Page 118 - Libro 2

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PART 2 — CEREBROVASCULAR
There are five primary criteria used to identify each vessel segment:
1. Approach: Each cranial window provides access to specific arteries only.
2. Sample volume depth: Each artery has a specific range of depths over which it courses.
3. Direction of blood flow relative to the ultrasound transducer.
4. The spatial relationship of one artery to another. For the anterior circulation, the reference vessel used to identify other arteries is the bifurcation of the terminal ICA.
5. Flow velocity: In general, the MCA 􏰀 ACA 􏰀 PCA 􏰁 BA 􏰁 VA. These relationships assist ves- sel identification and, when reversed, can be helpful in identifying pathological flow states.
See Table 7-2.17,18
Orbital Approach
Arteries identified through the orbital window in- clude the OA and cavernous carotid (siphon). The acoustic intensity is lowered to the manufacturer’s recommendations and close observation of the ALARA principle is used. The patient is asked to close his or her eyes gently and keep them shut until the end of the orbital exam to avoid getting ultrasound gel into the eye. A small amount of ultra- sound gel is placed on the transducer and or closed eyelid, and the transducer is placed gently over the center of the closed eyelid and aimed 15° to 20°, toward the midline, and without applying any pres- sure to the eye.
At sample volume depths ranging from 40 to 60 mm, the OA can be identified isolated away from the carotid siphon. The unique waveform of the OA has low velocities and, due to the higher resistance bed of the eye compared with the brain, typically have low diastolic flow. The OA is studied to deter- mine if flow is antegrade or retrograde, the latter be- ing indicative of an external carotid artery (ECA) to ICA collateral flow.10,11
By increasing the sample volume to 60 to 70 mm, flow can be detected in the carotid siphon, so named because of its tortuous course at this location re- sulting in flow directionality that may be toward, away, or bidirectional depending on the orientation of the vessel segment to the transducer (although normally, the physiological direction of flow is antegrade) (Fig. 7-7).10,11
Temporal Approach
The arteries identified through the transtemporal ap- proach include the MCA (M1 and proximal M2 seg- ments), the ACA (A1 segment), the terminal internal
carotid artery (TICA), and the PCA (P1 and proximal P2 segments). The ACOA and PCOA are usually only identified when they are carrying an increased vol- ume flow because they function as collaterals. Ample gel is applied to the transducer and to the patient’s skin to create a good interface for the transmission of the ultrasound.11
Finding the exact location on the temporal bone that allows the best ultrasound penetration can be challenging and will be facilitated by being system- atic in the exploration of this area and using small hand movements. To facilitate finding the temporal window, power is set at maximum and the sample volume is placed at a depth of 50 mm with the inten- tion of finding the MCA.
The exam is begun by placing the transducer in the posterior window, aiming the beam slightly anterior and superior, and using a circular motion scanning for an audible Doppler signal and visual spectral display. If using M-mode, employ the same manual technique while also observing the M-mode display for a red color band at depths ranging from 30 to 65 mm.19 If no or weak signals are obtained, the transducer is systematically moved to the middle and anterior or frontal locations and scanning is repeated until signal acquisition is accomplished.
Once a suitable acoustic window is identified, emphasis changes to identifying each arterial seg- ment. Initial signals directed toward the transducer at a depth of 50 mm most often arise from the MCA. The sample volume depth is then reduced in a step- wise manner, tracing the MCA distally in 2 to 5 mm increments. Distally, the main trunk of the MCA di- vides into the M2 segment and the branches course superior over the insula, have lower velocities, and flow direction may change to away from the trans- ducer (Fig. 7-8).
The sample volume is then increased to trace the MCA proximally to its origin at 55 to 65 mm depth. The bifurcation of the terminal ICA into the MCA and ACA serves as a reference landmark for the re- mainder of the study. When using a large sample volume size (5 to 10 mm), the ACA and MCA are frequently seen simultaneously as a bidirectional signal as long as they are oriented on the same axis. The M-mode display will also show bands of color— red at shallower depths and, past the TICA bifurca- tion, a blue band will appear indicating flow away from the transducer, usually the ACA. A word of caution is in order because, at this depth, flow away from the transducer is not necessarily the ACA. If the beam is pointing slightly inferiorly, the tortuous TICA may also reveal flow away from the transducer, making it important to always know how the beam is being aimed relative to the landmark bifurcation signal (Fig. 7-9).