Measurement of Doppler gate length using signal re-injection

Abstract

Signal re-injection provides a versatile way of assessing the performance of ultrasonic scanners. The technique was used to measure the Doppler gate length of Duplex ultrasonic scanners operating in the pulsed Doppler mode. Five scanners were assessed, each with two different gate lengths, and the nominal gate length was compared with that measured. For most scanners the nominal gate length was less than that measured, the difference (measured minus nominal) varying between -0.9 mm and +8.0 mm. However, incomplete knowledge of the scanner characteristics, in particular receiver focusing and automatic gain control of the audio signal, made the results difficult to interpret. The test equipment used a perspex block to provide a fixed delay between the emitted and re-injected ultrasonic pulse. A more accurate assessment would have required a re-injection system with a variable delay between the pulses.