Diagnostic ultrasound ( PDFDrive )

CHAPTER 26 The Extracranial Cerebral Vessels 937
A
B
FIG. 26.29 Abnormal Doppler Flow Caused by Tortuous Vessel. (A) Tortuous common carotid artery (CCA) displays color Doppler eccentric
jets of low (arrow). (B) Spuriously elevated velocity is caused by an eccentric jet in a tortuous proximal left CCA without any visible stenosis.
Patients with isolated severe or critical aortic stenosis may
demonstrate duplex waveform abnormalities, including prolonged
acceleration time, decreased peak velocity, delayed upstroke, and
rounded waveforms. 149 However, mild or moderate aortic stenosis
usually results in little or no sonographic abnormality. Tortuous
or kinked carotid arteries represent either congenital or acquired
changes in the carotid artery. he clinical signiicance of these
is debatable; however, the vascular tortuosity frequently results
in eccentric jets of high-velocity low, which may show elevated
velocities in the absence of signiicant stenosis 150-152 (Fig. 26.29).
Conversely, if the carotid bulb is capacious, a large plaque burden
may still fail to produce anticipated velocity increases. he relative
diference in area between the distal CCA and the residual patent
lumen of the large bulb is not suicient to produce a greater
than 50% velocity change. Some refer to this as nonstenotic
(homogeneous) plaque (Fig. 26.30). Frequently, an image/
Doppler mismatch alerts the examiner to potential pitfalls.
Causes of Image/Doppler Mismatch
Cardiac arrhythmia
Cardiac valvular disease, cardiomyopathy
Severe aortic stenosis
Hypotension or hypertension
Tandem lesions
Contralateral carotid stenosis
Nonstenotic plaque
Long-segment, concentric high-grade stenosis
Carotid dissection
Preocclusive lesion
Tortuous vessels
Calciied plaque, hypoechoic or anechoic plaque
Anatomic variants
Color Doppler can be used to overcome diagnostic dilemmas
in these situations, particularly when “cine loop” playback
capabilities are present. Cine loop allows the computer to store
portions of the previous color Doppler low recording for playback
at the real-time rate or frame by frame. his allows the clinician
to assess the illing of all parts of the vessel lumen. Obstructive
lesions in one carotid can afect velocities in the contralateral
vessel. For example, severe unilateral ICA stenosis or occlusion
may cause shunting of increased low through the contralateral
carotid system. his increased low artiicially increases velocity
measurements in the contralateral vessel, particularly in areas
of stenosis. 150,153-156 Conversely, a proximal common carotid or
innominate artery stenosis may reduce low, with consequent
reduction of velocity measurements in a stenosis that is distal
to the point of obstruction (tandem lesion) (see Fig. 26.22).
Velocity ratios that compare velocity values in the ICA to
those in the ipsilateral CCA can help avoid some pitfalls. 99 Of
particular value are the peak systolic ratio (PSV in ICA vs. PSV
in CCA) 104,157 and the end diastolic ratio (ICA/CCA EDV). 112
Grant et al. have shown that PSV ICA/CCA ratios are comparable
in accuracy to PSV values for determining the degree of ICA
stenosis. 109 Although the PSV and peak systolic ICA/CCA velocity
ratio have shown relative comparable sensitivities and speciicities,
at times the velocity ratio will more correctly identify the degree
of stenosis and the absolute velocity. Velocity ratios should always
be employed when unusually high or low CCA velocities or
signiicant asymmetry of CCA velocities is detected. Longsegment,
high-grade stenoses frequently will not demonstrate
the anticipated degree of ICA velocity elevation. In such situations,
the velocity ratio coupled with the gray-scale/color/power Doppler
appearance may provide insight into the actual degree of narrowing.
As discussed for spectral broadening, color and power
Doppler sonography are invaluable in the avoidance of pitfalls
related to spurious Doppler spectral traces.