Range ambiguity in pulsed doppler ultrasound: The ambiguity clarified?

Abstract

Pulsed Doppler ultrasound (PW) can be used to determine the location of frequency shifts within the cardiac chambers or great vessels. However, it is possible to record similar frequency shifts at sample volume locations distal to their original site; this is referred to as range ambiguity (RA). Eleven patients were studied with combined Doppler and two-dimensional echocardiography (2-D) to determine the circumstances in which RA occurs. Mapping of various flow patterns with PW was performed in each of four 2-D views beginning at 2 cm distances from the transducer and at subsequent 1-cm intervals until the maximal range of the PW was achieved. Range ambiguity was demonstrated only in the four-chamber view in patients with enlarged cardiac chambers or if an abnormal flow pattern was present. The site of origin of the ambiguous signals was dependent on the pulse repetition frequency (PRF) employed. Range ambiguity occurs more often when a relatively high PRF is used. Range ambiguity may be used for mapping of abnormal flow beyond the range of PW or for recording of high velocities at sample volumes far from the transducer without frequency aliasing.

A nonuniform sampled coherent pulsed Doppler ultrasonic velocimeter with increased velocity range

“…In situations that contain both fixed and moving structures in the measurable distance interval (0, dmax), as frequently occurs in the human body [3], it is possible to locate an interlaced sampling sequence, by adjusting the delay k, to avoid undesirable sample volumes. A graphical analysis is used here to explain this method.…”

“…Thus, interlaced sampling presents the following advantage compared to the DPRF method: the interlaced sequence can be continuously adjusted to avoid undesirable sample volumes, without changing the pulse repetition rate of the PDUV, and consequently, without changing the distance axis. To adjust the delay k, we can inspect the pulse-echo response, similarly to what is normally done when the DPRF method is used [3]. As an alternative procedure to adjust the delay k for systems at which pulse-echo response is not available, one can simply hear the Doppler-audio signals when the interlaced sequence is switched on.…”

“…Manuscript are superimposed to those from the desired distance. This physical phenomenon, known as range ambiguity [3], can be used to increase the maximum measurable velocity. By doubling the pulse repetition frequency (DPRF technique), the maximum measurable velocity is doubled, but the maximum measurable distance is reduced to d max = dmax/2.…”

“…It is possible to locate the sample volume at a distance d2 > d max , by adjusting the PDUV to measure the virtual distance d1 = d2 − d max , if there are no moving structures at d1, which is inside the range (0, d max ). This procedure is used in practice to increase the maximum measurable velocity of the conventional PDUV [2], [3]. However, with this procedure, the PDUV operator has no flexibility to avoid undesired sample volumes at d1, except by changing the pulse repetition frequency or repositioning the transducer, when this is feasible.…”