Diagnostic ultrasound ( PDFDrive )

926 PART III Small Parts, Carotid Artery, and Peripheral Vessel Sonography
throughout diastole (see Videos 26. through 26.17). he CCA
waveform is a composite of the internal and external waveforms,
but most oten the CCA low pattern more closely resembles
that of the ICA, and diastolic low is generally above the baseline
(see Video 26.18). Approximately 80% of the blood lowing from
the CCA goes through the ICA into the brain, whereas 20% goes
through the ECA into the head musculature. he relative decrease
in blood low through the ECA will cause it to have a generally
lower amplitude gray-scale waveform than in either the ICA or
the CCA. 15
Standard Examination
Virtually all state-of-the-art ultrasound equipment ofers color
and power Doppler, as well as gray-scale capabilities and pulsed
Doppler, for the carotid examination (Video 26.19). A rapid
color Doppler screen allows the detection of abnormal low
patterns, which allows the pulsed Doppler signal volume to be
placed in areas that are abnormal, especially those with highvelocity
jets (Video 26.20). hese high-velocity jets are located
in the region of and immediately distal to a high-grade stenosis
(Figs. 26.15 and 26.16). In cases where both gray-scale and color
and power Doppler images of an entire carotid artery are normal,
FIG. 26.15 Color Doppler Jet. High-velocity jet (arrow) or aliasing
color demonstrates the area of highest velocity in the area of
stenosis.
only representative spectral tracings from the CCA, ICA, and
ECA are necessary to complete the examination.
he standard Doppler spectral examination consists of traces
obtained from the proximal and distal CCA, carotid bulb, and
proximal ECA; samples in the proximal, middle, and distal ICA;
and a representative trace from the vertebral artery. Normal
velocities are higher in the proximal CCA and lower in the distal
vessel; normal ICA velocities tend to increase from proximal to
distal. In addition, blood low velocities are obtained immediately
proximal to, at, and just beyond regions of maximal visible stenosis
and at 1-cm intervals distal to the visualized plaque as far cephalad
as possible. Positioning the Doppler angle cursor parallel to the
vessel walls determines angle theta, used to convert frequency
information into velocity values (see Fig. 26.14B). Doppler angle
theta is deined as the angle between the Doppler transducer
line of sight and the direction of blood low. he ideal angle
theta is 0 degrees, as the cosine of this angle is 1, thus resulting
in the greatest possible detectable frequency shit. Because this
angle is rarely achievable in the clinical setting, angles from 30
to 60 degrees are considered acceptable for carotid spectral
analysis.
Certain schools of ultrasound use a technique in which the
Doppler angle is set at 60% and the transducer is “heel and toed”
to parallel the carotid artery for Doppler spectral analysis. In
our experience, it is frequently not possible to optimize cursor
placement in the midportion of the vessel using this technique
in tortuous vessels. herefore our technique involves selecting
the site of spectral analysis and paralleling the wall of the vessel
at that point, making certain that the Doppler angle does not
exceed 60 degrees. Although either technique can be used, results
obtained using these diferent methodologies can result in diferent
velocities. hus if the irst technique is used, a diferent set of
velocity criteria should be expected than if the second is used.
his is one of the factors responsible for the diferences in velocity
spectral criteria used in diferent laboratories (Fig. 26.16). When
angle theta exceeds 60 to 70 degrees, the accuracy of velocity
measurement declines precipitously (Fig. 26.17), to the point
that virtually no velocity change is detected at angle theta of 90
degrees. he entire course of the CCA and ICA should be
A
B
FIG. 26.16 Doppler Angle Theta Measurement. (A) Velocity obtained in the distal internal carotid artery with angle theta of 60 degrees is
higher than that obtained at 44 degrees (arrow). (B) However, the sample angle does not parallel the vessel wall at 60 degrees. Note central color
aliasing in the region of highest velocity (curved arrow).