Transcranial Doppler (TCD)

What is Transcranial Doppler (TCD) and How Does it Work?

Transcranial Doppler (TCD) is a diagnostic technique used to study blood flow in cerebral blood vessels. TCD employs high-frequency ultrasound to measure blood flow velocity and intensity through brain arteries, aiding in diagnosis and monitoring of numerous conditions including stroke, vascular disorders, ischemia, aneurysms, arteriovenous malformations, blood vessel diseases, and dementia.

Numerous physiological variables can affect blood flow velocity measured by transcranial Doppler. Some of these variables include age, sex, hematocrit, viscosity, carbon dioxide, temperature, blood pressure, and mental or motor activity. Therefore, it proves important to remember that during TCD studies, all measured differences in blood flow velocity should be interpreted within the context of these variables. All studies should be conducted with patients at rest—without speaking or limb movement.

How is Transcranial Doppler (TCD) Performed?

The TCD procedure is typically performed with the patient sitting or lying down, and the sensor positioned at various parts of the skull to capture desired brain vessels. The sensor then emits sound signals and records return signals from blood vessels. TCD requires no invasive procedures and proves generally safe for patients of all age groups, proving useful for early detection of neurological diseases and conditions, as well as providing necessary information for determining treatment and monitoring disease progress.

No special preparation proves needed for transcranial Doppler ultrasound.

Small amounts of water-soluble gel are applied to the skin over the area to be examined. Gel is usually applied to the back of the neck, above the buttock bone, in front of the ear, or over the eyelid. These represent locations of blood vessels supplying the brain. The device, resembling a microphone and called a probe, is held in place over the examination area. This probe then sends high-frequency sound waves through the brain and records return information about blood flow. Ultrasound signals convert into graphs or color images displayed on the screen. During the test, practically no discomfort occurs. You may feel slight pressure while the probe presses on your skin. Ultrasound generally lasts about 30 to 60 minutes.

Currently, two types of TCD equipment prove available: non-duplex devices (without imaging) and duplex devices (imaging). With non-duplex devices, arteries identify “blindly” based on Doppler sound shift and spectral display. Transcranial color duplex (TCCD) combines pulsed wave Doppler ultrasound with cross-sectional display of the insonated area, enabling artery identification relative to various anatomical locations.

Two Main TCD Limitations

Two main TCD limitations hinder its wider use. It depends heavily on the healthcare worker’s expertise using it, and the manual technique requires detailed three-dimensional knowledge of cerebrovascular anatomy and its variations.