Transcranial Doppler Studies

Transcranial Doppler studies provide information about the presence, quality, and changing nature of circulation to an area of the brain by measuring the velocity of blood flow through cerebral arteries. Narrowed blood vessels produce high velocities, indicating possible stenosis or vasospasm. High velocities may also indicate an arteriovenous malformation.

Purpose

• To measure the velocity of blood flow through certain cerebral vessels
• To detect and monitor the progression of cerebral vasospasm
• To determine whether collateral blood flow exists before surgical ligation or radiologic occlusion of diseased vessels
• To help determine brain death

Patient preparation

• Explain the purpose of the study to the patient (or to his family).
• Tell him that the test will be done while he lies on a bed or stretcher or sits in a reclining chair (or it can be performed at the bedside if he's too ill to be moved to the laboratory).
• Describe the procedure and tell the patient that it usually takes less than hour, depending on the number of vessels to be examined and any interfering factors (in a complete study, the middle cerebral arteries, anterior cerebral arteries, posterior cerebral arteries, ophthalmic arteries, carotid siphon, vertebral arteries, and basilar artery are studied).
• Tell the patient that fasting isn't required before the test.

Equipment

Transcranial Doppler unit, probe, conductive gel

Procedure and posttest care

• The patient reclines in a chair or on a stretcher or bed.
• A small amount of conductive gel is applied to the transcranial window (an area where bone is thin enough to allow the Doppler signal to enter and be detected); the most common approaches are temporal, transorbital, and through the foramen magnum.
• The technician directs the signal to ward the artery being studied and records the velocities detected; waveforms may be printed for later analysis.
• The Doppler signal can be transmitted to varying depths.
• When the study is completed, wipe away the conductive gel.

Precautions

• Be sure to remove turban head dressings or thick dressings over the test site.

Normal Findings

The type of waveforms and velocities obtained indicate whether or not pathology exists.

Abnormal findings

Although this test often isn't definitive, high velocities are typically abnormal and suggest that blood flow is too turbulent or the vessel is too narrow.

After the transcranial Doppler study and before surgery, the patient may undergo cerebral angiography to further define cerebral blood flow patterns and locate the exact vascular abnormality.

Interfering factors

• Failure to remove dressings over the test site (possible poor imaging)