Diagnostic ultrasound ( PDFDrive )

CHAPTER 26 The Extracranial Cerebral Vessels 939
FIG. 26.32 Aliasing of Doppler Waveform in the Region of a
High-Grade (80%-95%) Stenosis. Highest velocities are wrapped around
(arrow) and displayed below the zero-velocity baseline. ICA, Internal
carotid artery.
Another source of error in pulsed Doppler ultrasound analysis
is aliasing, which is caused by the inability to detect the true
PSV because the Doppler sampling rate, the PRF, is too low. he
maximal detectable frequency shit can be no greater than half
the PRF. With aliasing, the tips of the time velocity spectrum
(representing high velocities) are cut of and wrap around to
appear below the baseline (Fig. 26.32). If aliasing occurs, continuous
wave probes used in conjunction with duplex pulsed Doppler
can readily demonstrate the true peak velocity shit. Aliasing
can also be overcome or decreased by increasing the angle theta
(the angle of Doppler insonation), thereby reducing the detected
Doppler shit, or by decreasing the insonating sound beam
frequency. Increasing the PRF increases the detectable frequency
shit, but the PRF increase is limited by the depth of the vessel
as well as the center frequency of the transducer. 45,101,158 One can
also shit the zero baseline and reassign a larger range of velocities
to forward low to overcome aliasing. It is also valid to add
velocity values above and below the baseline to obtain an
accurate velocity value, provided that multiple wraparounds do
not occur, as seen in extremely high velocities. Aliasing may
sometimes be useful in color Doppler image interpretation, where
color-low Doppler aliasing can accent the severity of low
disturbances as well as deine the patent lumen.
Internal Carotid Artery Occlusion
Distinguishing between a string sign and a totally occluded
carotid artery has major clinical consequences. 88 Grubb et al. 159
showed that untreated preocclusive lesions carry about a 5% per
year risk for stroke. hus intervention in this patient population
is particularly important.
Carotid occlusion is diagnosed when no low is detected in
a vessel. Occasionally, transmitted pulsations into an occluded
ICA may mimic abnormal low in a patent vessel. he pulsed
Doppler cursor should be clearly located in the ICA lumen, and
arterial pulsatile low should be identiied. Close attention should
be paid to the direction of low and the nature of pulsations.
True center-stream sampling should be documented by transverse
scanning, and the sample volume reduced in size as much
as possible. Extraneous pulsations are seldom transmitted to the
center of the thrombus. 102
As a high-grade stenosis approaches occlusion, the highvelocity
jet is reduced to a mere trickle. It may be diicult to
locate the small residual string of low within a largely occluded
lumen using gray-scale imaging alone, particularly if the adjacent
plaque or thrombus is anechoic, making the residual lumen
diicult to visualize during real-time examination, or if calciied
plaque obscures visualization. In critical high-grade stenoses
(>95%), standard-sensitivity color Doppler settings may fail to
demonstrate a string of residual low. hus it is always prudent
to employ the slow-low sensitivity settings on color Doppler
to discriminate between critical stenoses and occlusions. 86,125,129
Alternatively, power Doppler ultrasound (with its increased
sensitivity to detecting low-amplitude, slow-velocity signals) may
be used to visualize a residual string of blood low (Fig. 26.33).
Color Doppler is 95% to 98% accurate in distinguishing highgrade
stenosis from complete occlusion on angiography when
appropriate technical parameters are employed. 160-162
he presence of a high-grade ICA stenosis or occlusion can
oten be inferred from inspection of the ipsilateral CCA on a
pulsed Doppler waveform or color/power Doppler image (see
Fig. 26.31). he pulsed Doppler waveforms in the ipsilateral
CCA and ICA proximal to a lesion frequently demonstrate an
asymmetrical, high-resistance signal with decreased, absent, or
reversed diastolic low, except when there are ECA collaterals
to the intracranial circulation (Fig. 26.34). he main intracranial/
extracranial collateral pathway exists between the orbital and
ophthalmic arteries. Other collateral pathways include the occipital
branch of the ECA to the vertebral artery and cervical branches
of the arch with the vertebral artery. Similarly, color or power
Doppler images may show a lash of color low in systole but a
conspicuous decrease or absence of color low in diastole, which
is asymmetrical compared to the contralateral side. he diagnosis
of carotid occlusion versus a string sign is made more accurately
with color and power Doppler than with gray-scale duplex
scanning and may obviate the need for angiography to conirm
a sonographically diagnosed ICA occlusion; however, if ambiguity
persists, CTA, MRA, or angiography can be helpful. 88,160,161
Internal Carotid Artery Occlusion
Internalization of ipsilateral ECA waveform
Absence of low in ICA by color Doppler, power Doppler,
or pulsed Doppler ultrasound
Reversal of low in segment of ICA or CCA proximal to
occluded segment
Thrombus or plaque completely illing lumen of ICA on
gray-scale, color Doppler, or power Doppler images
Dampened high-resistance waveform in ipsilateral CCA or
proximal ICA
Possibly signiicantly higher velocities in contralateral CCA
than in ipsilateral CCA