| 
Ultrasound
 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. [See
below.]
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.
|
Stroke Education
