268
Level 3
Advanced Topics
Fig. 64a
Standard color power Doppler showing the
intracranial vessels of the Circle of Willis.
Fig. 64b
Directional color power Doppler showing the intracra-
nial vessels of the Circle of Willis. Notice that there are no scale
markings on the power color bar.
Some systems nowoffer a directional color power Doppler technique,
overcoming one of the principal limitations of standard color power
Doppler. In order to present flow direction, the system uses phase
information in conjunction with the power information. Figure
64b illustrates directional power Doppler. Note that there are no
scales presented as the phase is used only to determine direction,
and not velocity.
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Fig. 65
Color power Doppler showing recanalized flow in an
occluded carotid artery. Note that power Doppler is advantageous
in this imaging situation both because of the relative angle insen-
sitivity and the better sensitivity for detection of low velocity flow.
Fig. 66
Color power Doppler image of the pericallosal artery in
a neonatal head.
Level 3: Advanced Topics
32. Understanding the Behavior of Color Wall
Filters
32.1 Digital Filtering
Predicting the behavior of the wall filters in color Doppler is much
more difficult than predicting the behavior of the wall filters for
spectral Doppler. Although the concept of the wall filters is the same,
the electronic implementation is very different. In contrast to spec-
tral Doppler, color Doppler wall filters are almost always designed
as pure digital filters. Usually these filters take on the form of an
infinite impulse response (IIR) filter. Since the number of samples
used in the filter must be small (usually around eight,corresponding
to the color packet size), the filters have a very gradual roll-off. As a
result, to achieve enough attenuation of the large clutter signals at
the lower frequencies, the wall filters must be designed to begin at-
tenuating at much higher frequencies than the wall filters for spectral
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