Chapter 23. Peripheral Vascular Disease

Peripheral vascular disease continues to be a leading cause of death and disability in the United States and the Western world.1 Advancements in imaging modalities have enhanced our ability to better understand both the anatomy and pathophysiology of vascular diseases. Health care practitioners with a clear understanding of the available imaging modalities and their utilization will be better positioned to optimize outcomes in patients with peripheral vascular diseases.

In this chapter, we present carotid, aortic, and lower extremity atherosclerotic peripheral vascular disease cases with computed tomography angiography (CTA) images that illustrate key points in diagnosis and treatment.

The recent and rapid advancement of computed tomography (CT) scanners from single-row to multidetector-row scanners has greatly enhanced our ability to image the body's vasculature. Wider detector arrays allow for more rapid image acquisition, using less contrast and less radiation dose. Improved spatial resolution of the new generation of scanners also provides more details of the vascular anatomy including plaque morphology characterization. Postprocessing of these rapidly acquired images with three-dimensional (3D) workstations allows viewing of images in multiple planes and with multiple viewing techniques.

CTA imaging has several advantages in peripheral arterial disease: (1) excellent spatial resolution; (2) exceptional sensitivity and specificity; (3) ability to assess postocclusion anatomy; (4) short examination time; (5) ability to accurately assess vasculature that has been revascularized (stented or bypassed); (6) imaging in multiple planes; (7) lack of interference with metal, including cardiac devices; and (8) patient preference and comfort. There is also the added benefit that CTA and magnetic resonance (MR) can potentially allow for the diagnosis of nonvascular abnormalities that may be clinically significant.

The main disadvantages potentially include: (1) nephrotoxic contrast agents; (2) exposure to radiation; and (3) potential "blooming" (partial volume) artifact from excessive calcium in the vascular system.

A 68-year-old man presents to the office of his primary care physician after experiencing several episodes of transient right hand paresthesias that last for less than 1 hour. He has a history of hypertension, dyslipidemia, and known coronary artery disease with three-vessel bypass 5 years prior to presentation. Physical exam is without focal neurologic deficits but does reveal bilateral carotid bruits. Blood pressure is 95/55 mm Hg. A duplex ultrasound (DUS) of the neck is ordered for evaluation.

Carotid DUS shows a "possible complete occlusion" of the left common carotid artery and "mild" disease within the right carotid system. The patient is referred to cardiology, and a CTA of the neck is ordered for further clarification. See Figs. 23–1, 23–2, 23–3.

Case Discussion

Stroke is the third leading cause of death in the United States, with an estimated 610,000 new strokes and 185,000 recurrent strokes diagnosed each year.1,2 Strokes are costly to diagnose and treat, with cost estimates of $73 billion for 2010.1 An estimated 15% to 30% of all strokes are related to atherosclerotic disease within the carotid bifurcation.3 The atherosclerotic process is postulated to be similar to that of coronary atherosclerotic disease involving areas of low shear stress, inflammation, lipid accumulation, and activation of enzymatic processes.4 The two main mechanisms of transient ischemic attack (TIA) or stroke caused by atherosclerotic disease within the carotid system are embolization and reduced blood flow in the setting of in-adequate collateral circulation or impaired vasoreactivity.5

The patient is a 68-year-old man with several risk factors for stroke and carotid disease who appears to be suffering TIAs in the form of stuttering focal right hand paresthesias. The neurologic symptoms that suggest TIA or stroke related to carotid artery disease are focal and typically involve the ipsilateral middle cerebral artery. TIAs are defined as transient episodes of neurologic dysfunction that are caused by a disruption of blood flow to the central nervous system that does not cause permanent injury and resolve within 24 hours. In the United States, the incidence of TIA is estimated at 200,000 to 500,000, and the prevalence is approximately 2.3%, or close to 5,000,000 people.6 At least 15% of all strokes are preceded by a TIA, underscoring the importance of patients and clinicians pursuing an aggressive evaluation and management strategy.7 The risk of stroke is highest in the 30-day period following a TIA, with the North American Symptomatic Carotid Endarterectomy Trial (NASCET) demonstrating a 20% stroke risk at 90 days, and other studies showing a 1-year mortality of 25%.8,9 These studies demonstrate the need for relatively urgent imaging for risk stratification in patients who have recently experienced TIAs. Factors that predict an upcoming stroke in patients experiencing a TIA are age >60 years, diabetes mellitus, focal weakness or speech impairment, and symptoms that last for longer than 10 minutes.10

TIAs related to carotid disease are usually caused by embolization of debris from the carotid arch or can be caused by low cerebral perfusion as a result of a significant carotid stenosis in the setting of inadequate collateral circulation and impaired vasoreactivity. Clinically, the low flow states usually present with brief symptomatic episodes, whereas embolic phenomena generally result in more prolonged symptoms. The risk of low flow TIAs resulting from total occlusion of the internal carotid is generally greatest at the time of occlusion but can also occur weeks to months later due to thrombotic embolization of the occlusive material, resulting in a delayed stroke.

The risk of stroke caused by carotid plaque is directly correlated with the degree of carotid stenosis.11 Carotid disease is progressive; patients with high-grade stenosis should be followed up every 6 months, and patients with moderate stenosis should be followed up annually. Plaque characteristics such as ulceration, degree of fibrous cap formation, and inflammatory factors also play a role. Active thrombotic plaque is usually found in patients who have suffered a stroke, whereas plaque from patients suffering TIAs or who are asymptomatic are much less likely to show active thrombus.12 Another important finding was that active thrombus was still found on plaques that were removed almost a year after symptom onset.

The patient's symptom of stuttering right arm paresthesias is a common and typical presentation of systemic hypotension leading to reduced cerebral perfusion in a patient with poor collateral circulation, although this could also have been a result of embolic phenomena. The patient was noted to have bilateral carotid bruits. Although assessment of bruits is recommended, the presence or absence of a bruit is a poor predictor of future stroke or significant carotid disease in asymptomatic patients.13 Bruits represent a hemodynamically significant lesion 30% of the time; however, in patients with hemodynamically significant lesions, a bruit was detected in only 50% of patients.14

The use of invasive arteriography and digital subtraction angiography is considered the gold standard when evaluating carotid artery disease. The incidence of stroke during the procedure is greater than 1% in most series, and there are additional morbidities such as access site complications, contrast nephropathy, and the exposure of patients and operators to radiation.15 Given these limitations, noninvasive imaging has taken on a larger role for screening and preoperative assessment of carotid disease. In this case, the initial imaging modality chosen was carotid DUS. The assessment is performed by evaluating peak systolic and end-diastolic velocities proximal and distal to the stenosis. The limitation of DUS is that its effectiveness is operator and patient dependent, with some studies estimating a misclassification rate as high as 25%.16 In general, it is felt that DUS is similar to MR and CT imaging in detecting complete occlusions but less effective in evaluating nonocclusive lesions.16 For stenoses in the 70% to 99% range, the sensitivity and specificity for DUS are 83% and 89%, respectively.17 The limitation of DUS in differentiating between complete and severe occlusions is important because procedural treatment is not indicated in complete occlusions. Given this, alternative confirmatory imaging modalities such as MR angiography (MRA) and CTA play a large role in patients who are likely to undergo carotid surgical or intravascular intervention. In the case presented, a CTA was chosen. This modality has a sensitivity of 80% to 95% and a specificity of 92% to 99% for lesions in the 70% to 99% range, with greater sensitivity and specificity noted in studies using multirow scanners.

In summary, CTA offers an excellent platform when carotid duplex is ambiguous, permitting visualization of aortic arch or high bifurcation pathology, reliable differentiation of total and subtotal occlusion, and assessment of ostial and tandem stenosis. Screening of the general population with imaging studies is not recommended but may be indicated in some patients with a higher prevalence of disease.

In our case, the CT showed the left internal carotid artery (ICA) to be severely stenotic but not occluded. The DUS showed a "possible complete occlusion," which is an absolute contraindication for surgical or interventional treatment approaches. Given the findings of CTA, our patient was referred for carotid endarterectomy (CEA).

Key Points

• Carotid artery atherosclerotic disease is a major cause of TIA and stroke in symptomatic and asymptomatic individuals.
• The risk of stroke is correlated with the degree of carotid stenosis.
• TIAs are a warning sign of future stroke; therefore, imaging and evaluation should take place promptly after a patient experiences symptoms.
• DUS is an effective screening test. Limitations include high variability, operator dependence, and difficulty in distinguishing whether there is a complete occlusion or severe stenosis.
• Screening for carotid artery disease is not recommended for the general public but can be helpful in risk stratifying high-risk individuals.
• CTA is an excellent modality to evaluate the carotid arteries that is both highly sensitive and specific.

The patient is a 67-year-old man with a history of coronary artery disease and multivessel percutaneous coronary intervention several years ago. He is sent for a DUS of his carotid arteries following the complaint of visual changes in his left eye, which shows a left carotid artery stenosis of 80% to 99%.

A CTA is ordered for confirmation and to help guide management. See Figs. 23–4 and 23–5.

The patient undergoes an endarterectomy of the left ICA, but his postoperative course is complicated by right-sided weakness. He returns to the operating department, where a large dissection is noted along the length of the left ICA. Attempts to repair are unsuccessful, and the patient is started on Plavix, aspirin, and Coumadin. He is transferred to the catheterization laboratory for urgent angiography, which verifies the dissection.

CTA of the neck is performed. See Figs. 23–6 and 23–7.

No intervention is performed, and he subsequently has marked improvement in his right-sided motor deficits after 2 weeks. At 8 months, he again experiences right-sided weakness. A CTA is performed. See Fig. 23–8.

The patient undergoes successful stenting of the left ICA. See Fig. 23–9. Three weeks later, he has a recurrent event. CTA reveals a separation of the stents. See Fig. 23–10. The stent separation is treated with repeat stenting with excellent results.

Case Discussion

This is a complex case of a 67-year-old man who is asymptomatic but undergoes CEA because of a severe left ICA stenosis discovered on a DUS screening test. CEA is the gold standard treatment for patients with symptomatic or asymptomatic carotid stenosis, with several randomized control trials showing reduction in ipsilateral stroke. The NASCET trial randomized patients who were symptomatic with >70% stenosis to CEA or medical management. At 18 months, the risk of stroke was 9% in the CEA arm and 25% in the medical management arm,8 with even greater benefit with more severe stenoses. A second cohort also found benefit in patients with 50% to 69% stenosis but no statistically significant difference in patients with less than 50% stenosis. The Asymptomatic Carotid Atherosclerosis Study (ACAS) evaluated asymptomatic patients with stenosis ranging from 60% to 99% and randomized them to CEA or medical management.18 At 5 years, the risk of stroke was 5% in the CEA arm and 11% in the medical arm.18 The European Carotid Surgery Trial (ECST) also showed benefit in treating asymptomatic patients with relatively severe stenosis.19

The Stenting and Angioplasty with Protection in Patients at High Risk for Endarterectomy (SAPPHIRE) trial compared carotid artery stenting (with mandatory distal protection) and CEA. This trial demonstrated noninferiority of CAS in high-risk patients who were asymptomatic or symptomatic.19 Stent fracture is a relatively rare complication that, in this case, was diagnosed with CTA. It was treated with repeat stenting with good result. In general, CTA is preferred over MR with stents that are newly placed.

Key Points

• CEA is indicated in patients with symptomatic carotid stenosis of >50% with greater benefit seen in patients with greater degrees of stenosis (NASCET), if the operative risk is less than 6%.
• CEA showed benefit in asymptomatic patients with stenosis >60% (ACAS).
• Carotid artery stenting is a safe and reasonable alternative to CEA and is preferred in patients who represent a high surgical risk.
• CTA is an excellent and safe modality to evaluate stent architecture and patency.

An 80-year-old man with a history of coronary artery disease, tobacco dependence, and hypertension is referred for evaluation of a pulsating mass in his abdomen. There is no history of pulmonary or renal disease. He denies family history of aneurysm. Physical examination is suggestive of a large abdominal aortic aneurysm (AAA) with findings of a pulsatile mass in the abdomen. An abdominal CTA is ordered. See Figs. 23–11 and 23–12.

Case Discussion

Aortic aneurysm remains an important clinical entity, with an estimated 15,000 deaths annually in the United States.20 An aneurysm is defined as a focal dilatation of the three aortic layers that is 1.5 times greater than that of the normal lumen. They are thought to be caused by collagen and elastin breakdown that is mitigated by inflammatory factors, eventually leading to weakening and distortion of the aortic wall. It is estimated that 25% of aortic aneurysms are thoracic and 75% are abdominal.20 The prevalence estimates of AAA are 4% to 7% in the US adult population, with a male-to-female ratio of at least 5:1.21 It is the tenth most common cause of death in patients older than age 55 years. Aneurysms are characterized as fusiform if the dilatation is relatively symmetrical and circumferential or as saccular if the outpouching of the aortic wall is focal. Thoracic and abdominal aneurysms are usually asymptomatic until the time of rupture, which is usually fatal. AAAs are further classified as suprarenal, juxtarenal, pararenal, or infrarenal. Juxtarenal aneurysms are in close proximity to the renal arteries, and pararenal aneurysms involve the origin of one or both renal arteries. The vast majority of AAAs are infrarenal.

Multiple imaging modalities can be used to image the aorta. CTA is particularly useful when endovascular intervention is being considered. Noninvasive imaging by CTA has become the mainstay for evaluating patients for endovascular aortic reconstruction (EVAR), allowing the clinician to "size" a particular device for a given anatomy.

The patient is an 80-year-old man who presented with a pulsating mass noted on physical exam, with subsequent CTA imaging showing a 7.5-cm infrarenal aortic aneurysm. Palpation of the abdomen for AAA has a sensitivity of 50%, with a specificity of 90% in a high-risk population.22 His risk factors are age, smoking, male sex, hypertension, and previous history of atherosclerotic disease. The risk of AAA in patients who smoke is six to seven times that of nonsmokers.23 The patient does not have a family history of AAA, but some studies show that 25% of those diagnosed with AAA will have a first-degree relative that is affected.20 The prevalence of AAA in patients 75 to 84 years of age is estimated at 12%.20 The large size of the aneurysm in this patient places him in a high-risk category for rupture, thromboembolic complications, or morbidity related to erosion of adjacent structures. For aneurysms greater than 5 cm, the 5-year risk of rupture is estimated at 25% to 40%, and for aneurysms in the 7- to 8-cm range, the 1-year rate of rupture is greater than 25%.24 Larger aneurysms can expand as much as 6 to 8 mm/year.24 In addition, his hypertension and continued use of tobacco independently increase the risk of rupture. The chance of surviving aortic rupture is estimated at 10%, with more than 75% dying before reaching the operating table.25

Treatment options include an open surgical approach, endovascular stenting, and watchful waiting on appropriate medical management. Despite understanding these risks, the patient opted for medical management. Control of blood pressure, lipids, and smoking cessation is imperative. It is a class I recommendation that AAA greater that 5.5 cm be surgically repaired to reduce the risk of rupture.20

A 71-year-old woman with a history of hypertension and family history of AAA is sent for a screening ultrasound, which shows a 4.8-cm AAA. A follow-up CT scan is ordered 6 months later for evaluation. See Fig. 23–13.

A decision is made to treat the patient medically with β-blockers and observation. Six months later, she presents to the emergency room because of a recent fall and lower back pain. A CTA done at this time reveals an AAA measured at 70 mm. Side-by-side review of the two CT scans shows no appreciable difference. This discrepancy occurs when the AAA is measured off axis in an axial view as opposed to centerline. See Fig. 23–14.

During her hospitalization, the patient reconsiders her treatment options and undergoes EVAR. See Fig. 23–15.

The patient remains asymptomatic, but a follow-up surveillance CTA at 6 months reveals a type II endoleak with an increase in the AAA diameter. See Fig. 23–16.

Case Discussion

The patient is a 71-year-old woman with hypertension, a significant smoking history, and a first-degree relative with an AAA. She was appropriately sent for a screening ultrasound, which showed a 4.9-cm AAA. A CT showed a 5.1-cm AAA. Although she met criteria for elective repair, she opted for medical management at that time. Class I recommendations include surveillance imaging with CT or ultrasound every 6 to 12 months for infrarenal aneurysms measuring 4 to 5 cm.20 Ultrasound screening is recommended in patients older than 60 who have a first-degree relative with an AAA or men age 65 to 75 who have been or are current smokers.

Six months later, the patient presented to the emergency room with a recent fall and back pain, and CT imaging at that time erroneously reported a 7.1-cm AAA. The source of error is the off-axis measurement of the AAA in the axial plane (axial slice artifact), which commonly overestimates the aortic diameter. The patient at this juncture decided to proceed to EVAR. A stent graft was placed through a femoral approach with excellent result. In EVAR, a graft is inserted into the aorta at the site of the aneurysm and deployed forming a seal with the aorta acting as a conduit for blood that excludes the aneurysm. After a period of time, the aneurysm will thrombose with eventual shrinking of the aneurysmal sac. Selection of appropriate patients for endovascular repair is based on risk of AAA rupture in comparison to operative risk, life expectancy, anatomic considerations, comorbidities, and patient preference. Unsuitable anatomy alone excludes as many as 50% of all AAAs for potential EVAR. Mortality estimates for elective open surgical repair are between 2.5% and 6%.25 The 2005 American College of Cardiology/American Heart Association guidelines recommend open repair for patients at low surgical risk and EVAR for high-risk patients. According to the guideline, EVAR may be considered in low-risk patients, although with less established evidence of benefit. A meta-analysis published in 2008 that included the Dutch Randomised Endovascular Aneurysm Management (DREAM) and EVAR studies evaluated 21,000 patients who underwent either open surgical or endovascular repair. There was a clear benefit shown in the EVAR group for 30-day mortality and postoperative complications and in long-term aneurysm-related mortality.25

After EVAR, the patient was placed on surveillance protocol with CT scans at 1, 6, and 12 months. Unfortunately, at 6 months, the surveillance CT showed a type II endoleak. Endoleaks are a result of persistent blood flow into the aneurysmal sac after device placement. Type I endoleaks occur at the proximal or distal attachment site and are a result of inadequate graft apposition with the aorta. Type II endoleaks result from flow into the aneurysmal sac from branch vessels. They are the most common form of endoleak. Type III and type IV endoleaks are the result of breakdown, leaks, or tears within the endograft. CTA imaging is an excellent tool to evaluate AAAs for potential EVAR repair as well as in surveillance protocols post-EVAR to assess for endoleaks. At the 1 year mark, the endoleak persists but the aneurysm has not increased.

Key Points

• Aortic aneurysms are clinically very important, with high mortality rates if left untreated.
• Aneurysms can be clinically silent, and clinicians must have a high clinical suspicion and have a command of the best use of imaging technology for early diagnosis.
• CTA imaging of the aorta is highly sensitive and specific.
• CTA imaging is the imaging modality of choice for evaluating aortic aneurysms that may potentially undergo surgical or endovascular repair.

The patient is a 66-year-old woman with a 25-year history of diabetes who presents with a nonhealing wound on her right foot that was sustained following minimal trauma. Physical exam revealed a 3-cm nonhealing ulcer on her right heal. There was a weak right femoral pulse and an absent popliteal pulse. The ankle-brachial index (ABI) was 0.6.

A CTA is ordered of the peripheral vasculature. See Figs. 23–17, 23–18, 23–19.

Case Discussion

Lower extremity peripheral arterial disease (PAD) is a common and growing problem. The Intersociety Consensus for the Management of Peripheral Arterial Disease (TASC II) consensus document estimates that 27 million patients are affected in the United States and Europe.26 The prevalence increases with age, with an estimated 10% to 14% of people above the age of 70 having an ABI of less than 0.9.26 Classic symptoms include rest pain and claudication. One study estimates that 3% to 6% of American men older than age 60 will experience intermittent claudication due to obstructive disease of the aortoiliac and lower extremities.27 Imaging methods used to assess lower extremity PAD include invasive angiography, DUS, MRA, and CTA. The rapid evolution of CTA technology has led to a dramatic increase in its use to evaluate disease and as a guide to treatment. Many physicians use CTA to determine a management strategy after PAD is diagnosed with physical exam or with noninvasive DUS. For interventionalists, CTA has become an important modality for mapping vascular anatomy and delineating location and severity of stenosis. This information is of paramount importance in selecting patients who are candidates for endovascular or surgical revascularization. Additional information including presence of aneurysms, popliteal entrapment, and cystic adventitial disease are also provided by CTA. In this group of patients, multidetector CT is noted to have high sensitivity (96%) and specificity (97%) and excellent interobserver agreement when compared with digital subtraction angiography (DSA) as the gold standard.28,29 One disadvantage can be the overestimation of the stenosis, which is seen with heavily calcified plaque due to partial volume artifact (blooming). In many cases, this can be overcome by using thin slice multiplanar reformation (MPR) images and routinely adjusting window and leveling setting in the presence of calcium. In addition, postprocessing filters (kernels) can be used, which improve edge detection at the cost of increased image noise.

Image acquisition with run-off studies may be challenging because the large areas covered by wide detector arrays can cause one to "outrun" the bolus. In essence, this means that the CT images are acquired before the contrast has had time to arrive at the area of interest. Routine rescan of the run-off vessels (popliteal trunk and anterior and posterior tibial and peroneal vessels) can be programmed into the run-off protocol on most machines to minimize the likelihood of this occurring.

Sensitivity of CTA for detection of stenosis more than 50% in the lower extremity including inflow and distal crural arteries is in the order of 90%. Another potential limitation of CTA is difficulty in analyzing the distal peroneal, anterior tibial, and posterior tibial vessels, especially in the presence of severe calcification. This information is very important when bypasses to these vessels are being considered. In this situation, MRA has been shown to be useful. In the authors' experience, these limitations can usually be overcome with good acquisition parameters and the utilization of 3D workstations that allow multiplanar imaging and thin slice MPR. CTA offers the clinician the ideal platform for analyzing postsurgical or catheter-based patients who present with a sudden change in their claudication symptoms.

Key Points

• MDCT provides nonvascular tissue and organ images and is the preferred modality when deciding on a treatment strategy.
• CTA is less invasive and faster, with less exposure to ionizing radiation, when compared with conventional DSA.
• CTA is the preferred modality in the presence of surgical clips, metal implants, and pacemakers.
• Limitations to run-off CTA include mistimed contrast and heavily calcified vessels below the knee.