Signals from dual gated TCD systems: curious observations and possible explanations

Dual gated transcranial Doppler (TCD) systems have the potential to distinguish definitively embolic events in the cerebral circulation from artefact. While developing such a system, three curious observations were made and are the subject of this article. The first was that the velocity of propagation of an embolus in an artery as calculated from the ultrasonic Doppler shift was, in many cases, dramatically different from the velocity calculated using a time of flight method. The second observation was that the durations of embolic signals measured in two gates were often very different, despite the received gates being identical in length. The third observation was that some emboli detected in the deeper gate did not appear in the more superficial gate, and, more surprisingly (as has recently been reported by another group), some emboli that had not previously appeared in the deeper gate appeared in the more superficial gate. It is hypothesised that all of these effects can be explained in terms of the geometry of the middle cerebral artery in relation to the interrogating ultrasound beam. None of these effects should detract from the usefulness of TCD, but will need to be widely appreciated if dual gated measurements of cerebral emboli are to be interpreted correctly.