Page 128 - Libro 2

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PART 2 — CEREBROVASCULAR
Figure 7-20 Color and spectral Doppler signals from the ACA (A1 segment); notice the surrounding B-mode image is usually not bright because the ACA usually courses above the boney processes.
Anterior Cerebral Artery
The short segment of the ACA can be challenging to identify with color flow, especially when the ves- sel is not oriented axially to the MCA. If not seen in tandem with the MCA, the following transducer adjustments are helpful: (1) twist the front end of the probe upward, (2) slide the probe upward, and (3) tilt the probe upward. The ACA does not lie in proximity to bone, and the surrounding B-mode im- age will not display the bright boney reflections seen surrounding the TICA. The precommunicating seg- ment of the ACA is short and ends at midline. The proximal postcommunicating segment can often be seen coursing toward the left of the screen (anterior). The ACA is blue on color Doppler but can change di- rection when acting as collateral. Take spectral Dop- pler waveforms in 5-mm increments. If the color box does not fill, try using the spectral Doppler at the anticipated location for the ACA (Fig. 7-20).
The ACA has a high incidence of anatomical vari- ation and may be hypoplastic or atretic, and most often exhibits asymmetries in the caliber between the right and left sides. When functionally absent, the crossover collateral ability (from one hemisphere to the other via the ACAs and ACOA) can be severely limited or absent.
Posterior Cerebral Artery
Locate the hypoechoic, butterfly-shaped midbrain using B-mode and place the color box around the midbrain, which is encircled by the PCAs. The P1 will be flowing toward the transducer (red) as will the proximal P2. Twist the transducer to open up the curving vessel. As the PCA curves around the mid- brain, the flow orientation is away from the trans- ducer (blue); this represents the postcommunicating or P2 segment.
To differentiate between the P1 and P2 segments, set the color box to include the TICA simultaneous- ly with the PCA. Draw an imaginary line between these two vessels and it will represent the PCOA. This artery can frequently be seen with color flow if the scale setting is low enough, even when it is not functioning as collateral. Flow in the ipsilateral PCA deeper than the imaginary line is the P1 segment, and superficial to it is the P2 segment. Obtain spec- tral Doppler signals along the PCA and document the highest velocity in the P1 and P2 segments (Fig. 7-21).
In normal anatomy the PCA originates from the BA, but in 18% to 27% of patients, it originates from the TICA either exclusively or in combination with the BA.6 This is referred to as a fetal origin and can frequently be seen with TCDI. Fetal origin is highly suggested when there is no communication between the BA and the PCA, which is evidenced by no color or spectral Doppler signals that can be obtained from the short P1 segment and a large vessel, originating
AB
Figure 7-21 A: Hypoechoic cerebral peduncle and P1 segment of the PCA. B: P2 segment of the PCA.