Method and device for determining variations in the position of a

Abstract

Variations in the position of a part of a living object within the measurement space of magnetic resonance apparatus are determined by an ultrasound device in which bursts of ultrasonic waves having a frequency F.sub.US are transmitted towards the object in response to start signals generated to the repetition frequency F.sub.R, and ultrasonic waves reflected by the object are received and an analog echo signal is detected which is converted into a digital echo signal made up of a sequence of echo pulses having a repetition frequency F.sub.US. The device includes a controller for generating a time-window, for determining whether a predetermined edge of an echo pulse occurs within the time-window, and for setting the time for generating the next time-window at 1/F.sub.R -1/2F.sub.US after an echo pulse has occurred within the time-window. In order to automatically generate a next time-window even in the absence of an echo pulse received in the previous time-window, an estimating circuit provides an estimated value for the time for generating the next time-window, which estimating circuit includes an envelope detector and a level detector for determining the time t.sub.L when the level of the envelope of the analog echo signal exceeds a predetermined level. The time for generating the next time-window is set at 1/F.sub.R -1/2F.sub.US after the time t.sub.L if no echo pulse has occurred within the time-window.