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a Chapter 3 Spectral Doppler: Basic Principles and Instrumentation 33
determine the maximum frequency shift envelopes.
Various Doppler indices are determined automatically
and displayed either spatially as color-coded superimpositions
over the gray-scale two-dimensional anatomical
image or as a statistical distribution histogram
of the indices in the scanned area (Fig. 3.16).
Although the reproducibility of the new modality has
been confirmed in relation to the umbilical artery, its
utility remains to be confirmed.
Doppler Display
During operation, the Doppler information is displayed
on a monitor screen. The details of the display
vary according to the type of instrumentation and
manufacturer. The CW Doppler instruments obviously
have simpler displays (Fig. 3.11) in comparison
with duplex Doppler systems (Fig. 3.12). During
real-time Doppler interrogation, the spectral display
scrolls from the right of the screen to the left with
time, as progressively newer spectral information is
added to the display. In addition to the spectral waveform,
a CW Doppler display also includes the zero
line, the frequency (or velocity) scale in the vertical
axis, and the time scale in the horizontal axis. The
spectral display area also includes the cursor lines for
measuring the peak systolic or end-diastolic frequency
shifts. Additional information is shown outside
the spectral display window but within the
screen area. It may include measures of the frequency
shift including the peak systolic and end-diastolic
components, Doppler indices, and various indicators
of the system function.
A duplex Doppler system display is complex because
of its extended function, encompassing both
imaging and Doppler modes. Moreover, M-mode capability
and both CW and PW Doppler modes are integrated
into these systems. With more advanced systems,
color Doppler flow mapping is also included.
Compared with the imaging and M-mode instruments,
these devices must convey to the operator a
far greater amount of information. To maintain userfriendliness,
the devices employ a software menu
approach to their system control, which is usually reflected
in the control panel display and the monitor
display, so the operator is guided through the multiplicity
of functions with relative ease. In the Doppler
mode the monitor screen should display not only the
spectra, the data of examination, and the patient's
identity but comprehensive Doppler information as
well, including transducer frequency, mode of Doppler,
depth and size of the sample volume, angle of
insonation, frequency or velocity mode, pulse repetition
frequency, and Nyquist limit. It is imperative to
indicate the filter and gain settings and to offer some
meaningful information on the power output of the
transducer. The latter is needed to ensure that the fetus
is not exposed to a high level of ultrasonic energy
when a lower output is sufficient to yield the desired
information. Most current instruments include realtime
energy output displays, the thermal index and
the mechanical index, which indicate to the operator
the potential for producing any biological effects in
soft tissue. This is discussed in Chap. 8. Both manufacturers
and users must be cognizant of these biosafety
considerations. A number of options are available
for archival storage of the Doppler information, including
video recording and hardcopy prints, thermal
printouts, photographic recording, and computer
storage. When choosing the appropriate storage modality,
many factors should be considered, including
the needs of an indivdual practice, cost-benefit issues,
archival integrity, and the ease of retrieval.
Summary
Fig. 3.17. Continuous-wave Doppler sonogram derived
from the umbilical arteries. It was obtained with a freestanding
Doppler device without duplex imaging guidance.
Vertical axis represents the magnitude of the Doppler shift
in kilohertz and the horizontal axis the time. Bottom: Peak
systolic (S), end-diastolic (D), and average frequency shift
(A) values. The indices (S/D, D/A) are also shown
The backscattered echo signals from a vascular source
consist of the carrier and the Doppler shifted frequencies
along with noise generated from various
sources. To obtain clinically useful information, it is
necessary to generate Doppler waveforms by systematically
processing these signals. The steps involve re-