It is also
important to realize that the maximum recordable velocities in any
jet relate to the frequency of the transducer used. A lower frequency
transducer increases the ability of a PW system to record high velocities
at any given range. Thus, aliasing will be encountered at lower
velocities with a 5 MHz transducer when compare to a 2.5 MHz transducer.
The main drawback of lowering the transducer frequency is a reduction
in the signal to noise ratio of the resulting Doppler data (and
thus the quality of the output). For this reason, most standard
PW Doppler systems operate at approximately 2.5 MHz.
 |
| Fig.1.29 |
The second, and most practical, method to overcome aliasing is to
take advantage of the range of velocity available in the opposite
channel by moving the baseline (also known as "baseline shift").
In Figure 1.29 almost the entire profile of a pulmonic insufficiency
jet can be seen when the baseline is at the bottom of the display.
In the left portion of this velocity spectrum, the top of the pulmonic
insufficiency profile is poorly seen. As the baseline is moved downwards
to the bottom of the display, the top of the spectral profile becomes
obvious. This baseline control may be called "zero shift" or "zero
off-set" on some systems. Note that use of the baseline shift control
doubles the Nyquist limit at any given depth.
2004