Nakagawa K et al. (2007) Does long-term continuous transcranial Doppler monitoring require a pause for safer use? Cerebrovasc Dis 24: 27–34

Transcranial Doppler sonography (TCD) is widely used for the monitoring of cerebral blood flow. Although no adverse effects have been reported with this technique, little attention has been given to the potential for extended sessions of TCD to cause thermal damage to the brain surface via the adsorption of ultrasound by tissue and skull bone. Now, researchers from the Jikei University School of Medicine, Tokyo, Japan, report the first in vivo animal study of local brain temperature changes during long-term TCD.

To monitor temperature change at the skull–brain interface (SBI), the researchers inserted thermocouples into 15 New Zealand White rabbits. Of these rabbits, 10 underwent 90 min of TCD (0.2 W; maximum output 2 MHz) via a probe placed on the parietal bone; the remaining five rabbits formed the control group and did not undergo TCD. SBI temperature and central brain tissue temperature (CBTT) were monitored throughout insonation. Mean CBTT peaked at 37.54 °C after 25 min of TCD (an increase of 1.86 °C from baseline), after which point it gradually decreased. SBI temperature showed a similar pattern of change, peaking after 20 min of TCD, with a maximum increase from baseline of 3.37 °C. The profiles of CBTT and SBI temperature measured in the TCD group were significantly different from those measured in the control group (P <0.0001 for both).

Although there are large differences in brain volume between rabbits and humans, cerebral blood flow per gram of brain weight is similar. On the basis of this similarity, the authors recommend a break in TCD monitoring every 30 min to avoid thermal damage to the brain.