Simple Continuous Wa Ve Motion Detection

In the simple case of a single reflector sited in a continuous sinusoidal ultrasound field (Figure 10.1) the measured echo signal will have both an amplitude related to the strength of the reflection and an associated phase-offset given by the frequency and time of flight of the sound wave. If the reflector is stationary, both the amplitude and phase of the returning signal will remain constant relative to the transmitted signal. As soon as the target moves, depending on the direction of the motion relative to the ultrasound beam, the characteristics of the returned signal will change. The general wisdom is that if a component of the motion is axial, directed along the sound beam, the phase of the detected signal will vary. Over time this change of phase is perceived by the detector as a shift in frequency (or Doppler shift) of the returned signal relative to the original. If a component of the motion is lateral, or perpendicular to the beam axis, there will be a change in the amplitude of the echo signal as the target passes through the ultrasound beam. Note, however, that a phase change may also occur as a target moves laterally across a sound beam even when there is no axial component to the motion.

Continuous Wave Ultrasound Beam Shape
Figure 10.1. Continuous wave (CW) and pulsed wave (PW) ultrasound fields; a target moving through the beams can cause variation in both amplitude (A) and phase (j) of the reflected signal. In the PWcase the time of flight (T) of the echo signal is also affected

This is due to the fact that the phase of the sound field from any diffracting aperture, even at a fixed axial distance, may vary with lateral position. Only when the target trajectory exactly follows the shape of the wavefront would no phase change be expected. Such a trajectory may actually require a small amount of axial motion.

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