Transtemporal examination

Using the transtemporal approach, the transducer is placed at the temporal bone cephalad to the zygomatic arch and anterior to the ear (Figure 9A.2). In this position, the hypoechoic butterfly-shaped cerebral peduncles, surrounded by the hyperechoic neighboring cisterns can be visualized in the axial plane on gray-scale B-mode image (Figure 9A.3). An imaging depth of 140–160 mm is recommended, to recognize the hyperechoic contralateral skull at the lower edge of the B-mode image. By slightly angling the transducer apically (about 10°) in the diencephalic plane,the third ventricle appears in the midline, with a thalamus on each side and the pineal gland posteriorly. In order to scan axial sections in the plane through the cella media, the transducer is tilted further cranially (by 30°) [16,17]. If the transducer is rotated from the axial plane by 90° the coronal plane can be displayed (B). Further details concerning B-mode brain parenchymal imaging are described in Chapter 24.

Figure 9A.2

Examination technique for intracranial vessels. Schematic image of the sectional planes for transtemporal (left), suboccipital (center) and transorbital insonation (right).

(With permission [12, p. 213].)

Figure 9A.3

View of the basal cerebral arteries with transtemporal insonation. (A) Magnetic resonance image of the anatomical position of the arteries of the circle of Willis in the same plane as in (B). (B) Schematic view in the axial plane with transtemporal insonation of the basal cerebral arteries. (A and B with permission [12, p. 218].) (C) Axial plane through the mesencephalon. Examination depth 130 mm. (+) = mesencephalic brainstem; (o) = midline; width of the contralateral lateral ventricle = 1.52 cm. (D) Color-coded duplex sonography of the middle cerebral artery = MCA (1), the A1 segment of the anterior cerebral artery = ACA (2), the A1 segment of the contralateral ACA (3), the A2 segment of the ipsi- and contralateral ACA (4), the posterior cerebral artery = PCA (5) and the contralateral PCA (6). Examination depth 140 mm. Large arrow: mesencephalic brainstem; small arrows: contralateral skull.

By color-coding the Doppler signal, the flow in the basal cerebral arteries can be imaged and the individual arteries identified on the basis of their anatomical position – with respect to the brainstem structures – and flow directions. The lesser wing of the sphenoid bone serves as a B-mode landmark for the terminal internal carotid artery and the siphon [18]. Vessels with flow toward the transducer are coded red while those with flow away from the transducer are coded blue [12]. Accordingly, assuming normal flow conditions, the transtemporal insonation displays the ipsilateral middle cerebral artery (MCA) in red and the two segments (A1 and A2) of the ipsilateral anterior cerebral artery (ACA) in blue.

Usually a long M1 segment of the MCA can be visualized because its proximal part runs in the axial plane. In elderly subjects. it can be recognized on B-mode image as a hyperechoic pulsatile structure located parallel to the lesser wing of the sphenoid bone. By using the color function of the device, the trifurcation and the larger branches can be visualized. It is often possible to simultaneously image the contralateral MCA (coded blue since the direction of flow is away from the transducer). Accordingly, the A1 segment of the contralateral ACA is coded red. The evaluation of the flow direction in the anterior cerebral artery is important for the assessment of the collateral circulation in extracranial and intracranial occlusive disease [18,19,20].

By slightly angling the transducer caudally, the terminal C1 segment of the internal carotid artery (coded red) ‒ usually displayed in the diagonal plane ‒ is visible. In the anterior coronal plane the course of the internal carotid artery can be easily followed in the carotid sulcus to its bifurcation into ACA and MCA (Figure 9A.4C,D). The carotid siphon can be visualized using the combined transtemporal axial and coronal approach [21].

Figure 9A.4

Coronal plane. (A) View of the magnetic resonance plane for the imaging of the coronal plane. (B) View of the corresponding magnetic resonance angiography. Arrows: C1 terminal segment of the ipsi- and contralateral internal carotid artery. (A and B with permission [12, p. 217].) (MRI, MRA images: By courtesy of Diagnoseklinik München.) (C) B-mode image in the coronal plane. The transducer is rotated by 90° in temporal position. 1,2 = ipsi- (1) and contralateral (2) frontal horns of the lateral ventricles; (+)= midline. Arrows: B-mode image of the carotid sulcus (carotid groove of the sphenoid bone). (D) Color-coded image of the terminal C1 segment of the internal carotid artery (ICA) in the anterior coronal plane. Arrows: ipsilateral ICA = coded red, contralateral ICA = coded blue. The spectral waveform of the middle cerebral artery (MCA) was recorded at the MCA/ACA bifurcation.

The posterior cerebral artery (PCA) winds posteriorly and laterally around the brainstem. It is not always possible to visualize it in the same plane as the MCA and the ACA. The transducer must be slightly tilted occipitally – this way it is sometimes possible to see the contralateral PCA as well, with inverse color coding.

Because of its curved course, usually the precommunicating P1 segment and the proximal (anterior) portion of the postcommunicating P2 segment of the PCA, with flow toward the transducer, are coded red, and the distal (posterior) P2 segment, with flow away from the transducer, is coded blue.

The posterior P2 segment and the basal vein of Rosenthal (BVR) can sometimes be imaged simultaneously in the diencephalic plane since they run relatively close to each other (see Chapter 22; Figure 9A.5). Here, there is a double Doppler signal: one for the PCA and one for the BVR [12,22].

Figure 9A.5

Examination in the diencephalic plane. Imaging of the P2 segment of the posterior cerebral artery (A) and of the basal vein of Rosenthal (B). Occasionally it is possible to simultaneously image the P2 segment of the posterior cerebral artery and the basal vein running side by side. The Doppler spectrum exhibits a mixed signal of the artery (a) and vein (b). Arrow: 3rd ventricle, (*) = ipsilateral thalamus.

A 90° rotation of the transducer at the level of the P1 segment of the PCA allows the visualization of the top of the basilar artery in the posterior coronal plane.

Transnuchal examination

The transnuchal (suboccipital) insonation is used for the examination of the proximal portion of the basilar artery and the intracranial (V4) segment of the vertebral arteries. The patient sits or lies in a lateral position on an examination bed, with the head slightly tilted forward. The transducer is positioned suboccipitally in the midline and pointed toward the nasion (Figure 9A.6). To make the orientation on the screen easier, it is helpful to first locate the echoreflective osseus border of the hypoechoic foramen magnum in the B-mode image [23,24]. After switching to the color mode, the two vertebral arteries can be easily recognized lateral to the great foramen. By changing the position of the transducer these arteries can be followed to their junction with the basilar artery at the depth of about 70–80mm. The two vertebral arteries and the basilar artery are coded blue due to their flow direction away from the transducer [25,26]. Because the vertebral arteries and the basilar artery have a tortuous course, it is difficult to visualize a long segment of the arteries. Under optimal examination conditions it is sometimes possible to visualize the “Y” configuration of the converging vessels. The posterior inferior cerebellar artery (PICA) branches off the V4 segment of the vertebral artery. It can be usually imaged well with red color-coding due to the flow toward the transducer.

Figure 9A.6

Imaging of the basal cerebral arteries by suboccipital insonation. (A) Schematic view of the sectional plane with suboccipital insonation of the terminal segments of the vertebral arteries and the basilar artery. (B) Transducer position for suboccipital (transforaminal) insonation. (C) Visualization of both vertebral arteries and the basilar artery by insonating through the great foramen, corresponding to the scheme shown in (A). (A, B and C with permission [12, pp. 213 and 221].) The basilar artery and the cranial segments of the vertebral arteries are coded blue, with flow away from the transducer. In their proximal V4 segment, both vertebral arteries are coded red – caused by their tortuous course in the great foramen region (large arrow). Small arrow: posterior inferior cerebellar artery (PICA) – coded red due to a flow in the opposite direction. The spectral waveform of the basilar artery (BA) was recorded at a depth of 83 mm. (1) = basilar artery. (D) The same view of both vertebral arteries and the basilar artery as in (C). The Doppler spectrum was recorded in the left vertebral artery. (2) = vertebral artery.

The imaging of the branches of the basilar artery is described in Chapter 9B.

Transorbital examination

Transorbital insonation allows visualization of the ophthalmic artery and the carotid siphon [27,28,29]. It is important to note that the acoustic energy must be reduced to a maximum of 10‒15% in order to prevent possible ocular injury. Transorbital insonation is described in detail in Chapter 25.