Transcranial Doppler (TCD)

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TCD insonation Ambulatory

Cardiovascular Doppler ultrasound has been around for more than 60 years. It has found widespread use in cardiology and in diagnosis of peripheral artery disease. However, the belief that the cranium was a prohibitive barrier to ultrasound Doppler insonation discouraged exploration of noninvasive transcranial Doppler techniques until the first paper appeared in 1982.

Transcranial Doppler (TCD) use relatively low frequencies (2MHz or below) and focused transducers. In most subjects, the temporal bone ultrasonic window(s) — see illustration Left — allows relatively good signals to be obtained from a variety of cerebral arteries and veins. In addition, and in practically all patients, the transorbital and transforamenal windows provide access to additional cerebral vessels. With the modern sector scan imaging Doppler it is relatively easy to identify the main arteries at the base of the brain, and then obtain the spectral display of the flow velocity waveform which is essentially the basis for diagnosis.

When we developed the TCD technique at the Neurosurgical Department in Bern, it was natural that the first application in patients was monitoring and diagnosis of cerebral vasospasm after subarachnoid hemorrhage (SAH).

It soon became evident that basing the evaluation purely on velocities in the spastic segments was not entirely satisfactory. We therefore started monitoring the flow velocity in the extracranial ICA to provide additional information. Fortunately, the submandibular approach allows assessment of relative changes in flow in the internal carotid artery (ICA) which supplies the anterior cerebral circulation. This provides additional information of the hemodynamic effect of the vasospam: when the flow in the ICA start to decline, it may soon become critical.

For patients with carotid artery disease, the TCD technique opened a window on the effect of the stenosis/occlusion on the cerebral circulation, in particular the damping of waveforms as well as the functioning of the collateral network on the base of the brain.

While the skull is an impediment to ultrasound, it also offers an advantage which has been a great benefactor for the use of TCD: a stable platform for mounting monitoring probes.

This has led to a paradigm shift in the use of ultrasound Doppler — a shift from being a relatively simple handheld diagnostic device — to a tool for long-term monitoring and the study of vasomotor reactivity, cerebral autoregulation, and functional coupling of blood flow responses to stimuli.

Using modern technology, the TCD device can be made very small and battery powered. It can be equipped with lightweight controllable probes so as to maintain signal during longterm monitoring in an ambulatory setting. A somewhat smaller device can even be used to monitor cerebral flow velocity during such activities as tennis.

The cerebral circulation is a very complex system, and the Doppler signal is often difficult to aquire and interpret. Education and training is important both for the sonographer and the physician. I have tried to convey my experience in transcranial Doppler and my understanding of cerebral hemodynamics in an interactive training app running on the ubiquitous Windows personal computer.

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Page Updated: 2019-10-12