Peripheral Vascular Disease Imaging

Peripheral vascular disease (PVD), or atherosclerosis of peripheral vessels, is the most common cause of symptomatic stenosis in the human vascular tree. The pathogenetic mechanisms that lead to PVD are similar to those of coronary artery disease (CAD). The risk factors are also similar and include a positive family history, cigarette smoking, diabetes, hypertension, dyslipidemia, advanced age, and physical inactivity, among others.[1, 2]  PVD, also known as arteriosclerosis obliterans, manifests as insufficient tissue perfusion resulting from atherosclerosis compounded by emboli or thrombi. The atherosclerotic process may gradually progress to complete occlusion of medium-sized and large arteries. it is estimated that 1% of persons older than 50 years in the United States have pulmonary arterial disease or chronic limb ischemia.[3]

Imaging modalities

The criterion standard for intraluminal obstruction is arteriography. Delay in performing arteriography in the setting of limb ischemia can result in delayed treatment. If time allows, arteriography can prove useful in discriminating thrombotic disease from embolic disease.

Doppler ultrasound studies are useful as primary noninvasive studies to determine flow status. Upper extremities are evaluated over the axillary, brachial, ulnar, and radial arteries. Lower extremities are evaluated over the femoral, popliteal, dorsalis pedis, and posterior tibial arteries.

Carotid duplex ultrasonography (US) is a noninvasive means by which to estimate the degree of cervical carotid stenosis.  US has the advantage of being available as a portable examination in the intensive care unit (ICU) or the coronary ICU. Carotid CT angiography (CTA) is a commonly performed imaging study in stroke centers.[3]  

Magnetic resonance imaging (MRI) may be beneficial because of its high visual detail. Plaques are easily seen, as well as the difference between vessel wall and flowing blood. MRI may be limited in the emergency setting because of its location and the technical skill required. Benefits of magnetic resonance angiography (MRA) include high diagnostic accuracy and the avoidance of exposure to ionizing radiation; drawbacks include limited availability and increased cost. Time-of-flight (TOF) MRA is useful for patients who cannot tolerate iodinated contrast agents used in CTA. Contrast-enhanced MRA using a gadolinium MR agent offers improved visualization in areas of high-grade stenosis where TOF MRA may falsely indicate a short-segment occlusion.[4, 5, 6, 7]

Quiescent interval slice-selective (QISS) MRA is a cardiac-gated technique described by Edelman et al for the evaluation of the lower extremities.[8]

Computed tomography (CT) can be of use in the emergency department because of its availability. Contrast studies are most useful for imaging arterial insufficiency. PVD often coexists with risk factors for contrast-induced renal failure. High-definition CT studies may help guide treatment decisions. Benefits of CT angiography (CTA) include rapid noninvasive acquisition, wide availability, high spatial resolution, and the ability to generate isotropic datasets on 64-detector-row and higher CT scanners; drawbacks include exposure to iodinated contrast and ionizing radiation.

Molecular imaging with radionuclide-based approaches may potentially provide a novel noninvasive assessment of biologic processes in PVD, such as angiogenesis and atherosclerosis. 

(Angiograms demonstrating PVD are shown below.)

Bilateral aortoiliac stenosis.

Superficial femoral artery stenosis causing claudication.

Left subclavian artery stenosis.

Left renal artery stenosis.

Percutaneous revascularization with techniques such as percutaneous transluminal angioplasty (PTA), a less invasive option in the management of peripheral vascular disease (PVD), has been furthered by the work of pioneers such as Dotter and Gruntzig.[9]  There has been steady growth in the use of PTA, and it has become the first-line therapy for PVD (see the images below).[10, 11]

Percutaneous transluminal angioplasty of superficial femoral artery stenosis, performed with a long balloon via a contralateral femoral approach.

Angiogram obtained after percutaneous transluminal angiography for superficial femoral artery stenosis.

Noninvasive and invasive modalities are used for diagnostic evaluation in iliofemoral disease. Noninvasive testing includes Doppler ultrasonography and magnetic resonance angiography (MRA).[12, 6]

Although digital subtraction angiography is the gold standard, duplex ultrasound has shown good accuracy in the detection of femoropopliteal lesions. Khan et al proposed 200 cm/s peak systolic velocity and a 2.0 velocity ratio to discern between < 70% and ≥70% stenosis in the femoropopliteal arterial segment.[13]

Doppler ultrasonography and evaluation of the ankle-brachial index

For Doppler ultrasonography, pneumatic cuffs are placed along the leg and are inflated to suprasystolic pressures. During controlled cuff deflation, a Doppler probe is placed over the dorsalis pedis artery or posterior tibial artery to detect the onset of flow. Normally, the systolic blood pressure in the leg is slightly higher than that in the arm; therefore, the normal ankle-brachial index (ABI) of systolic blood pressure should be 1.0 or slightly greater. An ABI of less than 0.95 is considered abnormal. Patients with leg claudication typically have an ABI of less than 0.8. In patients with ischemia at rest, the ABI is frequently less than 0.4.

Magnetic resonance angiography

MRA is another noninvasive approach for imaging the peripheral circulation. It does not involve the risk of intravascular catheterization or conventional contrast agents.[4, 5, 14]

Angiography

Invasive imaging with contrast arteriography is required when the diagnosis remains unclear or endovascular procedures are planned (see the images below).

Superficial femoral artery stenosis causing claudication.

Preangioplasty left superficial femoral artery angiogram in a middle-aged woman with severe left leg claudication and an ankle-brachial index of 0.5 on preadmission noninvasive assessment was recorded via a contralateral approach after sterile prepping and draping of the patient, administration of conscious sedation, the infiltration of local anesthetic (usually lidocaine 1% or 2% solution) at the right femoral access site, placement of an arterial sheath in the femoral artery, and advancement of the contra guide catheter over 0.035-in guidewire under fluoroscopic guidance. The tip of the guide catheter is taken beyond the aortobifemoral junction and positioned into the right iliac artery. An angiogram (as shown) is obtained after the guidewire is removed. The proximal end of the catheter is connected to a manifold and 4-8 mL of contrast agent is manually injected during cineangiographic recording. The image may be played in a loop, or a particular frame may be saved for use as a roadmap during angioplasty. Intravenous antithrombin agent, usually heparin, is administered as a bolus (generally 3000-4000 IU) before angioplasty. The patient's activated clotting time is monitored, with continuous monitoring of intra-arterial pressure, pulse oximetry, and heart rate.

Atherosclerotic disease that causes stenosis of more than 50% in at least 1 renal artery is encountered in 30% of patients with CAD, 38% of patients with abdominal aortic aneurysms (AAAs), and 39% of patients with iliofemoral disease (see the image below). In approximately one third of cases, renal artery disease is bilateral. About 11% of renal arteries with stenoses of greater than 60% progress to total occlusion within 2 years.[15, 16, 17]

Left renal artery stenosis.

In the past, captopril renography was used in the diagnosis of bilateral renal artery stenosis. The possibility of the development of renal atherosclerosis may be assessed just as accurately on the basis of clinical parameters, such as advanced age; female sex; the presence of atherosclerosis in other vascular beds; the recent onset of hypertension; smoking; the presence of abdominal bruits; an elevation in the creatinine level; and hypercholesterolemia. MRA has emerged as a useful noninvasive imaging method for diagnosing renal artery stenosis.

Flush abdominal aortography in patients undergoing coronary arteriography may be performed when the likelihood of renovascular disease is high. This imaging is usually performed by placing a pigtail catheter at the level of the first lumbar vertebra and injecting contrast material at a rate of 20 mL/s to achieve a total contrast-agent volume of 6-12 mL.

Subclavian and brachiocephalic artery disease

Patients with atherosclerotic disease of an upper extremity may develop symptoms of ischemia, but these occur less commonly than do symptoms of iliofemoral disease. Most patients with atherosclerotic obstruction of the subclavian or brachiocephalic arteries are asymptomatic. Usually, the condition is incidentally discovered when there is a difference between blood pressure measurements of a patient's arms or when evidence of obstructive disease is observed during angiography or during a noninvasive evaluation.

(An angiogram of subclavian artery stenosis appears below.)

Left subclavian artery stenosis.

Carotid artery disease

A diagnosis of carotid artery disease by means of physical examination alone is probably inaccurate. The anatomic diagnosis of carotid disease may be confirmed through noninvasive or invasive angiographic approaches (see the image below). Some authors advocate the use of duplex and transcranial Doppler ultrasonography as the first step in the evaluation of carotid disease; this approach is accurate in 90% of cases. The types of stroke include ischemic stroke, cardioembolic stroke, and others.

Left carotid artery stenosis.

MRA has emerged as a noninvasive means of visualizing the carotid, vertebrobasilar, and major intracranial vessels, but it provides less detail than contrast-enhanced modalities. When the combination of MRA and Doppler ultrasonography is used, however, nearly 100% specificity in defining the hemodynamic severity of carotid stenoses is achieved.

Although aneurysms may affect any arterial bed, infrarenal abdominal aortic aneurysms (AAAs) account for most arterial aneurysms. An AAA is defined as an aneurysm having a diameter that is greater than 3 cm. Most AAAs are incidentally discovered during abdominal ultrasonography or angiographic examinations performed for other indications.

The growth rate for AAAs is variable; the average growth rate is 0.3-0.5 cm/yr. Whether or not an aneurysm ruptures is most strongly related to its size.

Accurate sizing requires the performance of abdominal ultrasonography, computed tomography (CT) scanning, or magnetic resonance imaging (MRI).[18, 19, 20] Aortography is not a reliable means of determining the size of the aneurysm, because a laminated thrombus, if present, may cause the size of the aneurysm to be underestimated.

Guidelines

The American College of Radiology in its Appropriateness Criteria on pulsating abdominal masses noted that imaging studies are important in diagnosing the cause of a pulsatile abdominal mass and, if an AAA is found, in determining its size and involvement of abdominal branches. The ACR recommendations for diagnostic imaging and interventional planning and follow-up include the following[21, 22, 23]  :

• Ultrasound (US) is the initial imaging modality of choice when a pulsatile abdominal mass is present.
• Noncontrast computed tomography (CT) may be substituted in patients for whom US is not suitable.
• Contrast-enhanced multidetector CT angiography (CTA) is the best diagnostic and preintervention planning study, accurately delineating the location, size, and extent of aneurysm and the involvement of branch vessels.
• Magnetic resonance angiography (MRA) may be substituted if CT cannot be performed.
• After repair of AAA, CTA of the abdomen and pelvis and MRA of the abdomen and pelvis are appropriate follow-up procedures. 
• FDG-PET remains primarily a research tool but shows promise for assessing the metabolic activity of aneurysms.

The U.S. Preventive Services Task Force recommends a one-time screening for AAA with ultrasonography in men who are 65-75 years of age and have a history of smoking (ie, “ever smoker”: at least 100 cigarettes during lifetime). They also recommend selectively offering screening for mean 65-75 years of age who do not have a smoking history. They recommend against routine screeing in women who have never smoked and feel there is insufficient evidence to recommend screening in women who are 65-75 years of age and have a smoking history.[24, 25]