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28 APPLIED RADIOLOGY www.appliedradiology.com April 2010
There has been increasing reliance
on computed tomography angi-
ography (CTA) and magnetic
resonance angiography (MRA) for eval-
uation of the intracranial vasculature inpatients suspected of acute stroke. Digi-
tal subtraction angiography (DSA),
while traditionally the gold standard, is
invasive and associated with potential
complications. Further, DSA is not read-
ily accessible anytime day or night. In
the acute stroke setting, DSA is typically
reserved for patients undergoing thera-
peutic intervention with intra-arterial
thrombolysis or embolectomy.
The purpose of this article is to com-
pare the diagnostic accuracy of CTA,
MRA and DSA for evaluation of the
intracranial circulation in patients with
stroke. We will discuss the technical
parameters and potential artifacts that
must be understood so that an accurate
interpretation of each modality can be
made. We will structure our discussion
around an illustrative clinical case of a
patient who underwent CTA, MRA and
DSA within a 24 hour time frame.
Clinical case
A 62-year-old man presented to theemergency room with acute onset
diplopia, diffuse weakness and vomit-
ing. Given the concern for posterior-
circulation ischemia, a CT/CTA was
requested and performed within the first
hour of symptom onset (Figure 1). Ourstandard CT stroke protocol included
64-slice CTA from the base of the heart
through the cerebral vertex, as well as
delayed contrast-enhanced head CT
obtained approximately 1 min after CTA
(without the need for additional contrast
material because contrast has already
been injected for CTA). In this patient,
the initial CTA demonstrated lack of
opacification of the bilateral distal verte-
bral arteries and proximal basilar artery,
while the delayed contrast-enhanced
images demonstrated normal opacifica-
tion of the left distal vertebral artery and
proximal basilar artery.
Based on the clinical and imaging
findings, the patient received IV tissue
plasminogen activator (tPA) and
underwent 3-dimensional time of flight
(TOF) intracranial MRA approxi-
mately 4 hours following CTA to
reassess his vasculature after throm-
bolysis (Figure 2). The MRA demon-
strated absence of flow-related
enhancement in the bilateral distal ver-tebral and entire basilar artery, which
was discrepant with the initial CTA,
especially considering that there was
no change in the patients clinical
examination and no evidence of neuro-
logic deterioration between initial CTA
and follow-up MRA.
The patient then underwent DSA the
following day, approximately 24 hours
after the initial CTA, to reevaluate his
vasculature (Figure 3). Again, there
was no neurologic deterioration at the
time of the study. DSA demonstrated
occlusion of the right vertebral artery,occlusion of the left vertebral artery
proximal to the posterior inferior cere-
bellar artery (PICA), reconstitution of
the vertebrobasilar confluence distal to
the PICA, and occlusion of the remain-
der of the basilar artery with filling of
the basilar tip via the posterior commu-
nicating artery.
The important point about this case is
that CTA, contrast-enhanced CT, MRA
and DSA obtained within 24 hours of
each other, and without any change in
the patients clinical status, demon-
strated very different findings. Both
MRA and DSA appeared to demon-
strate occlusion of the basilar artery, as
well as occlusion of the bilateral distal
vertebral arteries. However, most of the
basilar artery appeared patent on CTA,
which showed occlusion limited to the
bilateral distal vertebral arteries and
only the most proximal aspect of the
basilar artery. Delayed contrast-
enhanced CT, in turn, appeared to
demonstrate patency of the distal leftvertebral and entire basilar artery.
One explanation for the discrepancy
is that the patients basilar artery
thrombosed between the time of CTA
and MRA/DSA. However, the absence
of any neurological changes between
examinations, as would be expected
with a typically devastating basilar
artery occlusion, weighs against this
Intracranial vascular imaging:Pearls and pitfalls
Jane J. Kim, MD, and Max Wintermark, MD
Dr. Kim is Assistant Professor of Clini-cal Radiology, Neuroradiology Section,San Francisco General Hospital, Uni-versity of California, San Francisco, CA;andDr. Wintermarkis Chief of the Divi-sion of Neuroradiology, University ofVirginia Health System, Charlottesville,VA.
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possibility. A more likely explanation for
the radiologic discrepancies lies in each
imaging modalitys way of capturing andassessing vascular flow.
Accurately reading intracranial CTA,
MRA and DSA to avoid pitfalls of inter-
pretation requires an understanding of the
technical parameters and potential arti-
facts underlying eachmodality.
CT angiography
The steady introduction of CT scanners
with ever - increas ing numbers
of detector rows has enabled greatercraniocaudal coverage at increasingly
faster scan times and better resolution.
Current state-of-the-art clinical stroke
CTA imaging relies on 64-slice CT, which
can scan from the aortic arch through the
intracranial vessels in a matter of 3 to 4 sec
at submillimeter isotropic resolution.
Acquisition time is significantly shorter
than single-slice CTA and 4-slice CTA.
This also means that less contrast material
is used and that contrast material has less
time to circulate within the vessels on 64-
slice CTA than on s ingle- or 4-s l iceCTA.
The entire duration of 64-slice CTA,
from start of contrast injection to con-
clusion of scanning, is approximately 20
to 25 sec. Because image acquisition only
requires 3 to 4 sec, most of this time is due
to the delay between when contrast is
injected and when scanning is begun.
This 15 to 20 sec delay is necessary to
ensure optimal opacif ication of the
cervical and intracranial vessels, and the
precise amount of delay can be deter-
mined by either timing bolus or by a bolus-tracking technique.13 We use the former
in our stroke CTA protocol.
The contrast-enhanced head CT that is
obtained after 64-slice CTA in our stroke
protocol utilizes contrast material that was
previously injected for the CTA, and does
not require that any additional contrast be
used. Contrast-enhanced CT is obtained
approximately 60 sec after CTA, which
a l lows addi t iona l t ime for contras t
material to circulate within the body as
compared with 64-sl ice CTA. In our
clinical example, CTA showed lack of
opacification of the distal left vertebral and
proximal basilar artery, both of which
appeared opacified on delayed contrast-
enhanced CT; only the distal right ver-
tebral artery appeared occluded on both
CTA and delayed CT. It is likely that slow or
low f low through the d is ta l le f t and
proximal basilar artery caused them to
appear occluded on fast 64-slice CTA,
while delayed contrast-enhanced CT
allowed for greater contrast circulation to
establish patency. Figure 4illustrat es a similar case in a different
patient.
These cases show that areas of sig-
nificantly delayed blood flow, secondary
to proximal vessel stenosis or distal
thrombosis, may be imaged by rapid 64-
slice CTA before adequate contrast
opacification has occurred. The scan can
30 APPLIED RADIOLOGY www.appliedradiology.com April 2010
INTRACRANIAL VASCULAR IMAGING
FIGURE 1. 62-year-old man with acute
diplopia, weakness and vomiting. Axial source
images from computed tomography angiogra-
phy (CTA) demonstrated no opacification and
apparent occlusion of bilateral distal vertebral
arteries (A) and very proximal basilar artery(B). The remainder of the basilar artery
demonstrated robust opacification (C) on
coronal reformatted images from CTA. Axial,
delayed contrast-enhanced CT (obtained
1 min after CTA) showed opacification of the
left distal vertebral artery (D) and proximal
basilar artery (E), both of which appeared
occluded on the initial CTA.
A B
C D
E
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www.appliedradiology.com APPLIED RADIOLOGY 31April 2010
outrun the contrast material circulation
in areas of slow flow, which are able to
opacify by the time delayed contrast-
enhanced CT is ob ta ined . Ar te r ia l
pseudo-occlusion is a type of flow artifactincurred by fast CT scanning.
CTA is reported to have high sensitivity
and spec if ic i ty for de tec t ion of
intracranial stenoses and occlusions. Two
recent studies indicated high sensitivity
and specificit y of CTA for detecting
moderate/severe stenosis (>97%); for
occlusion, the sensitivity and specificity
were even h igher (approaching
100%).4,5 The positive predictive value of
CTA for diagnosing intracranial occlu-
sion was 100% in these studies. Both of
these studies evaluated images obtainedon ear l ie r -genera t ion CT scanners
(single- and 4-slice in one study, 8- and 16-
slice in the other). The longer image-
acquisition time in these studies may help
to explain the lack of any false-positive
findings of occlusion. However, with
increased availability and adoption of
very fast CT scanning with 64-slice
scanners , the poss ib i l i ty of image
acquisition outrunning the contrast bolus
in cases of slow or altered flow should be
considered to prevent overestimating and
misdiagnosing occlusion.
MR angiographyPublished literature on 3-dimensional
intracranial TOF MRA shows equivalent,
to slightly lower, sensitivity of MRA as
compared with CTA for diagnosis of steno-
occlusive disease. Sensitivity ranges
between 70% to 100% for detecting
moderate/severe stenosis, and between
87% to 100% for occlusion; specificity is
close to 100% forboth.4,6,7
However, MRA has several importantlimitations. Unlike CTA and DSA, which
employ a contrast agent to directly image
the blood within a vessel, TOF MRA relies
on the magnetization of spins flowing into
an imaging slice and is prone to certain flow
artifacts.
TOF MRA uses a grad ien t echo
sequence and a rap id success ion of
INTRACRANIAL VASCULAR IMAGING
FIGURE 2. 3-Dimensional time of flight magnetic resonance angiography (MRA, A) obtained
on the same patient (4 hours after CTA and IV tissue plasminogen activator [tPA] treatment)
demonstrated absence of flow-related enhancement in the bilateral distal vertebral and entire
basilar artery. Compare this with findings on the initial CTA (B), which showed opacification of
the basilar artery. The patient did not have any change in neurological exam between initial
CTA and follow-up MRA.
FIGURE 3. Digital subtrction angiography (DSA) on the same patient after thrombolysis with
IV tPA and approximately 24 hours after initial CTA. Right vertebral artery injection, AP view
(A), showed occlusion of the distal right vertebral artery (arrow) and no opacification of the
basilar artery. Left vertebral artery injection, AP view (B), showed similar occlusion of the distal
left vertebral artery (black arrow) proximal to the posterior inferior cerebellar artery (PICA)
(white arrow). Left vertebral artery injection, lateral view (C), again demonstrated distal verte-
bral artery occlusion (black arrow), reconstitution of the vertebrobasilar confluence (arrow-
head) distal to PICA (white arrow), and occlusion of remainder of the basilar artery. Internal
carotid artery injection, lateral view (D), showed filling of the basilar tip (arrow) via the posterior
communicating artery. However, most of the basilar artery appeared occluded. This did not
change on late arterial and venous phases (not shown).
A B
A B
C D
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32 APPLIED RADIOLOGY www.appliedradiology.com April 2010
radiofrequency (RF) pulses to saturate
and suppress s ignal f rom stationary
background t is sue . Blood s i tua ted
outside the imaging plane remains rel-
atively unaffected by the pulses and flowsinto the imaging plane with magneti-
zation intact, generating br ight MR
signal (flow-related enhancement). As
such, ideal TOF imaging requires that
background tissue be completely sup-
pressed, and that inflowing blood be
completely unsaturated to generate
maximal s igna l . This is no t a lways
achievable in practice.
Background may not be entirely sup-
pressed with tissues that have short T1
relaxation times, such as fat, hematoma or
thrombus. These tissues quickly recovertheir longitudinal magnetization despite
the rapid succession of RF pulses, and are
able to generate bright MR signal that may
interfere with image interpretation and
obscurearteries.
Flowing blood may also experience
saturation effects by RF pulses. Slow
flowing blood spends more time within a
given imaging volume, making it sus-
ceptible to saturation by RF pulses. Areas
of slow flow due to proximal or distal
stenosis/thrombosis may have signal
loss and appear occluded.
Areas of turbulent blood flow near a
stenosis can also appear occluded due to
signal loss. The observation of signal loss
due to spin dephasing at stenoses with
turbulent flow has been described in both
phantom models and clinical studies.6,8,9
Finally, re trograde f low through
collateral vessels (in cases of vessel
occlusion) may not be depicted. TOF
imaging frequently uses a saturation band
to suppress all flow-related enhancement
opposite in direction to arterial flow,which typically eliminates unwanted
venous s ignal but may also have the
unintended consequence of suppressing
retrograde collateral flow. Knowledge of
these potential pitfalls is critical for
accurate interpretation of MRA. This is
espec ia l ly re levant to acu te s t roke
imaging , as pa t ien ts wi th ce rebra l
INTRACRANIAL VASCULAR IMAGING
FIGURE 4. 79-year-old woman with acute left hemiparesis. CTA shows apparent occlusion of
the entire right internal carotid artery (ICA) from its origin in the neck to the carotid terminus,
including at the skull base (A) and cavernous portion (B). Delayed contrast-enhanced images
(C,D), however, demonstrate opacification and patency of the right ICA at comparable levels.
DSA performed 4 hours after CTA confirms patency of the entire right cervical ICA (E) and
intracranial ICA (F). There is an abrupt occlusion of the proximal right M1 segment of the mid-
dle cerebral artery (black arrow, F). No other stenoses or occlusions were seen on the right,
and nonopacification of the r ight ICA on CTA was attributed to slow flow resulting from distal
M1 occlusion.
A B
C D
E F
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34 APPLIED RADIOLOGY www.appliedradiology.com April 2010
INTRACRANIAL VASCULAR IMAGING
thromboembolic disease are more likely
to have alterations in flow dynamics with
slow flow, turbulent flow and retrograde
flow.
Given the potential for artifactualsignal loss with TOF MRA in cases of
altered flow, it is not surprising that MRA
has a high false-positive rate for detection
of intracranial stenosis and occlusion.
Bash et al. compared CTA, MRA and
DSA findings in patients suspected of
acute cerebrovascular events, and found
that MRA had positive predictive values
of 65% for stenosis and 59% for occlu-
sion, compared with 93% and 100% for
CTA.4
In our clinical example, the basilar
ar tery appeared pa tent on CTA butoccluded on MRA obtained 4 hours later,
following thrombolysis. It is likely that the
distal vertebral artery occlusion resulted in
slow flow through the basilar artery and/or
re t rograde f low through pos te r ior
communicating arteries, causing arti-
factual signal loss on MRA. The complete
absence of any clinical deteriorati on
between CTA and MRA makes basilar
artery occlusion unlikely. MRA in this case
was falsely positive for occlusion, a
conclusion that can be reached given an
understanding of flow artifacts common
toTOF MRA.
Digital subtraction angiographyDSA is tradition ally considered the
gold s tandard for assessment of the
in tracran ia l vascula ture , and is the
modali ty with which CTA and MRA are
compared. In our clinical case, DSA
showed occlusion of most of the basilar
artery, with only minimal opacification
of the vertebrobasilar confluence and
basilar tip (the latter via posterior com-municating arteries on carotid injection).
However, CTA and delayed contrast-
enhanced CT had previously shown good
opacificati on of the basilar artery and
there had been no interval change in the
patients clinical status between CT and
DSA.
Given the CT f ind ings and s tab le
clinical examination, it is likely that DSAreflected a false-positive finding of
basilar ar tery occlusion. Bash et a l .
reported cases of false-positive DSA
findings of occlusion in 6 of 28 patients
(21%) who presented with symptoms of
acute stroke and underwent CTA, MRA
and DSA.4All 6 cases involved the pos-
terior circulation and occurred in very
slow- or low-flow states distal to a sig-
nificant stenosis, causing the vessel to
appear occluded on DSA but stenotic (and
otherwise opacified) on CTA. MRA was
also falsely positive for occlusion in 4 ofthese 6 cases. The conclusion of a false-
positive DSA finding was reached by
consensus re-evaluation of the data in all 6
cases.
The likely explanation for the dis-
crepancy between CTA and DSA findings
in our case, as well as the reported cases, lies
in differences in image acquisition time.
Angiographic runs are typically obtained
over 5 sec (at a 4 frames-per-second film
rate) and capture a single intracranial
circulation cycle. A single intracranial
circulation cycle, from opacification of the
carotid siphon to maximal opacification of
the cortical veins, lasts approximately 4 to 6
sec.10,11 While 64-slice CTA is extremely
fast, time from contrast injection to scan
completion is 20 to 25 sec and still leaves
more time for contrast material to circulate
through areas of severe stenosis than on
DSA. Delayed contrast-enhanced CT
obtained 60 sec after CTA allows even
more time for contrast material circulation
to opacify patent segments of the vessel
lumen on either side of stenotic or occludedareas.
ConclusionOur clinical case and l iterature review
highlights several important points about
intracranial vascular imaging with CTA,
MRA and DSA. C TA and MRA have
fairly high sensitivity/specificity for
detection of intracranial stenoses andocclusions, when using DSA as the gold
standard. However, flow-related arti-
facts can create the appearance of vas-
culature occlusion in each modality. The
mechanism by which th is occurs is
different.
In TOF MRA, signal loss can occur due
to slow, turbulent or retrograde flow; this
is well known and described extensively
in the literature. In contrast, flow-related
artifacts are not typically associated with
CTA or DSA. As acquis i t i on t imes
become increas ingly shor te r wi thgreaterdetector-row CT scanners, the
possibility of image acquisition out-
running the contrast bolus and producing
pseudo-occlusion must be considered.
Knowledge of these potential pitfalls is
usefu l and may prevent inaccura te
in te rpre ta t ion of imaging f ind ings
caused by dynamic alterations to blood
flow in patients with cerebrovascular
disease.
REFERENCES1. Bae KT. Test-bolus versus bolus-tracking tech-niques for CT angiographic timing. Radiology.
2005;236:369-370; author reply 370.
2. Cademartiri F, Nieman K, van der Lugt A, et al.
Intravenous contrast material administration at 16-
detector row helical CT coronary angiography: Test
bolus versus bolus-tracking technique. Radiology.
2004;233:817-823.
3. Hallett RL, Fleischmann D. Tools of the t rade for
CTA: MDCT scanners and contrast medium injection
protocols.Tech Vasc Interv Radiol.2006; 9:134-142.
4. Bash S, Villablanca JP, Jahan R, et al. Intracranial
vascular stenosis and occlusive disease: Evaluation
with CT angiography, MR angiography, and digital
subtraction angiography. AJNR Am J Neuroradiol.
2005;26:1012-1021.
5. Nguyen-Huynh MN, Wintermark M, English J, et al.
How accurate is CT angiography in evaluatingintracranial atherosclerotic disease? Stroke. 2008;
39:1184-1188.
6. Furst G, Hofer M, Steinmetz H, et al. Intracranial
stenoocclusive disease: MR angiography with
magnetization transfer and variable flip angle. AJNR