After completion of a CT scan or MRI of the head, ultrasound testing may be used to provide information on the condition of the carotid arteries in the neck and/or intracranial vessels. A number of different ultrasound tests for assessing the carotids are available.
B-mode imaging provides images of various levels, or planes, enabling the creation of a three-dimensional image of the carotid artery wall and surround structures. This technique provides information on the type and extent of arterial damage, but blood clots sometimes do not appear and the method cannot distinguish a severely narrowed from a completely occluded artery. To provide more complete information, B-mode imaging is often combined with another type of ultrasound, such as pulsed Doppler scanning. Together, the two sets are known as duplex ultrasound.
Doppler testing measures the speed of blood flow through an artery. Because atheromatous plaque narrows an artery, the velocity of blood increases past the point of plaque build-up. The greater the velocity, the narrower the blood vessel. Knowing the velocity of blood allows determination of whether the artery is narrowed and to what degree. When no velocity measurement can be made, the artery is usually considered to be completely occluded. Two types of Doppler ultrasound are used to obtain information on the velocity of blood flow in the carotids: (1) In pulsed doppler, the probe is placed over one spot on the neck over the carotid, and timed measurements are taken to determine the speed of blood flow in the artery; (2) In continuous wave Doppler, a probe is moved along the neck over the course of the carotid, and the velocity of blood passing along the vessel beneath the probe is averaged out.
Duplex ultrasound combines B-mode imaging and pulsed Doppler ultrasound to provide more detail on the condition of arteries than either test alone can provide. B-mode imaging provides information about arterial walls, while pulsed Doppler helps to orient the B-mode imaging and provides information about the arterial blood flow.
Early ultrasound studies are used in some centers to screen for intracranial and extracranial disease, but the effects of the results of these studies on early management are uncertain [Adams HP, et al, 1994]. A carotid ultrasound study is commonly used to evaluate patients with ischemic strokes, TIAs, and suspected carotid artery disease (CAD) who are potential candidates for carotid endarterectomy [Moore WS, et al. Guidelines for Carotid Endarterectomy. American Heart Association, 1995]. Pulsed Doppler has been accepted by some investigators as a satisfactory means of determining the severity of carotid artery stenosis. Duplex ultrasound, when performed in settings in which the results have been consistently well validated by comparison with angiography, is an accepted and accurate technique, but there is risk of calling a high-grade stenosis total occlusion (1% to 14% false-positive rate). The AHA Guidelines Committee concluded that it is premature to make a definitive statement since these techniques are still in evolution.
In transcranial doppler (TCD), a probe is placed over areas on the head to detect blood velocity and pressure in certain arteries at various depths in the brain. In the early hours after occlusive stroke, TCD allows the assessment of the location and extent of occlusions or atheromatous plaques in extracranial carotid and large intracranial vessels, including the middle cerebral and basilar arteries [Bruno A, 1993. Mohr JP. In: Barnett HJM, et al. Stroke Pathophysiology, Diagnosis, and Management. New York, Churchill Livingstone, 1992:15]. In addition, TCD may suggest arteriovenous malformation in some instances of brain hemorrhages and can be useful for early detection of vasospasm after acute aneurysm rupture.