It seems reasonable
to expect that if the problem of aliasing is caused by an insufficiently
high PRF, the way to reduce the problem is to find some method of
increasing the PRF. Recently, some PW Doppler systems have been
introduced which allow the operator to increase PRF above the Nyquist
limit and thereby reducing aliasing. High PRF Doppler can use multiples
of the PRF corresponding to the Nyquist limit at a given depth.
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| Fig.1.30 |
The basic principle of the so-called "high PRF" Doppler is illustrated
in Figure 1.30. In this example a given high velocity is located
near the mitral valve. If the pulse transit time to the jet and
then back to the transducer is one second then the PRF is one pulse
per second. Because the velocity being sampled exceeds the Nyquist
limit, aliasing is seen in the spectral display. High PRF systems
emit multiple pulses without waiting for the original one to be
received. In this example multiple pulses are emitted and received
each second. This results in an increase in PRF and a spectral display
that is not aliased.
The problem with this approach is that some of the range selectivity
used in precisely locating the sample volume is relinquished. As
this pulsing sequence is carried on over and over, some data is
returned to the transducer the points that are located by the increased
PRF in space. If other turbulence were located at any one of these
ranges the operator would not be able to tell where the high velocity
jet of interest was located, as data from all these volumes are
added together. This results in what is called "range ambiguity".
This method of increasing the PRF for the acquisition of high
velocity data has been described as programming the machine to
"think" that the high velocity jet is much closer to the transducer
than it really is. In reality, the machine knows exactly what
it is doing and the beginning operator is the one that is fooled.
When using the high PRF mode, it is best first to locate the high
velocity jet or turbulence using standard single gate PW Doppler,
and to ensure the absence of other areas of turbulence along the
path of the ultrasound beam. The high PRF mode can then be used
to record the unaliased Doppler signal.
2004