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34 D. Maulik: Chapter 3 Spectral Doppler: Basic Principles and Instrumentation
ception and amplification of the returning signals,
separation of the Doppler-shifted frequencies and determination
of the directionality of flow in relation to
the transducer, and spectral processing, which sorts
the frequencies of the signal and determines their
amplitude content. In addition, appropriate filtering
is performed to eliminate the low-frequency/high-amplitude
signals from tissue-sources and the high-frequency
noise signals from the device. Doppler interrogation
is performed by either CW Doppler transducers,
which do not have range resolution, or PW
transducers, which are capable of sampling a specific
vascular location. Each approach has significant advantages
and limitations. However, CW Doppler devices
are seldom used at present for fetal investigation.
Duplex Doppler ultrasonography combines twodimensional
imaging with the Doppler modality that
allows image-guided interrogation of vascular targets.
This capability is particularly useful for fetal investigation.
Of the various types of duplex transducers,
the convex sequenced array and the linear phased array
are commonly used for obstetrics and gynecology.
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