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Chapter 3
Spectral Doppler:
Basic Principles and Instrumentation
Dev Maulik
Spectral Doppler ultrasound velocimetry involves systematic
analysis of the spectrum of frequencies that
constitute the Doppler signal. This chapter presents a
general perspective on Doppler signal anlyses and describes
the spectral Doppler ultrasound devices commercially
available for clinical use. They include continuous-wave
(CW) Doppler, pulsed-wave (PW) Doppler,
and duplex Doppler devices. Within the realm of
obstetric usage, the application needs are diverse and
require various choices of equipment. For example,
fetal Doppler echocardiography requires advanced
duplex ultrasound instrumentation, which combines
the capabilities of high-resolution two-dimensional
imaging with the PW Doppler mode and an acoustic
power output appropriate for fetal application. For
umbilical arterial hemodynamic assessment, simpler,
substantially less expensive CW Doppler equipment
with a spectral analyzer may be sufficient. It is essential
therefore that one develop a basic understanding
of the implementation of Doppler ultrasound technology.
Doppler Signal Processing
As discussed in Chap. 2, the Doppler frequency shift
signal represents the summation of multiple Doppler
frequency shifts backscattered by millions of red
blood cells (RBCs). The RBCs travel at different
speeds, and the number of cells traveling at these
speeds varies as well. As the speed of the scatterers
determines the magnitude of the frequency shift and
the quantity of the amplitude, the Doppler signal is
composed of a range of frequencies with varying amplitude
content. Moreover, the total received Doppler
signal contains low-frequency and high-amplitude
signals generated by tissue movements and high-frequency
noise generated by the instrumentation. Obviously,
systematic processing is necessary before the
Doppler shifted frequencies can be used clinically.
The Doppler signal is processed in sequental steps
(Fig. 3.1), consisting of reception and amplification,
demodulation and determination of directionality of
flow, and spectral processing.
Fig. 3.1. Sequential steps of Doppler signal processing
Reception-Amplification
The returning signals are first received and amplified
by a radiofrequency (RF) receiving device. Amplification
is necessary, as the Doppler frequency shift generates
weak electrical voltage at the receiving transducer.
Demodulation
The total received and amplified echoes contain not
only the Doppler-shifted frequencies but also the carrier
frequency, which is the frequency of the incident
beam. During the next step, the Doppler-shifted
frequencies are extracted from the carrier frequency.
This process is known as demodulation. There are
various methods of demodulation [1], among which
the coherent phase quadrature procedure is commonly
employed. Phase quadrature implies that one
signal is one-fourth of a cycle out of phase or delayed
compared to the other. With this technique the incoming
signals are mixed with the direct and the