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a Chapter 3 Spectral Doppler: Basic Principles and Instrumentation 31
erates by sequentially triggering batches of piezoelectric
elements for scanning the beam; and (2) the
phased array, which electronically triggers all or most
of the piezoelectric elements as a group during a given
pulse. Time delays (known as phasing) are used in
consecutive pulses to form the scanning beam. For
both of these arrays, the beam is focused by appropriate
phasing, or time delay, of the piezoelectric elements.
A specific transducer may have mixed features;
for example, a linear sequenced array has phasic
operation for focusing, or an annular array focuses
electronically but scans mechanically. Currently,
the following electronic array transducers are most
frequently used in obstetric applications.
1. Convex sequenced array (Fig. 3.15). The piezoelectric
elements are arranged in a convex arc and are fired
sequentially in small groups as with the previous two
categories. They are also known as curvilinear, curved,
or radial array transducers (Fig. 3.15). The field of image
is sectorshaped. This design allows a small transducer
footprint with a wide field of imaging at depth
± a combination of features particularly usuful for obstetric
scanning. Furthermore, the wide field of view is
achieved without the grating lobe problem of the linear
phased array. With the phased convex sequenced array,
the focusing is achieved by phasing of the firing sequence.
Doppler insonation is performed by a batch
of phased elements that can be steered in the sector
field of exposure.
2. Linear phased array. This configuration is also
known as the phased or electronic sector array. As
the name implies, the piezoelectric elements are arranged
in a straight line, and the beam is formed by
the simultaneous phased firing of all the elements. Focusing
and steering is accomplished electronically by
the phased triggering of the elements. The field of image
created by a linear phased array is sector-shape.
Although these transducers do not often provide optimal
imaging for general obstetric applications, they
are useful for fetal cardiac imaging. The Doppler ultrasonic
beam is produced by a batch of elements that are
phased and can be steered within the scanned image
area to interrogate the target vessel.
Integration of the Doppler
and Imaging Functions in Array Transducer
We referred earlier in the chapter to the conflicting
needs of opitmal transducer frequency for imaging
and Doppler sonography. In many current duplex systems
the problem has been addressed by providing
dual-frequency transducers, a higher operating frequency
for imaging, and a lower frequency for Doppler
ultrasound. The latter improves the Doppler sensitivity
and sets a higher Nyquist limit, whereas the
former improves spatial resolution for imaging. For
example, the transducer has 3.5 MHz for imaging
function and 2.5 MHz for Doppler insonation. Such
an arrangement has been possible only since the ad-
Fig. 3.15. Convex sequenced array transducers. Left panel:
Example. Middle panel: Color flow image of the umbilical
circulation demonstrating placement of the Doppler sample
volume. The Doppler parameters are noted in the left
lower corner. Right panel: Doppler frequency shift waveforms
of the umbilical artery. The vertical cursors in this
panel define one cardiac cycle