Transient variations in the systolic pulsations in amplitude of intracranial echoes: their artifactual origin.

Variations in the amplitude of intracranial echoes that synchronize with systole are caused by movement of the reflecting interfaces. These movements result from propagation of the arterial pulse through the brain and vary with the arterial pulse pressure and the distance through the vascular tree for which the pulse propagates before being completely attenuated; this distance varies with the degree of arteriolar dilatation. The interpretation of such systolic fluctuations in echo amplitudes is subject to a number of limitations, and their recording is also subject to artifactual variations, usually resulting from the relative motion of the signal with respect to a fixed gate. Transient variation in the systolic fluctuations of echo amplitudes, as a result of various stimuli, can be recorded by a fixed gate but not simultaneously by a tracking gate. They would therefore appear to be artifactual and not indicative of regional changes in cerebral blood flow, which in any case would cause motion and amplitude changes in interfaces that were not confined to the region involved.