Transcranial Doppler Ultrasonography: Year 2000 Update

Abstract

In this update, the main clinical applications of transcranial Doppler ultrasonography are reassessed. A specific format for technology assessment, personal experience, and an extensive review of the literature form the basis of the evaluation. The document is approved by the American Society of Neuroimaging and the Neurosonology Research Group of the World Federation of Neurology.

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and
Reviews
Transcranial
Doppler
Ultrasonography:
Year
2000
Update
Viken
L.
Babikian,
MD
Edward
Feldmann,
MD
Lawrence
R.
Wechsler,
MD
David W.
Newell,
MD
Camilo
R.
Gomez,
MD
Ulrich Bogdahn,
MD
ABSTRACT
Louis
R.
Caplan,
MD
Merrill
P.
Spencer,
MD
Charles Tegeler,
MD
E.
Bernd Ringelstein,
MD
Andrei V. Alexandrov,
MD
In
this update, the main clinical applications
of
transcranial
Doppler ultrasonography are
reassessed.
A specific format for
technology
assessment,
personal experience, and
an
extensive
review
of
the literature form the
basis
of
the evaluation.
The
document
is
approved
by
the American Society
of
Neuroimaging
Received September 10, 1999 and in revised form December
27, 1999. Accepted for publication February 17, 2000.
From
the Departments of Neurology of Boston University
(VLB), and Harvard University (LRC), Boston, MA; Brown
University (EF), Providence, RI; University
of
Pittsburgh
(LRW), Pittsburgh, PA; University of Alabama
(CRG),
Bir-
mingham, AL; Wake Forest University (CT), Winston-Salem,
NC; University
of
Regensburg (UB), Regensburg, and West-
falische Wilhelms Universitat
(EBR),
Munster, Germany; Uni-
versity of Texas (AVA), Houston, TX; the
Department
of
Neurological Surgery of the University of Washington (DWN),
and the Institute of Applied Physiology and Medicine (MPS),
Seattle,
WA
Address correspondence to V.
L.
Babikian, MD,
Department
of Neurology, Boston University School
of
Medicine and Bos-
ton Veterans Administration, Medical Center, 150 South Hun-
tington Ave, Boston,
MA
02130, USA.
In assessing the use of transcranial
Doppler
ultrasound for
various uses, this article uses the words, "Effective," "Estab-
lished," "Promising," and "Investigational" in specific ways.
Please see the list
of
Definitions in the Appendix at the
end
of
this article.
The
peer review process was guided and adjudicated by
Walter
J.
Koroshertz, MD, Associate Editor, as both Editor-
in-Chief (Lawrence R. Wechsler,
MD)
and Co-Editor-in-Chief
(Viken
L.
Babikian,
MD)
are listed as authors.
The
article
is
an official statement of the American Society of
Neuroimaging and the Neurosonology Research
Group
of the
World Federation of Neurology.
and the Neurosonology
Research
Group
of
the World Federation
of
Neurology.
Key
Words: Transcranial Doppler ultrasonography, transcranial
color duplex ultrasonography, cerebral infarction.
Babikian
VL,
Feldmann
E,
Wechsler
LR
et
al.
Transcranial Dopp-
ler ultrasonography: Year 2000 update.
J Neuroimaging 2000;10:101-115.
Introduced to clinical use in 1982,1 transcranial
Doppler
ultrasonography (TCD) has
been
used extensively ever
since in a variety of clinical settings. A review of the tech-
nology and an assessment by the American Academy of
Neurology's Therapeutics
and
Technology Assessment
Subcommittee were published in Neurology in 1990?·3 Ex-
perience with
TCD
has expanded since, and the technol-
ogy has evolved considerably during
the
past decade.
Multi-channel instruments
that
allow for simultaneous bi-
lateral insonation are readily available today, as
is
new
software
that
allows for embolus detection.
The
ability to
store data, even real-time information, and to produce
hard copy results has also
been
improved
and
incorpo-
rated into
TCD
instruments.
In
addition, transcranial Du-
plex has emerged as a viable method for performing TCD.
These and
other
advances have considerably improved the
technology's diagnostic capabilities.
The
present review
constitutes an update of its clinical applications.
This assessment was performed by a panel of
11
ex-
perts, who were given the task of evaluating the clinical
utility of TCD. The
"Format
for an Assessment" devel-
oped by the American Academy of Neurology and used
for the assessment of single photon emission computed
tomography (SPECT, 4) and
other
technologies, was used.
Based on their personal experiences, solicited opinions,
extensive Medline searches,
and
literature reviews, this
document was generated
and
provided as an educational
tool. The document has
been
approved by the Board of
Directors of the American Society of N euroimaging and
the Neurosonology Research
Group
of the World Federa-
tion of Neurology.
ISCHEMIC CEREBROVASCULAR DISEASE
Intracranial atherosclerosis
is
responsible for up to 10% of
strokes and transient ischemic attacks. When extracranial
Copyright©
2000
by
the
American
Society
of
Neuroimaging
101

internal carotid disease and cardiac disease are excluded
as
the mechanism
of
these strokes and TIAs, it may be
important
for clinicians
to
identify intracranial arterial ste-
nosis, particularly when warfarin
is
considered a therapeu-
tic option. In these instances,
TCD
is often used as a
screening test to identify patients requiring invasive cere-
bral arteriography.
Intracranial
Arterial
Stenosis. Transcranial
Doppler
so-
nography criteria for
the
diagnosis
of
intracranial stenosis
include circumscribed flow velocity increase, distal signal
damping,
and
side-to-side differences in velocity. Mild
(<50%) stenoses are
not
reliably detected by these crite-
ria.
Extensive
correlation
of
specific velocity
values
against
percent
stenosis by angiography
is
lacking, and it
is
recognized that angiography has its own technical limita-
tions in quantifying intracranial stenoses. Criteria for oc-
clusion include no signal from
an
artery while signals are
present from
other
vessels insonated through
the
same
window, as well as signs
of
collateral flow.5•6
Arteries
where stenoses and occlusion can be reliably
detected
by
TCD
include
the
middle
cerebral
artery
(MCA)
Ml
segment; the internal carotid siphon;
and
the
intracranial vertebral, proximal basilar, and proximal pos-
terior cerebral arteries.7-
14
Performance of
TCD
against
angiography in the
anterior
circulation varies by center,
with an approximate sensitivity
of
80% to 90%, specificity
of
90%
to
95%, positive predictive value of 85%,
and
negative predictive value
of
98%. Performance of poste-
rior circulation
TCD
is
not
as well documented,
but
esti-
mated
at
the
80% sensitivity level, with limitations caused
by
more
variable
anatomy
and
by technical difficulty.
Transcranial
Doppler
sonography
can
be
given a positive,
type B recommendation as being
of
established value for
the detection
of
intracranial atherosclerotic lesions based
on
class II evidence.
Some clinicians combine
TCD
and
magnetic resonance
angiography
(MRA)
to identify intracranial stenoses. This
may eliminate
the
need
for cerebral angiography.
If
the
results
of
the
ultrasound screening are negative, evalua-
tion for intracranial stenosis
can
stop.
If
the results
of
the
ultrasound screening are positive,
MRA
can be used to
corroborate
the abnormality, even though it tends
to
over-
estimate the severity
of
stenosis.
If
ultrasound and
MRA
disagree, angiography is performed. However, there are
few prospective
data
that
TCD
and
MRA
in combination
can effectively replace angiography
at
this time for the
identification
of
intracranial atherosclerosis.
The
recently
launched
Stroke
Outcomes
and
Neuroimaging
of
Intracra-
nial Atherosclerosis
(SONIA)
study will provide some an-
swers to these concerns.
Extracranial
Internal
Carotid
Artery
Stenosis.
The
in-
tracranial hemodynamic consequences
of
internal carotid
102
Journal
of
Neuroimaging
Vol10
No
2
April
2000
artery stenosis can be detected with TCD. Reversal
of
the
direction
of
ophthalmic artery flow, presence
of
collateral
flow patterns,
and
altered pulsatility
of
the
MCA
all indi-
cate the presence of hemodynamically significant internal
carotid artery stenosis. Correlation
of
these variables,
both
singly
and
as a battery, has
been
performed by several
investigators, showing that
TCD
is 95% sensitive in iden-
tifying surgical disease
at
the
carotid
bifurcation.
15
•
16
Transcranial
Doppler
sonography can be given a positive,
type B recommendation as being
of
established value for
the evaluation of patients with internal carotid artery ste-
nosis based
on
class
II
evidence. Duplex ultrasonography
and
MRA
remain
the
tests
of
choice for imaging the com-
mon
carotid artery bifurcation.
Acute Brain
Infarction.
Cerebral angiography shows that
the
MCA
is
occluded in 76%
of
patients with acute inf-
arcts in its territory.
17
When
compared to angiography,
TCD
detects these occlusions with a sensitivity of 85%
to
95%
and
specificity
of
90%
to
95%.
18
•
19
Contrast-
enhanced
transcranial
color
duplex
ultrasonography
(TCDS) may
be
more
accurate
than
TCD
in detecting
these occlusions.
20
Absent
MCA
or
supraclinoid internal
carotid artery ultrasound signals,
"blunted"
or
unilaterally
decreased
MCA
velocities of less
than
30 cm/s indicate
arterial obstruction.21 -
23
Up
to
86%
of
intracranial occlu-
sions recanalize spontaneously within 2
weeksY·
21
Com-
bined
TCD
and
SPECT
studies show
that
a more rapid
recanalization
is
achieved with fibrinolytic agents such
as
streptokinase.
24
The
dosage
and
duration
of
thrombolytic
treatment
have
been
adjusted during
TCDS
monitoring
at
some centers.25
Transcranial
Doppler
sonography studies of patients
with acute stroke also have prognostic value. Major intra-
cranial arterial occlusions detected by
TCD
are associated
with
poor
neurologic recovery,21 -
23
whereas normal re-
sults
are
a
predictor
of
early improvement.
20
•
26
Early
thrombolysis-induced recanalization
is
associated with im-
proved outcome in some patients, particularly if lepto-
meningeal collaterals are present
27
-
29
; however, its effects
may occur
too
late
or
may
not
be
sufficiently extensive to
influence the clinical outcome.
24
In
patients with internal
carotid artery territory stroke, a logistic regression model
has shown that
TCD
findings, stroke severity
at
24 hours,
and lesion size
on
CT
are the only
independent
predictors
of outcome
at
30 days.
23
A cerebral perfusion index can
be
calculated using
the
combined
results
of
SPECT
and
TCD.
30
When
obtained during the 6 hours after stroke
onset,
the
cerebral perfusion index differentiates transient
deficit from cerebral infarction as accurately as clinical
stroke severity scores
on
admission.
30
In
patients studied
within 6 hours from stroke onset,
MCA
Ml
segment oc-
clusion as detected by
TCD
is
an independent risk factor
for spontaneous hemorrhagic transformation with a posi-

tive predictive value of
72%?
1 Transcranial
Doppler
so-
nography can
be
given a positive, type B recommendation
as being of established value for
the
evaluation
of
patients
with acute cerebral infarction based
on
class II evidence.
Microembolus
Detection.
Microembolic signals (MS)
detected by
TCD
correspond
to
embolic particles as they
pass through the cerebral circulation.
Formed
element
emboli (atheroma, thrombus
and
platelet-fibrin aggre-
gates) are believed
to
produce ultrasound signals
of
short
duration
and
increased intensity. They move within the
spectral envelope
and
with
the
direction
of
flow.
32
-
34
Ar-
tifacts are bidirectional, with maximum intensities
at
low
frequencies. Microembolic signals are detected
more
fre-
quently when retesting a particular individual
on
different
days
than
when performing prolonged monitoring during
a single session.
An
accurate and reliable characterization
of
embolus
size and composition is
not
yet possible using
current
technology.
The
ability to detect emboli is affected by
several factors including which
TCD
device is being used,
probe
frequency,
duration
of
recording
sessions, pro-
cessing speed of the ultrasound device,
and
the degree
of
overlapping of the fast-Fourier transforms spectral dis-
plays.35-37
Particle size and echogenicity determine
the
intensity detected by the transducer; however, there
is
con-
siderable overlap of intensities corresponding to particles
of
differing compositions and sizes. New software and hard-
ware capability such
as
the coincidence technique improve
the distinction between air and formed element emboli.
In
patients with carotid artery disease, MS are
more
prevalent and frequent in symptomatic
than
asymptomatic
patients. Most MS are asymptomatic,
but
are still believed
to be a
marker
of
risk for ischemic events.
Direct
com-
parison between studies is difficult because of differences
in diagnostic criteria, time span between MS detection
and
last symptom, treatment, recording conditions, and degree
of
stenosis.
38
-4
2
The
frequency of MS may vary with sur-
gical
or
medical therapy,
but
further studies are necessary.
Similar
data
are available in patients with atrial fibrilla-
tion, prosthetic valves,
and
other
cardioembolic condi-
tions.
In
all instances, the precise prevalence
and
response
to
treatment
of these MS remain
to
be
determined.
In patients with asymptomatic internal carotid artery
stenosis, the presence
of
MS is associated with increased
severity of stenosis.
The
presence of MS is also associated
with an increased risk
of
future symptoms of cerebral isch-
emia,38,43·44
and
correlates
with
irregularities
of
the
plaque's surface
but
not
with intraplaque hemorrhage.
40
Microembolic signal detection can
be
used
to
localize
an embolic source, identify high-risk patients with arterial
or
cardiac sources
of
embolism, monitor patients during
invasive procedures, and assess the effect
of
antithrom-
botic agents.
45
However, whereas
there
is
strong evidence
that
TCD
can
be
used
to
detect cerebral microemboliza-
tion, the clinical usefulness
of
identifying
the
passage
of
microemboli into the cerebral circulation remains insuffi-
ciently studied,
and
a definite recommendation can
not
be
given
at
this time.
Autoregulation
and
Vasomotor
Reactivity.
Cerebral
arterial
autoregulation
is
accomplished
by
resistance
changes
that
occur
at
the
level
of
cerebral arteries 400
f.Lm
in
diameter
or
less.
Transcranial
Doppler
sonography
evaluation of large basal conducting vessels, which remain
relatively constant in
diameter
during
moderate
pressure
fluctuations, can provide
an
index
of
relative flow changes
in response
to
small
blood
pressure changes
and
assess the
autoregulatory response
of
the
distal bed. However, small
changes in diameter may also occur in larger arteries, be-
coming sources
of
error.
Slowing
on
electroencephalography correlates with ve-
locity decreases
seen
after hyperventilation,
but
the
elec-
troencephalography response lags behind
and
persists for
3 minutes after hyperventilation.
46
-4
8
In
patients with se-
vere extracranial carotid disease, the restoration of veloc-
ity in response
to
a
drop
in pressure may
take
up to 60
seconds. This
is
the
case in
about
half of patients, with the
rest having a normal restoration
of
velocity, presumably
because
of
collateral flow.
49
-
51
Vasomotor
reactivity
can
be
assessed with
TCD
by
measuring changes in flow velocities in response to acet-
azolamide injection, hyperventilation,
or
C0
2 inhalation.
The
C0
2
method
of
testing is
preferred
as
there
is
less
risk,
the
effect is stronger, and
there
is
greater
certainty
regarding which arterial segments are affected.
In
healthy
patients, flow velocities may drop 35% with hyperventila-
tion and increase
50%
with hypercapnia.
The
response to
dilatory stimuli
is
reduced in
the
setting of severe carotid
disease because
the
resistance
bed
is
presumed
to
be
di-
lated
at
baseline.
Vasomotor
reactivity may
be
as low as
30% distal
to
occluded, symptomatic internal carotid ar-
teries, and it may approach
60%
in arteries
that
had
oc-
cluded asymptomatically.
52
-
54
Reactivity is also reduced
after intracranial
hemorrhage
and
head
trauma.
In
patients with severe extracranial internal carotid dis-
ease, diminished vasomotor reactivity has
been
associated
with
an
increased risk
of
stroke during the 3 months be-
fore
and
the
6 months after testing.
Impaired
reactivity has
been
observed to improve spontaneously following end-
arterectomy.
Testing for vasomotor reactivity has only limited clini-
cal applications
at
the
present
time. Identification
of
de-
creased reactivity may cause physicians inadvertently to
allow blood pressure
to
remain higher after internal ca-
rotid
artery
occlusion,
and
to
select
patients
with de-
creased reactivity for extracranial-intracranial bypass sur-
gery
or
endarterectomy.
52
-
54
Based
on
class
III
evidence,
Babikian
et
al: TCD
Ultrasonograpy
Update
103

TCD
can
be
given a type C recommendation as being
of
useful value for the evaluation
of
vasomotor reactivity.
Right
to
Left
Cardiac Shunts. Paradoxical embolism via
a
patent
foramen ovale is a recognized cause
of
stroke in
young adults.
55
·
56
Its clinical predictors, recurrence rate,
and
prevention have
been
extensively studied. 57
The
pres-
ence
of
an atrial septal aneurysm
and
of
right
to
left shunt-
ing are stroke risk factors.
When
compared
to
transesoph-
ageal contrast echocardiography using saline containing
air bubbles
or
echo-contrast enhancing agents,
the
sensi-
tivity
of
contrast
TCD
ranges between 70%
and
100% and
its specificity exceeds 95% in detecting right
to
left cardiac
or
pulmonary
arteriovenous shunts.
56
·
58
-
62
The
routine
performance
of
the
valsalva
maneuver
during testing can
improve sensitivity
and
specificity.
61
A thorough search
for evidence
of
peripheral
venous thrombosis
is
recom-
mended
in patients who have
been
found to have right
to
left shunts.
62
Based
on
available class
II
evidence,
TCD
is considered
of
established value in
the
evaluation
of
patients with ce-
rebral ischemia and suspected right
to
left cardiac shunts.
It
is given a type B recommendation.
SUBARACHNOID HEMORRHAGE
Vasospasm
of
the
cerebral vessels refers
to
a transient
contraction of
the
intracranial arteries, which can occur in
a variety
of
disorders affecting
the
central nervous system
and
can produce transient
or
permanent
neurologic dys-
function by inducing cerebral ischemia.
The
most common
clinical setting is following
spontaneous
subarachnoid
hemorrhage, frequently secondary
to
the
rupture
of intra-
cranial saccular aneurysms.
In
that
setting, vasospasm is
the
primary cause
of
delayed
ischemic neurologic de-
ficits63
and
mortality.
64
This has
led
to
prophylactic treat-
ments such as induced hypervolemia
or
hypertension, he-
modilution,
and
administering cerebral selective calcium
channel blockers.
65
-6
7
These
measures are
not
innocuous,
however,
and
their
administration
is
often
limited
to
symptomatic patients with severe spasm.
Nor
are they
completely effective,
and
often
other
experimental treat-
ments such as intracranial angioplasty
and
intraarterial
papaverine infusion are used in
the
treatment
of
vaso-
spasm in selected patients.
68
·
69
It
is therefore relevant
to
establish the diagnosis
of
vasospasm even before it be-
comes clinically symptomatic,
and
to follow its course
and
severity over time for optimal care
of
patients with sub-
arachnoid hemorrhage.
Vasospasm can also occur in patients with nonaneurys-
mal subarachnoid
hemorrhage
such as
that
occurring after
head
trauma,
and
in patients with meningitis
and
pre-
eclampsia
who
do
not
have
subarachnoid
hemor-
rhage?0-75
The
course
of
vasospasm in these conditions is
usually milder
but
it is
not
well understood,
and
clinical
104
Journal
of
Neuroimaging
Vol
10
No
2
April
2000
deterioration from vasospasm has
been
reported
in asso-
ciation with some
of
these conditions.7
6--
81
Conventional
and
digital subtraction cerebral angiog-
raphy are
the
standard methods to diagnose vasospasm,
but
are associated with significant morbidity. In spite of
several technical limitations, they remain
the
gold stan-
dard
to
determine
the
accuracy
of
TCD. Although vaso-
spasm can
be
angiographically detected in as many as 70%
of
patients, only as many as 40% show signs
of
cerebral
ischemia.
82
Thus, vasospasm often remains asymptomatic.
Factors
that
cause it
to
become symptomatic are
not
well
understood.
It
is assumed
that
severe vasospasm causing
marked
arterial narrowing and a critical drop
of
cerebral
blood flow
is
the
main immediate cause
of
cerebral isch-
emia. However, a specific critical severity
of
vasospasm
and
the
corresponding flow velocities
that
predict symp-
toms have
not
been
identified. Although
mean
MCA
flow
velocity values in
the
150-200 cm/s range are often con-
sidered clinically significant, values as great as 250 cm/s
can
be
tolerated without cerebral infarction.
83
In
most pa-
tients,
mean
flow velocity values greater
than
200 cm/s
reliably predict
the
presence
of
clinically significant vaso-
spasm.
84
Several factors, including age, intracranial pres-
sure (ICP),
mean
arterial blood pressure, hematocrit, ar-
terial
C0
2 content,
and
collateral flow patterns are known
to
affect flow velocities
and
should
be
taken
into account
when interpreting
TCD
studies.
The
effect
of
nimodipine
on
TCD
findings is reportedly minimal
but
has
not
been
fully investigated.
85
Most evidence
on
the
accuracy of
TCD
in detecting
cerebral vasospasm is based
on
patients with aneurysmal
subarachnoid hemorrhage.
There
is sufficient available
data
to
evaluate the accuracy
of
TCD
and to comment
on
its clinical usefulness.
65
·
84
·
86
-
88
It
should
be
noted
that
there
is
an ongoing controversy regarding the use-
fulness
of
TCD
in assessing distal segments of the intra-
cranial vasculature
89
and
in providing
data
that
alter treat-
ment
decisions.
90
·
91
Most
reports,
however,
indi-
cate
that
TCD
is useful in managing patients with vaso-
spasm.65·67·69·76·78·84·86--88·92-95
The
use of flow velocity ra-
tios improves the test's accuracy.
88
·
96
The
reported
sensitivity, specificity, and positive
and
negative predictive values for the detection
of
basal vessel
narrowing by
TCD
compared
to
cerebral angiography are
generally high.8
6--
88
·
96
Several
TCD
indices have
been
pro-
posed
to
improve accuracy. Available studies support the
use
of
TCD
as a useful tool in establishing the diagnosis
of
vasospasm. Transcranial
Doppler
sonography
is
most re-
liable in detecting vasospasm
of
the
MCAs (sensitivity
of
75% to 90%, specificity exceeding 90%) as well as the
vertebral
and
basilar arteries (sensitivity
of
77%, specific-
ity
of
79%
).
It
is
not
reliable in detecting vasospasm of the
anterior
cerebral arteries (sensitivity
of
15%, specificity
exceeding 95%) because
of
collateral flow patterns, and is

generally
not
useful for detecting distal vasospasm in
the
vertically oriented branches
of
the intracranial arteries dis-
tal
to
the basal cisterns.
87
·
89
·
97
These results may improve
with the introduction of TCDS.
98
Transcranial
Doppler
sonography can also be useful in
monitoring the temporal course
of
vasospasm. Daily
TCD
examinations showing rapid increases in velocity values,
especially during days 4
to
10 after hemorrhage, may iden-
tify patients at
an
increased risk for developing delayed
ischemic neurologic deficits.
92
·
93
For
optimal clinical care
in patients with subarachnoid hemorrhage, it has
been
rec-
ommended
that
cerebral blood flow studies
be
used
to
supplement
TCD
examinations
to
identify flow reductions
caused by vasospasm.
65
·
94
Based
on
the available class
II
evidence,
TCD
can
be
given a type B recommendation as being
of
established
value for the detection
of
cerebral vasospasm, with certain
limitations which are
inherent
to
the technology
and
the
portion
of
the vasculature affected by vasospasm.
ARTERIOVENOUS MALFORMATIONS
Arteriovenous
malformations
(A
VMs)
are
congenital
anomalies
of
the cerebral vasculature characterized by a
direct communication between arteries
and
veins, with
an
absence
of
vasomotor
arterioles
and
capillaries.
Their
clinical significance relates to their propensity to cause
intracerebral hemorrhage, seizures,
or
both. Transcranial
Doppler
sonography has
been
reported
as capable
of
de-
tecting A VMs.
99
-
105
When
compared
to
cerebral angiog-
raphy,
TCD
has little to offer as
the
initial
or
confirmatory
diagnostic tool.
Despite
this shortcoming, it
provides
physiologic information about A VMs
that
cannot
be
ob-
tained by
other
noninvasive means.
It
is therefore impor-
tant
to
examine
the
role
that
TCD
may play in
at
least
three clinical contexts.
First, A VMs are often supplied by distinct high-flow
shunts and are characterized by diminished
or
absent va-
somotor reactivity. These features
make
it possible for
TCD
to differentiate A
VM
feeding arteries from healthy
cerebral
vessels.
99
-
102
Second,
advanced
treatment
of
A VMs includes endovascular embolization, radiotherapy,
and surgical removal, and the criteria by which A VMs are
selected for a specific therapy continue
to
evolve. Trans-
cranial
Doppler
sonography may assist in discriminating
among different types
of
A VMs
10
6-
111
and
evaluating their
responses to these treatments.
112
-
118
Finally,
TCD
may
predict
the
hemorrhagic risk of individual A VMs based
on
their
hemodynamic
characteristics.
107
·
110
It
should
be
noted, however, that this remains controversial
and
re-
quires further investigation.
103
·
107
·
110
The
sensitivity and specificity of
TCD
in detecting me-
dium-sized and large A VMs
is
high,
but
it decreases when
evaluating small (
<2.5 em) malformations.
99
-
102
Hence,
early studies assessing the usefulness of
TCD
in the evalu-
ation of A VMs often focused
on
physiologic changes
that
characterize AVMs.
102
-
105
Once
the
AVM
feeding arteries
are identified,
TCD
is capable
of
assessing their vasoreac-
tivity
and
hemodynamic
characteristics.
104
·
108
·
110
Some
studies have addressed
treatment
selection as well as
the
longitudinal effects
of
therapy_l12
-
118
Evidence for
TCD
assessment
of
A VMs is based
on
clinical studies
of
restricted
patient
populations. Most
of
these studies
do
not
have
random
controls. Transcranial
Doppler
sonography was
not
consistently
compared
to
an-
giography in a blinded fashion
to
assess its accuracy.
It
should also
be
noted
that
although angiography is still
considered the gold standard, it is
not
without limitations.
Thus,
the
quality
of
evidence can
be
rated
as class III.
At
the present time
TCD
is
considered
an
unacceptable
screening test for
the
initial evaluation
of
patients sus-
pected
of
having A VMs (type D).
It
is effective in identi-
fying arteries
that
supply medium-sized
or
large A VMs
and in assessing the effects
of
staged embolization and/or
resection
of
these malformations (type C).
The
use
of
TCD
to
predict
the
risk
of
hemorrhage
from
an
individual
A
VM
is considered investigational (type C).
PERIOPERATIVE
AND
PERIPROCEDURAL
MONITORING
Carotid endarterectomy, cardiac surgery, cerebral angiog-
raphy,
and
angioplasty are associated with complications,
most
often
stroke.
Although
several
monitoring
tech-
niques have
been
used in
an
effort to reduce complications
from these procedures,
the
use
of
these techniques re-
mains controversial.
Transcranial
Doppler
sonography
monitoring during
the
procedures allows continuous real-
time
readout
of
velocity changes in the basal cerebral ar-
teries. Because the diameter
of
the
MCA
Ml
segment
remains relatively stable after the administration
of
anes-
thetic agents, changes in flow velocity reflect changes in
cerebral blood flow as long as fluctuations in arterial blood
pressure
and
arterial
C0
2
content
are
small. Transcranial
Doppler
sonography velocities correlate well with cere-
bral blood flow estimated by thermodilution
119
and
133
Xe-
non120
measurements. Thus,
MCA
velocity changes can
be
used
to
monitor changes in cerebral blood flow during
most stages of carotid endarterectomy
and
coronary artery
bypass grafting
(CABG).
In
addition,
TCD
has
the
unique
ability
to
detect MS
that
correspond
to
particulate emboli
or
microbubbles.
Carotid
Endarterectomy.
The
MCA
on
the side
of
ca-
rotid endarterectomy is the artery usually
monitored
by
TCD
because it is most directly affected by carotid blood
flow. Flow velocities change with a predictable
pattern
during
the
course
of
surgery.
During
carotid
cross-
clamping they typically decrease
and
in some cases drop to
zero.121 A large
decrement
in velocities
at
this stage sug-
gests hypoperfusion
or
inadequate
collaterals,
and
it has
Babikian
et
al: TCD
Ultrasonograpy
Update
105

been
considered an indication for
the
surgeon
to
insert a
shunt
or
to
pharmacologically raise
the
arterial blood pres-
sure
to
avoid ischemia. A precise
percent
decrease in flow
velocity from baseline
or
a velocity threshold
that
predis-
poses
to
cerebral ischemia has
not
been
established. How-
ever, a review
of
a large registry
of
patients found an
increased perioperative stroke rate in those with initial
decrements in
MCA
velocity
to
less
than
15%
of
baseline
values
and
with persistent decreases
to
less
than
40%
of
baseline after 5 minutes.
122
Another
study
of
240 carotid
endarterectomies suggested
that
a velocity reduction to
less
than
35%
of
baseline values
or
an absolute
mean
velocity
of
less
than
20 cm/s indicated
poor
collaterals.
123
Flow velocity changes also correlate with stump pressure
measurements following cross-clamping.
124
Transcranial
Doppler
sonography monitoring during shunting can also
be
useful in detecting kinking
or
thrombosis
of
the shunt.
The
hyperperfusion syndrome has
been
identified as a
cause
of
postoperative morbidity following carotid endar-
terectomy. Transcranial
Doppler
sonography monitoring
during the postoperative period helps detect this phenom-
enonY5
Increases in
MCA
velocities
to
more
than
150%
of preclamp values may identify risk
of
encephalopathy
or
hemorrhage.
Not
all ischemic events
that
occur during carotid end-
arterectomy are associated with
MCA
velocity changes.
This is expected, because it
is
believed
that
the
majority
of
perioperative events are embolic. Microembolic signals
most commonly occur during the dissection phase
of
sur-
gery and during shunting
and
unclamping_l26--
129
The
num-
ber
of
MS during dissection correlates best with new isch-
emic lesions seen
on
MRI
127
and
postoperative cognitive
deterioration.
129
Transcranial
Doppler
sonography has
confirmed
that
embolism from the operative site during
carotid
endarterectomy
is
the
principal cause
of
cerebral
infarction.
130
Information provided by
TCD
monitoring
during carotid endarterectomy helps
the
surgeon take ap-
propriate measures
and
reduce
the
risk of perioperative
strokeY
0
The
presence
of
more
than
50 MS/h during
the
early postoperative
phase
is
reported
to
be
predictive for
the development
of
ipsilateral focal cerebral ischemia in
some
studiesY
1·
132
This has led some investigators
to
treat
sustained postoperative embolism with dextran infusions.
Transcranial
Doppler
sonography monitoring is consid-
ered
possibly useful during
and
after carotid endarterec-
tomy.
It
can
be
given a positive recommendation (type
C)
based
on
class
III
evidence.
Heart
Surgery. Neurologic complications occur in
up
to
15%
of
patients undergoing
CABG,
and
include cerebral
infarction
and
encephalopathyY
3
-
135
Postoperative be-
havioral
abnormalities
noted
by
family
members
and
documented by neuropsychologic testing occur in as many
as 70%
of
patients.
106
Journal
of
Neuroimaging
Vol10
No
2
April
2000
The
mean
MCA
velocity typically decreases from a pre-
operative baseline after induction of anesthesia. This
is
followed by further changes during the initiation
of
car-
diopulmonary bypass (CPB)
136
·
137
and
after aortic cross-
clamping.U8
During
rewarming, velocities increase to
baseline levels
or
higher.
136
Velocity changes correlate
best with
temperature
and
arterial
C0
2 content.
136
·
138
In
most cases, velocity changes remain within a relatively
narrow range
and
do
not
correlate with neurologic com-
plications even in patients with coexisting carotid artery
disease.
139
Severe reductions in velocity during cardiac
surgery suggest cerebral hypoperfusion.
In
such cases,
augmentation
of
blood pressure may improve cerebral
blood flow. A sudden decrement in
MCA
velocity
on
one
side without change in blood pressure
or
other
physiologic
variables suggests an embolic event to the ipsilateral hemi-
sphere.
No
systematic attempts have
been
made to corre-
late velocity changes during cardiac surgery with neuro-
logic outcome.
In
addition
to
changes in velocity,
TCD
allows the mea-
surement
of
cerebral reactivity. During moderately hypo-
thermic CPB, the
C0
2 reactivity
is
typically preserved
but
the
MCA
velocity varies directly with cerebral perfusion
pressure, indicating loss
of
autoregulation.
140
There
are no
published studies correlating reactivity abnormalities to
neurologic outcome.
Cerebral microemboli manifested as MS during
TCD
monitoring are frequently
encountered
during
CABG.
Microembolism occurs in all phases
of
surgery,
but
is par-
ticularly frequent during aortic cannulation, aortic cross-
clamping,
and
removal
of
clamps.
141
High numbers of MS
have
been
correlated with postoperative neuropsychologic
abnormalities,
141
-
143
but
more
investigations are
needed
to confirm this finding. Although no association between
intraoperative MS
and
perioperative stroke has
been
re-
ported, surgeons using
TCD
during
CABG
respond to the
finding
of
MS by applying preventive measures.
The
com-
position
of
presumed emboli causing MS
is
also uncertain.
Particulate emboli, air microbubbles,
or
turbulent flow
may all occur during different phases of the procedure.
Transcranial
Doppler
sonography monitoring is consid-
ered
investigational
during
cardiac surgery (class III,
type C).
Cerebral
or
Cardiac
Angiography.
Cerebral angiogra-
phy
and
cardiac catheterization are occasionally compli-
cated by stroke, presumably because of dislodged embolic
material from the cardiac chambers, aortic plaques,
or
the
catheter
tip itself. Continuous monitoring during these
procedures allows detection of MS.
In
one report, MS
were detected during injection
of
contrast material and
during
catheter
manipulation
but
did
not
occur spontane-
ously.144
There
were no neurologic complications in the 42

patients studied. Thus, no conclusion regarding the rela-
tionship
of
MS
to
periprocedural stroke was possible.
Monitoring during angiography demonstrates
an
early
decrease followed by an increase in the
mean
MCA
ve-
locity ipsilateral to a bolus injection
of
contrast.
145
A ve-
locity increase can also occur in the contralateral MCA.
The
relationship
of
these changes
to
complications
of
an-
giography is
not
clear.
145
There
are no standards to determine the accuracy
of
TCD
during cerebral
or
cardiac angiography.
Data
sup-
porting the use
of
TCD
come from small case series (class
III). Transcranial
Doppler
sonography is considered an
investigational technique for monitoring during angiogra-
phy (type D).
INCREASED INTRACRANIAL
PRESSURE
AND
CEREBRAL
CIRCULATORY ARREST
Waveform changes on
TCD
can signal significant changes
in ICP. Transcranial
Doppler
sonography is particularly
useful in monitoring
ICP
in patients with bleeding diathe-
ses and
other
contraindications to invasive
ICP
monitor-
ing. Pulsatility changes
do
not
occur until cerebral perfu-
sion pressure
is
less
than
70
mm
Hg.
The
earliest sign of
increased
ICP
is
increased pulsatility, accompanied by
progressive reduction in diastolic velocities. Eventually,
the
mean
velocity begins to decrease.
146
,1
47
The
mecha-
nism behind these changes is believed
to
be
increased vas-
cular resistance.
An
extreme
ICP
elevation produces zero
diastolic velocity followed by an alternating flow
pattern
with retrograde diastolic flow. Eventually, the retrograde
diastolic flow disappears and only small systolic peaks per-
sist. This may be followed by no waveforms
at
all. Corre-
lation with angiography suggests
that
cerebral blood flow
disappears
at
the stage
of
reverberating flow.
146
·
147
These
changes are
not
specific and can be mimicked by any vasa-
constricting stimulus. Measurements
of
the absolute
ICP
value
cannot
be
performed
with
TCD,
but
changes in
TCD
parallel changes in ICP, assuming a constant arterial
C0
2 content and a constant degree
of
distal vasoconstric-
tion.
Brain death
is
a clinical diagnosis
that
can
be
supported
by
TCD
evidence
of
absent cerebral blood flow (flow ve-
locity)
at
all insonation sites.
The
specificity
of
TCD
for
the diagnosis
of
brain
death
is imperfect as absence
of
flow
may
be
transient, especially in conditions of extreme in-
tracranial hypertension.
148
·
149
Thus, monitoring should
be
maintained for
at
least
30
minutes
at
a normal body tem-
perature. Some patients with
TCD
findings
of
cerebral
circulatory arrest in the internal carotid artery circulation
are
not
brain dead
but
actually have minimal residual
brainstem activity.
The
advantage
of
using
TCD
in this
setting resides in the ability
of
the technique to detect
intracranial flow in patients with loss
of
brainstem func-
tions resulting from localized brainstem lesions
and
in sub-
jects with sedative
or
paralytic agent exposure
that
renders
clinical examination
of
brainstem signs difficult. Transcra-
nial
Doppler
sonography is useful in shortening
the
ob-
servation period before organ harvest,
and
it can be used
as a confirmatory laboratory test
to
the clinical diagnosis
of
brain death.
150
Doppler
sonographic criteria for cere-
bral circulatory arrest have
been
established.
151
Transcra-
nial
Doppler
sonography can
be
given a positive recom-
mendation (type B) as being
of
established value for the
evaluation
of
cerebral circulatory arrest associated with
brain death, based
on
class
II
evidence.
MIGRAINE
Flow velocities in the basal cerebral arteries drop during
migraine
152
-
156
and
cluster
headache
attacks.
157
Velocities
on
the
painful side
of
the
head
are approximately 10%
to
30%
lower
than
on
the nonaffected side.
152
·
153
·
156
When
taken
with
data
from
SPECT
studies showing no signifi-
cant changes in cerebral blood flow,
152
these findings sug-
gest
that
vasodilatation
of
basal cerebral arteries occurs
during migraine attacks.
Flow velocities during
nonheadache
days can
be
nor-
mal152·154·158
or
increased.
159
·
160
A difference between mi-
graineurs with
and
without
aura
regarding cerebrovascu-
lar vasoreactivity has also
been
reported.
161
·
162
Reactivity
decreases during migraine attacks.
163
Transcranial
Dopp-
ler sonography studies also show
that
sumatriptan has a
vasoconstrictive
effect
on
intracranial
basal
arter-
ies.152·164
This dose-dependent effect coincides with the
resolution
of
headaches.
The
effects
of
other
antimigraine
medications have also
been
studied.
154
·
159
·
165
Although
migraineurs have
been
extensively studied
with
TCD,
most published reports have considerable limi-
tations often caused by small study populations. Transcra-
nial
Doppler
sonography is considered
an
investigational
test for migraine
at
the
present
time. Based
on
the avail-
able class
III
evidence, it is given a negative, type D rec-
ommendation.
SICKLE
CELL
DISEASE
Cerebral infarction is a
common
complication
of
sickle cell
disease.
The
arterial vasculopathy in this condition con-
sists
of
a fibrous proliferation
of
the intima, which causes
stenosis.
It
can
be
monitored
by
TCD
identification
of
lesions
at
the distal internal carotid artery
and
at
proximal
segments
of
the
MCA
and
anterior
cerebral artery. A
mean
maximum velocity
of
200
cm/s
or
greater
in these
arterial segments is strongly associated with an increased
risk
of
stroke in neurologically asymptomatic children.
166
Periodic
red
blood cell transfusions in these patients re-
duce the risk
of
stroke.
167
-
169
A
September
19, 1997,
Clini-
cal
Alert
from the National
Heart,
Lung,
and
Blood In-
stitute recommended
that
sickle cell patients between the
ages
of
2
and
16
receive
TCD
screening.
167
Children
whose screens show normal results should be restudied
Babikian
et
al: TCD
Ultrasonograpy
Update
107

approximately every 6 months.
167
-
169
A
mean
MCA
ve-
locity
of
115 cm/s
is
believed
to
be normal in this popula-
tion. Based on class I evidence,
TCD
can
be
given a posi-
tive, type A recommendation as being effective for
the
evaluation
of
patients with sickle cell disease.
MENINGEAL
INFECTION
Angiography in patients with meningeal infection often
reveals obstruction
of
arteries
and
veins,
and
blood flow
studies have
demonstrated
reduced cerebral
blood
flow
with preserved vasomotor reactivity. Inflammatory changes
occur in the basal segments
of
large intracranial blood
vesselsP
0 Transcranial
Doppler
sonography studies have
demonstrated increased flow velocities in
the
acute stages
of
bacterial meningitis
170
·
171
and
cysticercotic
arteritisP
2
These changes
do
not
appear
to
occur with viral meningi-
tisP0
The
effect
of
increased intracranial pressure
on
flow
velocity in this
context
remains
insufficiently studied.
Transcranial
Doppler
sonography can
be
given a type C
recommendation as being possibly useful for
the
evalua-
tion
of
patients with meningeal infection
based
on
class
III
evidence.
CEREBRAL VEIN THROMBOSIS
The
straight sinus
and
the
deep
cerebral veins can be im-
aged in adults with
TCDS
as well as with conventional
TCD.
173
-
175
The
inferior sagittal sinus is less frequently
identifiable, and the superior sagittal sinus
is
occasionally
identified with
the
aid
of
an
echo-enhancing agent.
173
·
176
Angle-corrected,
mean
peak
systolic flow velocities within
cerebral veins
and
sinuses are 10-25 cm/s. Patients with
thrombosis display increased velocities,
177
often indicating
the
presence of collateral flow.
Transcranial color duplex ultrasonography
of
the ve-
nous system still poses significant technical difficulties.
Low flow velocities, as are found in cerebral veins and
sinuses, may
be
difficult
to
detect
during
routine
in-
sonation.
In
addition, signal intensity is often marginal,
necessitating
the
administration
of
ultrasound
contrast
agents.
The
use
of
transcranial ultrasound
to
image the
venous system
and
detect cerebral venous thromboses
is
considered investigational. Although
supported
by class
III
evidence, it is given a negative, type D recommenda-
tion.
DISEASES
OF
THE
BRAIN PARENCHYMA
The
ultrasonographic examination
of
brain parenchyma is
an
established
method
in perinatology.
In
adults, in whom
insonation through
the
fontanels
is
no
longer possible, fine
resolution images
of
brain
parenchyma
cannot
be
ob-
tained because
of
the
low-frequency, 2-MHz probe.
In
spite of these limitations,
TCDS
adds valuable informa-
tion in some parenchymal disorders.
In
healthy controls,
the
substantia nigra is either poorly
108
Journal
of
Neuroimaging
Vol
10
No
2
April
2000
visualized
or
not
detectable by TCDS.
In
patients with
severe Parkinson's disease, the echogenicity
of
the sub-
stantia nigra is distinctly increased.
It
has
been
suggested
that
TCDS
findings reflect the severity of gliosis and, pos-
sibly,
degenerationP
8 Patients with unipolar depression
have
reduced
echogenicity
of
the
raphe
nuclei of
the
brainstem.
179
·
180
Contrast-enhanced
TCDS
images
of
highly malignant
gliomas display atypical arterial and venous
Doppler
spec-
tra
indicative of the highly vascular nature of the tumors.
These atypical patterns are less common in low-
or
inter-
mediate-grade gliomas. 181
The
ability
to
monitor the pat-
tern
of
vascular supply
of
intracranial tumors may have
diagnostic as well as therapeutic relevance.
182
Transcranial
color duplex ultrasonography can also
be
useful intraop-
eratively, as
the
accuracy of
open
neurosurgical biopsies
may
be
improved by using B-mode ultrasound.
183
The
technique
is
especially
useful
during
glioma
sur-
gery.184·185
Based
on
the
available, class
III
evidence,
TCDS
is considered investigational in imaging brain pa-
renchymal brain disorders, and is given a positive, type C
recommendation.
TRAUMA
Cerebral ultrasonography may identify the site and extent
of brain injury in newborns.
186
A study of head-injured
children has disclosed
an
almost 100% correlation be-
tween brain CT,
MRI,
and
ultrasonographic findings.
187
Similar favorable results have
not
been
reproduced in
adults,
and
published studies remain preliminary.
Intracranial flow velocities are often increased within
72 hours
of
traumatic intracranial hemorrhage,
188
and may
be indicative
of
vasospasm in some patients.
It
is
still un-
clear
whether
impaired autoregulation detected after head
trauma
is
of
prognostic significance.
189
.1
90
The
effect
of
increased
ICP
on
TCD
measurements
is
reviewed else-
where in this article.
Based
on
the
available, class
III
evidence, the use
of
conventional
TCD
is considered of doubtful value in de-
tecting traumatic cerebral contusion and intracranial hem-
orrhage.
It
is given a negative, type D recommendation.
FUNCTIONAL DOPPLER SONOGRAPHY
Transcranial
Doppler
sonography may
be
used to assess
flow velocity changes induced by focal brain activation.
Changes
of
flow velocity elicited by hemisphere-specific
cognitive tests are detectable by unilateral
or
bilateral re-
cordings.191·192
Transcranial
Doppler
sonography can also
be
used in neuropsychologic testing
of
brain-injured pa-
tients and, in conjunction with functional MRI, may be
helpful in the evaluation
of
patients with dementia
193
or
cerebrovascular disease.
194
Functional
TCD
can be used
to
assess hemispheric language dominance.
195
Monitoring
of
patients
with
sleep
apnea
has
revealed
significant
changes in intracranial flow.
196

Table
1.
Summary
of
the effectiveness
of
transcranial
Doppler
for specific disorders
Application
Rating
Quality
of
evidence
Strength
of
recommendation
Ischemic cerebrovascular disease Established
Subarachnoid hemorrhage Established
Arteriovenous malformations Established
Perioperative monitoring Promising
Periprocedural monitoring Investigational
Cerebral circulatory arrest Established
Migraine Investigational
Sickle cell disease Effective
Meningeal infection Promising
Cerebral vein thrombosis Investigational
Experience with functional
Doppler
ultrasonography
remains limited.
There
are often
no
reference tests
to
as-
sess the quality
of
evidence regarding
the
use
of
TCD
in
this context. Published
data
are
rated
class III. Functional
TCD
is considered an investigational application
of
the
technology (type C).
ECHO-ENHANCED SONOGRAPHY
Echo-enhancing agents increase
the
intensity of
TCD
and
TCDS
signals.
197
~zoo
Published phase
II
and
III
trials have
addressed contrast dose,
method
of
administration,
and
diagnostic reliability.
197
•
198
Not
only
do
these agents per-
mit insonation of
the
basal cerebral arteries in nearly all
patients, they allow imaging
of
smaller,
more
peripheral
arterioles and veins. Although contrast agents are admin-
istered intravenously, their side effects have
been
lim-
ited.Z00
Contrast agents have
not
yet
been
released for routine
TCD
use in the
United
States. Some are available in
Eu-
Appendix
1.
class
II
class
II
class III
class III
class III
class II
class
III
class I
class
III
class
III
type B
type B
type C
type C
typeD
type B
typeD
type A
type C
typeD
ropean
countries.
Based
on
the available class
III
evi-
dence, the use
of
contrast agents during
TCD
testing is
considered investigational (type C).
TECHNICAL CONSIDERATIONS
Information provided by
TCD
can clearly
be
useful in the
management
of
patients with specific neurologic disorders
such as ischemic cerebrovascular disease
and
subarach-
noid
hemorrhage
(Table
1).
The
noninvasiveness
and
availability
of
TCD
in
the
intensive care unit constitute
clear advantages,
and
an
increasing body
of
literature sup-
ports its use during some surgical procedures.
Transcranial
Doppler
sonography has limitations, and
several technical considerations should
be
taken
into ac-
count
when interpreting its findings.
For
example, flow
velocity estimates provided by
TCD
assume a 0-30° angle
of
insonation.
In
approximately 25%
of
insonated arteries,
however,
the
insonation angle exceeds 30°
?01
Because
most instruments
do
not
allow a direct
measurement
of
Definitions
Effective. Producing a desired effect
under
conditions
of
actual use.
Established. Accepted as appropriate by the practicing medical community for
the
given indication in
the
specified patient
population.
Promising. Given
current
knowledge, this technology appears to be appropriate for
the
given indication in the specified patient
population.
As
more
experience and long-term follow-up are accumulated, this interim rating will change.
Investigational. Evidence
is
insufficient to determine appropriateness; warrants further study.
Use
of
this technology for given
indication in the specified patient population should
be
confined largely to research protocols.
Quality
of
Evidence Ratings
Class
I.
Evidence provided by
one
or
more
well-designed, randomized, controlled clinical trials.
Class II. Evidence provided by
one
or
more well-designed clinical studies, e.g., case control,
cohort
studies.
Class III. Evidence provided by
one
or
more
expert opinions, nonrandomized historic controls,
or
case reports.
Strength
of
Recommendations Ratings
Type A. Strong positive recommendation, based on class I evidence
or
overwhelming class
II
evidence when circumstances
preclude randomized clinical trials.
Type B. Positive recommendation, based on class
II
evidence.
Type
C.
Positive recommendation, based on strong consensus
of
class III evidence.
Type D. Negative recommendation, based on inconclusive
or
conflicting class II evidence.
Type E. Negative recommendation, based on evidence
of
ineffectiveness
or
lack
of
efficacy, based on class II
or
class I evidence.
Modified from Assessment
of
Brain SPECT.
Report
of
the Therapeutics and Technology Assessment Subcommittee
of
the American Academy of
Neurology. Neurology
1996;46:278~285.
Babikian
et
al: TCD
Ultrasonograpy
Update
109

the angle, this introduces
an
error
in flow velocity values
obtained by
TCD.
Measurement
of
the angle is clearly
improved by using TCDS, and angle-corrected velocities
are higher
than
uncorrected ones.201 -
203
In
addition, inad-
equate
temporal
bone
windows limit
transcranial
in-
sonation
in
5%
to
20%
of
patients.
204
Other
imaging
methods
have to
be
used in these patients.
Like
other
ultrasound techniques,
TCD
is
operator
de-
pendent.
For
this reason, it should
be
performed
and
in-
terpreted
by those with
adequate
background, training,
and
experience with this diagnostic modality. Recommen-
dations for certifying physicians who
perform
or
interpret
TCD
studies have
been
made
by the American Academy
of Neurology and the
American
Society of Neuroimaging,
and guidelines for training during neurology residency
have
been
endorsed
by these organizations
and
the Asso-
ciation of University Professors in Neurology
205
. Neuro-
sonology certification for physicians
and
neurovascular
laboratory accreditation are recommended.
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