 |
| Fig.1.26 |
The Nyquist limit defines when aliasing will occur using PW Doppler
(Fig.
1.26). The Nyquist limit specifies that measurements of frequency
shifts (and, thus, velocity) will be appropriately displayed only
if the pulse repetition frequency (PRF) is at least twice the
maximum velocity (or Doppler shift frequency) encountered in the
sample volume.
It is obviously desirable to use as high a PRF as possible for
recording abnormally elevated velocity jets. The problem is that
the maximum PRF is limited by the distance the sample volume is
placed into the heart.
 |
| Fig.1.27 |
The closer the sample volume is located to the transducer the
higher the maximum PRF that can be used. Conversely, the farther
the sample volume is placed into the heart the lower the maximum
PRF becomes. This occurs because the distance (and therefore pulse
travel time) to and from the sample volume is much shorter in
the near field and, therefore, pulse roundtrip transit time is
much less when compared with greater distances. An example of
the spectral velocity display with the sample volume located in
the near field is shown in Figure
1.27. The maximum velocity that can be recorded without aliasing
is 2.50 m/sec in either direction (Fig.
1.27 left panel arrow). When the sample volume is positioned
farther into the field the maximum possible velocity in either
direction is reduced to .70m/sec (Fig.
1.27, right panel arrow).
 |
| Fig.1.28 |
Note that with the Doppler system used in the previous example,
the scale of the spectral display automatically changes. In some
systems, the scale is fixed and the size of the spectral tracing
will alter.
Figure 1.28 demonstrates mild aliasing when a high velocity
jet is in the near field (A). Progressively more severe aliasing
with distortion of the full profile occurs if this same jet is
encountered at progressively increasing distances from the transducer
face (B, C and D). At point D the aliased profile is so distorted
as to be unrecognizable.
In a practical sense, the Nyquist limit is a descriptive term
which specifies the maximum velocity that can be recorded without
aliasing. This limit is controlled by two factors: depth into
the tissue and transducer frequency
When dealing with valvular heart disease, most abnormal jets
exceed 1.5 to 2.0 m/sec. Therefore, Doppler beginners should not
expect to record easily the full flow profile of these abnormal
jets using PW Doppler. In fact, most beginners should simply attempt
to recognize the presence of aliasing. With experience, it will
become easier to identify an aliased signal. The recognition of
aliasing on the audio output is difficult.
2004