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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


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


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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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


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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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.

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3. Hallett RL, Fleischmann D. Tools of the t rade for

CTA: MDCT scanners and contrast medium injection

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4. Bash S, Villablanca JP, Jahan R, et al. Intracranial

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