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Special Arterial Anatomy and Ultrasound Anatomy 25
Fig. A2.27 TCCS, transtemporal approach, axial plane. Left: Color-
mode imaging in a stroke patient with impaired transtemporal bone
window—color signals of the circle of Willis are, apart from the P1and P2-PCA segment barely visible. Right: Color-mode image after
intravenous administration of 1 mL Sonovue. Excellent images of all
basal cerebral arteries are seen except for the ipsilateral M1-MCA
(arrows), confirming M1-MCA occlusion. Note the blooming effect
and aliasing phenomenon artificially enlarging the insonated vessels.
prove signal-to-noise ratio—help to overcome this problem. Currently available in a large number of countries are
the galactose and palmitinic acid-based Levovist and the
perfluorocarbon-based Sonovue. Using these substances,
intracranial vessel visualization is improved and detection
rates reach 90%, even in the elderly population (Gerriets et
al. 2002). An example of color-signal improvement after
contrast application is given in Figure A2.27.
Insonation through a bone window is comparable to
peeking through a key hole. 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 or moved caudally and aiming
upward to see thalamus or lateral ventricles.
Through the transtemporal bone window five main axial
(Fig. A2.28)andtwocoronal(seeFig. A2.34 below) insonation planes can be distinguished. Within these planes
different structures such as bone, parenchyma, or cerebrospinal fluid (CSF) can be assessed on conventional B-mode
imaging and used as landmarks for intracranial orientation
and insonation of main vessels. Similar to magnetic resonance (MR) and computed tomography (CT) imaging the
TCCS examination should be started using the axial planes.
We recommend beginning in the midbrain plane as most
vessel segments can be identified there and the probe can
be placed perpendicular without relevant inclination. The
midbrain appears as a butterfly-shaped hypoechogenic
structure surrounded by the hyperechogenic basal cisterns
(Fig. A2.29). By lowering the insonation angle by approximately 10° the upper pontine plane is displayed, and by
another 10° thelowerpontineplane(Figs A2.30, A2.31).
Ultrasound landmarks are anteriorly the sphenoid and
posteriorly the petrosal bone forming the middle temporal
Fig. A2.28 Schematic drawing of the fiveaxial 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. A2.29 Midbrain plane. Left: Probe position. Right top: Corresponding MR contrast-enhanced T1-weighted image, axial plane.
Right bottom: Corresponding TCCS B-mode image.
fossa and the hypoechogenic cerebellum. Pointing the
transducer 10° upward from the midbrain plane, the thalamic plane is displayed with both hypoechogenic thalami
embracing the third ventricle and the hyperechogenic
pineal gland behind the third ventricle (Fig. A2.32). Further increasing the insonation angle by 10–20° reveals the
cella media plane with angular cut of the hypoechogenic
lateral ventricles (Fig. A2.33).
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 allows to differentiate more precisely between the terminal ICA, the beginning of the MCA,
2 Vascular Anatomy and Structure of Ultrasound Examination26
Fig. A2.30 Upper pontine plane. Left: Probe position. Right top:
Corresponding MR contrast-enhanced T1-weighted image. Right
bottom: Corresponding TCCS B-mode image.
Fig. A2.32 Thalamic plane. Left: Probe position. Right top: Corresponding MR contrast-enhanced T1-weighted image. Right bottom:
Corresponding TCCS B-mode image.
and the ACA. The posterior coronal plane can be used to
analyze the distal BA and to distinguish the proximal PCA
from the SCA (Figs A2.34, A2.35). As B-mode reference
points, the hypoechogenic vessel sheath of the proximal
ICA segments may be used in the anterior coronal plane
and the hyperechogenic prepontine cistern 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 and— although rarely used—via the transfrontal
bone window. Whereas insonation energy in the transtemporal approach is up to 90 % absorbed by the bony
structures, 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 concordance
withtheUSFoodandDrugAdministration(FDA)recom-
Fig. A2.31 Lower pontine plane. Left: Probe position. Right top:
Corresponding MR contrast-enhanced T1-weighted image. Right
bottom: Corresponding TCCS B-mode image.
Fig. A2.33 Cella media plane. Left: Probe position. Right top: Corresponding MR contrast-enhanced T1-weighted image. Right bottom: Corresponding TCCS B-mode image.
mendations for insonation of the orbit and eye—should not
be higher than 0.26.
Internal Carotid Artery
Anatomic details: According to its anatomic course, the
intracranial ICA is divided into three parts and six subsegments (Fig. A2.36). Provided that a sufficient bone window is present, all intracranial segments can be insonated
by TCCS in combined axial and coronal planes. If the axial
planeisused,thetransducerhastobefocusedbetween
theupperandlowerpontineplanes(seeFigs A2.30,
A2.31). For coronal insonation the probe is turned 90°
upward without changing its position over the preauricular transtemporal bone window (Fig. A2.34).
Special Arterial Anatomy and Ultrasound Anatomy 27
Fig. A2.34 Left: Illuminated skull demonstrating location and best
transducer position over the transtemporal bone window for the
anterior coronal plane (yellow) and posterior coronalplane (orange).
Right: 3D TOF MRA, lateral MIP. Insonation field of the anterior
coronal plane (yellow) and the posterior coronal plane (orange).
Fig. A2.36 Schematic drawing of the intracranial part of the ICA.
(Adapted from Schünke et al. 2006.)Extracranial part: Pars cervicalis.
Intracranial parts: Pars petrosa: C6 segment. Pars cavernosa: C5–C3
segments. Pars cisternalis: C2 and C1 segments.
Fig. A2.35 MR T2-weighted weighted image, coronal plane. Left:
Anterior coronal plane comprising distal ICA, and proximal MCA and
ACA segments. Right: Posterior coronal plane comprising distal BA
and proximal PCA segments.
Fig. A2.37 Top: 3D TOF MRA, axial source image, lower pontine
plane, image rotated by 90° to correspond with the ultrasound
image. Horizontal part of the C6 segment (arrows). Bottom: TCCS,
transtemporal approach, axial lower pontine plane. Right: Colormode image demonstrates the horizontal part of the C6 segment.
Left: Doppler spectrum analysis (flow velocity: 69/31 cm/s).
C6 Segment
Anatomic details: The most proximal segment is C6 with
its ascending and horizontal part within the petrosal bone,
leaving the skull at the foramen lacerum (Fig. A2.37). Anatomic length including the vertical and horizontal parts:
25–35 mm.
Position and vessel identification: Only the horizontal part
of this segment can be visualized in the axial lower pontine
plane at the deepest point of the skull base (Fig. A2.37)
with a flow direction away from the transducer. In subjects
with a good transtemporal bone window it is visible in up
to 86 % over a length of 14 ±4 mm (Eggers et al. 2007a).
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).
C5 Segment
Anatomic details: This segment is the ascending part up to
the beginning of the carotid siphon (Fig. A2.38).
Position and vessel identification: The C5 segment can
best be visualized in one of the coronal planes (Jurgita et
al. 2002) (Fig.A2.34, A2.35). Because of an unfavorable
insonation angle, blood flow velocity measurements are
usually not recommended (Fig. A2.38).
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).
2 Vascular Anatomy and Structure of Ultrasound Examination28
Fig. A2.38 Top: MR contrast-enhanced T1-weighted image, coronal
plane, image rotated 90° to correspond with the ultrasound image.
Vertical part of the C5 segment (arrows). Bottom: TCCS, transtemporal approach, coronal plane. Right: Color-mode image demonstrates the C5 segment. Left: Doppler spectrum analysis (flow velocity: 31/13 cm/s).
Fig. A2.40 Top: MRI, T2-weighted image, coronal plane, image rotated 90° to correspond with the ultrasound image. Note the flow
void in the C1/2 segment (arrows). Bottom: TCCS, transtemporal
approach, anterior coronal plane. Right: Color-mode image demonstrates a flow signal in C1/C2 toward the probe. Left: Doppler
spectrum analysis (angle-corrected flow velocity 113/36 cm/s).
Fig. A2.39 Top: MRI, 3D TOF MRA, axial source image, upper pontine plane, image rotated 90° to correspond with the ultrasound
image. C3/C4 segment (arrows). Bottom: TCCS, transtemporal approach, axial upper pontine plane: Right: Color-mode image demonstrates the C-shaped part of the carotid siphon. Left: Doppler
spectrum analysis (angle-corrected flow velocity: 50/25 cm/s).
Fig. A2.41 A Oscillation maneuver. B Schematic drawing of the ori-
gin of the OA from the carotid siphon (arrows). (Adapted from
Schünke et al. 2006.) C, D TCCS, transtemporal approach, upper
pontine plane. C Doppler spectrum demonstrates lowflow velocities
(top) and a positive oscillation phenomenon on mild oscillation of
the ipsilateral optic bulb (bottom). D Color-mode image demonstrates a small red-coded OA signal toward the probe—anterior and
slightly laterally to the carotid siphon (arrows).
C3/C4 Segment:
Anatomic details: Along with the C2 segment, both segments form the carotid siphon, which can have a variable
appearance. Its shape can vary from a simple C-like to a
tortuous S-shaped vessel course (Ta b le A2.2). Elongated
vessel courses are more frequently seen in subjects with
higher age. Anatomic length including C5: 30–50 mm.
Position and vessel identification: The C3/C4 segment can
be visualized in the axial, upper pontine plane (Fig. A2.30).
It is visible in almost all subjects with good insonation
quality. Exact measurements of flow velocities are difficult
as angle correction is impaired by the vessel course
(Fig. A2.39). The highest measured velocity should be recorded. In cases of a missing transtemporal bone window,
visualization of the carotid siphon via the transorbital
approach can be attempted.
Normal values: Flow velocities: see table Table A2.7
(p. 52).
Special Arterial Anatomy and Ultrasound Anatomy 29
0–20 years 21–50 years 51–74 years
Normal 100 74 41
Omega 0 24 51
Tor tu ous 0 2 8
C1/C2 Segment
Anatomic details: Both segments form the rising cisternal,
most distal part of the vessel—also called terminal ICA
(TICA)—before it bifurcates to form the carotid T. Anatomic
length: 13–18 mm.
Position and vessel identification: The C1/C2 segment
usually follows a mediolateral course which allows good
color imaging in the coronal plane (Figs A2.34, A2.35). It is
visible over a length of 10 ±3mmin 100% ofcaseswitha
patent bone window (Eggers et al. 2007a). Often the C1/C2
segment follows a straight course. In these cases anglecorrected flow velocity measurement may be obtained
(Fig. A2.40).
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).
Ophthalmic Artery
Anatomic details: After arising from the C2 or C3 segment
oftheICAtheOArunsthroughtheopticcanalintothe
orbital socket, where it branches into segments that supply theeye and face(Fig. A2.41).The main stem has a mean
diameter of 0.8–1.2 mm. A dual origin with a dominant
contribution via the middle meningeal artery has been
observed in 2.4 % of cases. In 1.2 % of cases the OA arose
solely from the middle meningeal artery (Hayreh and Dass
1962). The OA can be partially insonated during its intracranial course via the transtemporal bone window and
during its course through the orbital socket via the transorbital insonation approach.
Tab l e A2. 2 Anatomic variants of the carotid siphon in relation to age (%) (adapted from Huber
1982)
Intracranial Ophthalmic Artery
Position and vessel identification: For intracranial insonation the transtemporal upper pontine imaging plane is
chosen and the carotid siphon is visualized (Fig. A2.30).
The color-mode signal of the OA can then be identified in
approximately 90 % of cases with a patent transtemporal
bone window about 5–10mm anterior of the carotid siphon and about 5 mm medially of the lesser wing of the
sphenoid bone (Schreiber et al. 2006). To maximize color
gain the PRF has to be reduced, i. e., optimized for detection of low flow velocities. The insonation depth may vary
between 55 mm and 70mm. The OA shows a Doppler
spectrum with flow direction toward the transducer. Vessel identification can be confirmed by slight manual oscillation of the ocular bulb (two fingers applied flat on the
closed eye). This results in a positive oscillation phenomenon within the Doppler profile (Fig. A2.41).
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).
Extracranial Ophthalmic Artery
Position and vessel identification: For extracranial insonation of the OA within the orbital socket using the transorbital approach the same transducer may be used but
with a maximally reduced insonation power (FDA recommendations for insonation of the eye: MI < 0.26). The OA
can be identified in a depth between 35 mm and 50 mm as
a color and Doppler signal toward the probe within the tip
of the orbital socket (Fig. A2.42) However, in cases of an
elongated vessel course, flow directions away from the
transducer might occasionally be seen.
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).
2 Vascular Anatomy and Structure of Ultrasound Examination30
Fig. A2.42 A Transducer position for transorbital OA insonation.
CAVE: Reduce insonation energy below an MI of 0.26. B 3D TOF
MRA, axial source image, post contrast: Note the contrast filling of
the OA in the tip of the orbital socket (arrows). C B-mode image of
the orbital socket. D Doppler spectrum analysis shows a pulsatile
flow (flow velocity: 28/7 cm/s). E Color-mode image with a typical
OA signal toward the transducer.
Fig. A2.44 Anatomical variants of M2 branches. (Adapted from
Huber 1982.) A Tri fur cat ion: 2 5 %. B Lateral pseudo-bifurcation:
18 %. C Early temporal M1 branch: 6 %. D Lateral bifurcation: 48 %.
E Medial bifurcation: 3 %.
Fig. A2.43 Schematic drawing of the MCA segments, coronal view.
(Adapted from Huber 1982.) M1 segment: Pars horizontalis, M2
segments: Pars insularis, M3 segments: Pars opercularis, M4 segments: Pars corticalis.
Fig. A2.45 Top right: MRI, 3D TOF MRA, axial MIP, midbrain plane,
image rotated 90° to correspond with the ultrasound image. Demonstration of the M1-MCA and two M2 branches corresponding with
the TCCS images (transtemporal approach, axial midbrain plane).
Topleft and bottom: Anatomic MCA variations asvisualized by TCCS.
Middle Cerebral Artery
Anatomic details: The MCA is divided into four segments
(Fig. A2.43). Provided that a sufficient bone window is
present, the entire horizontal M1 segment and the proximal horizontal and insular M2 segments can be insonated
by TCCS in combined axial and coronal planes. The MCA in
its M1 segment shows a constant and symmetrical and
often horizontal course. In advanced age the main stem of
the MCA frequently reveals a descending course (Ta b l e
A2.3). Anatomic variants such as hypoplasia are extremely
rare (< 1%). The M1 segment has a mean caliber of 2.7 mm
(range 1.5–3.5 mm) and a mean length of 16 mm (range
5–24mm).Withinadepthof35–45 mm it then separatest
into a variable number of M2 branches. In approximately
5–10% of cases an early strong temporal branch might
origin from the proximal M1-MCA segment which can be
the source of incorrect interpretation in assumed M1-MCA
occlusion (Fig. A2.44).
Position and vessel identification: For insonation the
transtemporal approach either in the axial midbrain plane
or in the anterior coronal plane can be used. Routinely, the
axial insonation plane is used. For axial evaluation of the
M2 origin and assessment of more distal M2 and M3 segments within the lateral fissure the probe has often to be
Special Arterial Anatomy and Ultrasound Anatomy 31
slightly turned upward towards the thalamic and cellamedia plane (Figs A2.45, A2.46). For coronal insonation the
anterior coronal plane has to be used. It additionally permits a good visualization of the the A1-ACA segment and
the C1/2-ICA segment, distinguishing the distal ICA from
0–20 years 21–50 years 51–74 years
Rising 71 15 2
Horizontal 28 47 2
Descending 1 38 96
the proximal M1, as well as the visualization of a prominent early temporal branch if it is present (Fig. A2.47).
Normal values: Flow velocities: see Tab l e A2.7 (p. 52). For
M2-MCA segments No systematic values have been reported.
Tab l e A2. 3 Anatomic variants of the
M1-MCA segment in relation to age
(%) (adapted from Huber 1982)
Fig. A2.46 TCCS, transtemporal approach, axial midbrain plane.
Right: Color-mode signal of the M1-MCA segment. Left: Doppler
spectrum analysis of the M1 segment (flow velocity: 86/53 cm/s).
Fig. A2.47 Left: MRI, T2-weighted image, coronal plane, rotated 90°
counterclockwise. Flow void in the C5-ICA segment, C1/C2-ICA
segment and ipsilateral M1-MCA segment. Note an early temporal
branch (arrow). Right: TCCS, transtemporal approach, anterior coronal plane. Corresponding color-mode image demonstrating blood
flow in the distal C1/C2-IC A segment, partly in the carotid siphon, in
the A1-ACA segment,and the proximal M1-MCA segment (ipsi- and
contralateral). Note the red and blue color-codes indicating flow
directions toward and away from the transducer. Note also an early
temporal branch (arrow).
2 Vascular Anatomy and Structure of Ultrasound Examination32
0–20 years 21–50 years 51–74 years
Horizontal
and mild rise
Sharp rise 23 3 0
Descending 5 20 25
Loops and
coils
72 69 60
0815
Tab l e A2. 4 Anatomical variants of A1-ACA
segment courses in relation to age (%)
(adapted from Huber 1982)
Fig. A2.48 Schematic drawing of the ACA segments, sagittal view.
(Adapted from Huber 1982.) A1 segment – Pars precommunicalis,
A2 segment – Pars infracallosa, A3 segment – Pars precallosa, A4
segment – Pars supracallosa.
Anterior Cerebral Artery
Anatomic details: The ACA can be divided into four seg-
ments (Fig. A2.48). The ACA in its A1 segment may show
considerable variations in its course (Ta b l e A 2.4). In the
majority of cases, both A1 segments are of equal size. If
asymmetry is present, usually the left side is the larger one.
Hypoplasia is seen in DSA in one A1 in about 4 %. The A1
segment has a mean caliber of 2.1 mm (range 0.8–3.8 mm)
and a mean length of 14mm (range 8–19 m m). T he A 2
segments begin after the ACoA is emitted (Figs A2.49,
A2.50). Both A2 segments usually lie close together.
Position and vessel identification: Provided that a suffi-
cient bone window is present, the entire A1 segment and
the proximal A2 segments can be insonated by TCCS, preferably in the axial plane (Figs A2.49, A2.50). For axial
evaluationofthecompleteA1segmenttheprobemight
need to be turned upward or downward depending on the
anatomic variant (Table A 2 . 4 ). Occasionally, one A1 segment isaplastic or hypoplastic,in which case the contralateral A1 segment supplies the blood to both A2 segments.
As the two A2 segments are usually running closely together within the interhemispheric fissure, most current
ultrasound systems will often show a single color-mode
A2-ACA signal only (Fig. A2.49). Provided that the trans-
temporal insonation conditions are good, both A2 segments can be visualized in up to 30 % of cases. An unpaired
Special Arterial Anatomy and Ultrasound Anatomy 33
Fig. A2.49 Top: Schematicdrawing, TOFMRA and TCCS color-mode
image (transtemporal approach, axial midbrain plane) of a symmetric A1-ACA variant. Bottom: Respective image series with right-sided
A1-ACA hypo-/aplasia (arrows). Note the color visualization of both
A2-ACA segments.
A2, however, occurs only in about 2% of cases. For coronal
insonation, the anterior coronal plane is used (see M1MCA and C1/2-ICA).
Normal values: Flow velocities: see Table A 2.7 (p. 52). For
A2-ACAsegments no systematicvalues have beenreported.
Anterior Communicating Artery
Anatomic details: The ACoA connects both ACAs to form
the anterior part of the CW. Furthermore, it divides the
ACA into its A1 and A2 segments. In up to 58 % of cases, the
ACoA is a singular arterial vessel, in more than 20 % it is
doubled, and occasionally it may even have a reticular
structure. Frequently it has a large caliber, ranging from
0.1mm to 3 mm. It is very short, its length ranging from
1 mm to 3 mm.
Position and vessel identification: In healthy individuals
the ACoA can rarely be detected. This changes as soon as
the vessel becomes a collateral in case of high-grade extracranial carotid artery stenosis or occlusion (for further
details, see also Chapter 5, “Collateral Pathways,” p.101). It
canthenbeidentifiedbyitsstronglydisturbedflowprofile
and position between both distal A1 segments. Under
physiological conditions, occasionally a bidirectional low
flow profile may be found in singular cases in projectionof
the ACoA.
Vertebral Artery
V4 Segment
Anatomic details: After passing the foramen magnum the
VA may show considerable variations in its intracranial V4
segment with regard to vessel length and vessel course
(Fig. A2.51). The intracranial caliber of the VA corresponds
to the extracranial caliber, i. e., a hypoplastic VA in its
Fig. A2.50 TCCS, transtemporal approach, axial midbrain plane:
Color-mode image and Doppler spectrum analysis of a normaltype ACA variant (flow velocity: 72/35 cm/s). Note the single A2ACA signal, despite the presence of two A2-ACA segments.
Fig. A2.51 Schematic drawing of the posterior intracranial circulation showing the regular, symmetric anatomy of the posterior circulation. (Adapted from Schünke et al. 2006.) Image rotated 180
degrees to correspond with the ultrasound anatomy presentation.
Note the origin of the PICA from the V4-VA segment, the origin of
the AICA from the proximal BA and the SCA from the distal BA. VA =
vertebral artery; PICA = posterior inferior cerebellar artery; AICA =
anterior inferior cerebellar artery; BA = basilar artery; SCA = superior
cerebellar artery; PCA = posterior cerebral artery.
extracranial course will continue to be so into its intracranial segments. Sometimes a VA does not merge into the
BA but ends as the PICA. A small vessel bridge between the
distal V4 segment and the BA, however, may be present. A
PICA termination without any connection to the BA has
been reported in 0.2 % of cases, only (Yasargil 1984). In our
own experience the prevalence of the above variant seems
to be higher. Differentiation between the ending of the
contralateral dominant VA and the beginning of the BA
might then become difficult.
2 Vascular Anatomy and Structure of Ultrasound Examination34
Fig. A2.52 Left: Transducer position for transforaminal VA and BA
insonation. Patient in supine body position, head turned to one side.
Top: Transducer position for lower transforaminal insonation plane.
Bottom: Transducer position for upper transforaminal insonation
plane. Right: MR T2-weighted image, midsagittal plane: Yellow lines
indicate direction of the upper transforaminal insonation plane
aiming for the frontal bone at the level of the frontal eminence.
Orange lines indicate direction of the lower transforaminal insonation plane aiming for the nasion.
Fig. A2.54 A CTA, 3D reconstruction (occipital skull removed), image rotated 180° to correspond with the ultrasound image. The
course of the right and left VAs is symmetric from the extracranial
V3 segments (arrows) to the intracranial V4 segments, which join to
form the BA. Note the PCA originating from the top of the BA
(arrow). B TCCS, lower transforaminal insonation plane. Corresponding color-mode image of the right V3 (arrows) and V4 segment and the proximal BA. C TCCS, upper transforaminal insonation
plane. Corresponding distal V4-VA, BA and one PCA or SCA (arrow).
Position and vessel identification: VA and BA are stu d i e d
via the transforaminal approach with the transducer
placed in or near the midline in a skin impression between
the occipital bone and the atlas. In the majority of patients
it will be sufficient to keep the patient in the supine body
position with the head rotated by 30to 45° tothe left or the
right side and in an anteverted position. Only occasionally
it might be necessary to have the patient lie on one side or
be investigated in a sitting position. If both VAs merge to
Fig. A2.53 Left: Skull base with foramen magnum and clivus. Foramen: mean length: 35 mm, range: 30–41 mm; mean width: 30 mm,
range 21–38 mm. Clivus: mean anteroposterior length: 45 mm,
range 37–52 mm (Lang 2001). Right: Corresponding B-mode ultrasound image delineating the hypoechogenic foramen magnum and
the hyperechogenic clivus.
form the BA, identification of both V4 segments is easy to
achieve. However in cases with hypoplasia or elongated
vessel courses interpretation should be cautious, especially as the VA might be mistaken for the PICA and vice
versa.
Using the transforaminal approach, two axial insonation
planes should be distinguished with a transversely held
probe (Fig.A2.52). The lower transforaminal insonation
plane (transducer pointing toward the root of the nose)
allows visualization of the VA from the extracranial V3 to
the intracranial V4 segment and the proximal BA. In this
plane, the PICA can be visualized and sometimes the anterior spinal artery, too.The uppertransforaminalinsonation
plane (transducer pointing toward the frontal eminence)
permits insonation of the distal V4 and proximal to mid BA
segments. We recommend starting routine insonation
with the latter plane as the BA is easily identified in most
cases. For this purpose, it is important to start with B-mode
imaging (insonation depth: 10 to 12cm) depicting the
hypoechogenic foramen magnum and the hyperechogenic
clivus before using the color mode to find the best position
for detection of the BA confluens in about 60 to 80 mm
(Fig. A2.53). From there both VAs can be followed to their
proximal segments, adjusting the probe position continuously for the best insonation quality. The V3 segment
appears as a bidirectional color-signal due to loop in its
course at level of the arch of the atlas (Fig. A2.54). In
patients with straight vessel courses the V4 segment is
insonated with a flow away from the probe (Fig. A2.55).
Cases with marked elongation however, may have a variable flow direction that may impede vessel identification.
Normal values: Flow velocities: see Ta b l e A2.7 (p. 52).