Critical limb ischemia (CLI) is the most severe manifestation of peripheral arterial disease (PAD) and presents with severe, chronic rest pain or ischemic skin lesions (ulcer or gangrene). It is the major cause of amputation of ischemic limbs in the United States; the annual incidence of CLI is 500 to 1000 cases per million people. Revascularization by endovascular or open surgical technique is recommended, depending on the severity of the disease.

Many imaging modalities can identify an arterial lesion that is a candidate for endovascular or open surgical technique. Angiography is the gold-standard diagnostic test but it is a high-risk, invasive, and costly procedure (Figure 1).

Computed tomography angiography (CTA), using multidetector computed tomography (MDCT) technology and magnetic resonance angiography (MRA), is highly accurate in diagnosing PAD, equivalent to angiography. CT acquisition is rapid and less prone to motion artifacts than MRA is, but its disadvantage is exposure to doses of radiation and the use of iodinated contrast to enhance the vessel visualization. Also, CTA produces streak artifacts from heavy calcification or metallic materials, resulting in a limited evaluation of vascular patency (Figure 2).

MRA produces an image without contrast or interference from calcium, but its disadvantages are it is time-consuming and expensive, it cannot be performed on patients with metallic devices or on patients who might experience claustrophobia from a closed area, and it might produce false-positive results (Figure 3).

Duplex ultrasound is widely available for the screening and diagnosis of vascular lesions.  It is safe without risk of radiation exposure, easily repeatable, relatively cost-effective, and causes minimal discomfort. Several studies have shown that duplex ultrasound has good sensitivity, specificity, and diagnostic agreement with angiography in assessment of lower-extremity arterial disease in critical limb ischemia. The peak systolic velocity (PSV) quantified by spectrum analysis from spectral pulsed-wave (PW) Doppler ultrasound was the most common criterion used for diagnosis and categorizing the severity of the disease in most previous studies. The duplex scanning protocol begins with B-mode and color Doppler ultrasound to identify the lesion and assess blood flow before sampling the Doppler signal by spectral PW Doppler ultrasound. The waveform feature analysis (triphasic or monophasic, acceleration time, spectral broadening, turbulence, or direction) and velocity measurement can be obtained at the proximal, distal, and overall points of the lesion when suspicion of abnormality exists. Using mutual interpretation of the findings from B-mode, color, and spectral PW Doppler ultrasound (waveform analysis and velocity measurement) is the key concept of using duplex ultrasound for the diagnosis of vascular disease and PVD (Figure 4).