Transcranial Doppler TCD

Transcranial Doppler (TCD) ultrasound provides rapid, noninvasive, real-time measures of cerebrovascular function. TCD can be used to measure flow velocity in the basal arteries of the brain to assess relative changes in flow, diagnose focal vascular stenosis, or to detect embolic signals within these arteries. TCD can also be used to assess the physiologic health of a particular vascular territory by measuring blood flow responses to changes in blood pressure (cerebral autoregulation), changes in end-tidal CO₂ (cerebral vasoreactivity), or cognitive and motor activation (neurovascular coupling or functional hyperemia).

What is Transcranial Doppler ultrasound?

TCD is a noninvasive and painless ultrasound technique that uses sound waves to evaluate blood flow (blood circulation) in and around the brain. There is no special contrast or radiation involved in the test.

Physicians recommend this test to determine if there is anything happening in the blood vessels that is affecting blood flow to the brain. Transcranial Doppler ultrasound is often the test of choice for:

• Vasospasm following a ruptured brain aneurysm
• Sickle cell anemia to determine a patient’s stroke risk
• Ischemic stroke
• Intracranial stenosis or blockage of the blood vessels
• Cerebral microemboli
• Patent Foramen Ovale a hole in the heart that doesn’t close properly after birth

At Alabama Clinics, TCD ultrasounds are used to evaluate the 13 major arteries supplying blood flow to the brain, but there are other TCD studies used to identify specialized problems related to brain blood flow:

• Embolic detection monitoring
  This test detects any free-floating particles that may be in the bloodstream. These particles can be a significant source of stroke risk.
• Agitated saline bubble study
  This test is specific for identifying whether there is a passageway or hole between the right and left chambers through which blood enters the heart. This hole usually closes after birth. If it doesn’t close, it can be a source of increased stroke risk in certain patients.
• CO₂ vasomotor reactivity study
  This noninvasive study looks at whether the small vessels that regulate blood flow to the brain are working properly. When a small amount of carbon dioxide (CO₂) and oxygen are inhaled, similar to holding one’s breath, these small vessels should widen and increase blood flow to the brain. When a person hyperventilates, the vessels should shrink and slow blood flow to the brain.

Types of TCD test

Two types of TCD equipment are currently available: non-duplex (nonimaging) and duplex (imaging) devices. In non-duplex devices, the arteries are identified “blindly” based on the audible Doppler shift and the spectral display. Specific vessel identification is based on standard criteria, which includes the cranial window used, orientation of the probe, depth of sample volume, direction of blood flow, relationship to the terminal internal carotid artery, and response to various maneuvers such as the common carotid artery compression and eye opening and closing.

The imaging B-mode transcranial color-coded duplex (TCCD) combines pulsed wave Doppler ultrasound with a cross-sectional view of the area of insonation, which allows identification of the arteries in relation to various anatomic locations. The color-coded Doppler also depicts the direction of the flow in relation to the probe (transducer) while recording blood flow velocities. In TCD, the angle of insonation is assumed < 30 degrees (as close to zero as possible) to minimize the Doppler shift measurement error. However, in TCCD, the angle of insonation can be measured and used to correct the flow velocity measurement. More recently, a more advanced technology, called the power motion-mode TCD (PMD/TCD), has also become available that provides multi-gate flow information simultaneously in the power M-mode display. It uses several overlapping sample volumes to simultaneously display flow signals. PMD/TCD appears to simplify handling of the TCD by facilitating the temporal window location and alignment of the incident signal to allow cerebral blood flow velocity recordings through multiple vessels.

Test

There is no preparation necessary for the TCD ultrasound. However, it is important that the patient stay quiet and still during the test.

Since TCD includes sampling over the closed eyelid, patients should leave or remove hard contacts during the test. TCD will not be performed on patients who have had invasive eye surgery within the previous month, including laser sight correction, implants or cataract removal.

Then the technician will place a small microphone-like device, known as a transducer, over those sites and listen to the blood flow. The transducer collects the sound waves and the computer uses those sound waves to create pictures that represent blood flow speed and resistance within the vessels.

Depending on the type of measurement needed, the test lasts at least 45 minutes.