Imaging in Lower-Extremity Atherosclerotic Arterial Disease

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The first step in imaging assessment of a patient with lower-extremity atherosclerotic arterial disease is to record pulse-volume (plethysmography) and blood pressure measurements in the upper and lower extremities to compare the pressures. An ankle-brachial index (ABI) is determined. This is usually measured by dividing the highest systolic measurement in the lower extremity by the measurement in the upper extremity on the same side. An ABI of less than 0.95 is a strongly predictive sign of lower-extremity perfusion compromise. This noninvasive test provides information regarding the intravascular blood flow at different sites of the leg (upper thigh, lower thigh, above the ankle) as a waveform. Triphasic readings are normal and change to biphasic or monophasic in the diseased state. ^{[1, 2]}

Doppler ultrasonography (US) has become the second line in the evaluation of lower-extremity arterial disease. Doppler US findings provide good information about the anatomy and physiology of the vessels. Spectral Doppler ultrasonography 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. ^{[3, 4, 5, 6, 7]}

CT angiography (CTA) and MRA are quite comparable in visualizing vessels and can provide much of the diagnostic information of the lower-extremity arteries that can be obtained with conventional angiography. A prospective study comparing CTA, MRA, and digital subtraction angiography (DSA) showed similar diagnostic accuracy. Therapeutic confidence in the scale of 0-10, 10 representing the highest confidence, showed similar confidence level of MR, 7.7; CT, 8.0; and angiography, 8.2.

Images of lower-extremity atherosclerosis are provided below.

CTA provides higher resolution and can scan the entire volume quickly. It allows evaluation of arterial calcium, vascular stents, volume rendering, and imaging unstable patients. MRA is a fast procedure, providing much of the diagnostic information that can also be derived from catheter angiography with less risk. MRA is generally used in young patients and in patients with contrast allergy or renal insufficiency. MRA should not be used in patients with pacemaker and other implants. MRA is not effective in unstable and uncooperative patients. Generally, CTA is the preferred imaging modality for planning endovascular interventions for abdominal and thoracic aortic aneurysms and lower-extremity arteries. ^{[8, 9, 10]}

Digital subtraction arteriography is the most accurate test used to define the anatomy and degree of pathology, but it is indicated only when intervention is considered. It is commonly performed using isosmolar, nonionic, iodinated contrast agent with ionizing radiation.

Digital subtraction arteriography can be performed using CO₂. CO₂ is the only proven safe contrast agent in patients with contrast allergy and renal failure. CO₂ is currently used for aortogram and runoffs, primarily in patients with allergies and renal failure. In nearly 100% of patients, the entire runoff can be imaged with CO₂ and with the addition of small volumes of iodinated contrast medium. The buoyancy and low viscosity of CO₂ provides better filling than iodinated contrast of collateral arteries. The plastic bag system is useful as multiple CO₂ injections are required to complete iliac arteriograms and runoff. If used correctly, the bag system is safe without air contamination. Recently, AngioDynamics discontinued manufacturing the plastic bag system (AngioFlush 111 fluid collection system and AngioFlush 111 fluid management system).

Arterial access is made to the common femoral artery contralateral to the extremity with the symptoms. A 4-F OmniFlush or a Shepherd hook catheter is advanced into the aortic bifurcation and into the contralateral common iliac artery, and iliac arteriograms are performed in the AP and ipsilateral oblique projections with the injection of 20-30 cc of CO₂. The catheter is then further advanced into the common femoral or the superficial femoral artery for CO₂ runoff.

If the infrapopliteal arteries are poorly visualized, the intra-arterial administration of nitroglycerin will improve filling of the peripheral arteries. If the distal arteries are still poorly seen, a 3-Fr microcatheter is advanced coaxially into the popliteal artery for CO₂ injection. Because of the buoyancy of CO₂, elevation of the feet will improve distal arterial filling with CO₂. For the ipsilateral runoff, the catheter is positioned in the external iliac artery for CO₂ injection. If CO₂ has to be injected into the distal artery for improved arterial filling, the catheter should be changed from the retrograde to antegrade placement using a reversed hook catheter.

CO₂ is useful for most interventional procedures of the iliac and lower-extremity arteries in patients with contrast allergy and renal failure. The advantages of the gas are the lack of allergy and renal toxicity, injection of unlimited volumes of CO₂, and the low viscosity that permits the injection of CO₂ between the needle or catheter and the guidewire.

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. ^[11]

Doppler US is a valuable diagnostic test; it is inexpensive and widely available but does not offer detailed description of the length, severity, or type of the diseased portion of the vessel, all of which help in planning surgical or endoluminal intervention. Although vascular mapping can be performed to evaluate the iliac vessels and the femoropopliteal arterial segments, it is time and labor consuming (with examinations sometimes requiring as long as 2 h). It is also operator dependent.

Arteriography remains the most accurate and informative test. Arteriography is the criterion standard, but it is considered an invasive diagnostic method. This examination is associated with complications such as hematoma at the puncture site; complications due to radiation exposure, intimal flap dissection, or arterial wall rupture; and nephrotoxicity due to the intravenous contrast material (which poses greater risk because of the common association of LEPAD with renal arterial disease and renal disease). Therefore, arteriography is preserved for preoperative evaluation only.

MRA is a rapidly developing and a promising study that may replace diagnostic angiography in the future. MRA is noninvasive, it does not require the use of ionizing radiation, and the contrast agent used is relatively non-nephrotoxic. This modality is associated with limitations such as its cost, its availability, the limited depiction of small vessels, its contraindications, and the possible overestimation of the degree of stenosis.

Magnetic resonance angiography (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. ^{[8, 12, 13, 9, 14, 15, 16]}

Magnetic resonance images of lower-extremity atherosclerosis are provided below.

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.

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.

Spectral Doppler ultrasonography 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. ^{[3, 4, 5, 6, 17]}

Ultrasound images of lower-extremity atherosclerosis are provided below.

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.

Lower-extremity peripheral arterial disease (LEPAD) is 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.

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.

Gabriel et al concluded that the use of duplex ultrasound arterial mapping (DUAM) to identify and define lesions in lower extremity arteries preoperatively was particularly successful for intravascular procedures. ^[18]

Lower-extremity arterial disease occurs in 2 types: acute ischemia and chronic ischemia. 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).

Angiographic images of lower-extremity atherosclerosis are provided below.

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 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.

When the disease occurs at the level of the common femoral artery , 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.

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, the peroneal artery often is patent and supplies blood through the perforating branches to the foot.

A combined pattern is most common in patients with severe ischemia. It usually involves the femoropopliteal portion, along with the anterior and/or posterior tibial arteries.

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.

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Chadi Chahin, MD Consulting Staff in Vascular and Interventional Radiology, Glendale Adventist Medical Center

Chadi Chahin, MD is a member of the following medical societies: American College of Radiology, Society of Interventional Radiology

Disclosure: Nothing to disclose.

Barry Rose, MD Former Associate Professor of Radiology, Northeast Ohio Medical University- NEOMED

Barry Rose, MD is a member of the following medical societies: American College of Radiology, American Medical Association, American Roentgen Ray Society, Ohio State Medical Association, Radiological Society of North America

Disclosure: Nothing to disclose.

Sam Stuhlmiller, MD Consulting Staff, Department of Radiology, Aultman Hospital

Sam Stuhlmiller, MD is a member of the following medical societies: American Roentgen Ray Society, Radiological Society of North America

Disclosure: Nothing to disclose.

Kyung J Cho, MD, FACR, FSIR William Martel Emeritus Professor of Radiology (Interventional Radiology), Frankel Cardiovascular Center, University of Michigan Health System

Kyung J Cho, MD, FACR, FSIR is a member of the following medical societies: American College of Radiology, American Heart Association, American Medical Association, American Roentgen Ray Society, Association of University Radiologists, Radiological Society of North America

Disclosure: Nothing to disclose.

Anthony Watkinson, MD Professor of Interventional Radiology, The Peninsula Medical School; Consultant and Senior Lecturer, Department of Radiology, The Royal Devon and Exeter Hospital, UK

Anthony Watkinson, MD is a member of the following medical societies: Radiological Society of North America, Royal College of Radiologists, Royal College of Surgeons of England

Disclosure: Nothing to disclose.

Imaging in Lower-Extremity Atherosclerotic Arterial Disease

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