Diagnostic ultrasound ( PDFDrive )

30 PART I Physics
A
B
FIG. 1.44 Wall Filters. Wall ilters are used to eliminate low-frequency noise from the Doppler display, but high wall ilter settings may result
in interpretation errors. Here the effect on the display of low-velocity low is shown with wall ilter settings of (A) 100 Hz and (B) 400 Hz. In
general, wall ilters should be kept at the lowest practical level, usually in the range of 50 to 100 Hz.
of the target (Nyquist limit), aliasing results (Fig. 1.46). When
PRF is less than twice the frequency shit being detected, lower
frequency shits than are actually present are displayed. Because
of the need for lower PRFs to reach deep vessels, signals from
deep abdominal arteries are prone to aliasing if high velocities
are present. In practice, aliasing is usually readily recognized.
Aliasing can be reduced by increasing the PRF, by increasing
the Doppler angle (thereby decreasing the frequency shit), or
by using a lower-frequency Doppler transducer.
Doppler Angle
When making Doppler measurement of velocity, it is necessary
to correct for the Doppler angle. he accuracy of a velocity
estimate obtained with Doppler is only as great as the accuracy
of the measurement of the Doppler angle. his is particularly
true as the Doppler angle exceeds 60 degrees. In general, the
Doppler angle is best kept at 60 degrees or less because small
changes in the Doppler angle above 60 degrees result in substantial
changes in the calculated velocity. herefore measurement inaccuracies
result in much greater errors in velocity estimates than
do similar errors at lower Doppler angles. Angle correction is
not required for the measurement of Doppler indices such as
the resistive index, because these measurements are based only
on the relationship of the systolic and diastolic amplitudes.
Sample Volume Size
With pulsed wave Doppler systems, the length of the Doppler
sample volume can be controlled by the operator, and the width
is determined by the beam proile. Analysis of Doppler signals
requires that the sample volume be adjusted to exclude as much
of the unwanted clutter as possible from near the vessel walls.
Doppler Gain
As with imaging, proper gain settings are essential to achieving
accurate and reproducible Doppler measurements. Excessive
Doppler gain results in noise appearing at all frequencies and
may result in overestimation of velocity. Conversely, insuicient
gain may result in underestimation of peak velocity (Fig. 1.47).
A consistent approach to setting Doppler gain should be used.
Ater placing the sample volume in the vessel, the Doppler gain
should be increased to a level where noise is visible in the image,
then gradually reduced to the point at which the noise irst
disappears completely.
OPERATING MODES:
CLINICAL IMPLICATIONS
Ultrasound devices may operate in several modes, including
real-time, color Doppler, spectral Doppler, and M-mode imaging.
Imaging is produced in a scanned mode of operation. In scanned
modes, pulses of ultrasound from the transducer are directed
down lines of sight that are moved or steered in sequence to
generate an image. his means that the number of ultrasound
pulses arriving at a given point in the patient over a given interval
is relatively small, and relatively little energy is deposited at any
given location. In contrast, spectral Doppler imaging is an
unscanned mode of operation in which multiple ultrasound pulses
are sent in repetition along a line to collect the Doppler data. In
this mode the beam is stationary, resulting in considerably greater
potential for heating than in imaging modes. For imaging, PRFs
are usually a few thousand hertz with very short pulses. Longer
pulse durations are used with Doppler than with other imaging