Thirty Years of Transcranial Doppler: What has changed?
Prof. Rune Aaslid, PhD
This year (2011) marks the 30th anniversary of the first application of transcranial Doppler (TCD). In the summer of 1981, I was fortunate to be invited to do research at the Department of Neurosurgery in Bern by Prof. Helge Nornes—a pioneer in the study of cerebral hemodynamics using electromagnetic flowmetry and intraoperative Doppler. Before accepting the position, I decided to try to get Doppler signals from the intracranial vessels using a 2MHz pulsed Doppler system that I happened to have on loan in my laboratory (the Doppler was used for developing a full scale physical model of the left ventricle and aorta for the Norwegian Underwater Institute).
After searching the temporal region where the bone was assumed to be thin, a distinct Doppler sound came from a depth of 5cm, and hyperventilation made its frequencies drop markedly. The signal simply had to originate from a cerebral artery.
Arriving in Bern we immediately started to explore the new technique, determining which intracranial vessels were possible to investigate, and how to identify them. Normal values were established, and the first patients were soon investigated.
Cerebral vasospasm is a dreaded complication of subarachnoid hemorrhage that can lead to brain infarction and in severe cases, death. Angiography was at that time practically the only modality to diagnose the arterial narrowing caused by vasospasm. In Bern we were lucky to have a neuroradiologist, Prof. Peter Huber, who was an expert in this field and who had developed a technique to accurately measure the diameter of the intracranial arteries. A clear inverse relationship between the degree of angiographic spasm and the TCD velocities was found, and monitoring of cerebral vasospasm remains one of the most useful and widespread applications of the technique.
A reasonably good relationship between clinical vasospasm (patient has ischemic symptoms) and the TCD velocity in the spastic segment has also been documented by many studies. But frequently, patients with high velocities (>200 cm/s) do not have clinical symptoms. Since TCD was introduced, the change to very early aneurysm surgery combined with aggressive treatment (3H therapy and angioplasty) has allowed many patients to come through the vasospasm period without symptoms and ischemic damage. We have also found it useful to monitor the velocity in the extracranial internal carotid artery where it enters the skull base. A patient, in whom intracranial spasm velocities remain high, and, at the same time, those in the ICA drop, is at danger for ischemia.
In the early years, TCD was also applied to the evaluation of the intracranial collateral circulation in carotid stenosis. In the 1990’ties the transcranial color imaging Doppler was introduced. As this instrumentation became more sensitive, it is currently preferred in many centers for the routine examination. The advantage is that the image makes artery identification easier for the operator. However, the diagnosis is still based on spectral analysis of the Doppler signal like in the conventional TCD examination.
Due to its portability and small probe, conventional TCD is the preferred instrumentation for monitoring during surgical procedures and in the ICU. It is also the only practical equipment for monitoring embolic events.
Recent developments in micro electronics have facilitated development of a small TCD instrument suitable for ambulatory monitoring during normal daily activities. Running on battery power, continuous monitoring and recording of the raw Doppler signal for extensive time periods (8 hours) is possible. This is especially important for emboli detection, as the events typically come in clusters so that longer monitoring time is required to avoid false negatives. A lightweight and comfortable probe fixation has been designed on the basis of a spectacles frame, with 2-axes robotic probe control so that the signal can be maintained even during sporting activities. This instrumentation hopefully will be available soon commercially and open up for the development of new diagnostic and monitoring TCD applications.