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a Chapter 3 Spectral Doppler: Basic Principles and Instrumentation 25
Pulsed-Wave Doppler
Ultrasonography
With the PW Doppler setup, a single transducer crystal
emits pulses of short bursts of ultrasound energy
(Fig. 3.8). Between the pulses the same crystal acts as
the receiving transducer. As the velocity of sound in
soft tissues is virtually constant, the time interval between
transmission and reception of the ultrasound
beam determines the distance, or range, of the target
area from the transducer. Thus the location of the
target area can be selected by varying this time delay.
This process is known as range gating.
A simplified description of the pulsed Doppler ultrasound
system is presented: As mentioned above,
the same transducer crystal acts as both the transmitter
and the receiver of ultrasound signals. A master
oscillator generates a reference signal at the resonant
frequency of the transducer. An electronic gate controls
the transmission of the signal to the transducer,
so a pulse of only a few cycles reaches it. The rate at
which the pulses are generated (pulse repetition frequency)
is determined by an electronic device called
the pulse repetition frequency generator. The generator
also controls the delay gate, which in turn controls
the range gate and therefore the time interval
between the ultrasonic transmission and reception.
As described above, a demodulating system extracts
the Doppler frequency shifts from the carrier frequency.
The Doppler signals thus produced are
stored, integrated, and updated with the signals from
the next pulse. After appropriate filtering, the signals
are now ready for audio output and spectral analysis.
A more complete review of the pulsed Doppler system
has been presented by Evans and associates [1].
Doppler Sample Volume
The three-dimensional region in the path of the
transmitted ultrasound beam from which the frequency
shift signals are obtained is called the Doppler
sample volume (DSV). For the CW Doppler system,
the totality of the superimposed region represents
the DSV and is therefore of no relevance. For
the PW Doppler system, the range-gated zone from
which the backscattered echoes are received constitutes
the sample volume. The DSV is an important
consideration for PW Doppler applications because
its location and axial dimension can be controlled by
the examiner. The examiner therefore has the ability
to interrogate a target area of appropriate location
and size and to obtain more precise spatial velocity
information. This subject is discussed further below.
The length of the DSV along the ultrasonic beam
axis is known as its axial dimension. The lateral measurable
limits of the beam, perpendicular to the beam
axis, define its transverse dimension, or width. The
shape resembles a pear or a teardrop (Fig. 3.9). The
axial dimension is determined by the pulse length or
duration as sensed by the receiving transducer and is
therefore dependent on the duration of the transmitted
ultrasonic pulse and the time window during
which the receiver gate is open. As the last two factors
are amenable to controlled variations, the axial
dimension of the DSV can be altered by the operator.
Many current duplex systems allow sample length
variations, from 1.5 to 25.0 mm.
The transverse dimension of the DSV is determined
by the ultrasonic beam width, which approximates the
diameter of the transducer face in the near field. In the
Fig. 3.8. Pulsed-wave Doppler transducer
Fig. 3.9. Doppler sample volume and the factors controlling
its dimensions