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General Arterial Anatomy

Extracranial Arterial Anatomy

Intracranial Arterial Anatomy

General Structure of Arterial Ultrasound Examination

Special Arterial Anatomy and Ultrasound Anatomy

Extracranial Arteries

Intracranial Arteries

General Venous Anatomy

Intracranial Venous Anatomy

Extracranial Venous Anatomy

General Structure of Venous Ultrasound Examination

Special Venous Anatomy and Ultrasound Anatomy

Intracranial Veins and Sinuses

Extracranial Veins

General Arterial Anatomy

The analysis of the arterial vascular system of the brain and its pathologies has been the main focus of extracranial and transcranial ultrasound examinations for many years. The increasing quality of the available ultrasound systems, especially for intracranial examination, has led to more detailed visualization even of peripheral vessel segments. This consequently requires constant adaptation of examination techniques, including thorough knowledge and particularly a spatial sense of extracranial and intracranial anatomy. In all applied diagnostic techniques, including ultrasound, anatomic descriptions should be as precise as possible: e.g., for spatial orientation the approved notation should be used (Fig. A2.1). All vessels can be described and classified by their origin, course, caliber, and length, and are subdivided into different segments within the angiographic nomenclature. Comprehensive knowledge of cerebrovascular anatomy should also include awareness of anatomic variations. The terminology used here is a synopsis of several reference textbooks whose data are derived from postmortem anatomic examinations and angiographic studies (Huber 1982, Lang 2001, Osborn 1999, Yasargil 1984). Information from special reports is cited separately in the text. In particular, we attempt to elaborate specific anatomic definitions which are useful for ultrasound purposes, giving extra consideration to variations in anatomic nomenclature.

Extracranial Arterial Anatomy

The common carotid arteries (CCAs) and vertebral arteries (VAs) are the four vessels securing the blood supply of the brain in a healthy individual. In ~70% of subjects the brachiocephalic trunk (BCT), left CCA, and left subclavian artery (SA) arise separately from the aortic arch. The right CCA originates from the BCT (Fig. A2.2). However, in up to 8% of cases the left CCA also originates from the BCT. Other variations have also to be considered (Fig. A2.3). The CCA then usually continues without branching up to the level of the thyroid cartilage, i.e., the cervical 4/5 level, where it separates into the internal carotid artery (ICA), which supplies the brain, and external carotid artery (ECA), which supplies the face and neck. However, the level of bifurcation may vary greatly and may be found more proximal in elderly subjects (Fig. A2.4).

Fig. A2.1 Anatomic nomenclature used for spatial orientation. Red: axial plane; blue: sagittal plane, green: coronal plane. (A) Anterior, (B) posterior.

At the carotid bifurcation the CCA widens and the dilatation continues into the proximal segment of the ICA. This segment is called the carotid bulb or carotid sinus. The diameters of the CCA, ECA, and ICA and the relationships between them vary greatly. Analyzing 1.969 nonaffected carotid bifurcations on DSA, the following ratios were reported: ICA/CCA 0.63 ± 0.11 (0.44–0.86), ICA/ECA 1.20 ± 0.29 (0.75–1.83), and ECA/CCA 0.55 ± 0.12 (0.34–0.79) (Schulz and Rothwell 2003). Ethnic differences have also been reported. In a study of 153 angiograms, African Americans had a proportionally smaller ICA and larger ECA compared with whites and Caribbean Hispanics. The carotid bulb revealed more or less the same diameter as the CCA (Koch et al 2009a). Others found a mean carotid bulb–CCA ratio of 1.19 ± 0.09 (Williams and Nicolaides 1987) (Fig. A2.5 and Fig. A2.6). Loops, coiling, and tortuosities of the ICA are also common findings, the latter particularly in elderly people. Hypoplasia or aplasia of the ICA is rare, occurring in less than 0.001% of the population (Given et al 2001). It can also occur bilaterally (Viglianesi et al 2010) and affect the CCA (Drazin et al 2010). The left and right ICAs develop symmetrically and usually lie in a dorsolateral position in relation to the ECA. However, variations of their courses are frequent so that the vessel location should not be the only criterion of identification (Fig. A2.7). The ICA rises to the base of the skull without branching but may show elongations during its course before entering the skull through the carotid canal. The ECA also originates symmetrically on both sides but quickly branches into the superior thyroid, lingual, facial, maxillary, superficial temporal, and occipital arteries (Fig. A2.8). The first branch, the superior thyroid artery, is frequently visible on ultrasound examination and can be used for differentiating between the ECA and ICA as it arises in more than 70% as the first branch of the ECA. In ~20% the vessel may start at the bifurcation and in up to 10% from the CCA (Gupta et al 2014; Fig. A2.9).

Fig. A2.2 Postprocessed contrast-enhanced 3D MRA, arterial phase, coronal MIP. Aortic arch and normal variant of brachiocephalic trunk (BCT). MCA = middle cerebral artery; ICA = internal carotid artery; BA = basilar artery; VA = vertebral artery; ECA = external carotid artery; CCA = common carotid artery; SA = subclavian artery.

Fig. A2.3 Schematic drawing of normal anatomy and main variants of the aortic arch. (A) 70% of cases—normal type with direct origin of BCT, left CCA, and left SA from the aortic arch. (B) 15% of cases—left CCA originates from the right side of the aortic arch. (C) 8% of cases—left CCA originates from the BCT. (D) 5% of cases—left VA originates directly from the aortic arch. 1 = brachiocephalic trunk; 2 = right subclavian artery; 3 = right vertebral artery; 4 = right common carotid artery; 5 = left common carotid artery; 6 = left vertebral artery; 7 = left subclavian artery (adapted from Huber 1982, data from Toole 1974.)

Fig. A2.4 CTA, 3D reconstruction (mandible removed) of the arteries of the neck. (A) Left lateral view. Note the dilatation and small atherosclerotic changes of the ICA at the carotid bifurcation (arrows). (B) Anterior view. Note the tortuous course of the superior thyroid artery (arrow) and of the proximal CCA (arrows). (C) Right lateral view. Note the clavicle obscuring the view of the proximal CCA segment. Note also the proximal (arrow) and the distal (arrows) V3-VA loop.

Fig. A2.5 Normal anatomy of carotid bifurcation. (A) schematic drawing. (B) DSA, selective CCA injection. lateral view, 90° counterclockwise rotated to correspond with the ultrasound image. (C) Duplex sonography, B-mode, longitudinal plane of the same subject demonstrating a wide bulb (6.8 mm) compared with the ICA (4.0 mm) and CCA (5.8 mm).

Fig. A2.6 (A–C) Variations of the carotid bulb. Contrast-enhanced MRA, lateral view. Note the variability of the carotid bulb and its relationship to the midpart and distal ICA (ICA in A–C on the right side).

The VA originates from the SA as its first branch, which itself derives from the BCT on the right side and from the aortic arch on the left side. In ~5% of cases the left VA originates directly from the aortic arch. In contrast with the ICA, the VAs show considerable differences between the right and left sides, with the frequent finding of a dominant left-side artery. Using ultra-sound and a lumen threshold diameter of 2.3 mm, VA hypoplasia has been reported in 7.8% on the right side and 3.8% on the left (Jeng and Yip 2004). In their extracranial course the VAs are divided into different segments. V0 is the point of origin, V1 is the extraforaminal segment of the VA before it enters the transverse foramen of the transverse process which occurs in ~93% at the sixth vertebra (Uchino et al 2013), V2 is the segment during its intraforaminal course up to the exit from the transverse foramen of the second cervical vertebra (axis, C2). Here V3 begins with its tortuous course and several loops allowing head mobility, also referred to as the atlas loop, before it enters the dura mater becoming the V4 segment. First V3 runs posteriorly and vertically between C2 and C1 for ~10 mm, forming a proximal loop. Then it enters the transverse foramen of C1 (atlas) and turns medially in a horizontal course within the VA sulcus of the atlas forming a distal loop, then it runs in an oblique course ventrally entering the foramen magnum. Here the intracranial V4 part starts after penetrating the atlanto-occipital membrane, the dura, and arachnoid mater (Fig. A2.10 and Fig. A2.11). It is important to know, that unlike the extracranial ICA, the VA has numerous segmental connections to muscular branches of the ECA, of which the occipital artery at the atlas loop is the most important one. These may be activated as collaterals in case of proximal occlusive VA processes.

Intracranial Arterial Anatomy

The ICA enters the skull through the carotid foramen at the apex of the petrosal pyramid, where it follows an intrapetrosal course within the petrous carotid canal running in an upward, forward, and medial direction (petrosal segment C6). It leaves the base of the skull via the foramen lacerum vertically along the side of the sphenoid bone (ganglionic segment C5). A persistent trigeminal artery, defined as a fetal-type connection between the C5–ICA segment and the upper third of the basilar artery (BA), may be found in up to 0.6% of all cases (Uchino et al 2000, 2003). From C5 the ICA turns upward and forms the carotid siphon (segments C2–C4) within the cavernous sinus (C3/C4). Here small hypophyseal and tentorial branches spring off. It enters the subarachnoid space (segment C2) and rises to its terminal part (segment C1) where it bifurcates into the middle cerebral artery (MCA) and the anterior cerebral artery (ACA). This termination is often referred to as the “carotid-T” because of its shape (or the “terminal” or “top” of the carotid). Before this, it gives off several branches: anterior at the C2/C3 level the ophthalmic artery (OA) and posterior at the C1/C2 segment the posterior communicating artery (PCoA) followed by the anterior choroidal artery (AChA) (Fig. A2.12). In up to 10% of cases the AChA rises from the MCA or from the PCoA. The A1–ACA segment turns medially and slightly upward. It becomes the A2 segment at the point where the anterior communicating artery (ACoA), connecting both ACAs, originates. It then turns sharply upward to supply the ACA territory of the brain. The initial and horizontal M1 segment of the MCA may be symmetric but variations are not uncommon. It commonly bifurcates after 1–2 cm into two or more insular M2 branches, which supply the MCA territory (Fig. A2.13).

Fig. A2.7 Schematic demonstrating the spatial relationship of the ECA and ICA (Hennerici and Neuerburg-Heusler 2006).

Fig. A2.8 Schematic of the ECA and its branches (Adapted from Schünke et al 2006; drawing: Karl Wesker.)

Fig. A2.9 Thyroid artery variants (arrows), ultrasound fusion imaging technique (Esaote MyLab Twice). Left: CTA. Right: Corresponding color-mode ultrasound image. (A) Most frequent variant with origin at the ECA (70%). (B) Rare variant with CCA offspring (10%).

Fig. A2.10 CTA, 3D reconstruction (mandible removed) of the arteries of the neck to illustrate the posterior circulation. (A) Anterior view with numbered cervical vertebras (1 atlas, 2 axis). Note the right dominant VA. The V2-VA segment starts bilaterally at C6. (B) Right lateral view. Note the intraforaminal V2-VA segments as well as the proximal loop between axis and atlas (arrow) and distal loop between atlas and occiput (arrows) at the V3-VA level. (C) Posterior view. Note the distal loop of the V3-VA segment at the atlanto-occipital joint before entering the foramen magnum (arrows).

Fig. A2.11 Postprocessed contrast-enhanced 3D MRA, arterial phase, coronal MIP of the vertebrobasilar circulation. Red: Definition of VA segments from V0 to V4. Blue: Definition of cervical transverse processes from C1 to C6.

Fig. A2.12 Left: Schematic drawing of intracranial branches of the ICA (adapted from Lasjaunias et al 2001). The dotted line indicates the border between extra- and intradural compartment. Intradural OA origin occurs in 92.5%, extradural origin in 7.5% of cases. Right: DSA, ICA injection, lateral view showing the corresponding anatomic situs. ACA = anterior cerebral artery; AChA = anterior choroidal artery; ICA = internal carotid artery, MCA = middle cerebral artery, OA = ophthalmic artery, PCoA = posterior communicating artery.

Fig. A2.13 3D TOF-MRA, 3D volume-rendered reconstruction of the cerebral arterial circle (circle of Willis). (A) Anterior view. (B) Left lateral view. ACA = anterior cerebral artery; BA = basilar artery; ICA = internal carotid artery, MCA = middle cerebral artery PCA = posterior cerebral artery; VA = vertebral artery.

Posterior Circulation

The VAs demonstrate considerable variation in length, course, and caliber. However, the intracranial caliber in general resembles the extracranial caliber. Both VAs enter the skull through the foramen magnum by piercing the atlanto-occipital membrane, the dura, and the arachnoid mater (V4 segment). Before merging to form the BA each VA gives off a posterior inferior cerebellar artery (PICA) which divides the VA into a proximal and a distal V4 segment. In people with VA hypoplasia, the VA can terminate partially or completely as the PICA, which then might not connect or contribute to the BA blood flow at all. The VA segment distal of the PICA up to the vertebrobasilar confluence can be separated from the pre-PICA segment and defined as the post-PICA V4 segment; sometimes it is also called the V5 segment. The BA is in almost all cases a well-developed vessel which may become rather elongated with increasing age (Fig. A2.14). In bilateral FT-posterior cerebral artery (PCA), however, the BA appears hypoplastic. Apart from numerous small arteries providing the brain stem with blood it also gives off the paired anterior inferior cerebellar arteries (AICAs) and, near the basilar head, a pair of superior cerebellar arteries (SCAs). Finally it bifurcates into the two P1 segments of the PCA. The P1-PCA segment is short. It becomes the P2 segment at the point of origin of the PCoAs.

Fig. A2.14 Variations of the vertebrobasilar arteries. (A) CTA, 3D reconstruction: “Ideal” anatomic variant with two similar-sized VAs merging in a V shape into the BA. (B) MRI, T2-weighted image, coronal plane: Note the moderate elongation of the BA. (C) CTA, coronal MIP reconstruction: Note a left dominant VA and a hypoplastic right VA merging into a distinctly elongated BA.

Fig. A2.15 Cerebral arterial circle (circle of Willis) and peripheral arteries: Anatomic preparation. Proximal vessel segments and circle of Willis shown in red. Note the fine distribution of the leptomeningeal peripheral vessels which show diameters comparable to the proximal vessel segments. (With kind permission from Prof. Bogusch, Center for Anatomy, Charité—Universitätsmedizin Berlin, Germany.)

Fig. A2.16 Cerebral arterial circle (circle of Willis): Nomenclature and anatomic data of the basal cerebral arteries. All values (except for the ACoA) are given as: minimum—mean—maximum. L = length (mm), W = width (mm). (Adapted from Duus et al 2005; data: Lang 2001, Schulte-Altedorneburg et al 2000, Yasargil 1984.) Additional data not shown: ophthalmic artery diameter 0.8–1.2 mm; anterior choroidal artery diameter 0.5–1.5 mm.

The anterior and posterior circulation connect via the paired PCoAs and the unpaired ACoA to form a circle known as the cerebral arterial circle or circle of Willis (CW), from which all further main arteries (ACA, MCA, and PCA) emerge (Fig. A2.15). Owing to the lack of a valvular system, blood flow through this circle can follow the direction of need. However, “perfect” anatomy with a completely developed CW is found in less than 30% of the population (Fig. A2.16).

Variations of the CW are the rule and a good knowledge of these is required for exact interpretation of findings during ultrasound examination. Functional assessment of communicating artery patency, which is probably more significant than the anatomic data alone, can be performed using transcranial ultrasound during extracranial carotid compression. Hoksbergen and coworkers (2000b) analyzed ACoA and PCoA function during an ipsilateral CCA compression maneuver. A functionally patent ACoA was assumed if CCA compression led to a reversed flow in the ipsilateral A1-ACA segment. A functionally patent PCoA was assumed if the flow velocity in the ipsilateral P1-PCA segment increased more than 20% over baseline levels (Hoksbergen et al 2000b). Using this approach they found the absence of increased flow in the P1-PCA segment, i.e., impaired PCoA function, to be the most common variant in 61% of cases (45% unilateral, 16% bilateral). A unilateral fetal-type PCA, in which the PCA derives its blood directly from the ICA, resulting in a flow reduction or cessation in the vessel considered to be the PCoA during CCA compression, was seen in 13% of subjects. Impaired function of the ACoA was found in 4% of cases, while the A1-ACA segment was absent in 1% of cases (Fig. A2.17). A combination of the above variants can occur but represents a less frequent finding. A limitation of this study is that the compression tests could only be performed for a few seconds, so no conclusions can be drawn regarding the potential adaptation of collaterals over time (for further details see Chapter 5, “Collateral Pathways”). A bilateral fetal-type PCA may also be observed.

Fig. A2.17 Cerebral arterial circle (circle of Willis): Anatomic variations (arrows) as assessed by functional TCCS (according to Hoksbergen et al 2000b). (A) Normal type with complete circle of Willis (29%). (B) Impaired ACoA function (4%). (C) Unilateral impaired A1-ACA function (1%). (D) Unilateral impaired PCoA function (45%). (E) Bilateral impaired PCoA function (16%). (F) Unilateral impaired P1-PCA function corresponding to fetal-type variant of PCA (13%). (G) Bilateral impaired P1–PCA function (none). All data refer to subjects, not hemispheres. Combined variants are rare. A1 = A1 segment of the anterior cerebral artery; ACoA = anterior communicating artery; AICA = anterior inferior cerebellar artery; BA = basilar artery; ICA = internal carotid artery; M1 = M1 segment of the middle cerebral artery; OA = ophthalmic artery; P1 and P2 = P1 and P2 segment of the posterior cerebral artery; PCoA = posterior communicating artery; PICA = posterior inferior cerebellar artery; SCA = superior cerebellar artery; VA = vertebral artery.

General Structure of Arterial Ultrasound Examination

For transcranial insonation slight elevation of the head may be more comfortable for the patient, and 1–3-MHz transducers are used to enable the ultrasound beam to penetrate through the bone. However, compared with extracranial ultrasound, these low insonation frequencies reduce the spatial resolution of the ultrasound image, impeding the evaluation of vessel wall structures. Transcranial color-mode insonation is used to assess the vessel course, but is not sensitive enough for reliable evaluation of vessel lumen. Doppler analysis allows determination of flow direction and flow velocity. Angle correction of flow velocities is problematic because of the mostly tortuous course of intracranial arteries, especially in elderly people. In addition, often only a small segment of each vessel can be insonated. There is ongoing debate about whether or not to use angle correction. To improve reproducibility we recommend to measure the maximal achievable flow velocity without angle correction. In cases with a stenosis the corresponding insonation depth should be documented. If needed, angle-corrected velocities can be noted as additional data, provided the sample volume can be positioned in a satisfactory long vessel segment aligned with the direction of the vessel in the color-mode image (Nedelmann et al 2009b). For a detailed discussion see Chapter 1, “Doppler Shift and Flow Velocity” under “Ultrasound Principles”).

The aim of the color-coded ultrasound investigation is to visualize the anterior and posterior circulations from the vessel origin at the aortic arch up to the main intracranial arteries. Even under optimal insonation conditions, however, this goal cannot be achieved for the origin of the CCA, which lies deep behind the clavicle, and for the vertical segments of the C6-ICA, which run within the petrosal bone of the skull base. Except for these locations, all other major brain-supplying arteries are accessible to ultrasound.

Special Arterial Anatomy and Ultrasound Anatomy

The following section is ordered according to the blood flow from the heart into the brain, i.e., from the proximal cervical to the distal intracranial vessels. Instructions for insonation focus on color-coded duplex ultrasound only. Reference data of reported normal values for flow velocities are summarized in Table A2.2 and Table A2.3.

Extracranial Arteries

Position and vessel identification: The CCA can frequently be visualized over a length of more than 5 cm proximal to the carotid bifurcation. The best insonation results are obtained if the head of the patient is turned away from the insonated side by 10–20°. Insonation should be performed in a cross-sectional as well as a longitudinal insonation plane over the whole visible length of the artery (Fig. A2.18). Differentiation from the internal jugular vein (IJV) is easy as the latter can be completely obstructed by applying slight pressure on to the neck (e.g., with the transducer). The origin of the CCA usually cannot be insonated as it lies behind and below the clavicle. Insonation is started in the B-mode. Longitudinal and axial planes are evaluated to analyze vessel wall configuration, thickness of intima media, or plaque formation (Fig. A2.18). Color-mode insonation and Doppler spectrum analysis are then performed (Fig. A2.19) (see also Videos A2.1 and A2.2).

Normal values: Blood flow is usually laminar and not turbulent. For flow velocities, see Table A2.2.

Fig. A2.18 Top left: Transducer position for axial CCA, ICA, and ECA insonation. Top right: Transducer position for longitudinal CCA, ICA, and ECA insonation. Bottom: Extracranial duplex, B-mode images of the CCA. Left: Axial plane. Right: Longitudinal plane. Note the close anatomic relationship of the CCA and the internal jugular vein (IJV). Note the spatial orientation used in all extracranial images: Left—distal vessel segment, Right—proximal vessel segment.

Fig. A2.19 Extracranial duplex, color-mode images of the CCA. Top left: Axial plane. Top right: Longitudinal plane. Bottom: Doppler spectrum analysis and color-mode image of the CCA (flow velocity 77/31 cm/s).

Internal Carotid Artery (ICA)

Anatomic details: At the carotid bifurcation the CCA divides into the ICA, which supplies the brain, and the ECA, which supplies the face and neck. The appearance of the carotid bulb and the spatial relation between the two vessels varies. The two main anatomic variants are illustrated in Fig. A2.20. In most cases, the ICA runs laterally. With increasing age the course of the ICA becomes more elongated. The diameter of the ICA ranges between 4 mm and 5 mm (Yazici et al 2005). Agenesis of the ICA is rare and several types of collateral pathways have been described (see also Case 42). In the most common type the cervical segment of the ICA is absent and the supraclinoid part and the MCA are fed by the PCoA (Uchino et al 2015).

Position and vessel identification: For ICA identification, the transducer is held in a position similar to the CCA insonation (Fig. A2.18). We recommend starting with a longitudinal or transverse approach by visualizing the CCA, following its course cranially, and examining the carotid bulb and the ICA–ECA bifurcation, rather than trying to identify an isolated distal vessel segment. If the longitudinal plane is used it is possible to visualize both ICA and ECA at the same time only in about one-third of cases. Otherwise the vessels are insonated individually, beginning at the transition from the CCA to the ICA. At that point, the ECA can usually be found if the distal part of the transducer is moved medially while the proximal part of the transducer rests above the CCA (Fig. A2.21). Once the proximal ICA is identified, the vessel is followed in the longitudinal plane as far distal as possible so as not to overlook vessel coiling or kinking. In presumed ICA dissection axial examination may allow further distal insonation of the ICA. Routine documentation in the longitudinal plane with Doppler spectra should include the ICA at its origin and the most distal part as well as any relevant pathology, if present. Table A2.1 lists the criteria for differentiating between the ICA and ECA.

The sequence of examination (B-mode followed by color-mode and Doppler spectrum analysis) is the same as for the CCA (Fig. A2.22). At the origin of the ICA, where the vessel widens to form the carotid bulb, the following peculiarity is frequently observed in the color mode and the Doppler spectrum analysis: As the blood flow in this region is less laminar and often disturbed, there may be an apparent flow reversal near the vessel wall with reversed color-code and blood flow direction. However, this is normal and present in almost all subjects (Middleton et al 1988). It should therefore not be interpreted as a pathologic finding as it is caused by the physiologic, highly variable, widening of the carotid bulb (Fig. A2.23) (see also Videos A2.1 and A2.2).

Table A2.1 Ultrasound identification criteria for the extracranial ICA and ECA ICA ECA

Fig. A2.20 CT angiography, 3D reconstruction, frontal view. Examples of the two most common anatomic ICA–ECA variants. Left: ~70% of cases—ICA lateral and/or posterior in relation to the ECA. Right: Remaining cases—ICA medial in relation to the ECA.

Fig. A2.21 Top left: Transducer position for longitudinal CCA and ICA insonation. Top right: Transducer position for longitudinal CCA and ECA insonation. Bottom left: Extracranial duplex, longitudinal plane, B-mode image of the CCA and ICA. Bottom right: Extracranial duplex, longitudinal plane, B-mode image of the CCA and ECA. Schematics indicate probe movements for insonation of the ICA and ECA together with the CCA.

Fig. A2.22 Extracranial duplex. Left: B-mode (top) and color mode (bottom) images of the ICA and ECA just above the carotid bifurcation. Right: axial plane, B-mode (top) and color-mode (middle) imaging of the ICA and ECA; (bottom) Doppler spectrum analysis of the ICA (flow velocity 75/31 cm/s).

Fig. A2.23 Extracranial duplex, longitudinal plane. Top left: B-mode image of a wide carotid bulb. Top right: Color-mode image of the carotid bulb. Note the apparent flow reversal (blue-coded) caused by nonlinear flow within the bulb. Bottom: Doppler spectrum analysis and color-mode image of the carotid bulb. Note the retrograde flow component in the Doppler spectrum.

Normal values: Within the carotid bulb the blood flow may be disturbed and may show low flow velocities. The flow pattern becomes laminar in the more distal ICA. The Doppler spectrum is less pulsatile than the spectrum in the ECA. For flow velocities, see Table A2.2.

External Carotid Artery (ECA)

Anatomic details: Unlike the ICA, which ascends without branching to the base of the skull, the ECA main stem quickly divides into branches that supply the face and neck. Its caliber is usually smaller than that of the ICA. The first ECA branch is the superior thyroid artery which can frequently be seen turning caudally toward the thyroid gland (see Fig. A2.4, Fig. A2.9, Fig. A2.24). Branches that can also be visualized by duplex ultra-sound and that are routinely assessed within a clinical context are the occipital artery and the superficial temporal artery. Other branches that may be more difficult to detect by ultrasound are the ascending pharyngeal artery, the lingual artery, the facial artery, the posterior auricular artery, and the maxillary artery.

External Carotid Artery: Main Stem

Position and vessel identification: See ICA (Videos A2.1 and A2.2). Main stem duplex imaging is demonstrated in Fig. A2.24. A useful differentiation criterion is the analysis of the Doppler spectrum of ECA and ICA during digital tapping of the preauricular superficial temporal artery, which reveals a stronger response in the ECA. The frequently detectable first branch—the superior thyroid artery—can also be used to identify the ECA. Its Doppler spectrum reacts positively to gentle digital tapping over the thyroid gland (Fig. A2.24). Also, the second branch of the ECA, the lingual artery, may be detected with a highly pulsatile flow which decreases upon pressing the tongue against the hard palate. Release of pressure leads to transient reactive hyperemia and subsequently to a pronounced increase in flow velocity (Video A2.3).

Normal values: Doppler spectrum analysis frequently shows a highly pulsatile flow compared with the ICA (Fig. A2.24). For flow velocities, see Table A2.2.

Superficial Temporal Artery (STeA)

Anatomic details: The main stem of the vessel ascends in the preauricular region and quickly divides mainly into a frontal and parietal branch, with variable vessel courses further ascending toward the temple and forehead (Fig. A2.25).

Fig. A2.24 Extracranial duplex, longitudinal plane. (A) ICA Doppler spectrum during preauricular superficial temporal artery tapping demonstrates a weak oscillation phenomenon. (B) Distinctly positive oscillation phenomenon in the ECA Doppler spectrum (flow velocity 68/12 cm/s). (C) Doppler spectrum of the superior thyroid artery. Note the positive oscillation phenomenon during slight digital tapping over the thyroid gland.

Position and vessel identification: Insonation is performed in the color mode with an insonation depth as low as possible (ideally 1.5 cm). The main stem is best identified in the preauricular region with a typical pulsatile external flow signal. According to the vessel’s course the probe position needs to be adapted to display the maximum amount of the artery. In normal findings the color signal completely fills the vessel lumen. A hypoechoic vessel wall thickening is indicative of vessel wall vasculitis (Fig. A2.25). In patients with suspected vasculitis the branches of the STeA should be insonated as far as possible using the color mode to improve the detection rate of a regional hypoechoic vessel wall (so-called “dark halo” phenomenon). For better spatial resolution of the STeA vessel wall a linear transducer with transmission frequencies of 12–15 MHz is recommended (Video A2.4).

Normal values: For flow velocities, see Table A2.2.

Occipital Artery (OccA)

Anatomic details: The vessel has a variable course, ascending behind the ear (Fig. A2.26).

Position and vessel identification: For identification of the OccA, the transducer can be positioned in an axial plane cranially to the vertebral V3-VA segment and over the mastoid bone (see also “V3 segment” under “Extracranial Vertebral Artery” below). Insonation starts in the color mode with an insonation depth of ~2 cm. Doppler spectrum analysis of the OccA reveals a highly pulsatile arterial signal and may have variable flow directions. The transducer has then to be turned according to the vessel’s course to display as much as possible of the artery and to analyze the flow pattern (Fig. A2.26; Video A2.4). Detection rates of 100% have been reported (Tee et al 2013).

Fig. A2.25(A) Transducer position for STeA insonation. (B) CTA, 3D reconstruction, lateral view. STeA: Main-stem course, ascending preauricular branches (arrows), and distal segments proceeding over the temporal and frontal bone. (C,D) Doppler spectrum analysis and color-mode imaging of the STeA (flow velocity 40/13 cm/s).

Fig. A2.26 (A) Transducer position for OccA insonation. (B) CTA, 3D reconstruction, right posterior oblique view: OccA main-stem course and branches ascending over the mastoid bone (arrows). (C,D) Doppler spectrum analysis and color-mode imaging of the OccA (flow velocity 10/2 cm/s).

Normal values: For flow velocities, see Table A2.2.

Subclavian Artery (SA)

Anatomic details: The anatomic variants of the aortic arch and its branches, including the SA, are described in “General Arterial Anatomy” above. Important SA branches to assess in a clinical context are the axillary and brachial arteries in addition to the VA.

Subclavian Artery: Main Stem

Position and vessel identification: Insonation of the SA is usually impaired as the vessel lies behind the clavicle and conventional longitudinal or cross-sectional images are frequently difficult to obtain. We routinely approach the SA using a linear transducer while following the VA longitudinally or in mixed planes via the V1 segment to its origin from the SA (Fig. A2.27 and Fig. A2.28). Holding the probe laterally, the distal SA becomes visible usually with a flow away from the probe but it may show in its most proximal part a flow toward the probe. Pointing the probe in a medial direction reveals the proximal SA with a flow toward the probe. In impaired insonation conditions (e.g., a short neck or a greater than usual neck circumference) the use of a sector array might be helpful (e.g., see Fig. A2.39). Vessel identification is achieved by color-mode visualization and Doppler spectrum analysis. In many cases only a short vessel segment of the SA proximal and distal to the VA origin can be insonated.

Normal values: The Doppler spectrum profile normally shows a triphasic pulsatile flow signal with high velocities. Compression of the BrA leads to a marked reduction of systolic flow velocity (Fig. A2.27). No systematic values have been reported.

Fig. A2.27 (A) Transducer position for SA and proximal longitudinal V0/V1-VA insonation. (B) Extracranial duplex: B-mode imaging of the V0/V1-VA and SA. (D) Color-mode image of the same segment. (C) Doppler spectrum analysis of the SA with its typical triphasic pattern (153/−26 cm/s).

Fig. A2.28 (A) Transducer position for SA and V0/V1-VA insonation. (B) DSA, left SA injection, anteroposterior projection. Note the tortuous VA origin which may lead to misinterpretation of the VA origin as SA.

Fig. A2.29 (A) Transducer position for proximal longitudinal AxA insonation. (B) Color mode, axial insonation. Note the flat corresponding vein (blue) (C,D) Longitudinal insonation. (E,F) Doppler spectrum. Note the typical triphasic flow spectrum at rest (left) and the marked blood flow velocity increase during repeated hand grips (right).

Position and vessel identification: The AxA can easily be insonated in the axillary fossa in a longitudinal or an axial insonation plane (insonation depth 0.5–1.5 cm). As with the brachial artery the axial approach permits the fastest vessel identification and is also sufficient to evaluate the flow profile. For angle-corrected flow assessments the transducer has to be turned into the longitudinal insonation plane (Fig. A2.29; Video A2.5).

Normal values: The Doppler spectrum profile shows a triphasic pulsatile flow signal. For flow velocities, see Table A2.2.

Brachial Artery (BrA)

Position and vessel identification: The BrA can easily be insonated in the antecubital fossa in a longitudinal or an axial insonation plane (insonation depth 2–3 cm). The latter approach permits the fastest vessel identification and is also sufficient to evaluate the flow profile. For angle-corrected flow assessments the transducer has to be turned into the longitudinal insonation plane (Fig. A2.30; Video A2.5).

Normal values: The Doppler spectrum profile shows a triphasic pulsatile flow signal. For flow velocities, see Table A2.2.

Extracranial Vertebral Artery

Anatomic details: Unlike the carotid arteries, both VAs demonstrate considerable variations in length, caliber, and vessel course. Differences between the left and right side are common. There is no clear definition of VA hypoplasia: In general it is assumed if the diameter of a VA is less than 2–2.5 mm or the diameter equals less than 50% of the contralateral VA. Alternatively, comparative volume flow measurements can be used to determine the differences between the two sides, and a cut-off of less than 40 mL/min may be used to define hypoplasia (Seidel et al 1999). Hypoplasia may be present in up to 10% of cases and is more frequently seen on the right side. Extracranially, the V0–V3 segments can be evaluated. VA examination should begin with the V2 segment as it is most easily identified.

Fig. A2.30 (A) Transducer position for axial BrA insonation. (B) Corresponding color-mode image. (C) Transducer position for longitudinal BrA insonation. (D) Corresponding color-mode image. (E) Doppler spectrum analysis of the BrA at rest. Note the triphasic Doppler spectrum similar to that of the SA. (F) Doppler spectrum of the BrA shortly after repeated hand grips. Note the hyperemic response with marked flow velocity increase and decreased pulsatility.

Fig. A2.31 Left: Transducer position for longitudinal V2-VA insonation. Right: CTA, 3D reconstruction, right anterior oblique view. Distal V1 and V2 segments of the VA. The sixth transverse process separates the V1 from the V2 segment (arrows).

Anatomic details: The V2-VA segment starts at the point where the VA enters the transverse foramen. In 90% of cases this occurs at the level of the sixth vertebra (Fig. A2.31). The V2-VA segment ends when the vessel leaves its intraforaminal course at the second vertebra.

Position and vessel identification: The V2-VA segment can be insonated in nearly 100% of cases. This is best done with the patient’s head in a straight position, chin slightly elevated. The transducer is held lateral to the larynx, aiming in a posterior and slightly medial direction toward the transverse foramen (Fig. A2.31). If starting from the CCA insonation, the probe is moved into an upright position until the transverse processes become visible (Video A2.6). Standard B-mode, color-mode, and Doppler spectrum analyses are usually done in the longitudinal insonation plane because of the small vessel size and the deeper location compared with the ICA. Routine examination should start with conventional B-mode to identify the transverse processes of the cervical vertebrae. A characteristic image pattern is found with a small visible vessel segment framed by the acoustical shadow of two transverse processes (Fig. A2.32 and Fig. A2.33). Then color-mode and Doppler spectrum analysis are performed (Video A2.6). Diameter, flow direction, and flow velocity within the V2 segment are routinely documented. We recommend measuring the diameter using B-mode but care must be taken not to include the outlining of the vertebral vein into the calculation. In case of doubt, measuring can be done in color mode avoiding color aliasing. In deeply located V2-VA segments it is helpful to reduce the PRF and switch off the beam steering of the color window. Furthermore, tilting the probe by slightly pressing the caudal end of the transducer into the skin of the neck optimizes color filling and facilitates angle correction in Doppler spectra assessment. In cases of impaired VA visualization we recommend first trying to examine the contralateral side where vessel identification might be easier. This will in particular be the case in VA hypoplasia, where the contralateral side is frequently hyperplastic. Once one VA is identified, contralateral insonation is easier as both VAs are located at a similar depth and the same transducer position can be applied on both sides. In case of a pathologic V2-VA finding, or if the clinical question suggests involvement of the posterior circulation, the routine measurement of the V2-VA in one intervertebral segment is extended to all VA intervertebral segments and the V0/V1 as well as the V3 segments. In individuals with a slender neck the V2-VA segment can also be seen in a transverse insonation plane (Fig. A2.34; Video A2.7).

Fig. A2.32 Extracranial duplex, V2-VA insonation in the longitudinal insonation plane. (A) B-mode image, dominant VA, diameter 4 mm. (B) Color-mode image. Note the VA color-mode signal interruption by the transverse processes. (C) Doppler spectrum of the dominant VA (flow velocity 58/17 cm/s, PI = 1.5).

Fig. A2.33 Extracranial duplex, V2-VA insonation in the longitudinal insonation plane. (A) B-mode image, hypoplastic VA, diameter 1.9 mm. (B) Color-mode image. (C) Doppler spectrum of the hypoplastic VA (flow velocity 23/3 cm/s, PI = 2.6). Note the higher PI compared with a normal-sized VA (Fig. A2.32) typically seen in VA hypoplasia. The detectable diastolic flow indicates brain perfusion and argues against a distal flow obstruction.

Fig. A2.34 (A) Transducer position for transversal V2-VA insonation. (B) Extracranial duplex, B-mode image, V2-VA insonation in the longitudinal insonation plane, 90° counterclockwise rotated. The yellow lines indicate the V2-VA segment between the fifth and sixth cervical transverse processes leading to image C. (C) Extracranial duplex, color-mode image, transversal plane as indicated in B. Note the prominent VA (arrow) with the corresponding vein and the CCA (arrowhead) with the IJV.

Fig. A2.35 CTA, 2D coronal MIP. (A) Anatomic situs of VA origin from both SAs in typical deep location. (B) In reality the VA origin is hidden by the clavicle.

Normal values: Pulsatility of the Doppler spectrum might vary from high pulsatility caused by low diastolic flow in a distinctly hypoplastic artery (Fig. A2.33) to low pulsatility with high velocities comparable with the ICA flow in a large vessel (Fig. A2.32). Even in a hypoplastic vessel a preserved diastolic flow component as indicator of remaining brain perfusion should be present. In this case the VA often ends as PICA. For flow velocities, see Table A2.2.

V0/V1 Segment

Anatomic details: Anatomic variations of the VA origin have already been described in “General Arterial Anatomy” above. As it lies behind the clavicle, insonation may be difficult (Fig. A2.35). Larger ultrasound studies have demonstrated insufficient V0–VA visualization in 6–14% on the right side and in 14–40% on the left side (Trattnig et al 1993). The difference is mainly explained by the deeper origin of the left VA and the 5% of variants in which the left VA directly originates from the aortic arch. Apart from impaired insonation conditions caused by the deep vessel localization, other factors may also contribute to insufficient visualization. The VA originates at the SA apex in only ~50% of cases (Fig. A2.36A). In the remaining cases the vessel originates deeper in the aortic arch or at the apex but from the posterior wall, and in a minority even from the inferior SA wall which cannot be insonated by conventional ultrasound (Trattnig et al 1993). A tortuous VA origin may be a further restricting factor. In elderly stroke patients, a tortuous proximal VA can be expected in up to 50% of cases (Fig. A2.36 B, C).

Fig. A2.36 Variations of the VA origin (encircled). (A) Straight but very deep (i.e., proximal) VA origin. (B) Marked elongation of the VA origin. (C) Kinking of the VA origin.

Fig. A2.37 (A) Transducer position for V0/V1-VA and SA insonation in the longitudinal plane. (B) Contrast-enhanced MRA, coronal view. Note the straight VA origin (arrow). Here the proximal VA and SA can concomitantly be visualized in a longitudinal examination plane. In elderly patients and elongated vessels (see Fig. A2.35) an axial insonation plane may facilitate the V0-VA segment evaluation. (C,D) Extracranial duplex, longitudinal insonation plane. Color-mode image and Doppler spectrum analysis of the VA origin (flow velocity 53/17 cm/s).

Position and vessel identification: Insonation of the V0/V1–VA segment is started at the V2 segment, with the linear transducer used for the examination of the carotid arteries applying the color mode. From there the vessel is followed along its proximal extraforaminal V1 path. Use of beam steering, for example by changing the linear beam into a partial sector field, if available, and beam steering of the color-mode field facilitates the identification of the proximal VA down to its origin at the supraclavicular fossa with the transducer pointed caudally and slightly ventrally (Fig. A2.37). In the case of tortuosities the course of the proximal VA may be visualized in the transverse plane. If identification of the VA is required because the vessel could not be followed continuously to its origin, confirmation can be achieved by digital tapping of the ipsilateral VA at the atlas loop. Tapping is easily done with one finger over the palpable groove behind the mastoid (Video A2.8). A typical modulation of the signal—similar to the tapping maneuver for the STeA—identifies the VA (Fig. A2.38). Compared with the V2 segment, the distal V1 segment runs rather superficially with a more inclined course, and it may therefore be examined even if the visualization of the V2 segment is insufficient. This may also help to clarify if a missing diastolic flow component in the V2 segment is “real” or just be caused by an unfavorable insonation angle. In difficult insonation conditions, the patient may be asked to reach toward their knee with the hand on the side of insonation (Alexandrov 2013). Also, a sector probe may facilitate the detection of the VA origin. As with the use of the linear probe, we recommend starting with the identification of the V2–VA segment and following the vessel in a longitudinal plane down to the SA (Fig. A2.39). Unlike the V0–VA segment, the V1–VA can be insonated in more than 90% of subjects (Kuhl et al 2000).

Normal values: Directly at the origin of the VA Doppler spectra might appear mildly turbulent and the pulsatility higher compared with the distal VA segments, without pathologic significance. This phenomenon is caused by the perpendicular branching offof the vessel from the SA. For flow velocities, see Table A2.2.

V3 Segment

Anatomic details: The V3 segment is a tortuous vessel segment with a complex three-dimensional anatomy (Ulm et al 2010). It encircles the atlas and the atlanto-occipital joint first in a posterior-vertical direction and then in a medio-horizontal direction, aiming for the foramen magnum. Two loops with a comma-shaped appearance can be distinguished. After leaving the transverse foramen of the second vertebra, the proximal loop runs in a vertical orientation between C2 and C1. The distal loop winds round the atlanto-occipital joint (Fig. A2.40 and Fig. A2.41).

Position and vessel identification: Ultrasound insonation of the V3 segment traditionally focuses on the distal segment with its loop, also called the “atlas loop.” Insonation rates of 76% on the right side and 86% on the left have been reported (Trattnig et al 1990). In VA hypoplasia, confident identification may be difficult. For insonation the patient is asked to turn their head and the transducer is positioned in a transverse position just below the mastoid bone with a slight anterior and rostral orientation (Fig. A2.42). Pure B-mode imaging may fail to detect the artery. However, in color mode the segment appears with a flow away from the probe or, if slightly tilted downward, comma-shaped with a bidirectional color image, which makes confusion with the ICA unlikely (Fig. A2.43). Insonation depth varies according to the circumference of the neck but the vessel segment is usually detected in 1.5–4 cm of depth. The proximal V3 loop is best insonated with the linear probe following the V2 in the longitudinal plane (Fig. A2.44). The distal V3 segment can also be examined using the transcranial probe via the transforaminal approach (Video A2.9; see also “V4 and Distal V3 Segment” under “Vertebral Artery” later in this chapter).

Fig. A2.38 Extracranial duplex, longitudinal insonation plane. Color-mode image and Doppler spectrum. Top: In case of doubt about VA identification—typical transients during VA tapping at the atlas arch confirms VA identity. Bottom: A distal V1-VA segment with a normal diameter (3 mm) and a flow pattern without diastolic flow is indicative of distal VA occlusion proximal of the PICA origin. Note that the signal here is quite similar to a normal SA flow signal. A distal VA occlusion should not be too readily assumed from V2-VA derived flow profiles, as a missing diastolic flow there might be attributable to an unfavorable angle of insonation, e.g., facilitated by vessel hypoplasia.

Fig. A2.39 Extracranial duplex, longitudinal insonation plane, sector probe. Top: B-mode (left) and color-mode image (right). The sector transducer may facilitate the examination of the VA origin in patients who are obese, have a short neck, or with a deep location of the VA origin. Left: Proximal V2 in B-mode imaging. Right: V0–1 and SA in color mode. Bottom left: SA Doppler spectrum with its typical triphasic flow pattern (flow velocity 85/-42 cm/s). Bottom right: Color-mode image of the VA origin and SA.

Fig. A2.40 Postprocessed contrast-enhanced 3D MIP MRA, arterial phase. Vertebrobasilar anatomy. Left: Posteroanterior view. Middle: 45° rotation to the left. Right: 90° rotation to the left. Note the tortuosity of the V3-VA.

Fig. A2.41 Left: Drawing of the vertebrobasilar anatomy (adapted from von Reutern and von Büdingen 1993). Right: DSA, left VA injection, posteroanterior projection. Note the tortuous V3 course in both images and the two prominent loops: The proximal V3 loop (red circles) at the exit of the transverse foramen (C2) and the distal V3 loop (yellow circles) with its horizontal course behind the atlanto-occipital joint before entering the foramen magnum.

Normal values: Assessment of flow velocity has to allow for the fact that exact angle correction is impeded by the vessel course. We therefore recommend documenting the highest flow velocities without angle correction. As no systematic values have been reported, the available data from the V2 segment should be used as a guide for flow velocity evaluation (Table A2.2).

Fig. A2.42 Top left: Transducer position for distal V3-VA loop (atlas loop) insonation in a slightly oblique axial plane. Bottom left: Proximal V3-VA loop insonation in longitudinal plane. Right: CTA, 3D reconstruction, right posterior oblique view of the distal V2- and both parts of the V3-VA segment: the atlas loop (arrows) and the proximal V3 loop (white arrow). Note the OccA (dotted arrow), which should not be mistaken for the V3-VA (see Fig. A2.26).

Fig. A2.43 Top left: Postprocessed contrast-enhanced 3D MIP MRA, arterial phase, axial MIP: V3 distal atlas loop. Top right: Extracranial duplex, axial insonation plane. Corresponding color-mode image. Note the straight horizontal vessel course behind the atlanto-occipital joint with a flow away from the probe (red) before entering the foramen magnum. Bottom: Extracranial duplex, Doppler spectrum analysis of the distal V3-VA (flow velocity 39/19 cm/s and 33/13 cm/s).

Fig. A2.44 Left: Postprocessed contrast-enhanced 3D MIP MRA, arterial phase, axial MIP, image rotated 45° counterclockwise to correspond with ultrasound plane: Proximal V3 loop. Right: Extra-cranial duplex, longitudinal insonation plane, B-mode image (top) and color-mode image (bottom). Note: the loop located between C2 and C1 appears similar to the atlas loop between C1 and skull base.

Fig. A2.45 Skull of a woman (aged 59), illuminated by flashlight from within. Note the transtemporal, transforaminal, transorbital, and transfrontal bone windows. (With kind permission from Prof. Schnalke, Berlin Museum of Medical History, Charité—Universitätsmedizin Berlin, Germany.)

Intracranial Arteries

A sector transducer with transmission frequencies of 1–3 MHz is required for intracranial vessel analysis using TCCS. The majority of ultrasound systems use 2- or 2.5-MHz transducers. Despite the use of low frequencies, insonation in adults can only be performed in regions where the skull is naturally thin: i.e., where a bone window is present (Fig. A2.45).

The dimension of each bone window is individually different and depends on several factors. Kollár and coworkers (2004) correlated the thickness of the temporal bone and quality of the transtemporal bone window. The reported thickness ranged from 0.7 mm in excellent transtemporal ultrasound conditions to 6 mm in poor conditions. The best insonation results come from the transtemporal bone window, which yields the majority of information about the intracranial blood circulation, and are achieved if the transducer is positioned in front of the external acoustic meatus just above the zygomatic arch. Transtemporal ultrasound penetration and subsequently insonation quality worsen with increasing age. In addition, gender differences at older ages are observed with worse insonation conditions in women. Finally, brain atrophy, whatever its cause, is also associated with worsening transtemporal insonation conditions.

Hoksbergen et al studied 112 European vascular patients aged over 60 years. Using the transtemporal window they found no signal in 1% of men and 23% of women while all signals (defined as M1-MCA, A1-ACA, and P1- and P2-PCA) were present in 83% of men and 40% of women. During transforaminal insonation 6% of men and 5% of women revealed no flow signal, whereas assessment of both V4-VA and BA was possible in 91% and 79%, respectively (Hoksbergen et al 1999). Of 198 ethnic South Asian stroke patients (mean age 64 years, 70% males) an inadequate acoustic bone window was seen in 16.2% of cases (De Silva et al 2007). A prevalence as high as 23% has been reported for Thai stroke patients (Ratanakorn et al 2012). For clinical practice, the patency of the acoustic bone window can be divided into five categories:

In spite of ongoing technical and system improvements, the patency of the bone window will probably remain crucial. The development and use of ultrasound contrast agents—consisting of stabilized microbubbles that can be injected intravenously to improve signal-to-noise ratio—help to overcome this problem. Currently available in a large number of countries is the sulfur hexafluoride-based agent SonoVue (Bracco Imaging), sold as Lumason in the United States. Using echo-contrast agents, intracranial vessel visualization is improved and detection rates reach ~90%, even in the elderly population (Gerriets et al 2002). In another study at least the main stem of the MCA was visible after the administration of echo-contrast agents, which was not possible before in 12% of subjects (Postert et al 1998). An example of color signal improvement after contrast application is shown in Fig. A2.46 and Video A2.10. (For further details concerning ultrasound contrast, see also Chapter 1, “Harmonic Imaging and Ultrasound Contrast Agents” under “Imaging Modalities, Parameters, and Settings.”)

Fig. A2.46 TCCS, transtemporal approach, axial plane. Left: Color-mode imaging in a stroke patient with impaired transtemporal bone window; color signals of the A1-ACA and M1-MCA (arrows) are not visible, apart from the P1- and P2-PCA segments. Right: Color-mode image after intravenous administration of 1 mL Sono-Vue. Excellent images of all basal cerebral arteries are seen except for the ipsilateral M1-MCA (arrows), although the contralateral M1-MCA becomes visible (arrow), confirming M1-MCA occlusion. Note the blooming effect and aliasing phenomenon artificially enlarging the insonated vessels. (A = anterior, P = posterior)

Like MRI and CT, TCCS uses well-defined examination planes to achieve reliable and reproducible images of parenchyma and vessels. As with MRI and CT, axial planes are the standard examination planes. Coronal planes are helpful for visualizing vessel segments with a vertically orientated course and sagittal planes may be used for vessels with a rostrocaudal course (Stolz et al 1999b). Fusion imaging with MRI facilitates the recognition of planes and vessel assignment (for methodological aspects see Chapter 1, “Ultrasound Fusion Imaging” under “Imaging Modalities, Parameters, and Settings”).

Applying TCCS through the transtemporal bone window, five main axial insonation planes (Fig. A2.47; see also Videos A2.11–A2.13 for transtemporal insonation with and without fusion imaging) and two coronal insonation planes (see Fig. A2.54 and Video A2.14) can be distinguished. Within these planes different structures such as bone, parenchyma, or cerebrospinal fluid (CSF) within the cisterns and fissures can be assessed on conventional B-mode imaging and used as landmarks for intracranial orientation and insonation of main vessels. We recommend beginning in the midbrain plane as most vessel segments of the CW can be identified there and the probe can be placed perpendicular. The best temporal acoustic window (i.e., the thinnest temporal bone) is usually found preauricular above the zygomatic arch. From here the probe is moved in small circles to pick up the best B-mode image. The brighter the whole image, the closer the probe is to the optimal window. The hypoechoic midbrain should easily be distinguished from its surrounding hyperechoic basal cisterns (Fig. A2.48). From here, only minimal probe movements are needed for further image optimization, controlled by one or two fingers of the transducer-holding hand that are kept in contact with the patient’s skin during insonation.

Fig. A2.47 Transducer position and schematic drawing of the five axial insonation planes in a coronal T2-weighted MR image. 1 = midbrain plane; 2 = upper pontine plane; 3 = lower pontine plane; 4 = thalamic plane; 5 = cella media plane.

Fig. A2.48 Midbrain plane. Top left: Probe position. Top right: Corresponding MR ce-MP-RAGE images rotated by 90° to correspond with ultrasound images, axial plane. Bottom left: Corresponding TCCS B-mode image. Bottom right: Color-mode images demonstrating M1-MCA, M2-MCA, A1-ACA, P1-PCA, P2-PCA, and BA tip.

Fig. A2.49 TCCS, axial B-mode image of the midbrain plane. Top: Optimal midbrain positioning in the center of the ultrasound image, facilitating anatomic orientation and optimizing vessel visualization. Bottom: Nonrecommended midbrain positioning.

Fig. A2.50 Upper pontine plane. Top left: Probe position. Top right: Corresponding MR ce-MP-RAGE images rotated by 90° to correspond with ultrasound images, axial plane. Bottom left: Corresponding TCCS B-mode image. Bottom right: Color-mode images demonstrating ICA siphon.

For a standardized insonation approach, the mid-brain should be centered within the ultrasound monitor (Fig. A2.49). By lowering the insonation angle by ~10° the upper pontine plane is displayed, and another 10–20° decrease displays the lower pontine plane at the border to the medulla oblongata (Fig. A2.50 and Fig. A2.51). Ultrasound landmarks are anteriorly the sphenoid and posteriorly the petrosal bone forming the middle temporal fossa and—especially for the upper pontine plane—the hypoechoic pons and cerebellum as well as the hyperechoic fourth ventricle. Pointing the transducer 10° upward from the midbrain plane, the thalamic plane is displayed with both hypoechoic thalami embracing the third ventricle and the hyperechoic pineal gland behind the third ventricle (Fig. A2.52). Further increasing the insonation angle by 10–20° reveals the cella media plane with angular cut of the hypoechoic lateral ventricles (Fig. A2.53).

Fig. A2.51 Lower pontine plane. Top left: Probe position. Top right: Corresponding MR contrast-enhanced MP-RAGE images rotated by 90° to correspond with ultrasound images, axial plane. Bottom left: Corresponding TCCS B-mode image. Bottom right: Color-mode images demonstrating C6-ICA.

Fig. A2.52 Thalamic plane. Top left: Probe position. Top right: Corresponding MR contrast-enhanced MP-RAGE images rotated by 90° to correspond with ultrasound images, axial plane. Bottom left: Corresponding TCCS B-mode image. Bottom right: Color-mode images demonstrating M2/M3-MCA transition (arrow), A2-ACA (arrows), and P2/P3-PCA transition (arrowhead).

Fig. A2.53 Cella media plane. Top left: Probe position. Top right: Corresponding MR contrast-enhanced MP-RAGE images rotated by 90° to correspond with ultrasound images, axial plane. Bottom left: Corresponding TCCS B-mode image. Bottom right: Color-mode images demonstrating insular M3-MCA branches.

Fig. A2.54 Left: Illuminated skull demonstrating location and best transducer position over the transtemporal bone window for the anterior coronal plane (yellow) and posterior coronal plane (orange). Right: 3D TOF-MRA, lateral MIP. Insonation field of the anterior coronal plane (yellow) and the posterior coronal plane (orange).

Transtemporal coronal planes may be the first choice to analyze craniocaudally orientated vessels and may help to render stenotic lesions more precisely. The anterior coronal plane in particular facilitates the complete analysis of the intracranial ICA and makes it possible to differentiate more precisely between the terminal ICA, the beginning of the MCA, and the ACA. The posterior coronal plane can be used to analyze at least the distal BA and to distinguish the proximal PCA from the SCA but may also visualize the entire course of the BA and even parts of the ipsi- or contralateral V4-VA. Also, the C6-ICA segment can be visualized in the posterior coronal plane (Fig. A2.54 and Fig. A2.55). As B-mode reference points, the hypoechoic vessel sheath of the proximal ICA segments may be used in the anterior coronal plane and the hyperechoic prepontine cistern and clivus in the posterior coronal plane.

The proximal intracranial posterior circulation is examined through the transforaminal window, i.e., through the foramen magnum. Further access paths are via the transorbital or the transfrontal bone window (although the latter is rarely used). In the transtemporal approach up to 90% insonation energy is absorbed by the bony structures, but the transorbital approach requires reduction of the insonation energy to prevent side effects to the human eye. Therefore the former approaches allow a mechanical index (MI) of up to 1.5, the latter—in accordance with the US Food and Drug Administration (FDA) recommendations for insonation of the orbit and eye—should not be higher than 0.26.

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