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324
ICA
EC
Chapter 5 · Extracranial Cerebral Arteries
. Table 5.9 Multiparametric approach using a set of primary and secondary criteria for grading the severity of internal carotid artery (ICA)
stenosis using duplex ultrasound (Modied according to Neale etal. 1994; Faught etal. 1994; Moneta etal. 1995; AbuRahmaetal.1998;
Grant etal. 2003; and Arning etal. 2010). Later Investigators using the NASCET criteria for grading ICA stenosis propose lower threshold
velocities: PSV of 130 cm/s for 50% stenosis and 230 cm/s for 70% stenosis (AbuRahma et al. 2011; Jahromi et al. 2005)
Distal degree (NASCET) (%) 10 20–40 50 60 70 80 90 Occlusion
Local degree (ECST) (%) 45 50–60 70 75 80 90 95 Occlusion
Primary
criteria
5
Secondary
criteria
Number of plus signs indicates diagnostic relevance (compared with other criteria)
ACA anterior cerebral artery, CCA common carotid artery, EDV end-diastolic velocity, ICA internal carotid artery, PSV peak systolic velocity
1. B-mode image +++ +
2. Color duplex image + +++ + + + + + +++
3. PSV at the site of maximum
stenosis (cm/s)
4. PSV in the poststenotic
segment (cm/s)
5. (Beginning) collateralization
(periorbital arteries/ACA)
6. Decrease in diastolic velocity in
the prestenotic segment (CCA)
7. Abnormal ow in the poststenotic segment
8. EDV at the site of maximum
stenosis (cm/s)
9. Confetti sign (+) ++ ++
10. Carotid stenosis index (ICA/CCA
PSV ratio)
<100 120–160 >200 >250 >300 >350–
400
>50 <50 <30 No ow
(+) ++ +++ +++
(+) ++ +++ +++
+ + ++ +++ (+)
<50 <90 <100 >100 >100
≥2 ≥3 ≥4 ≥4.5
200–
500
No ow
signal
signal
A
A
C
B
CCA
D
. Fig. 5.23 Stenosis criteria for grading internal carotid artery (ICA)
stenosis. A=peak systolic velocity (PSV) within the stenosis (the most
reliable parameter based on scientic evidence); C=peak end-diastolic
velocity (EDV) in the stenosis; A/B=ratio of intrastenotic PSV in the ICA
and PSV in the common carotid artery (CCA); C/D=ratio of peak EDV in
the stenosis and peak EDV in the CCA
. Table 5.10 Pitfalls in carotid stenosis grading due to other
factors aecting peak systolic velocity (PSV)
Stenosis grading Factor/Source of error
Underestimation
of stenosis
(of risk of
embolism)
Overestimation of
stenosis
Episode of low blood pressure
Proximal stenosis, e.g., aortic stenosis
Very-high-grade stenosis, subtotal
occlusion
Long stenosis
Tandem stenosis (additional distal/
intracranial stenosis)
Very eccentric stenosis
High blood pressure at time of
examination
Hyperperfusion (contralateral carotid
occlusion or very-high-grade stenosis)
Very pulsatile ow (medial sclerosis in
diabetes mellitus)
Small or contracted vessels
Very short stenosis
ab
cd
700
Flow velocity (cm/s)
100
Local degree of stenosis (%)
5.6 · Ultrasound Criteria, Measurement Parameters, andDiagnostic Role
325
5
600
500
400
300
200
100
0
10 20 30 40 50 60 70 80 90 100
0
Poststenotic flow velocity in ICA
120
cm/s
90
60
30
N = 105
0
40
50
Degree of stenosis (%)
60
70 80 90
Local diameter reduction (%)
r = 0.89
100
Peak systolic velocity (cm/s)
600
Intrastenotic PSV
480
360
240
120
0
30
Peak systolic velocity (cm/s)
600
480
360
240
120
0
30
40
40
50
50
60
60
70 80
Degree of stenosis (%)
£ 0.5 cm
> 4 cm
70 80 90
90 100
1–2 cm
. Fig. 5.24 a Internal carotid artery (ICA) stenosis: relationship between angiographic stenosis and intrastenotic peak systolic velocity (PSV)
for calculating the distal degree of stenosis (. Fig.5.7b) (From Moneta etal. 1995); dierences in plaque morphology are one factor contributing to scatter (see . Fig.5.27). b Flow velocity (PSV) as a function of stenosis severity (local degree). Intrastenotic PSV increases continuously
with luminal diameter reduction (according to the continuity equation) and degreases again in very-high-grade stenosis/subtotal occlusion due
to friction loss, especially when a long carotid segment is stenosed. c Distribution of poststenotic PSV as a function of local diameter reduction.
Poststenotic PSV tends to decrease when the degree of stenosis is at least approx. 85% (According to Görtler etal. 1994). d Relationship between
the length of stenotic plaque (length of stenosis) and expected maximum PSV as a function of stenosis severity. When a long high-grade stenosis is present, friction loss may lead to a lower PSV in the stenotic jet than would be expected from the degree of stenosis. This association is
especially important when luminal narrowing is due to dissection. However, the actual decrease in PSV to be expected also depends on plaque
conguration and has not been investigated systematically (According to Widder 2004)
Color duplex ultrasound performed with properly
adjusted settings (gain, PRF) allows qualitative evaluation
but no grading of carotid artery stenosis. e color mode
may help in dierentiating plaque from owing blood in the
patent lumen but reliable determination of the degree of
luminal narrowing is not possible due to inherent methodological limitations, especially in transverse orientation (see
7 Sect. 1.2.3). Color ow images may also facilitate dieren-
tiation of subtotal and total occlusion.
Poststenotic PSV should be measured as far crani-
ally as possible (distal to the stenosis jet). It is decreased in
high- grade stenosis (poststenotic PSV >50 cm/s in <70%
stenosis (NASCET) versus <30cm/s in >90% stenosis) (see
. Table5.9). Comparison of poststenotic PSV with the con-
tralateral side is advisable.
Presence of collateral circulation can point to carotid
stenosis. Abnormal ow (reversed ow direction, reduced
ow) in the supratrochlear artery is a sign of high-grade
ICA stenosis. Collateralization is more reliably detected
intracranially using transcranial duplex ultrasound. A variety of arteries can be recruited to bridge a stenosed carotid
artery; therefore, identication of a single collateral with
increased ow taken alone is not a reliable indicator of
carotid stenosis. In most examinations, searching for collaterals is not necessary or helpful for estimating stenosis
severity.
ICA
ICA
ab
Chapter 5 · Extracranial Cerebral Arteries
326
III
I
II
CCA
CCA
5
B
A
. Fig. 5.25 a Unilateral high-grade internal carotid artery (ICA) stenosis or occlusion (left diagram) leads to an increase in flow velocity in the
contralateral common and internal carotid arteries if they function as the primary collateral pathway; this must be borne in mind in stenosis
grading (diagrams from Widder and Görtler 2004). b Because end-diastolic velocity (EDV ) decreases with the duration of diastole, stenosis
grading on the basis of EDV is dependent on a patient’s heart rate (as well as other systemic factors such as blood pressure at the time of measurement, contralateral occlusion, wall elasticity). As a consequence, resistance indices, which incorporate EDV, also vary with heart rate (see
. Fig. 1.28c, d)
Secondary Stenosis Criteria (
z
Reduced diastolic ow velocity in the common carotid
artery (CCA)
: Flow velocity in the CCA, particularly during
diastole, decreases when higher-grade ICA stenosis is present. Pulsatility increases prestenotically, and the CCA ow
prole becomes more like that of the ECA (externalization).
e increase in pulsatility is determined not only by the
degree of ICA stenosis but also by the number of ECA
branches recruited as collaterals: the stenosis-related increase
in pulsatility is damped more markedly when more ECA
branches provide collateral ow. e ECA waveform thus
becomes less pulsatile, i.e., more like that of the ICA (internalization), which in turn aects the ow pattern in the CCA.
Turbulent ow is oen more conspicuous acoustically
(like foodsteps on gravel) than visually in the waveform.
Transition from normal laminar to turbulent ow occurs at
sites of stenotic narrowing when the Reynolds number
exceeds 2000 and in the poststenotic segment (sudden widening of the lumen with ow separation). e occurrence of
turbulent ow depends not only on stenosis severity but also
strongly on plaque conguration and surface.
Intrastenotic end-diastolic velocity (EDV) increases
with the severity of carotid stenosis. An EDV of >50cm/s
suggests that at least 50% stenosis is present with velocities
>80–100cm/s suggesting high-grade stenosis (>70%). is
parameter is especially useful for quantifying subtotal occlusion, in which PSV is more dicult to determine (Carpenter
et al. 1996). Note, though, that EDV increases with the
patient’s heart rate because a higher heart rate means a
shorter end-diastolic phase (. Fig.5.25b).
. Table5.9)
Perivascular tissue vibration occurs in the so tissues
around higher-grade carotid stenosis. is phenomenon can
cause what is known as the confetti sign in color duplex
imaging (adequate gain and PRF) or can be auscultated
because the tissues vibrate at audible frequencies.
Carotid stenosis index or ratio of PSV in the ICA to that
in the ipsilateral CCA (ICA/CCA PSV ratio): Absolute ow
velocities are inuenced by physiologic and abnormal systemic factors (hypertension, aortic valve stenosis, medial
sclerosis, contralateral occlusion, intracranial collateralization). e eect of these factors can be reduced by calculating
the ratio of PSV in the stenosis to PSV in the prestenotic segment of the same vascular territory (Moneta et al. 1993;
Howton etal. 2008; Carpenter etal. 1995). However, as the
ECA also arises from the CCA, the ratio will be altered if the
ECA is stenosed or acts as a collateral in the presence of ICA
stenosis (
Ratio of intrastenotic to poststenotic systolic velocity
(PSV) in the ICA
degrees of stenosis (risk of embolism) can be associated with
dierent intrastenotic and above all poststenotic velocities,
depending on the length of the stenosis (. Fig.5.24d). In a
long (high-grade) stenosis, friction losses result in a lower
intrastenotic PSV.To account for the resulting error in stenosis grading, it has been proposed to calculate the ratio of
intrastenotic to poststenotic velocity, preferably using
intensity- weighted mean ow velocities rather than PSV.A
cuto of 5 is assumed to indicate higher-grade carotid stenosis (Ranke etal. 1999; . Table5.11). is parameter is considered unreliable and is used very rarely.
III
II
I
. Figs.5.23 and 5.25).
(see 7 Sect. 1.2.3 and . Fig. 1.48): Identical
PI » 2
PI » 1
PI » 0.7
5.6 · Ultrasound Criteria, Measurement Parameters, andDiagnostic Role
327
. Table 5.11 Sensitivity, specicity, and accuracy of duplex ultrasound in internal carotid artery (ICA) stenosis grading using dierent
sonographic criteria (angiography as reference standard). Some studies report local (ECST) degrees of carotid stenosis, while others report
distal (NASCET) degrees of stenosis (see . Fig.5.9b). Some authors provide no information on the method of stenosis grading used
Author Year N Method (parameter) Results
5
Degree of
stenosis (%)
Faught etal. 1994 70 130 93
Polak etal. 1992 50 125 83
Huston etal. 2000 915 50 130 70 1.6 92 90 90 91 91
915 70 230 3.2 86 90 83 92 89
Soulez etal. 1999 70 94 81 62 98
60 2.9 94 80 72 96
AbuRahma
etal.
Grant etal. 2000 70 225 90
Carpenter
etal.
Hood etal. 1996 457 70 230 100 78 97 88 94 93
Carpenter
etal.
Browman
etal.
Moneta etal. 1995 176 60 260 70 3.2–
Neale etal. 1994 60 70 270 110 96 91 93
Moneta etal. 1993 70 325 130 4 83 90 80 92 88
Eckstein etal. 2001 68 70 (ECST) 180 92 42 82 67 79
Finkenzeller
etal.
1998 50 140 92 95 97 89 93
60 150 65 82 97 96 86 90
70 180 96 85 95 91 92 92
1996 110 70 210 94 77 68 96 83
110 70 70 92 60 73 86 77
110 70 3.3 100 65 65 100 79
1995 60 230 98 87 88 98 92
60 40 97 52 86 86 86
60 2.0 97 73 78 96 76
60 230 40 2.0 100 100 100 100 100
1995 75 70 175 91 60
2008 21 50 120 50 1.5 Correlation with IADSA, r=0.852 (Pearson)
70 200 100 2.0
PSV
(cm/s)
EDV
(cm/s)
PSV
ratio
3.5
Sensitivity (%)
84 94 92 88 90
Specicity (%)
PPV
(%)
NPV
(%)
Accuracy (%)
EDV end-diastolic velocity, IADSA intra-arterial digital subtraction angiography, NPV negative predictive value, PPV positive predictive
value, PSV peak systolic velocity
Stenosis Grading Using the Primary
z
and Secondary Criteria
Degrees of Carotid Stenosis
(. Table5.9, . Figs.5.20, 5.21,
5.22, 5.23, and 5.24):
5 Low-grade stenosis (<20–40% by NASCET criteria) and
plaque morphology are evaluated by B-mode ultra-
sound. While a slight increase in intrastenotic ow
velocity may be apparent in the Doppler waveform or
color duplex images, the increase is not hemodynamically relevant and precludes quantication.
5 Moderate stenosis (50% NASCET/70% ECST) is associ-
ated with a PSV of up to 200cm/s. Whether turbulent
328
Chapter 5 · Extracranial Cerebral Arteries
ow occurs depends on plaque conguration. e
carotid stenosis index may be increased above 2, while
the other parameters are unaected or the changes are
not diagnostically meaningful.
5 Moderate to high-grade stenosis (60% NASCET) causes
local ow acceleration with color aliasing and a PSV of
up to 250cm/s. ere may be turbulent ow and
beginning signs of perivascular tissue vibration.
5 High-grade stenosis (70% NASCET) is associated with
aliasing, a PSV of up to 300cm/s, and beginning eects
5
on secondary criteria (collateral ow, supratrochlear
artery). Turbulent ow is apparent, the confetti sign
may be seen, and intrastenotic EDV is increased
(>100cm/s).
5 High- to very-high-grade stenosis (80%) leads to even
more marked changes compared with high-grade
stenosis (PSV of 350–400cm/s, carotid stenosis index of
>4). Poststenotic velocity is decreased.
5 Subtotal occlusion (90%) is associated with a further
increase in intrastenotic PSV; on the other hand,
intrastenotic PSV may be lower due to greater friction
loss (PSV range of 200–500cm/s). With very high
receive gain, it is oen possible to capture the low-amplitude, high-frequency jet components. e diagnosis of
subtotal ICA occlusion is corroborated by a decrease in
poststenotic ow velocity (<30cm/s) and an increase in
CCA pulsatility.
5 Occlusion of the ICA is characterized by the absence of
ow signals (color duplex and waveform) throughout
the extracranial ICA.Sonographic evaluation should
extend to the skull base in order not to miss pseudoocclusion (see, however, persistent primitive hypoglossal
artery (PPHA),
7 Sect. 5.6.1.3.1). In addition, ow in the
CCA is decreased and becomes more pulsatile; and
collateral circulation can be demonstrated.
overestimation in these situations, a higher PSV cuto of
140–150cm/s should be used to discriminate between lowgrade and hemodynamically signicant (>20–40%) carotid
artery stenosis (modied according to AbuRahma et al.
1995). Not taking these factors into account will lead to falsepositive results and overestimation of carotid stenosis (Horrow etal. 2000; Busuttil etal. 1996).
In the study of Busuttil etal. (1996), duplex ultrasound
overestimated the degree of carotid stenosis in patients with
severe contralateral disease, falsely suggesting high-grade
stenosis in 27% of cases (using angiography as the reference).
Following unilateral carotid endarterectomy (CEA), PSV on
the unoperated side decreased on average by 36cm/s. Other
investigators reported a 20–40% higher PSV due to compensatory ow in patients with over 90% contralateral stenosis
(Henderson etal. 2000).
e compensatory increase in PSV is determined not
only by the severity of contralateral stenosis but also by the
contribution of the ICA to collateral circulation and by the
recruitment of other collaterals (ipsilateral ECA and supratrochlear artery, posterior circulation).
In another study including 107 patients with asymptomatic
50–99% contralateral carotid artery stenosis (PSV >125cm/s),
a postoperative duplex examination showed a mean decrease
in PSV of 48 cm/s (10%) and a mean decrease in EDV of
36 cm/s (19%) (Abou-Zamzam et al. 2000). e authors
concluded that patients with severe bilateral carotid stenosis
should be restudied with duplex scanning after the rst
operation before undergoing CEA of the contralateral side.
Pitfalls in carotid stenosis grading
(. Table 5.10)
include:
5 Acoustic shadowing due to long calcied plaque (>2cm)
precludes measurement of intrastenotic PSV (rotation of
transducer to avoid areas of acoustic shadowing, see
. Fig.5.4)
5 Plaque conguration: very eccentric plaque with little
Critical Appraisal of PSV: The Main Criterion of Carotid
z
Stenosis
e parameters for grading internal carotid artery (ICA) stenosis outlined above have accuracies of 83–97% using intraarterial angiography as the gold standard. Several studies
found good interobserver agreement both for grading ICA
stenosis (kappa=0.7) and for identifying candidates for surgery (kappa=0.72; Griths etal. 2001). When the degree of
carotid stenosis determined using absolute parameters provides no denitive basis for recommending surgery, eects of
systemic conditions such as hypertension or hypercirculation (fever, hyperthyroidism) should be considered
. Table 5.10). Medial sclerosis in long-standing diabetes
(
mellitus leads to pulsatile ow with a larger systolic component and a smaller diastolic component. Contralateral carotid
artery occlusion (. Fig. 5.25) or high-grade stenosis and
multiple-vessel disease with vertebral artery involvement
may also lead to articially elevated ow velocities (Busuttil
etal. 1996) in the carotid system; another factor to be considered in interpreting carotid blood ow velocities is collateralization (see . Figs. 5.68 and 5.69 (both Atlas)). To avoid
hemodynamic eect (relatively low PSV) but high risk of
embolism due to plaque thickness (. Figs.5.15, 5.27,
and 5.57e (Atlas))
5 Presence of bilateral higher-grade stenosis (PSV
overestimates stenosis because collateral function results
in higher PSV than expected on the basis of the degree
of stenosis alone;
. Figs.5.68 and 5.69 (both Atlas)) or
presence of tandem stenosis (PSV underestimates ICA
stenosis, even if the second stenosis is in the aorta)
5 PSV underestimates the degree of long stenosis
(. Fig.5.24d)
5 High bifurcation: carotid bulb and proximal ICA cannot
be insonated adequately (switching to a curved-array
transducer with small footprint may help)
5 Mistaking occlusion for pseudo-occlusion in individuals
with relling of the ICA through a PPHA (. Fig.5.31)
5 Recanalization of ICA occlusion (. Fig.5.30)
5 Carotid aneurysm with thrombosis as source of embo-
lism (. Fig.5.72 (Atlas))
5 Carotid dissection (. Figs.5.44 and 5.45)
5 Vasculitis (. Fig.5.46)
5.6 · Ultrasound Criteria, Measurement Parameters, andDiagnostic Role
329
5
Normal blood ow velocity is higher in the thinner vessels of
slim patients or arteries narrowed by other factors such as
temporary vascular contraction compared with the velocities
determined in a general population. is will then translate
into higher PSV in a stenotic segment (. Fig.5.49 (Atlas)).
As already mentioned, a long high-grade stenosis (in particular a stenosis of >3 cm) will cause a less pronounced
increase in PSV than a shorter stenosis of the same degree
(. Fig.5.24d). e thresholds dened in investigations using
angiography as the gold standard are usually based on the
most common stenosis length of 1–2cm. According to the
Hagen-Poiseuille law, ow resistance also depends on the
length of the narrowed segment. A very short stenosis will
cause a more marked increase in PSV, and this is why the
length of the stenosis has to be considered in stenosis grading
as well (although no detailed study-based data exist).
An intracranial stenosis of the carotid territory occurring
in tandem with an extracranial ICA stenosis reduces extra-
cranial ow velocity, resulting in a less marked increase in
PSV across the extracranial stenosis (stenosis mismatch). A
tandem lesion with high-grade intracranial stenosis may be
suggested:
5 if ow velocity in the distal extracranial ICA is markedly
lower than would be expected from the degree of
upstream stenosis and
5 if ow is more pulsatile than expected (
. Table5.10).
Some authors prefer measurement of end-diastolic velocity
for stenosis grading, arguing that this parameter is less
aected by the patient’s blood pressure at the time of the
examination. Nevertheless, data from comparative studies
still show PSV to be the most reliable velocity parameter for
stenosis grading since EDV varies with the patient’s heart
rate and other systemic factors (. Fig.5.25b). Repeated measurement of constant blood ow in the same vessel at increasing heart rates would yield increasingly higher EDVs due to
shortening of the cardiac cycle, resulting in an articially
lower resistive index (RI).
e eects of systemic factors such as hypertensive episodes or greater pulsatility due to reduced wall elasticity can
be minimized by calculating the ratio of PSV in the ICA to
that in the ipsilateral CCA (ICA/CCA PSV ratio or carotid
stenosis index) for stenosis grading. is ratio can be determined as a supplementary parameter whenever absolute PSV
suggests a borderline stenosis and it is assumed that the measurement was inuenced by systemic factors.
e compilation of studies in
. Table 5.11 shows that
accuracy rates of over 90% can be achieved in stenosis grading on the basis of the hemodynamic parameters derived
from duplex ultrasound (using angiography as the gold standard). Hence, the accuracy of duplex imaging is comparable
to the interobserver variability between two radiologists
evaluating the same angiograms (. Tables 5.12 and 5.13).
Taken together, published data indicate that ultrasound
achieves consistently good results with sensitivities and specicities of approximately 90% in detecting greater than 70%
carotid artery stenosis (relevant for identifying surgical can-
. Table 5.12 Agreement between two independent
radiologists in identifying and classifying hemodynamically
signicant carotid artery stenosis on angiography
Author/year Agreement between two indepen-
dent radiologists (%)
Croft etal. 1980 88
Moneta etal. 1993 93
. Table 5.13 Accuracy of angiography in comparison with
pathologic workup of surgical specimens
Author/year Accuracy of angiography compared
with pathology (%)
Croft etal. 1980 79
didates). For carotid stenoses of 50–70%, several studies and
a meta-analysis of 41 studies investigating dierent imaging
modalities in comparison with intra-arterial digital subtraction angiography (IADSA) found sensitivities for duplex
sonography that were 5–30% lower but specicities of over
90% (Wardlaw etal. 2006). An explanation for these results is
not apparent from the meta-analysis, but the use of dierent
criteria (threshold velocities) for dening hemodynamically
relevant stenosis (50% stenosis or greater), systemic factors
(blood pressure, wall elasticity), and dierent hemodynamic
eects of eccentric versus concentric plaques (
. Fig.5.27b)
appear to be contributing factors.
e question regarding the most valid velocity parameter
for the grading of carotid artery stenosis– PSV, EDV, or ICA/
CCA PSV ratio– still remains open. Published data yield no
uniform picture and the heterogeneity of study designs
makes results dicult to compare.
A study of the ICA PSV/CCA PSV ratio determined by
duplex ultrasound in more than 300 carotid artery examinations with receiver operating characteristic (ROC) analysis
found good accuracy for the detection of 70–99% NASCET
stenosis using a cuto of 4 (Moneta etal. 1993). Other investigators achieved good accuracies with dierent cutos
(. Table5.11).
Nevertheless, PSV has turned out to be the most reliable
velocity parameter in detecting and quantifying high-grade
carotid artery stenosis, showing consistently high accuracy
in many studies (Arning etal. 2003; Lal etal. 2004; Lewis
and Wardlaw 2002) (. Table5.11). If the ultrasound exami-
nation is technically adequate, patients in whom highergrade stenosis is diagnosed can be scheduled for surgery
without additional imaging tests for stenosis grading (Grant
etal. 2003; Lewis and Wardlaw 2002). e decision to recommend carotid endarterectomy (CEA) always involves
weighing the predicted risk of future vascular events against
perioperative and postoperative morbidity/mortality. Various attempts at dening the best PSV cuto for identifying
330
Chapter 5 · Extracranial Cerebral Arteries
hemodynamically relevant stenosis (>50%) by means of
ROC curve analysis in studies using angiography as the reference standard show that a higher PSV improves specicity,
albeit at the cost of sensitivity; conversely, a lower cuto
velocity improves sensitivity but lowers the specicity of the
method (see . Fig. 6.9: ROC analysis for determining the
cuto velocity for renal artery stenosis; . Fig. 2.19: for profunda femoris stenosis). is situation is impressively illustrated by a study of Moneta etal. (1995), who investigated
dierent PSV cutos for identifying 60–99% carotid artery
5
stenosis in a larger patient population. A PSV cuto of
200cm/s yielded high sensitivity of 93% but poor specicity
of 76% (84% accuracy), while a cuto of 300cm/s resulted in
low sensitivity of 78% and high specicity of 95% (87% accuracy). e best compromise was to use a PSV cuto of
260cm/s, which yielded the highest accuracy of 88% with
86% sensitivity and 91% specicity.
Using a combination of PSV >260 cm/s and EDV
>70cm/s, Moneta etal. achieved 84% sensitivity, 94% specicity, 92% positive predictive value, and 90% accuracy in
discriminating 60–99% stenosis. Similar results were
obtained with an ICA/CCA PSV ratio>3.2 (
. Table5.11). In
. Fig. 5.26 Severe kinking (acute angle) of the internal carotid artery
(ICA) can cause stenosis. However, in a kinked segment, stenosis grading is limited because Doppler angle correction is dicult to accomplish, and additional parameters such as turbulent ow need to be
used to corroborate the diagnosis. In the example, a PSV of 135cm/s is
measured in the kinked distal ICA.This PSV suggests 30–40% stenosis
(by NASCET criteria, equivalent to 50–60% ECST stenosis)
asymptomatic patients, the statistical benet of prophylactic
CEA is smaller, and the number needed to treat to prevent
one stroke is higher than is the case for patients with symptomatic carotid stenosis. For this reason it has been proposed
that velocity thresholds with a higher positive predictive
value be used in asymptomatic patients. For an intrastenotic
PSV cuto of 290cm/s combined with an EDV of 80cm/s,
the authors reported a 95% positive predictive value for
60–99% asymptomatic ICA stenosis (angiography).
Technical advances and the advent of high-resolution
transducers led to improved sensitivities and specicities of
90–95%
in correctly identifying hemodynamically signicant carotid artery stenosis. e correlation of intra-arterial
angiography and color-coded duplex imaging is 0.8–0.9
(Faught etal. 1994; Sitzer etal. 1993).
Despite these good results, some caution is in order in
view of the range of PSV cutos proposed for dening hemodynamically relevant stenosis (Elgersma etal. 1998) and the
scatter apparent in
. Fig. 5.24a. In one study, PSV values
ranging from 50 to 530cm/s were measured for 70% angiographic stenosis (Hunink etal. 1993). is variation cannot
be fully explained by measurement errors and failure to take
the hemodynamic eects of dierent plaque congurations
(. Fig.5.27) into account.
Because carotid stenosis grading based on PSV cutos
alone is prone to errors, the German Society of Ultrasound in
Medicine (Deutsche Gesellscha für Ultraschall in der Medizin, DEGUM) advocates a multiparmatric approach using
the set of primary and secondary criteria discussed above
(7 Sect. 5.6.1.2.1 and . Table5.9). Several decades of sonographic and vascular surgical experience from the clinician’s
perspective conrm that this sonographic approach allows
reliable preoperative carotid stenosis grading (Khaw 1997).
e practice is dierent in North America, where carotid
ultrasound examinations are performed by sonographers
and PSV alone is used for stenosis grading. e dierent philosophies necessarily result in dierent recommendations
regarding PSV cutos: while the Radiological Society of
North America (RSNA) recommends one standardized PSV
cuto to detect all carotid stenoses >70%, the DEGUM advocates a more exible approach based on PSV measurement in
conjunction with additional parameters (
. Fig.5.24b).
Despite the variation in ow velocities measured for a
given angiographic degree of stenosis and despite the pitfalls
in determining PSV discussed above, it is safe to conclude
that there is good overall agreement between hemodynamic
stenosis quantication by duplex ultrasound and angiographic stenosis grading (. Tables 5.10, 5.11, 5.12, and 5.13).
Another aspect worth mentioning here is that a study
investigating ultrasound machines from dierent manufacturers in a phantom model of predened ow velocities
found dierences in ow velocity measurements on the order
of 5–10% (Fillinger et al. 1996). is is another issue that
tends to be overlooked when discussing dierences in
reported scientic data.
Atherosclerotic elongation of the ICA leads to tortuosity,
kinking, and coiling due to the limited space available
between the carotid bulb and the base of the skull. Such
changes typically do not require treatment and are oen incidental ndings that impair duplex ultrasound evaluation.
Only kinking stenosis, especially when symptomatic, should
be operated on (see
. Fig.5.51 (Atlas)).
Even severe kinks or coils will produce a stenosis only if
the artery takes a sharp turn; they may however impair ow
velocity measurements due to the diculty of achieving an
adequate Doppler angle. erefore, indirect criteria such as
turbulent ow
must be considered as well (. Fig.5.26). In
severe ICA kinking, the degree of luminal narrowing may
vary with dierent functional positions of the cervical spine.
=plaque
CD
=lumen
a
b
5.6 · Ultrasound Criteria, Measurement Parameters, andDiagnostic Role
331
5
Most stenoses at the origin of the ICA (bulb), the most
common site of carotid stenosis, are due to atherosclerosis.
Plaque
A
Distal carotid stenosis is rare and typically has other underlying causes such as bromuscular dysplasia, wall dissection
(usually due to trauma), or kinking.
B
A
C
B
Collateralization also aects the risk of embolism in
steno-occlusive carotid disease. Intrastenotic PSV in an
80–90% stenosis is lower when there is good collateralization
D
Plaque
as opposed to the same degree of stenosis in a patient with
poor collateral pathways. Lower intrastenotic velocities
reduce the risk of embolism because wall shear stress is lower.
e poststenotic ICA diameter is another prognostic fac-
tor
. When a high-grade ICA stenosis develops slowly, the
plaques
Eccentric plaquesConcentric
decrease in blood supply is compensated for by the recruitment of collaterals. As a result, carotid blood ow on the side
of stenosis is decreased. A subgroup analysis of the ECST
shows that a reduced poststenotic ICA lumen has important
prognostic implications for patients with high-grade stenosis
(Rothwell etal. 2000). In this analysis, patients with postste-
50% diameter
75% area
50% diameter
50% area
50% diameter 50% diameter
<50% area >50% to <75%
area
notic narrowing of the ICA dened as an ICA/CCA ratio of
<0.42 (which usually means <3mm) had a two thirds lower
stroke rate than patients with similar stenosis severity but
without narrowing at 5-year follow-up. e authors assume
that poststenotic narrowing may be protective as blood ow
distal to the stenosis is insucient to carry emboli to the brain.
For the reasons outlined before, color ow imaging (color
duplex or power mode) is inaccurate for stenosis quantication (diameter reduction in longitudinal plane or
sectional area reduction in transverse plane
) on the basis
cross-
of the width of color-coded ow relative to the lumen diameter at the site of maximum stenosis. e inherent methodological limitations (angle dependence of the ow signal) can
be overcome by using the B-ow technique, which allows
better dierentiation of owing blood from the vessel wall or
plaque because of its superior discrimination of echoes from
mobile and stationary reectors. A small study (21 patients)
evaluated the performance of dierent imaging techniques in
discriminating 50–95% stenosis (by NASCET criteria) compared with intra-arterial DSA, which served as the gold standard. In this study,
B-ow imaging (see . Fig.5.86) showed
the highest correlation (Pearson) with DSA (R= 0.94), followed by contrast- enhanced MRI (1.5 Tesla, standard coil;
R = 0.9117), color duplex ultrasound, and reconstructed
contrast-enhanced CT (both R= 0.85) (Finkenzeller et al.
2008). Note, however, that plaque evaluability in both
B-mode and B-ow imaging crucially depends on plaque
morphology, in particular on the presence of calcication.
e accuracy of stenosis grading with B-ow imaging
decreases with the severity of calcication.
Inherent Methodological Dierences Between
z
Ultrasound and Angiography
. Fig. 5.27 a Eccentric luminal narrowing can vary widely in appear-
ance and suggest dierent degrees of stenosis, depending on the
imaging plane (angiography and B-mode ultrasound). The diagrams
illustrate how the same plaque can lead to dierent estimates of the
degree of stenosis caused by it: A: 0%, B: approx. 50%, C: approx. 70%,
D: 100%. This source of misinterpretation results from the reduction of
the three-dimensional vascular lesion to the two-dimensional imaging
plane. Color duplex facilitates evaluation of eccentric plaques, which,
if possible, should be assessed in transverse and longitudinal orientation. b Plaque morphology (concentric–eccentric) can lead to variable
degrees of cross-sectional area reduction despite identical diameter
reduction. Eccentric plaques are thicker than concentric ones, even
when they cause the same degree of stenosis. Because they protrude
into the lumen, eccentric plaques are exposed to greater shear stress
(longitudinal pulsation) and are thus more susceptible to rupture and
embolism. These two factors (reduction to two planes and dierences
in plaque conguration) explain the discrepancies in stenosis quantication between duplex ultrasound and angiography when eccentric
plaques are present. Dierences in plaque conguration lead to
problems when stenosis grading on the basis of hemodynamic criteria
with duplex ultrasound is compared with angiography using diameter
reduction as a morphologic criterion. For instance, a 50% diameter
reduction measured angiographically and indicating a 50% stenosis
corresponds to a 50% stenosis based on hemodynamic grading
(expressed in terms of cross-sectional area reduction) when the plaque
is eccentric, but to a 75% stenosis when the plaque is concentric. With
duplex ultrasound, stenosis grading is based on hemodynamic criteria
derived from the Doppler waveform. A cross-sectional area reduction
of 75% results in a PSV (local grading method) of approx. 240cm/s
(PSV ratio of 4) versus 120cm/s (PSV ratio of 2) for 50% area reduction
(in peripheral arteries)), while the angiographic diameter reduction is
50% in both cases (local degree of stenosis). PSV ratio=prestenotic
PSV/intrastenotic PSV; degree of stenosis=(1–1/PSV ratio)×100. The
degree of stenosis determined hemodynamically is a more adequate
marker of the severity of blood ow reduction (in relation to cross-area
reduction). This is also relevant in evaluating peripheral or renal artery
stenosis (see . Fig. 2.17)
e gold standard, angiography, also has pitfalls– even when
the stenosis is evaluated in dierent projections and quantication is done in the plane with the most severe luminal narrowing. Independent interpretations of the same angiograms
by two radiologists show variation in accuracy ranging from
80% to 93%.
from the fact that the 3D plaque is projected onto the 2D
lm, where the resulting degree of narrowing varies with the
imaging plane (. Fig.5.27a). In DSA, stenosis grading is also
Errors in angiographic stenosis grading arise
332
Chapter 5 · Extracranial Cerebral Arteries
inuenced by the concentration of contrast medium. Moreover, plaques of dierent conguration which produce the
same diameter reduction produce dierent reductions in the
cross-sectional area. Hence, they dier in hemodynamic relevance, giving rise to dierent increases in PSV (. Fig.5.27b).
Angiography is based on the distal method of carotid stenosis grading. With this method, luminal narrowing of the
ICA bulb on the order of 30% merely means that the lumen
of the bulb is reduced to the width of the distal ICA, and no
stenosis is diagnosed (. Fig.5.10).
5
In an analysis of 1001 angiograms of the ICA, 34% of stenoses were classied as 70–99% stenoses by ESCT criteria
versus only 16% by NASCET criteria (distal grading method)
(Rothwell etal. 1994).
cranially to the level of the bifurcation. Dierentiation
between subtotal and total ICA occlusion is important as surgery is usually only recommended for subtotal occlusion.
Unfortunately, the ingrowth of vessels into the connective
tissue of an organized occlusion makes it dicult to distinguish the two. us, the depiction of color-coded ow signals
in the ICA segment near the skull base indicating a patent
lumen is the decisive criterion for establishing the dierential
diagnosis. Contrast-enhanced ultrasound (CEUS) may be
helpful in dierentiating subtotal or pseudo-occlusion from
true occlusion of the ICA.
Adjusting the scanner settings (low wall lter, low pulse
repetition frequency, high gain) is important for detecting
slow ow and small blood volumes in pseudo-occlusion.
With adequate settings, the absence of ow distal to the ath-
Stenoses of the Common Carotid Artery (CCA) and
z
External Carotid Artery (ECA)
Compared with the ICA, the ECA has a more pulsatile ow
prole with a smaller diastolic component. Stenosis typically
occurs at the ECA origin but becomes clinically relevant only
in the presence of concomitant ICA occlusion and maintenance of brain perfusion via extracranial branches such as
the supratrochlear artery or, very rarely, in patients with
multiple-vessel disease and globally reduced perfusion of the
brain with collateral pathways involving ECA branches. Due
to the higher pulsatility of blood ow in the ECA, the PSV
cuto should be higher than for the same degree of stenosis
in the ICA; therefore, we may safely assume that a PSV of
250–300 cm/s suggests higher-grade ECA stenosis. Consequently, there is no need for ROC analysis to determine cutos for grading ECA stenosis.
Stenosis of the CCA is rare. e most common sites of
CCA stenosis are the origin of the artery from the aortic arch
or from the brachiocephalic trunk and the distal CCA segment just below the bifurcation. Between these two sites,
CCA stenosis can be graded using the intrastenotic-toprestenotic PSV ratio (see
. Fig.5.38a). In patients with sub-
total or total CCA occlusion, branches of the ECA (such as
the superior thyroid artery) can be recruited to ensure blood
ow to the ICA.Depending on the course of the collateral
pathways, the ECA will show variable retrograde lling. e
hemodynamic stenosis criteria for the CCA are the same as
for the ICA.
erosclerotic lesion is the most reliable evidence of ICA occlusion. Since no vessels arise from the ICA extracranially, the
Doppler waveform sampled far distal to the bulb (near skull
base) provides the most reliable information for dierentiation, especially since there are no structures interfering with
the Doppler signal (such as calcied plaques). Blood ow
distal to a subtotal occlusion oen assumes a venous character with a slow systolic velocity in spectral Doppler analysis.
Using these criteria, color duplex ultrasound has a positive
predictive value of 92.5–96.7% (Kirsch etal. 1994).
B-mode imaging is not very reliable in detecting ICA
occlusions, especially recent ones, when intraluminal structures are still absent. Once transformation of the thrombus
has occurred, occlusion may be identied in the B-mode by
the presence of internal echoes from connective tissue in the
shrunken lumen. However, an occluded artery is oen dicult to distinguish from surrounding tissue.
When the ICA is occluded, even greater care is required
so as
not to confuse the ICA and the ECA. In ICA occlusion,
the ECA supplies the brain (. Fig.5.61 (Atlas)) through the
supratrochlear artery, resulting in a less pulsatile Doppler
waveform with a larger diastolic component (. Figs.5.28 and
5.29). In this setting, the identity of the ECA must be con-
rmed by rhythmically tapping the temporal artery and
looking for the transmitted pulsation (
. Fig. 5.28). is is
important to avoid making a mistake and localizing the
occlusion to the ECA. Moreover, the examiner should be
aware that, because ECA occlusion is typically short, the distal ECA tends to be relled via collaterals. In this situation,
5.6.1.3 Occlusion
Occlusions of the carotid territory are chiey due to local
thrombus formation secondary to stenosing atherosclerosis
at the origin of the ICA.As there are no arteries emptying
into or arising from the extracranial portion of the ICA
(except in individuals with a persistent primitive hypoglossal
artery (PPHA); see 7 Sect. 5.6.1.3.1 and . Fig. 5.31), an
occlusion will extend to the level of the next branching in the
petrous bone or intracranially into the ophthalmic artery.
Embolic carotid occlusion mainly aects the distal, intracranial segments of the internal carotid system. In the most
severe form, there will be to-and-fro ow in the Doppler
waveform (thump pattern), and the thrombus may extend
the waveform from the postocclusive ECA segment becomes
more like the ICA waveform (. Fig.5.30).
Recanalization following ICA occlusion is very rare, and,
if it occurs, it is generally associated with thromboembolic
occlusion. It is characterized by ow signals in a tortuous and
thin artery. Oen, hypoechoic areas are seen near the wall of
the shrunken lumen throughout the extracranial ICA
(. Fig.5.30). Spectral Doppler analysis will not reveal a stenosis jet but typically only slow ow and greater pulsatility
compared with the normal ICA.
In the rare cases of CCA occlusion, the ICA may be
relled by branches of the ECA recruited as collaterals (e.g.,
superior thyroid artery; see . Fig.5.63 (Atlas)). e corre-
5.6 · Ultrasound Criteria, Measurement Parameters, andDiagnostic Role
333
. Fig. 5.28 a Image of the carotid bifurcation showing occlusion of the internal carotid artery (ICA) (absence of ow in the color ow image and
Doppler waveform). The Doppler waveform from the external carotid artery (ECA) shows a rather large diastolic ow component and pulsatility
as in the ICA, indicating that the ECA supplies the brain. Tapping of the temporal artery with transmission of this eect into the waveform proves
that the waveform is actually from the ECA. b The normal common carotid artery (CCA) has a mixed waveform (combining features of ICA and ECA
blood ow). When the ICA is occluded, as in the example shown, pulsatility in the CCA becomes more like that in the ECA (left part of waveform)
and ow velocity is reduced. The Doppler tracing was obtained by moving the transducer from the CCA to the ECA (while maintaining a constant
Doppler angle). In this patient, the contribution of the ECA to collateral ow is small
5
ICAICA
ICA
ECAECA ECA
ab c
. Fig. 5.29a–c Diagrams of Doppler waveforms illustrating normal and abnormal ndings in the carotid territory. a Normal ow patterns–
internal carotid artery (ICA): low-resistance ow with little pulsatility and large diastolic ow component; external carotid artery (ECA): pulsatility
higher than in the ICA (the ECA mainly supplies skin and muscles and therefore faces a higher peripheral resistance) but lower than in the extremity arteries, due to the ECA’s supply to the glands; common carotid artery (CCA): mixed type, pulsatility intermediate between that of the ICA and
ECA, which it supplies. b High-grade stenosis or occlusion of the ICA– ow in the CCA becomes increasingly pulsatile, approaching the ow pattern of the ECA (as the inuence of the ICA on the CCA waveform recedes or disappears). If the ECA additionally assumes collateral function and
supplies the brain via the supratrochlear artery, the ECA waveform will become less pulsatile (low peripheral resistance). ICA waveform distal to
high-grade ICA stenosis: characteristic postocclusive ow with delayed systolic upslope and decreased pulsatility, i.e., larger diastolic ow component and reduced peak systolic velocity (PSV). The dierence is less striking than the change from triphasic to monophasic ow in extremity arteries because the ICA is a low- resistance artery and already has a monophasic ow prole under normal conditions. c ICA occlusion– with the CCA
exclusively supplying the ECA, its ow increases in pulsatility and the waveform becomes similar to that of the ECA.The ECA may partially supply
the brain (e.g., via the supratrochlear artery), reected in a less pulsatile ow pattern with a larger diastolic component
sponding Doppler waveform will show retrograde ow in the
proximal ECA with reversal to forward ow, but with a highly
postocclusive character, in the ICA.
Bypass graing is indicated only in multi-vessel disease
with reduced global cerebral perfusion, which may manifest
as borderzone infarction.
5.6.1.3.1 Persistent Primitive Hypoglossal Artery
A persistent primitive hypoglossal artery (PPHA) is an
embryonic communication between the anterior and posterior cerebral circulations, which normally resolves and obliterates during early embryonic development. PPHA is rare
and is typically detected incidentally by angiography. e