Page 79 - Libro 2

4 — The Extracranial Duplex Ultrasound Examination
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Figure 4-29 Turbulent arterial flow demonstrated with mosaic color Doppler pattern.
in recent generations of ultrasound machines. How- ever, the diagnostic relevance of these methods for the carotid duplex evaluation remains secondary to Doppler velocities. The main benefit from using the color Doppler and power Doppler modalities is the rapid identification of flow disturbances and deter- mining the location and direction of high velocity jets. When set with appropriate sensitivity, the flow disturbances associated with poststenotic turbulence are readily demonstrated with color Doppler imag- ing (Fig. 4-29). Power Doppler is particularly helpful in detecting extremely low flow velocities, including “string sign” flow.
There are several adjustments that can be made to optimize the color Doppler image and maintain proper sensitivity. Whenever possible, the color Dop- pler scale should be set high enough so that no color aliasing is present during any phase of the cardiac cycle and low enough so that color fills the patent lu- men with even the lowest velocities (Fig. 4-30). This may not be feasible when a wide range of velocities is present, and in this setting, color aliasing can be used to identify the sites of high velocity flow. The color Doppler transmit frequency can be adjusted to provide better resolution or penetration, depending on vessel depth.
Smooth, single color in the low-to-medium tone range indicates laminar flow. When the flow velocity exceeds the color Doppler scale, aliasing occurs with brighter tones of color progressing to the opposite
color (e.g., red to blue). Turbulent flow produces a typical “mosaic” color Doppler pattern (Fig. 4-31). Regardless of the color-flow characteristics, Doppler spectral waveforms must always be used to classify the severity of disease.
The power Doppler modality displays flow based on the amplitude of the Doppler signal rather than the frequency shift and thus does not give any in- formation on flow direction. This representation of blood flow is relatively independent of the angle of insonation. The main advantage of power Doppler is its ability to detect low-flow states.
VERTEBRAL ARTERY STENOSIS
Although the vertebral artery cannot be visualized in its entirety due to acoustic shadowing as the vessel courses through the transverse processes of the cer- vical vertebrae, the proximal vertebral artery is usu- ally evaluated during a routine carotid duplex scan. Normal vertebral artery flow has the same pattern as the ICA, with a low resistance pattern and antegrade (toward the brain) flow throughout the cardiac cycle. Waveform characteristics include a brisk systolic ac- celeration, sharp peak, and relatively high diastolic flow (Fig. 4-32). A proximal vertebral artery stenosis will produce abnormal dampened waveforms dis- tally with delayed acceleration, a rounded peak, and possibly poststenotic turbulence. Vertebral artery stenoses generally occur at the origin of the vessel