o_1977r8vv9vk1ts2ms0kd8pksa.pdf

72 D. Maulik
1. Transducer for transmitting the ultrasound
beam and receiving the echoes for both imaging and
Doppler analysis.
2. Receiver, which receives and amplifies the incoming
signals from the transducer for further processing
for gray-scale tissue imaging and for Doppler
color flow mapping.
3. Echo information for tissue imaging is processed
and converted to digital format. It is stored in
the digital scan converter for subsequent integration
with color flow mapping.
4. Backscattered echoes for Doppler processing are
first converted from analog (electrical voltage variations)
data to numeric or digital data.
5. Digitized signals are then subjected to filtering
to remove noise generated by stationary and slowmoving
tissue structures. The filter is known as the
moving target indicator.
6. Filtered data are then analyzed by the autocorrelator
to determine the Doppler phase shift. The autocorrelator
output consists of three types of information:
Doppler mean frequency shift, variance, and
Doppler amplitude (power or energy). These data are
fed to the digital scan converter, where they are integrated
with the tissue image information.
7. Doppler-related data are color-coded by the color
processor and the combined gray-scale tissue image,
and the Doppler color map is sent to the video
display via digital-to-analog conversion.
The above description is only a general outline.
The implementation of color Doppler sonography involves
highly complex technology and proprietary engineering
innovations ± information not accessible in
the public domain.
Transducers for Doppler Color
Flow Mapping
The various types of duplex transducer are discussed
in Chap. 3. Among them electronic array systems are
used in most devices for color flow mapping. Mechanical
transducers do not offer as optimal a platform
for Doppler color flow implementation as does
simultaneous tissue imaging and Doppler interrogation;
and other advanced features, such as beam
steering cannot be performed with these transducers.
With these devices one must freeze the tissue image
before using the Doppler mode. These limitations
preclude their use for obstetric Doppler sonography.
Color Doppler flow mapping has been implemented
using linear sequenced array, convex sequenced
array, linear phased array, and annular phased array
transducers. In a linear sequenced array transducer,
the scan lines are perpendicular along the length of
the transducer face, producing a rectangular field.
Although this configuration provides the optimal
imaging approach, it is not optimal for Doppler
imaging of vessels or flow channels located across the
beam path. This problem can be mitigated by the hybrid
sequenced and phased systems in which the
Doppler beam can be steered independent of the direction
of the imaging beam, producing a more favorable
angle of insonation. As all scan lines are parallel
to one another, they incur the same angle with a given
flow axis. Beam steering reduces the effective aperture
and increases the beam thickness; it may compromise
the sensitivity and lateral resolution. These
transducers are advantageous for peripheral vascular
imaging, but they are not particularly useful for obstetric
or gynecologic applications.
A modification of the linear sequence design, the
convex sequenced array offers distinct advantages
over the previous design for obstetric scanning because
of its smaller footprint. It also offers a wider
field of imaging at depth, and the wider field of view
is achieved without the grating lobe problem of the
linear phased array. The angle of Doppler insonation
is better achieved in the convex than in the linear array
despite some of the disadvantages of the former,
as previously discussed.
The linear phased array offers a sector-shaped
field of image and is useful for cardiologic applications.
These transducers, however, are not optimal for
fetal imaging applications. They do not provide the
wide angle of view at depth and may produce side
lobe problems, resulting in spurious flow depiction.
Annular array transducers are seldom used for color
flow obstetric applications.
Color Mapping
Color flow mapping is based on color-encoding each
pixel representing the averaged mean Doppler shift.
The color is used to represent the direction, magnitude,
and flow characteristics of the sampled circulation.
These parameters are qualitative rather than
quantitative. The color scheme is based on color classification,
which is derived from the fundamental
properties of light perception composed of hue, luminance,
and saturation.
Color Classification
Hue is the property of light by which the color of an
object is classified as the primary colors of red, blue,
green, or yellow in reference to the light spectrum.
The basic classification is based on the presence or
absence of hue. Those colors with hue are termed
chromatic colors and include red, orange, yellow,
green, blue, and so on. Those without hue are called