Chapter 21. Transcranial Doppler Examination

Introduction

Transcranial Doppler (TCD) ultrasound examination of blood flow velocity through the brain can be used to detect cerebral arterial vasospasm, steno-occlusive disease of the cerebral arteries, or the absence of blood flow (in the determination of brain death).

Definitions and Terms

• ▪  Doppler ultrasound: A technique based on the principles of sound travel and reflectance in tissues—transmitted sound waves change their frequency when they come into contact with moving blood cells and the frequency change is known as the Doppler effect.
• ▪  Blood flow velocity: Measurement of the speed (Figure 21-1) and direction of blood flow (as distinct from total blood flow).
• ▪  Continuous wave Doppler: Continuous measurement of sound pitch changes relating to blood flow velocity through vessels, around obstructions, or through narrowings.
• ▪  Duplex Doppler: Provides a picture of the vessels and information about the speed and direction of blood flow in the vessels.
• ▪  Color flow Doppler: Velocities are mapped into colors and overlaid on the image of the blood vessels.
• ▪  Acoustic window: A portion of the skull that permits transmission and reception of ultrasound through the skull—the temporal, transorbital, suboccipital, and submandibular windows are typically used.

Techniques

• ▪  The probe is applied lightly to the skin over the acoustic window using ultrasound gel to improve contact and sound wave transmission (Figure 21-2).
• ▪  Indications
  • —Evaluation of intracranial vascular disease
  • —Monitoring of vasospasm (increased blood flow velocity through narrowed vessels)
  • —Detection of cerebral emboli
  • —Evaluation of vertebrobasilar vascular disease
  • —Detection of arteriovenous malformations

Clinical Pearls and Pitfalls

• ▪  Normal cerebral arterial velocities decrease with age, which is phenomenon related to decreasing cardiac output.
• ▪  TCD temporal bone windows are not as good in females as in males.
• ▪  Arterial velocities increase in anemia, as viscosity decreases.
• ▪  Arterial velocities are increased in fever.

Suggested Reading