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a Chapter 6 Sonographic Color Flow Mapping: Basic Principles 75
tal scan converter. When both imaging and flow mapping
fields are completely scanned, the completed
frame is displayed with the flow image superimposed
on the tissue image.
2. Imaging and flow data are synchronously
sampled for each line similar to the previous
approach but are displayed sequentially as each line
is interrogated rather than when a complete frame is
formed.
3. Scanning for tissue imaging is performed first
followed by scanning for flow. Flow information is
then superimposed on the tissue image.
The composite image frames generated by the above
methods are remarkably similar, although there are differences
related to the superimposition technique that
may compromise the ease of visual perception or interpretation
of the hemodynamic information. For example,
separate frame formations for the tissue and color
flow images may produce a hemodynamically incoherent
appearance. Similarly, line-by-line sequential image
formation may lead to mixing of consecutive individual
frame components, which is seen during frame-byframe
review of the images stored in the memory.
It should be noted that whereas only one pulse per
scan line is sufficient for tissue imaging, 3±32 pulses
are needed for color Doppler imaging ± which is
therefore a more time-consuming process. One way
to minimize the demand on time is to restrict Doppler
flow sampling to an area smaller than the total
available field. This color window is superimposed on
the gray-scale tissue image, and its size can be manipulated
by the operator. A small color window improves
Doppler sensitivity and temporal resolution.
Therefore it is advisable to restrict the size of this
window to the area of interest for Doppler interrogation.
Persistence
Interpolation algorithms may be used to produce
temporal averaging of color Doppler information. The
process of averaging is known as persistence, as it allows
more prolonged display or lingering of the color
image. It produces a smoother color Doppler image
at the cost of losing some detail. Most devices allow
selection of different degrees of averaging or persistence.
As the color Doppler image remains displayed
longer with a higher persistence setting, areas of circulation
with a lower flow velocity become more visible
in the color map. The application of persistence
control may therefore be useful when color mapping
such slow-flow conditions as are encountered in the
ovarian, placental, and fetal splanchnic circulations.
Similarly, as the blood flow velocity declines near the
vessel wall because of viscous drag, persistence may
produce a more complete outline of a vascular image.
Frame Rate
Frame rate is the number of image frames produced
per second. For Doppler color flow mapping, the rate
varies from 10 to 60 frames per second. As discussed
above, Doppler color flow interrogation places great
demand on time. The frame rate (FR) is directly affected
by the pulse repetition frequency (PRF); it is
inversely influenced by the scan line density (SLD) in
the color field and the number of pulses per scan line
or the ensemble length (EL):
FR ˆ PRF=…SLD EL†
Therefore the frame rate of color imaging can be
changed by manipulating these factors, which, however,
alters other imaging parameters due to their interdependence.
For example, a higher frame rate can
be achieved by increasing the PRF, which also limits
the depth of interrogation. Similarly, a reduction in
the line density not only increases the frame rate, it
compromises the spatial resolution; a decline in the
number of samples per line also reduces the Doppler
sensitivity. These trade-offs should be taken into consideration
when ensuring optimal color imaging for a
given situation. Thus a low frame rate improves image
quality by producing better sensitivity and spatial
resolution. Slow flows are better detected with a low
frame rate. The latter also allows more effective highpass
filtering, which results in more effective elimination
of motion artifacts, such as ghosting. A low
frame rate decreases temporal resolution, however, so
asynchronous events may be displayed in the same
frame. As expected, the problem becomes worse with
a high heart rate, as observed in the fetus. A high
frame rate improves the temporal resolution, provided
the line density and number of pulses per scan
line remain adequate to maintain an acceptable spatial
resolution and Doppler sensitivity, respectively.
M-Mode Color Frame Formation
In Doppler color M-mode, multigated Doppler sampling
is performed on a single scan line sampled
1,000 times per second. The unidimensional tissue
and color flow images are scrolled (usually from right
to left on the video screen) and therefore are displayed
as a function of time (Fig. 6.6). In the timemotion
format, the vertical axis represents tissue
depth, and the horizontal axis represents time. As
with gray-scale M-mode insonation, the spatial location
of the beam path is ascertained using two-dimensional
color flow imaging. Because of the high
sampling rate from a single line, color M-mode insonation
provide high Doppler sensitivity and temporal
resolution, and it allows reliable timing of the flow