o_1977r8vv9vk1ts2ms0kd8pksa.pdf

24 D. Maulik
nomena. The autoregression method acts as a digital
filter and assumes the signal at a given time to be the
sum of the previous samples. In contrast to power
quantification of the discrete frequencies in the FFT
analysis, autoregression allows power quantification
of all the frequencies of the spectrum. Moreover, it
produces cleaner spectra and better frequency resolution
than those generated by FFT processing.
We have evaluated autoregressive (AR), moving
average (MA), and autoregressive moving average
(ARMA)-based spectral processing approaches as
possible alternatives to the FFT analysis [10]. Compared
to the FFT processing, these parametric techniques
produce cleaner spectra. The AR model estimates
maximum frequency well, and the MA model
indicates velocities at which most scatterers (erythrocytes)
travel. The ARMA shows the potential of producing
superior spectral analysis by combining the
advantages of the AR and MA models. These parametric
methods depend on appropriate system identification
of the data.
Continuous-Wave Doppler
Ultrasonography
Fig. 3.7. Continuous-wave Doppler transducer
A continuous-wave (CW) Doppler transducer continuously
transmits and receives ultrasonic signals. A CW
Doppler system essentially consists of a double element
transducer for both emitting and receiving signals, a
radiofrequency receiver amplifier for the acquisition
and amplification of the electrical voltage signals from
the receiving transducer, a voltage generator that provices
the driving frequency of the emitting transducer
and the reference frequency for the demodulator, a demodulator
that extracts the Doppler shifted frequencies
from the total echo signals, and a direction detector
that completely separates the signals of the forward
flow from those of the backward flow. Details of these
components of Doppler signal analyses and subsequent
spectral processing are described at the outset of this
chapter. The transducer assembly encases two piezoelectric
elements; one continuously emits ultrasound
and the other continuously receives the backscattered
echoes (Fig. 3.7). For most transducers, the emitting
and receiving elements are either rectangular or halfcircle
in shape. There may also be a concentric arrangement
with a central circular element surrounded
by a ring element. The elements are positioned in such
a manner that their faces sustain a suitable angle to
each other, and consequently there is an overlap between
the transmitted ultrasound field generated by
the emitting element and the sensitive zone for the receiving
element.
A CW Doppler setup does not have range discrimination,
and any movement within the sensitive region
causes backscattering and consequently Doppler
shift of the incident beam. If the beam interrogates
more than one vessel, the Doppler signals from all
the vessels merge to form a composite display. The
only discrimination that exists is that the signals from
a deep location are more attenuated because of the
longer transmission path through tissues. A CW system
is sensitive also to the movements of nonhemodynamic
objects in its path, such as pulsating vascular
walls or moving cardiac structures. These sources
of Doppler shift produce high-amplitude/low-frequency
signals, which may overwhelm a receiver±amplifier
of limited dynamic range. This situation leads
to loss of important hemodynamic signals. The problem
can be resolved by ensuring an extended dynamic
range of the receiver and by use of an appropriate
high-pass filter, as described above.
Continuous-wave Doppler ultrasound instruments
are used extensively in obstetrics for nonvelocimetric
applications, such as fetal heart rate detection and external
electronic fetal heart rate monitoring. For velocimetric
applications, CW devices with a spectral
analyzer are used for insonation of the umbilical arteries
and often the uterine arteries. Despite the inherent
absence of range discrimination, Doppler signals
from the umbilical or uterine arteries can be obtained
with relative ease. Moreover, the equipment is
substantially less expensive than the duplex pulsed
Doppler devices and requires less expertise to operate.
However, CW Doppler instruments are infrequently
used at present for fetal surveillance.