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Transcranial Doppler in the Evaluation of Cerebrovascular Disease


Transcranial Doppler (TCD) ultrasonography is the only noninvasive real-time examination that adds important physiologic information to anatomic imaging studies when evaluating blood flow in major intracerebral vessels. It is considered an extension of the clinical neurologic examination, which includes ultrasound of the extracranial vessels. Introduced by Aaslid and colleagues in 1982 as a technique using ultrasound technology to record velocity measurements of cerebral arteries through the temporal bone, its original application was limited to the detection of cerebral arterial vasospasm following subarachnoid hemorrhage. Since then, TCD has rapidly evolved to an important and relatively inexpensive imaging modality with a broad range of clinical applications established by the Clinical Practice committee of the American Society of Neuroimaging ( Box 1 ).

BOX 1
Clinical Applications of Transcranial Doppler Ultrasonography

  • Evaluate need for blood transfusions in sickle cell disease (children)

  • Detect, localize, and quantify cerebral embolism

  • Determine stroke pathogenic mechanism

  • Determine presence of intracranial occlusive disease, embolization, shunting, and impaired vasomotor reactivity

  • Detect progression and regression of stenoses during follow-up

  • Monitor vasospasm in subarachnoid hemorrhage

  • Monitor surgery affecting the cerebrovascular circulation (CABG, repairs of ascending aorta)

  • Periprocedural or surgical monitoring during carotid endarterectomy or stenting

  • Monitor evolution of brain death

CABG, Coronary artery bypass graft.

TCD has the advantage of being the most convenient method to evaluate the intracranial vasculature at the patient’s bedside because it allows measurements in both the acute setting and for prolonged periods. However, TCD has its limits; it is highly operator dependent and requires in-depth training and an understanding of cerebrovascular physiology, anatomy, and pathology. Moreover, there is a 15% rate of inadequate imaging through temporal windows, which is commonly observed in Asian, African American, and elderly female populations.

Technique

A TCD examination is performed with the patient in the supine position. Carotid duplex imaging in most cases should be performed as part of the patient’s evaluation, because intracranial hemodynamic properties may be altered with extracranial carotid disease.

TCD is performed through four ultrasound windows: the transtemporal, which provides the most intracranial circulatory information of all windows; the transorbital, for evaluating the ophthalmic artery and the siphon of the internal carotid artery (ICA); the submandibular window, for imaging the proximal intracranial portion of the ICA; and the suboccipital window, for evaluating the basilar and vertebral arteries.

Clinical Indications

Cerebral Ischemia

Acute Cerebral Ischemia

TCD can evaluate up to 16 intracranial arterial segments for the detection of normal, stenotic, or occluded vessels. Because TCD provides real-time information regarding the direction and velocity of blood flow and the hemodynamic significance of intracranial or extracranial stenotic or occluded lesions, it has the ability to assist in defining the pathogenic mechanism of stroke and the collateral circulation (which often is dormant under normal circulatory conditions). This information is useful to correlate intracerebral hemodynamics with information obtained from other noninvasive tests and brain imaging studies such as computed tomography (CT) or magnetic resonance imaging (MRI).

Two recent studies have validated the diagnostic accuracy of TCD versus CT angiogram for evaluating arterial occlusive disease in the setting of acute cerebral ischemia. The yield of TCD is greatest the closer in time it is performed to the onset of stroke symptoms, and anatomically it has higher yield for the anterior (sensitivity, 70%–90%; specificity, 90%–95%) than for the posterior circulation (sensitivity, 50%–80%; specificity, 90%–96%).

As reported in the Stroke Outcomes and Neuroimaging of Intracranial Atherosclerosis (SONIA) trial, TCD (and magnetic resonance angiography) can reliably exclude intracranial stenosis (negative predictive value >80%). However, if abnormal findings are encountered, a confirmatory test is required to identify a stenotic lesion.

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