Anatomy and Structure of Ultrasound Examination


Vascular Anatomy and Structure of Ultrasound Examination

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

Anterior Circulation

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).

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).

Posterior Circulation

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

Anterior Circulation

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).

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.

Cerebral Arterial Circle (Circle of Willis) and Its Variations

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.

General Structure of Arterial Ultrasound Examination

Each radiologic imaging modality, such as DSA, CTA, and MRA, provides different aspects of anatomic information, reflecting its individual methodical basis which may be more closely related either to the real vessel anatomy or to vessel hemodynamics. Ultrasound with its unique methodical principles provides a specific view of vessel anatomy and cerebral hemodynamics, sometimes limiting its diagnostic yield but often providing additional or new insights into brain perfusion. A comparison of DSA, CTA, and MRA makes it possible to appreciate especially the knowledge of intracranial vessel anatomy that can be gained by ultrasound techniques.

All the blood vessels that supply the brain are best studied with the patient in a comfortable supine position with the examiner investigating from behind the patient’s head. According to the normal arterial flow direction, extracranial examination should be done prior to the intracranial insonation. To facilitate topographic orientation it is advisable to always start insonation in B-mode before color-mode imaging is used. Especially for transcranial insonation this approach finally leads to more rapid detection and identification of intracranial vessels. The sonographer should also be comfortably positioned. Ideally, the investigating arm should rest on the examination couch while one or two fingers of the hand holding the transducer are in contact with the patient’s skin near the ear or the auditory canal. Just enough ultra-sound gel should be applied to the lens of the transducer for the insonation; excessive amounts should be avoided. This approach will lead to optimal and stable guidance of the transducer even if the insonation should take longer than expected. It will also facilitate faster repositioning of the probe, e.g., after unplanned study interruptions.

For extracranial ultrasound, the head of the patient is preferably only slightly elevated, or not at all, to ensure maximum insonation space at the neck. Blood vessels are routinely insonated using 5–10-MHz transducers in the longitudinal and axial insonation planes and information is obtained regarding vessel wall alterations (B-mode), a potential lumen reduction (B-mode and color mode), and flow direction, as well as flow velocity (Doppler mode). Extracranially we recommend always obtaining and documenting angle-corrected flow velocities if a straight vessel course can be followed.

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.

Video examples for the insonation of all relevant extra- and intracranial arteries are available online in the Thieme MediaCenter. Images

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

A linear transducer with transmission frequencies between 5 and 10 MHz is sufficient for extracranial vessel analysis in most cases. Small and superficial arteries are easier to insonate with frequencies higher than 10 MHz. A sector transducer may facilitate the insonation of the SA and the origin of the VA. The pulse repetition frequency (PRF) should always be adapted to the target vessel. Deep located and small arteries require a low PRF setting.

Common Carotid Artery (CCA)

Anatomic details: Apart from their variation in origin (see “General Arterial Anatomy” above), the CCAs are extremely stable and bilaterally symmetric blood vessels with hardly any anatomic variation. Elongated vessel courses can be seen with increasing age. The CCA has a mean caliber of ~6–7 mm (Yazici et al 2005).

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 Images A2.1 and A2.2).

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

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 Images A2.1 and A2.2).

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 Images 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 Images 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).

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 Images 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 Images A2.4). Detection rates of 100% have been reported (Tee et al 2013).

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.

Axillary Artery (AxA)

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 Images 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 Images 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.

V2 Segment

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 Images 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 Images 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 Images A2.7).

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. A2.37 (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 Images 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 Images A2.9; see also “V4 and Distal V3 Segment” under “Vertebral Artery” later in this chapter).

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).

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:

  1. Excellent—all main segments of the basal intracranial arteries are visible throughout their entire length.
  2. Good—the intracranial arteries can be reliable judged.
  3. Fair—a complete assessment is not possible.
  4. Bad—only parts of certain vessels are visible.
  5. Missing—no vessel signal is available.

In most ultrasound systems the quality of the B-mode image correlates well with the visibility of the CW, i.e., the quality of the color-mode image. However, sometimes these findings may also differ, with missing color-mode signals despite a reasonable B-mode visualization of parenchymal structures such as the midbrain. Transcranial color-mode quality can be optimized by reducing the depth of insonation, the size of the color window, and the PRF. The latter may lead to “dirty” images with a profound color aliasing phenomenon. Although this might seem to impair the detection of circumscribed stenoses and the analysis of flow direction, we recommend use of this strategy to obtain good spatial information on the insonated vessel segment as a “road map” and to prove the vessel’s integrity. Doppler spectrum analysis of vessel segments should be performed as a second step, which then allows exact assessment of flow velocity and flow direction. The latter is facilitated by using the ultrasound system’s “triplex examination mode,” which allows real-time examination with B-mode, color mode, and Doppler mode simultaneously active. The triplex mode especially facilitates the examination of elongated vessels through different insonation planes—a constellation which is true for most of the intracranial arteries and veins. Unfortunately, many ultrasound systems suffer a substantial reduction of image quality as soon as the triplex mode is activated, which is probably due to limited calculation power. Even so, the triplex mode helps to quickly identify the required vessel segment and the system may then be switched into the duplex mode as required, e.g., to analyze a turbulent flow or raised flow velocity in more detail.

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 Images A2.10. (For further details concerning ultrasound contrast, see also Chapter 1, “Harmonic Imaging and Ultrasound Contrast Agents” under “Imaging Modalities, Parameters, and Settings.”)

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 Images A2.11–A2.13 for transtemporal insonation with and without fusion imaging) and two coronal insonation planes (see Fig. A2.54 and Video Images 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.

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).

Insonation through a bone window is comparable to peering through a keyhole. Depending on the region of interest, the position of the transducer might need to be adjusted, e.g., moved cranially but aiming downward to visualize basal structures (as if looking through a keyhole and trying to see the floor behind the door). Alternately, the probe might need to be moved caudally but aiming upward to see thalamus or lateral ventricles (like trying to see the ceiling in our keyhole example).

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.

Jun 20, 2018 | Posted by in NEUROSURGERY | Comments Off on Anatomy and Structure of Ultrasound Examination
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