Diagnostic ultrasound ( PDFDrive )

CHAPTER 26 The Extracranial Cerebral Vessels 927
A
B
FIG. 26.17 Incorrect Doppler Angle Theta. Velocity obtained in the internal carotid artery (ICA) with angle theta of 60 degrees (A) is less
accurate than the velocity obtained from the same area of the same ICA with an angle theta of 70 degrees (B).
(bandwidth) and thus quantitate spectral broadening. he validity
of these measurements remains unproved, however, and further
correlative studies are needed to document the relationship of
quantitative spectral broadening parameters to speciic degrees
of stenosis. 104 Most tables no longer include a measurement for
spectral broadening when grading carotid stenosis. Nevertheless,
a visible gestalt of the amount of spectral window obliteration,
as well as color Doppler heterogeneity, provides a useful, if not
quantitative, predictor of the severity of low disturbance.
FIG. 26.18 High-Grade External Carotid Artery (ECA) Stenosis.
Elevated velocities and visible narrowing. Spectral broadening is
present. Color Doppler spectral broadening is also seen.
interrogated with a consistent angle theta between the transducer
and the vessel maintained throughout the examination, when
possible. Generally, only the origin of the ECA is evaluated because
occlusive plaque is less common here than in the ICA and is
rarely clinically important. A stenosis of the ECA should be
noted because it may account for a worrisome cervical bruit
when the ICA is normal. 30
Spectral Broadening
Atheromatous plaque projecting into the arterial lumen disturbs
the normal, smooth laminar low of erythrocytes. he RBCs
move with a wider range of velocities, so the spectral line becomes
wider, illing in the normally black spectral window. his phenomenon,
termed spectral broadening, increases in proportion
to the severity of carotid artery stenosis 102-104 (Fig. 26.18, Video
26.21). Some duplex machines allow the operator to measure
the spectral spread between the maximal and minimal velocities
Pitfalls in Interpretation
Pseudospectral broadening can be caused by technical factors,
such as too high a gain setting. In such cases the background
around the spectral waveform oten contains noise. Whenever
spectral broadening is suspected, the gain should be lowered to
see if the spectral window clears. Similarly, spectral broadening
caused by vessel wall motion can occur when the Doppler sample
volume is too large or positioned too near the vessel wall.
Decreasing the size of the sample volume and placing it midstream
should eliminate this potential pitfall.
Altered low patterns can normally be found at certain sites
in the carotid system. For example, it is normal to ind low
separation at the site of branching vessels, such as where the
CCA branches into the ECA and ICA. 105 Flow disturbances also
occur at sites where there is an abrupt change in the vessel
diameter. For example, low disturbances and bizarre waveforms
caused by low separation may be encountered in a normal
carotid bulb where the CCA terminates in a localized area of
dilation as it divides into the ECA and ICA 15 (Fig. 26.19).
he tendency for spectral broadening increases in direct
proportion to the velocity of blood low. For example, spectral
broadening can be observed in a normal ECA, vertebral arteries,
and CCA supplying circulation contralateral to an occluded
contralateral ICA. Increased velocity may also account for the
disturbed low that is sometimes observed in the normal extracranial
carotid arteries of young athletes with normal cardiac
outputs or in patients in pathologically high cardiac output states.
It is also seen in arteries supplying arteriovenous istulas
and malformations. 15,106,107 Postoperative spectral broadening