Diagnostic ultrasound ( PDFDrive )

CHAPTER 26 The Extracranial Cerebral Vessels 933
tissue structures, which lack a detectable phase or frequency
shit, are assigned an amplitude value and displayed in a
gray-scale format with lowing blood in vessels superimposed
in color. Color assignments depend on the direction of blood
low relative to the Doppler transducer. Blood low toward the
transducer appears in one color and blood low away from the
transducer is in another. hese color assignments are arbitrary.
Color saturation displays indicate the variable velocity of blood
low. Deeper shades usually indicate low velocities centered
around the zero-velocity color-low baseline. As velocity
increases, the shades become lighter or are assigned a diferent
color hue. Some systems allow selected frequency shits to be
displayed in a contrasting color, such as green. his green-tag
feature provides a real-time estimation of the presence of
high-velocity low.
Setting the color Doppler scale can also be used to create an
aliasing artifact corresponding to the highest-velocity low within
a vessel (see Figs. 26.16B, 26.18, and 26.21). hese high-velocity
jets pinpoint areas for spectral analysis. Color assignments are
a function of both the mean frequency shit produced by moving
RBC ensembles and the Doppler angle theta. If the vessel is
tortuous or diving, angle theta between the transducer and vessel
will change along the course of the vessel, resulting in changing
color assignments that are unrelated to the change in RBC
velocity. he color assignments will reverse in tortuous vessels
as their course changes relative to the Doppler transducer, even
though the absolute direction of low is unchanged. Portions of
a vessel that parallel the Doppler beam when angle theta is 90
degrees will have little or no frequency shit detected, and no
color will be seen.
Optimal Settings for Low-Flow Vessel Evaluation
Color Doppler low studies should be performed with optimal
low sensitivity and gain settings. Color low should ill the
entire vessel lumen but not spill over into adjacent sot tissues.
he pulse repetition frequency (PRF) and frame rates should
be set to allow visualization of low phenomena anticipated in
a vessel. Frame rates will vary as a function of the width of
the area chosen for color Doppler display and for depth of the
region of interest. he greater the color image area is, the slower
the frame rate will be. he deeper the posterior boundary of
the color image is, the slower the PRF will be. Color Doppler
sensitivity should be adjusted to detect anticipated velocities,
Optimization of Color Doppler Low-Flow
Vessel Evaluation
Use low pulse repetition frequency
Use Doppler angle of less than 60 degrees
Increase gain setting
Increase power setting
Decrease wall ilter
Increase persistence
Increase dwell time
such that if slow low in a preocclusive carotid lesion is sought,
low-low settings with decreased sampling rates are employed.
However, the system will then alias at lower velocities because of
the decrease in PRF. In addition to changes in the PRF, optimization
of the Doppler angle, increases in gain and power settings,
decrease in the wall ilter, increase in persistence, and increase
in ensemble or dwell time can be used to optimize low-low
detection.
Flowing blood becomes, in efect, its own contrast medium,
with color or power Doppler outlining the patent vessel lumen.
his allows determination of the true course of the vessel, facilitating
positioning of the Doppler cursor and thus providing more
reliable velocity determinations. Furthermore, color Doppler
facilitates Doppler spectral analysis by rapidly identifying areas
of low abnormalities. he highest velocities in the region of and
immediately distal to a stenosis are seen as aliasing high-velocity
jets of color. Color Doppler ultrasound facilitates placing the
pulsed Doppler range gate in the region of these most striking
color abnormalities for pulsed Doppler spectral analysis. he
presence of a stenosis can be determined by color Doppler changes
in the vessel lumen as well as by visible luminal narrowing.
Although color Doppler can be used to determine the presence
of hypoechoic plaque, it cannot be used optimally to determine
the area of patent lumen in transverse projection because the
optimal angle for measuring the area or diameter of narrowing
is at 90 degrees to the long axis of the vessel, which is the
worst angle for color Doppler imaging. Gray-scale assessment,
power Doppler, or B-low imaging should be used to assess the
diameter/area of the patent carotid lumen (see Fig. 26.12). If a
stenosis produces a bruit or thrill, the resultant perivascular
tissue vibrations may be seen as transient speckles of color
in the adjacent sot tissues, more prominent during systole 124
(Fig. 26.24).
Comparisons of color Doppler ultrasound with conventional
duplex Doppler sampling techniques and angiography have shown
relatively similar accuracy, sensitivity, and speciicity. 125,126
However, color Doppler ofers many beneits, including a reduction
in examination time by pinpointing areas of color Doppler
abnormality for pulsed Doppler spectral analysis. 126 Branches of
the ECA are readily detected, facilitating diferentiation from
the ICA. he real-time low information over a large crosssectional
area provides a global overview of low abnormalities
and allows ready determination of the course of a vessel. Furthermore,
color Doppler improves diagnostic conidence and
reproducibility of ultrasound studies, thereby avoiding many
potential diagnostic pitfalls.
he laminar blood low is disrupted in the region of the carotid
bifurcation where there is a normal transient low reversal
opposite the origin of the ECA (see Fig. 26.19). Color Doppler
displays this normal low separation as an area of low reversal
located along the outer wall of the carotid bulb, which appears
either at early systole or in peak systole and persists for a variable
period into the diastolic part of the cardiac cycle. 127,128 his low
reversal can produce some strikingly bizarre pulsed Doppler
waveforms; however, the color Doppler appearance readily
discerns the nature of these waveform changes. Furthermore,
the absence of this low reversal may be abnormal and may