o_1977r8vv9vk1ts2ms0kd8pksa.pdf

606 I. Zalud
differences between vessel architecture in benign and
malignant ovarian growths [22, 30]. Improved detection
and classification of tumor architecture after instillation
of contrast agents might contribute to better
diagnostic accuracy.
Pitfalls and Artifacts
It is well known that artifacts can be observed during
ultrasound examinations. The same is true for color
and pulsed-wave Doppler images of blood flow [31,
32]. Recognizing these artifacts is important so as to
avoid image misinterpretation and, when possible, to
overcome them by modifying the technique, the unit
settings, or both. Most manufacturers have introduced,
and many diagnostic units have obtained, color Doppler
scanners. However, as with any new technology,
color scanning is being used by many without a complete
understanding of how the instrument can be optimally
adjusted to provide the best possible diagnostic
information. The examiner must have good working
knowledge of the instrument controls that affect the
color display and how these controls interact with each
other. In addition, knowledge of Doppler physics, the
basics of image display formats, and the hemodynamics
of blood flow is essential to optimize color
and pulsed-wave Doppler examinations.
Two basic colors ± red and blue by convention ± are
assigned to represent the direction of blood flow relative
to the ultrasound transducer. This rule assumes
an ideal situation, where the artery and vein lie parallel
to the skin surface and are straight, long tubes with
blood flow parallel to the vessel walls. In reality, vessels
do not follow these rules. They do not lie parallel to the
skin surface. They are branched and tortuous, and their
diameters often change. The flow pattern can be complicated.
The combination of these features makes interpretation
of the direction of blood flow difficult under
ideal circumstances. The most important tip for
determining the direction of flow is always to know
where the ultrasound beams are coming from and
how they are intersecting the vessels.
The length of the color or pulsed-wave Doppler bar
represents the range of frequencies obtainable at a designated
pulsed repetition frequency (PRF). The PRF is
adjustable but is limited by the depth of the image. The
Doppler bar is divided into maximum positive frequency
and minimum frequency, with each assigned
relative color or pulsed-wave Doppler assignments.
The color shading can be selected by the operator in
most systems. Once a ªcolor mapº is chosen, it should
remain constant for each type of examination. The
range of the Doppler scale is displayed as either the frequency
or the velocity. The velocity values are calculated
using a standard angle, usually 08. The numbers
displayed are not quantitative values because most vessels
examined to not lie parallel to the probe. These values
are used, rather, in a qualitative manner; that is, as
the numbers increase, the PRF increases and the ability
do detect higher frequencies without aliasing is increased.
The color frequency does not represent a peak
frequency but a mean or mode frequency. The frequency
displayed by the color is a single representative
frequency between the maximum and minimum frequencies,
as would be seen on a spectral waveform.
These values should not be used for peak frequency
or velocity measurements as is traditionally done with
spectral waveforms.
The PRF should be changed constantly throughout
the examination to accommodate the changing velocity
patterns that may be present in the observed pelvic
vessels. As a general rule, one should be aware
that if the PRF is set too high the lower frequencies
may not be detected; and if the PRF is set too low
the higher frequencies are aliased. Color flow is essentially
a guide for pulsed-wave Doppler exploration
of solid tissue perfusion (i.e., uterus and ovary and
tumor vascularization). Basically, if there is no color
flow, waveform analysis is not done. The presettings
of the instrument may be the reason for false-negative
Doppler findings.
Aliasing refers to generation of one of the most
common Doppler artifacts. It is a low-frequency component
in the signal spectrum when the PRF of the
instrument is less than two times the Doppler signal
frequency. Doppler spectral waveforms display the
aliased frequencies beginning at the opposite end of
the scale and moving toward the zero baseline. Color
Doppler sonography shows aliasing in the same manner:
The aliased frequencies are displayed as the opposite
color within the flow. In the display, dark color
next to dark color usually indicates a directional
change, but this color pattern also may occur with severe
aliasing. Here the aliased frequencies are so high
they go through the opposite color and cross the zero
baseline into the dark shade of the original color. It
is displayed as an apparent directional change and
may be falsely interpreted. Aliasing occurs in areas
where the frequency is increased owing to vessel obstruction
or the angle of the vessels approaching 08
relative to the transducer. Therefore aliasing is often
mistaken for true flow phenomena (e.g., turbulence
or eddying) rather than a display artifact. Increased
PRF, adjusted zero baseline, and decreased image
depth can help to avoid aliasing.
The colors displayed can be chosen by the operator
in most instruments. Usually manufacturer-preset
maps are available, as is a method for creating personally
selected maps. Experimentation with different
maps for different studies is recommended to allow
the operator to choose the most demonstrative colors