The sector-shaped image on the screen obtained from the phased array probe is oriented with the index mark on the upper left aspect of the image. The examiner ideally stands behind the patient’s head, although any position that allows the elbow to rest on the patient’s bed or be held close to the examiner’s side is acceptable; arm fatigue can be expected to develop rapidly. Insonation is initially performed through the transtemporal acoustic window, with the transducer placed immediately anterior to the upper border of the tragus and superior to the posterior zygomatic arch (Fig. 15.3). The index mark on the probe is directed anteriorly and rotated at a 10–20° angle superior to the axial plane (Fig. 15.4) with the transducer itself tilted at a 10–20° angle cephalad to the axial plane (Fig. 15.5). If the desired image is not obtained, the probe can be moved within the approximate area delineated by the circle in Fig. 15.3 until the desired initial grayscale image is obtained. If no recognizable anatomical features are seen, it is possible that the patient does not have an acoustic window on that side.

The single most important anatomical feature to identify at this point is the midbrain (Fig. 15.6), at a depth of about 5–8 cm, with the “Mickey Mouse ears” of the crus cerebri seen turned on their side. This is comparable, but not identical, to the image obtained with computed tomography (CT) imaging at this level (Fig. 15.7). Once this plane of imaging is obtained, other prominent anatomical features can be rapidly identified, including the lesser sphenoid wing of the temporal bone and the basal cisterns, particularly the interpeduncular and quadrigeminal cisterns (Figs. 15.6 and 15.7). The normal anatomical distribution of the major intracranial arteries and the circle of Willis is shown in Fig. 15.8. Once the midbrain is identified on the grayscale image, the color box is positioned over the expected anatomical location of the vessel(s) of interest, with “red” typically depicting flow toward and “blue” flow away from the probe. The most important intracranial artery to identify is the middle cerebral artery (MCA), seen as a curved vessel extending out toward the probe with flow directed toward the probe (depicted in red in Fig. 15.9) from a point anterior and lateral to the midbrain that is the point of bifurcation of the internal carotid artery, at about 4–6 cm depth. The A1 segment of the anterior cerebral artery (ACA) is seen extending from the internal carotid artery (ICA) bifurcation in the opposite direction, anteriorly, and toward the midline, with flow away from the probe (depicted in blue in Fig. 15.9). The ipsilateral (blue) and contralateral (red) A1 segments appear to meet in the midline (where the anterior communicating artery is typically not visualized) with the two A2 segments (which can often not be individually distinguished) extending anteriorly from this point, in the midline, with flow away from the probe (blue in Fig. 15.9). Minimal downward (caudal) angulation of the probe from this plane will reveal the distal internal carotid artery (ICA) following its entry into the skull, in continuity with proximal MCA, with flow directed toward the probe. Placing the color box around the midbrain itself, in the original plane of insonation, will reveal the posterior cerebral artery (PCA) curving around the crus cerebri and the midbrain from its point of origin (the bifurcation of the basilar artery) in the interpeduncular cistern. The ipsilateral PCA P1 segment demonstrates flow toward the probe (red in Fig. 15.9), which changes to flow away from the probe (blue) in the P2 segment following the completion of the bend in the PCA around the crus cerebri.

Increasing the depth of insonation to 13 cm or greater and extending the color box will sometimes reveal the contralateral intracranial vessels, depending on the quality of the acoustic window. This is most feasible when performing insonation through a large decompressive craniectomy defect. When visible, the contralateral ACA A1 segment and PCA P2 segments are seen with flow toward the probe (red), while the contralateral MCA and PCA P1 segment are seen with flow away from the probe (blue), as seen in Fig. 15.9. An attempt at interrogation of the contralateral vessels may be the only recourse when a patient has a viable transtemporal acoustic window on only one side.

To visualize the vertebrobasilar circulation, the patient’s head is flexed forward and the transducer firmly positioned in the suboccipital region with use of copious conductive gel to permit acoustic transmission through the hair at the nape of the neck. The probe is angled so that the long axis of the probe passes through the nasion, permitting insonation through the foramen magnum (Fig. 15.10). The most important structure seen on the grayscale image will be the foramen magnum, a large circular hypoechoic structure in the midline between about 2–7 cm of depth (Fig. 15.11). The color box is placed so that it covers the foramen magnum and the expected location of the basilar artery deeper to the foramen, within the cranial vault. The two vertebral arteries are visualized on either side of the foramen magnum, with flow away from the probe (blue in Fig. 15.11), extending intracranially to a point of confluence at about 6–8 cm of depth, beyond which the basilar artery is seen extending distally into the cranial vault, also with flow away from the probe (blue in Fig. 15.11). This confluence typically results in the “Y” shape seen in Figs. 15.11 and 15.12. The distal bifurcation of the basilar artery is typically not visualized with TCCS.

Once the vessels of interest are identified, pulse Doppler is selected with a sample volume size of about 3–10 mm. The sample volume is then placed on the arterial segment of interest (Fig. 15.13). Since the anatomical orientation of the artery can be clearly visualized, an effort must be made to obtain an insonation angle as close as possible to zero degrees for flow toward the probe and 180° for flow away from the probe (Figs. 15.13 and 15.14). When this is not possible, most POC machines with TCCS capability permit angle correction through manual delineation of the direction of flow within the vessel. The machine then corrects the calculated velocity using a formula that includes the angle of insonation. Using angle correction is generally not recommended when the angle of insonation is greater than 60° or when a sufficiently long arterial segment is not available to accurately depict the direction of flow. Once the sample volume is appropriately positioned (and angle correction performed if so desired), the spectral Doppler reading is obtained (Fig. 15.15). The Doppler frequency shift recorded within the sample volume is converted to flow velocity, depicted on the y-axis, against time, on the x-axis. One complete cardiac cycle on the spectral Doppler waveform is then traced out and the machine calculates the parameters of interest, including the peak systolic velocity (PSV), end-diastolic velocity (EDV), the mean cerebral blood flow velocity (mCBFV – otherwise known as the time averaged peak or TAP), and the pulsatility index {PI = (PSV-EDV)/mean flow velocity or MFV}.

When the blood flow velocity within an arterial segment is relatively high relative to the set color scale, aliasing may occur. Aliasing is visible as an abrupt change in color duplex examination within a segment of the artery (but not the entire artery) from the expected color appropriate to the direction of flow to the opposite color: red to blue or vice versa. The presence of aliasing can be clinically useful, since regions of aliasing are more likely to represent higher velocities and can be preferentially sampled when looking for vasospasm (Fig. 15.12). Most aliasing does NOT represent vasospasm; however, and if so desired, aliasing can be eliminated by increasing the range of the color scale.

Once the intracranial vessels have been interrogated, measuring the velocities in the distal cervical internal carotid arteries, immediately prior to their entry into the skull, should be performed when evaluating for vasospasm. These extracranial ICA (exICA) velocities are used to calculate the Lindegaard ratio (mCBFV_MCA/mCBFV_exICA). The transducer is placed in the submandibular region at the angle of the jaw and directed upward and slightly medially, toward the ipsilateral eye (Fig. 15.16). The exICA is a linear (sometimes tortuous) vessel with flow away from the probe (blue in Fig. 15.17) and is typically medial to the internal jugular vein, which is distinguished by the presence of flow toward the probe (red) and a venous pattern of pulsation. The most important distinguishing feature of the exICA, compared to the external carotid artery, is the presence of substantial diastolic flow in the exICA. As with intracranial vessels, obtaining an angle of insonation as close to 0° or 180° as possible is important; otherwise, perform angle correction.

The most important parameter obtained from TCCS is the mCBFV, since most angiographic correlation to vessel caliber has been performed with this parameter. The range of “normal” velocities used at the University of Michigan is shown in Table 15.1.