eMedicine Specialties > Radiology > Vascular/Interventional

Lower-Extremity Atherosclerotic Arterial Disease: Imaging

Author: Chadi Chahin, MD, Staff Physician, Department of Radiology, Aultman Health Foundation/Mercy Medical Center
Coauthor(s): Barry Rose, MD, Associate Professor of Radiology, Northeastern Ohio Universities College of Medicine; Program Director, Department of Radiology, Aultman Hospital; Sam Stuhlmiller, MD, Consulting Staff, Department of Radiology, Aultman Hospital
Contributor Information and Disclosures

Updated: Feb 9, 2007

Radiography

Findings

Conventional radiographs of the lower extremities are not useful in screening or confirming LEPAD. Conversely, lower-extremity arterial calcifications are common incidental findings. The presence and extent of arterial wall calcification is not correlated with the clinical symptoms.

Computed Tomography

Findings

Currently, contrast-enhanced arterial CT is evolving in the diagnosis and treatment of lower-extremity vascular disease. Disadvantages of conventional CT include the use of ionizing radiation and the requirement for contrast materials.

Magnetic Resonance Imaging

Findings

Recently, MRA has emerged as a safe and noninvasive alternative to conventional angiography in the diagnosis of lower-extremity vascular disease. Using MRA studies, a radiologist should be able to detect signs of narrowing (stenosis), dilatation (aneurysm) in the vessel, or a complete interruption of flow, and he or she should be able to compare the results in both legs.

Initial reports used 2-dimensional (2D) and 3-dimensional (3D) time-of-flight (TOF) MRA with limited success. These methods rely on the detection of flow-related phenomena to produce angiographic images. Images were degraded by patient motion (primarily due to long scanning times, which may been > 1 h). Other causes of poor image quality include turbulence, pulsating arteries, saturation, and poor signal-to-noise ratios (SNRs). However, the TOF is still considered a better technique than contrast-enhanced MRA for evaluating infrapopliteal vessels because MRA depends on blood flow in the immediate vicinity of the region imaged.

Contrast-enhanced 3D MRA has become the method of choice. The technique relies on the detection of contrast enhancement in the vascular lumen to produce findings that are comparable to those of conventional catheter angiography. The current technique uses the bolus-chasing method material in which vessels are imaged sequentially as contrast flows distally. Multiple overlapping fields of view are used, and images are obtained in the coronal or sagittal planes (usually in 3 coronal stations). This technique also uses subtraction to improve the resultant vascular images by suppressing the background and reducing the volume averaging.

Images demonstrate the contrast-enhanced anatomy of the arterial lumen. Stenosis is depicted as areas of narrowing, and occlusion is depicted as areas of absent signal intensity. Ulceration and aneurysm can also be defined.

Use of bolus-chasing MRA enables radiologists to establish protocols for different studies by adjusting the bolus dose and time, the infusion rate, the region of interest, the section thickness, and the position in the imaging plane by considering the purpose of the study, the patient's condition, and the equipment available.

Bolus-chasing MRA is rapidly evolving for many reasons such as the technology revolution that made equipment widely available, improvements in technical capabilities (eg, increased field strengths, dedicated coils, increased SNRs, decreased repetition times, improved bolus-detection techniques, MR SmartPrep technique). With these changes, along with the increased familiarity and confidence of referring physicians with this new modality, bolus-chasing MRA will replace conventional catheter angiography.

False Positives/Negatives

The value of the study can be increased by using newer magnetic resonance devices with higher resolution, by using gadolinium-based contrast agent, and by having the expertise to interpret the images. However, even when all available resources are used, false-positive and false-negative results may still be encountered, especially in patients who have undergone previous interventions. For example, indwelling stents can cause severe artifacts and may render findings inaccurate or nondiagnostic.

To date, no established data are available; however, multiple studies reveal a sensitivity and specificity of more than 90% with the bolus-chase technique. The rate is slightly better in evaluating the iliac, femoral, and popliteal segments.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Ultrasonography

Findings

Spectral Doppler US and color-flow vascular imaging supplement gray-scale US in identifying blood vessels, confirming the direction of blood flow, and detecting vascular stenosis or occlusion.

Gray-scale US illustrates the anatomy of the scanned region, usually in vertical or longitudinal planes. Blood flow and its speed and direction are detected by measuring the Doppler shift originating from a sample volume inside the artery.

Doppler shift is measured by processing the returning Doppler signals by using the fast–Fourier transform spectrum analyzer that sorts the data into individual components and displays them as a function of time on velocity scales (displayed in real time). Blood velocity and frequency shift are directly related mathematically. Therefore, faster movement of the red blood cells (RBCs) and the larger numbers of cells in the vessel moving toward the transducer result in the representation of greater velocity on the screen; this is depicted as brighter colors.

In addition, data are displayed as waveforms. Different blood vessels have unique flow characteristics that can be recognized by the Doppler spectral waveforms produced. Two major waveforms are identified, ie, high resistance and low resistance ones. The type of waveform is determined by the type of vessel and by vessel compliance. The waveform is also defined by its monophasic, biphasic, or triphasic pattern. Triphasic patterns are found most often in the lower extremities.

Using gray-scale technique, a significant atherosclerotic vascular lesion can be detected only by thickening of the vessel wall or segmental narrowing of the lumen (which usually represents plaque or mural thrombus). Aneurysms and intimal flaps may also be identified.

LEPAD often diagnosed by using US, which depicts a change in the flow pattern on Doppler spectrum imaging. Proximal to the lesion, the flow pattern is normal. At the stenosis, the peak systolic velocity increases in proportion to the degree of stenosis. The diastolic portion of the Doppler waveform depends on the artery distal to the lesion and the severity of the lesion. Diastolic flow may be significantly increased or absent. Systolic velocity distal to the lesion is equal or lower than the velocity proximal to the stenosis.

The peak systolic velocity is affected less by distal vasodilation than by diastolic velocity (which also affects the collateral vessels that develop because of the decreased blood supply). Therefore, the peak systolic velocity is the preferred Doppler velocity parameter to be measured by using the Doppler spectrum at the site of a suggested stenosis. Vascular stenosis may also be reflected as a change of the waveform from triphasic to biphasic or monophasic.

The absence of a flow signal may represent occlusion, vascular calcifications, or technical error. Thrombosis is usually seen as echogenic material in the artery. Large collateral branches are likely to indicate high-grade stenosis or more distal occlusion.

A thorough examination provides information about the entire common femoral, superficial femoral, and popliteal arteries. Examination of the deep femoral and tibial vessels is usually limited.

False Positives/Negatives

Multiple published studies evaluated the femoropopliteal segment. The reported sensitivity was more than 85%, and the specificity was more than 92% in detecting segmental arterial lesions.

Angiography

Findings

Arteriography is the most accurate method with which to identify the diseased segment of the artery because it can reveal stenosis, dilatation, occlusion, plaque ulceration, or thrombotic material (which is usually manifested by filling defects).

Lower-extremity arterial disease occurs in 2 types, ie, acute ischemia and chronic ischemia.

Acute ischemia

Acute ischemia usually results from a thromboembolic event. Usually, the patient complains of an acute onset of symptoms, especially severe pain. Angiographic findings in this setting are cutoff or runoff with few or no collaterals, which may occur at any level, although these are more common at the level of tibial or foot arteries.

Chronic ischemia

Chronic ischemia occurs in as many as 80% of patients with lower-extremity arterial disease. The occlusive process demonstrates different patterns according to the level of the lesion and the pattern of the disease.

• At the level of the common femoral artery: When the disease occurs in this location, a collateral circulation is derived from the common iliac-iliolumbar artery, the internal iliac–superior gluteal artery, the internal iliac–inferior gluteal artery, or the external iliac–deep iliac circumflex artery. All of these branches reconstitute with the profunda femoris artery to supply blood to the lower extremity.
• At the level of the superficial femoral artery: Disease may occur at the level of proximal, middle, or distal third. In all cases, the collateral circulation from the profunda femoris artery to the popliteal artery carries blood to the lower limb.
• At the level of the profunda femoris artery: The profunda femoris artery is almost always spared in atherosclerotic occlusive disease. If it is affected, it has a great significance in the planning of future interventions.
• At the level of the popliteal artery: Depending on the level of obstruction, the geniculate arteries form collaterals to supply blood to the anterior and posterior tibial distal to the level of obstruction.
• At the level of the anterior or posterior tibial arteries: In this location, the peroneal artery often is patent and supplies blood through the perforating branches to the foot.
• Combined pattern: This finding is most common in patients with severe ischemia. It usually involves the femoropopliteal portion, along with the anterior and/or posterior tibial arteries.
• At the level of the foot vessels: The arterial network of the foot (the foot or plantar arch) forms a continuous circuit that supplies blood to all segments, even in the presence of arterial occlusion.

• Search for CME/CE on This Topic »