 |
| Fig.1.34 |
Understanding
the Doppler controls is very important because improper adjustment
of these controls can increase or decrease the quality of the Doppler
recordings. A summary of the important controls for Doppler examination
of the heart is given in Figure 1.34. The schematic drawing of the
control panel is generic, and Doppler users should be able to find
these controls on their system by the same or a similar name.
In this basic model, the eight controls are divided into three categories.
First is the group of controls that influence the quality of the
Doppler recording (Doppler gain, gray scale, and wall filter). This
group is of importance in both CW and PW examinations. Second are
the controls that change the appearance of the graphic display (scale
factor and baseline position) and also apply to both CW and PW examinations.
The third group are of use only for PW Doppler since they relate
to the sample volume (cursor, sample depth and angle).
Doppler Gain
 |
| Fig.1.35 |
The most important Doppler control is the overall gain. Rotating
between higher and lower settings will alter the strength of the
Doppler signals from the audible output and will be perceived by
the operator as a change in the volume of the sound. Figure 1.35
shows the range in appearance of the velocity spectral recording
for excessively high, correct and low gain settings.
As with any ultrasound system, it is prudent to use the lowest gain
or power setting that allows the recording of adequate signals.
More detailed examination of the recording in Figure 1.35 shows
a normal aortic spectral trace obtained from the suprasternal window.
Systolic flow velocity toward the transducer is depicted as an upward
profile and is laminar in appearance. The first two profiles show
a gain setting that is too high. This results in excessive background
noise that makes identification of the clear outline of systolic
flow difficult and produces an overflow in the opposite channel
represented below the baseline (this is called "mirroring" or "crosstalk").
The third profile (Figure 1.35, arrow) is set at an optimal gain
setting and displays a clear systolic envelope of flow with minimum
background noise, while the fourth profile demonstrates an incomplete
spectrum due to an improperly low gain setting.
 |
| Fig.1.36 |
The practical use of correct gain setting is again shown in Figure
1.36. In this CW examination from the ventricular apex, tricuspid
insufficiency is encountered as a systolic movement of the velocity
spectrum away from the transducer. In the complexes without adequate
gain the full velocity profile is not well seen (Figure 1.36, open
arrows). It is not until the gain is increased to an adequate level
that true spectral broadening and the true peak velocity are noted.
A beginner should first detect some flow signal and then run through
all of the possible gain settings to become familiar with the effect
of too much or too little gain.
2004