The transorbital window can be used when assessing the carotid siphon if no temporal window is available. Terminal segments of the vertebral arteries and basilar artery can be insonated via the suboccipital window. The tracing is assumed to be obtained at an optimal angle of 0 degrees. Hence scanning other arterial segments is not as important as scanning the MCA, except in specific clinical scenarios. However, because the MCA is the main cerebral artery that supplies blood to 60% to 80% of the hemisphere, its acute blood flow changes may reflect hemodynamic alterations in the entire hemisphere. The temporal window also allows for recording the internal carotid artery and the anterior and posterior cerebral arteries. Two to three spectra are recorded, and the most powerful signal at the highest mean blood velocity is used for measurement. After switching to the spectral Doppler mode, the 4- to 5-mm–wide Doppler sample gate is placed on the initial part of the MCA, and the recording is optimized by slightly tilting and sliding the transducer to obtain the cleanest and highest-velocity spectrum, which indicates that the axis of the vessel and the Doppler beam are closely aligned. Color-scale settings should be optimized according to the blood velocities. The middle cerebral artery (MCA) is coded red, with blood flow circulating toward the transducer.
3 The circle of Willis, which is depicted on color mode, projects anteriorly ( Figure 5-1). The transducer is placed just anterior to the ear’s tragus and above the zygoma, and the axial grayscale view of the base of the brain depicts the hypoechoic “butterfly-shaped” cerebral peduncles and the echogenic “star-shaped” suprasellar cistern (reference landmarks). TCDI transducers are sector or phased-array low-frequency (1.8-4.0 MHz) dedicated probes with small footprints that facilitate insonation of the intracranial arteries via the temporal window.
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