An Introduction to Cerebrovascular Disease

Cerebrovascular disease is a major cause of cardiovascular morbidity and mortality and results from carotid and vertebral stenosis in the setting of atherosclerotic disease. When left untreated, progression of this disease can lead to occlusion, embolization, or plaque rupture, causing neurologic sequelae such as transient ischemic attack or stroke leading to potential permanent neurologic dysfunction and sometimes even death.

Angiography was the initial diagnostic test of choice for cerebrovascular atherosclerotic disease. This invasive study provided anatomic definition of any lesions but required selective catheterization of the great vessels and predisposed patients to risks of periprocedural stroke, contrast nephropathy, and access site complications. The modern era of cerebrovascular diagnostics instead utilizes duplex ultrasonography as a minimally invasive tool, capable of assessing not only anatomy but vessel hemodynamics with the use of spectral Doppler imaging.

The Role of Duplex Exam in Carotid Disease

Duplex exam of the carotid arteries is normally performed with the patient in a supine position and the sonographer at the patient’s head. Imaging conventions stipulate positioning of the probe such that the head of the patient is at image left for longitudinal views, and the patient’s right is at image left on transverse views as if viewed standing at the foot of the patient. The patient should be at rest for at least 5 minutes before beginning any examination in order for blood flow to reach a physiologic resting state.

Images can be obtained in a variety of positions and from a variety of angles, allowing the sonographer to visualize different portions of the circulation. The standard position is the posterolateral projection, in which the transducer is placed longitudinally along the vessel at an angle of 45 degrees from the horizontal. Spectral Doppler and color-flow data are readily obtained from this position.

Longitudinal brightness-mode view of carotid artery.

The transverse position enables the sonographer to follow the carotid artery in a transverse plane along its entire course in the neck, which is useful for initial identification of the carotid, its branch points, and position relative to the jugular vein. Other positions of the probe either in more anterior or posterior positions can help with visualization in patients with very distal disease or with large or thick necks.

Transverse brightness-mode view of common carotid artery.

Normal Carotid Imaging and Flow

The common carotid artery (CCA) lies deep to the sternocleidomastoid and jugular vein. The artery and vein can be differentiated by direction of flow on color Doppler as well as by the tendency of the vein to collapse with external ultrasound probe compression.

The normal spectral Doppler waveforms differ between the different components of the carotid system. The common carotid generally has medium pulsatility on spectral Doppler imaging, with peak systolic velocities (PSV) around 90 cm/s; however, velocities tend to decrease from proximal to distal, and a velocity immediately preceding the carotid bulb of 10-20cm/s lower than velocities measured proximally is not abnormal. The carotid bulb itself tends to have turbulent flow rather than laminar, causing pockets of retrograde flow on color Doppler. For this reason, peak systolic velocity measurements of the common carotid artery should be obtained approximately 2cm proximal to the carotid bulb [1].

Normal common carotid artery duplex.

The external carotid artery (ECA) displays many of the characteristics of a high resistance vessel, including a high pulsatility waveform. Velocities vary widely between patients but peak systolic velocities around 77 cm/s have generally been accepted as
normal [1].

The internal carotid artery (ICA) is a lower resistance vessel and displays low to medium pulsatility on spectral imaging with no or minimal reversal of flow. Peak systolic velocities over 100cm/s are generally accepted to be abnormal; however, anatomic variations such as vessel kinking and tortuosity can occasionally elevate velocities in the absence of true disease.

Normal external carotid artery duplex.

Normal internal carotid artery duplex.

Measuring Carotid Stenosis

Several different methods have been utilized in the past to measure carotid stenosis. Most of these were developed using invasive angiography and, although currently rarely used for diagnosis of carotid stenosis, are still considered the “gold standard” for lesion measurement and are used to validate ultrasound criteria. Although ultrasound plaque can be visualized and qualitatively analyzed using duplex ultrasound, vessel diameter measurement can be subjective and may often underestimate degree of stenosis. Validation studies comparing angiographic findings with duplex imaging have shown the utility of spectral Doppler velocity measurements in accurately and reliably documenting carotid stenosis.

Internal Carotid Artery Stenosis

The NASCET angiographic stenosis criteria [2] is used for reference in most North American centers and studies today, and is the standard used to validate existing ultrasound criteria for carotid stenosis. Measurement of degree of stenosis by duplex is assessed using a set of three criteria: internal carotid artery peak systolic velocity, end diastolic velocity (EDV), or the ratio of the ICA PSV to the CCA PSV as measured 2cm below the carotid bulb.

Degree of Stenosis	ICA PSV (cm/s)	ICA/CCA PSV Ratio	ICA EDV (cm/s)
< 50%	< 125	< 2.0	< 40
50-69%	125 – 230	2.0 – 4.0	40 – 100
> 70%	> 230	> 4.0	> 100

Criteria for duplex diagnosis of internal carotid stenosis [6].

Criteria may vary slightly by institution.

Utilization of multiple criteria may prevent errors in interpretation based on a single measurement. For example, patients with decreased cardiac output may have lower systolic velocities overall, affecting the ICA PSV; however, the ratio will continue to report a valid measurement. Likewise, in a situation where a tandem common carotid lesion (in addition to the internal carotid lesion) increases the PSV in the common carotid and lowers the ratio, the use of ICA PSV and/or EDV may continue to provide accurate inference about the lesion severity.

Internal carotid artery stenosis. In this case, the ICA/CCA ratio was approximately 7, meeting all three criteria for a severe (>70%) stenosis.

Common and External Artery Stenosis

Common carotid occlusion is simple to detect using duplex ultrasound; however, no consensus on stenosis criteria exists due to the lack of published data and its uncommon occurrence compared to the internal carotid distribution. Some authors have advocated a stenotic/distal ratio of greater than two to suggest moderate disease, and a ratio of greater than four to suggest severe disease [3]. Others have advocated for the use of a peak systolic velocity of greater than 182 cm/s, developed in a study comparing duplex to CT angiography, although these criteria have not been externally validated [4]. Normal changes in flow dynamics throughout the course of the common carotid and the absence of ultrasound windows for imaging the proximal left common carotid also contribute to the diagnostic uncertainties.

Quantitative evaluation of external carotid artery stenoses is likewise difficult, due to lack of published data and low clinical significance of disease in this vascular distribution. Lesions should be analyzed using duplex as part of a comprehensive examination but results reported qualitatively. Analysis of external carotid flow can be useful for determining lesions in neighboring vessels, such as internal or common carotid occlusion.

Ultrasound for Screening and Surveillance

Screening for asymptomatic cerebrovascular stenosis is an area of some controversy. Screening has been advocated as a tool for early detection of carotid stenosis and identification of patients who may be at high risk, with potential benefit from carotid intervention. Evidence from several multicenter trials using ultrasound criteria to enroll patients have demonstrated the need for strict protocol and quality control [5, 6]. The utility of duplex as a mass screening tool is dependent on the identification of thresholds that increase the sensitivity of the test for severe stenoses, resulting in fewer false negatives. Several studies have identified a peak systolic velocity of 230 cm/s as a reasonable threshold for determining 70% stenosis, and this has been suggested as a suitable screening threshold as well [5,6].

In addition, any benefit of a cerebrovascular screening study depends on the expected risk reduction of any intervention based on screening results. The original studies validating intervention in asymptomatic patients showed absolute risk reductions at 5 years of 5-6%, but this number remains in question with continuing improvements in medical management of asymptomatic patients and the lack of recent data [5,6].

The benefit of surveillance of patients with asymptomatic stenosis also remains uncertain as data on risk and progression of those with varying degrees of stenosis determined by ultrasound remain limited. Patients with peak systolic velocities between 175 and 260 cm/s may represent a group at higher risk for future neurologic event, but this has not yet been definitively shown [7].

Conclusions

Cerebrovascular duplex ultrasound for carotid disease is a powerful tool that has become an invaluable resource in the decision making process. Duplex ultrasonography is able to provide both anatomic and hemodynamic information about the state of a vessel, allowing health care providers to make informed decisions regarding intervention for stroke prevention.

Basics of Extracranial Carotid Artery Duplex Ultrasound

Basics of Extracranial Carotid Artery Duplex Ultrasound