS demonstrates comparable accuracy to CTA but offers advantages in reducing the use of contrast agents and exposure to radiation. IVUS can be a valuable adjunct to CTA, particularly in patients with poor femoral artery diameters or extensive artifacts on CTA [23]. Additionally, IVUS has shown strong intra- and inter-observer agreement in measuring parameters such as lumen CSA, EEM CSA, luminal diameter, and reference vessel diameter, further supporting its utility in peripheral arterial procedures [21].

Compared to angiography, IVUS can more precisely determine the type (e.g., soft, fibrous, calcified, or mixed) and morphology (e.g., shape, length, volume, eccentric or concentric) of plaques [24]. The introduction of virtual histology IVUS, which analyzes additional low radiofrequency signals, allows for an enhanced assessment of the histological composition of arterial plaques [7]. Both the type and location of the plaque influence the choice of procedure and the outcome of EVT. IVUS has been utilized to assess lesion volume and composition before and after atherectomy, as well as to predict the risk of amputation [25,26]. According to the plaque echo intensity characteristics on IVUS, hypoechoic plaques, typically laden with a higher lipid content, are prone to exhibit no or slow blood flow post-intervention, necessitating increased vigilance during the procedure. Furthermore, IVUS enables the differentiation between superficially calcified plaques, which may undergo more aggressive pre-dilatation, and deeply calcified plaques, where simpler pre-dilation techniques or a cutting balloon are preferred [27].

Angiography underestimates the degree of calcification within a vessel because it only reveals calcification on one or both sides of the arterial wall and fails to accurately identify intimal or medial calcification [22]. In contrast, IVUS is more sensitive in detecting the presence and severity of vessel wall calcification [16]. The characteristics of calcification identified by IVUS have been strongly linked to the patency rate following EVT [28]. Fujihara et al. [29] demonstrated that a calcification angle greater than 180°, as assessed by IVUS, was associated with a smaller postoperative minimum lumen area, indicating challenges in successfully dilating the lesion. Similarly, a prospective multicenter study on femoropopliteal lesions found that a smaller calcification angle detected by IVUS correlated with higher vascular patency after PTA [18].

The echogenicity of older, mechanized thrombi on IVUS resembles that of fibrous plaques, while fresh, acute thrombi appear hypoechoic due to their high erythrocyte concentration and low fibrin content [7]. Although IVUS identifies more lower-extremity arterial thrombi than angiography, its capacity to differentiate stages of thrombus development is constrained by its lower resolution [30].

IVUS, as an adjunctive imaging technique, enhances the safety and efficacy of EVT by providing accurate lesion characterization and guiding stent deployment to optimize therapeutic strategies (Fig. 3) [31]. Although PTA or stenting can improve vessel patency in arterial occlusive lesions, they also carry risks such as early thrombosis or stent migration due to inadequate expansion, and intimal hyperplasia or vessel perforation from overexpansion. The ability of IVUS to precisely assess vascular structure and lesion characteristics not only enables a precise pre-procedural diagnosis but also guides the selection of appropriate interventions and assists in the accurate deployment of endovascular devices. Compared to angiography, several studies have shown that incorporating IVUS during endovascular procedures for lower-extremity arterial disease improves long-term clinical outcomes, including higher primary patency rates, lower complication rates, and reduced restenosis rates [20,32,33]. There was no significant difference between the two approaches in terms of technical success or procedure time. During lesion debulking or atherectomy for peripheral arterial occlusive disease, IVUS helps orient the cutting blade to ensure adequate plaque removal and prevent vessel perforation, leading to improved revascularization rates and fewer complications [17,34]. IVUS also aids in identifying patients who may benefit more from atherectomy than from standard PTA. In cases of IVUS-guided true lumen reentry, a satisfactory clinical success rate was achieved regardless of the catheter used [35-37]. Mori et al. [38] demonstrated that a higher percentage of intraplaque routes resulted in higher primary patency and a restenosis rate of only 9% during follow-up. Meanwhile, IVUS-guided percutaneous bypass could be feasible for treating patients with an uncrossable superficial femoral artery [39]. In patients with pre-existing renal insufficiency, IVUS can avoid the use of nephrotoxic contrast agents [13]. More importantly, IVUS-guided interventions were associated with lower target lesion revascularization, which improved outcomes and reduced costs in the long run, highlighting IVUS as an overall cost-saving strategy [40].

Several studies have demonstrated that IVUS can identify previously inadequate stent deployments that were considered sufficiently expanded by angiography, influencing changes in the treatment plan [41,42]. For instance, research on EVT for atherosclerotic aortoiliac occlusive disease found that IVUS detected inadequately deployed stents, which angiography had deemed adequately dilated, in about 40% of patients [43].

Higher-level dissection is associated with increased rates of target lesion revascularization and decreased lumen patency rates [44]. Precise imaging within the vessel wall is essential to accurately assess the extent and severity of vessel dissection (Fig. 4) [24]. Angiography often underestimates both the presence and severity of vascular dissections following EVT, typically resulting in the selection of treatment segments that are too small. IVUS not only identifies more severe dissections than those detected by angiography but also reveals dissections that angiography does not detect [19]. The ability of IVUS to diagnose dissections has been demonstrated in most arteries of the lower extremities [11,17,19,36,37]. A prospective study that employed atherectomy and angioplasty for treating femoropopliteal artery lesions found that IVUS detected 3.55 times as many vascular dissections as angiography, with about one-third of these dissections involving the intima or media [45]. Currently, there are no standardized criteria for classifying dissections in lower-extremity arteries. Furthermore, it remains unclear which dissections identified by IVUS should be managed and whether IVUS should be used to guide the repair of these dissections. Therefore, large-scale samples and prospective clinical studies are still required.

The risk of restenosis following EVT has been associated with IVUS findings observed during the intervention. These findings provide valuable information for postoperative follow-up and surveillance [14,15]. Factors such as a large distal plaque burden, minimal lumen area, and a significant maximal dissection angle post-intervention on IVUS have been identified as predictors of restenosis after EVT [18,46-48].

An appropriate clinical evaluation of patients before intervention procedures is crucial to identify those who might benefit from treatment, as iliac vein compression can often be asymptomatic. IVUS reveals findings that are not detectable by single-plane venography and has proven to be superior in assessing the morphology of stenosis (Fig. 5). Indeed, IVUS is more effective at detecting obstructive lesions than venography, which often fails to recognize severe obstructions [50,52,53]. When compared to venography, CTA provides a diagnostic capability similar to that of IVUS for identifying lower-extremity venous stenosis or obstruction [55,57]. In cases of iliac vein obstruction, IVUS can detect detailed intraluminal and mural features such as mural thickness, trabeculation, frozen valves, and external compression, thus effectively preventing blind stenting [49]. Additionally, IVUS can distinguish between acute thrombosis and iliac vein compression lesions, allowing for the selection of appropriate treatment based on the lesion's characteristics [58]. A fresh thrombus requires thrombolysis, whereas iliac vein compression may necessitate PTA/stent placement. IVUS also enables the quantification of thrombus volume, providing a precise clinical basis for the aspiration of iliac vein thrombi.

IVUS is more accurate than venography in determining the appropriate stent diameter, length and number. A comparative study of venogram and IVUS in diagnosing iliac vein obstruction revealed that IVUS identified more significant lesions that were not detected by conventional venography. This led to changes in treatment approaches and an increase in the number of stents used [53]. Meanwhile, IVUS can clearly identify the anatomical structures where the lesion is located, enabling more accurate stent deployment. Another study highlighted that IVUS more effectively identified maximal area stenosis, its anatomical location, the ilio-caval confluence, and the distal landing zones [59].

IVUS is instrumental in identifying significant complications, such as grading the severity of residual lesions, stratifying thrombus, and ensuring proper stent apposition. Traditionally, venography has been regarded as the gold standard for assessing the effectiveness of pharmacomechanical thrombectomy. However, it tends to overestimate thrombus removal due to factors such as contrast agent stasis within the thrombus, obstructive anatomy that masks underlying venous lesions, reduced visibility in obese patients, and the limitation of providing only two-dimensional views of three-dimensional structures [51]. IVUS excels in imaging the interface between blood, thrombus, and the lumen wall, and it can diagnose external compression and residual stenosis. This capability allows it to identify inadequate thrombolysis that might be missed when guided solely by venography [51,60]. IVUS helps to visualize residual obstruction areas that are poorly depicted by venography and is more accurate in evaluating the apposition of adjacent stents [56]. Raju et al. [61] demonstrated that stent over-dilatation guided by IVUS resulted in more durable improvements in caliber and better clinical outcomes (Fig. 6). Additionally, IVUS is useful for longitudinal assessments post-EVT when noninvasive imaging is not indicated. This includes assessing the long-term patency of iliac vena cava stents in patients with chronic venous insufficiency [62], evaluating the midterm patency of stents in nonthrombotic iliac venous lesions [54], and routinely monitoring for iliac vena cava stent obstruction [63].