Journal of Computer Assisted Tomography
14(1):32-35, January/February
© 1990 Raven Press, Ltd., New York

MR Imaging of Basilar Artery Occlusion

Laurie Knepper, José Biller, Harold P. Adams, Jr., William Yuh, Tony Ryals, and

John Godersky

Abstract: We studied six patients with basilar artery occlusion using magnetic
resonance imaging. Two patients also had arteriography. All had signs consis-
tent with pontine infarction; three had a “‘locked in’’ syndrome. Magnetic
resonance imaging (five done within 24 h of onset of progression of neurologic
symptoms) revealed a linear structure of increased signal intensity in the pon-
tine cistern on Tl-weighted parasagittal images and absence of flow void phe-
nomena. Hyperintense signals at various brain stem levels corresponding to
the course of the basilar artery were noted on T2-weighted images. Multiple
parenchymal abnormalities were noted in the brain stem (six), cerebellum
(four), occipital lobes (two), and thalamus (two). Magnetic resonance per-
formed early in the course of basilar artery occlusion is a high-yield, safe
procedure and might eliminate the need for arteriography. Index Terms: Brain,
blood flow—Arteries, occlusion—Arteries, basilar—Magnetic resonance im-

aging.

Magnetic resonance (MR) imaging is superior to
CT in imaging posterior fossa structures. It pro-
vides better visualization of arteries, veins, and si-
nuses, particularly near bony structures (1,2). Mag-
netic resonance has been reported to be superior to
CT in detecting brain stem and cerebellar infarction
(-6). Although transcranial Doppler and dynamic
CT might prove in the future to be useful, definitive
diagnosis of acute vertebral or basilar artery occlu-
sion still requires cerebral arteriography.

We previously reported MR findings in three pa-
tients with basilar artery occlusion who were stud-
ied within 24 h after onset or latest progression of
neurologic symptoms (7). Occlusion of the basilar
artery, as shown by the absence of flow void on
axial MR, was seen as early as 90 min after the
onset of symptoms. To further evaluate the useful-
ness of MR in the diagnosis of basilar artery occlu-
sion, we describe the course and MR findings of six
additional patients and provide arteriographic cor-
relation of MR abnormalities.

From the Departments of Neurology (L. Knepper, J. Biller,
and H. P. Adams, Jr.), (Division of Neuroradiology) (W. Yuh
and T. Ryals), and Neurosurgery (J. Godersky), College of Med-
icine, University of Iowa, Iowa City, IA. Address correspon-
dence and reprint requests to Dr. J. Biller at Department of
Neurology, University of lowa Hospitals and Clinics, Iowa City,
TA 52242, U.S.A.

32

MATERIALS AND METHODS

Six patients with presumed basilar artery occlu-
sion were admitted between October 1987 and July
1988. All had complete neurological and medical
examinations by at least two of the authors. All had
CT within 24 h of stroke onset. The CT images were
obtained without contrast enhancement using a
Siemens DRH unit (Iselin, NJ, U.S.A.) with 8 mm
thick axial slices. Five had MR examinations within
24h, and the other within 72 h of onset of stroke. All
MR examinations were performed with a 0.5 T
Picker International superconductive unit (High-
land Heights, OH, U.S.A.). Parasagittal T1-
weighted [repetition time (TR) 350-600 ms, echo
time (TE) 20-26 ms] (T1W) and axial and coronal
T2-weighted (TR 2,000-2,300 ms, TE 80-100 ms)
(T2W), 5 mm thick images were obtained. Extra
pulse sequences with various slice thickness and
oblique angle cuts were also obtained in some
cases. Two patients had arteriography, including
arch and vertebral artery injections. Neuroradio-
logic studies were reviewed independently by a
neuroradiologist and two neurologists.

The MR criteria used to diagnose infarction, de-
scribed previously, included an area of decreased
signal intensity on T1W images and an area of in-
creased signal intensity on T2W images in the same
location (4-6). The MR criteria indicating basilar
BASILAR ARTERY OCCLUSION: MR FINDINGS 33

artery occlusion were the presence of a linear struc-
ture isointense with the brain stem in the pontine
cistern on T1W parasagittal or coronal images and
the lack of flow void phenomena with an area of
hyperintensity on T2W axial images corresponding
to the course of the basilar artery (7).

RESULTS

Six patients, four women and two men, were
evaluated. Their mean age was 64 (range 32-83)
years. All had arterial hypertension, two had diabe-
tes mellitus, two had coronary artery disease, and
three had a previous history of tobacco use. One
patient had two previous infarctions (one left occip-
ital, one left cerebellar) and one had a previous ep-
isode of amaurosis fugax. Table 1 summarizes the
clinical presentation and neuroradiologic studies of
these patients. One patient (Case 3) died 2 months
following hospital discharge. Postmortem examina-
tion was not obtained. The two patients who had
arteriography are described below.

CASE REPORTS

Case 4

A 32-year-old woman developed dysphagia, dysarthria,
bilateral visual loss, and decreased hearing over a 4 day
period, at which time she was transferred to the Univer-
sity of Iowa Hospitals. On admission she was obtunded
and mute, but able to follow simple commands. She had

a left gaze paresis, skew deviation, and impaired vertical
gaze with horizontal and rotatory nystagmus. Corneal re-
flexes were depressed bilaterally. She had bifacial pare-
sis, an absent gag reflex, and a flaccid right upper extrem-
ity. Muscle stretch reflexes were absent throughout and
she had bilateral Babinski signs. Both CT and MR were
done within 24 h and arteriography 5 days after stroke
onset. Magnetic resonance demonstrated bilateral basis
and brachium pontis and left cerebellar infarctions (Figs.
1 and 2). Angiography confirmed a basilar artery occlu-
sion (Fig. 3).

Case 5

A 63-year-old woman with a history of diabetes melli-
tus, coronary artery disease, congenital Duane syn-
drome, and right eye blindness presented after the sudden
onset of nausea, vomiting, vertigo, and right arm and leg
weakness. On examination she had a right relative affer-
ent pupillary defect, bilateral ptosis with decreased ocular
abduction, and spontaneous right beating horizontal
nystagmus. She had a right hemiparesis, hemisensory
deficit, and a right Babinski sign. The CT scans showed a
calcified sphenoid-sellar mass. Magnetic resonance, done
within 24 h of symptom onset, showed a suprasellar-
sphenoid mass, a left pontine infarct, and absence of flow
void in the area of the basilar artery (Figs. 4 and 5). Ar-
teriography done the day following admission showed a
basilar artery occlusion at the level of the anterior inferior
cerebellar artery (Fig. 6). Eight days after the onset of
symptoms, she had resection of her craniopharyngioma
tumor. Postoperatively she developed complex partial
seizures and progression of her neurologic deficits to a
flaccid asymmetric quadriparesis with bilateral Babinski
signs, thalamic infarction, and extension of her pontine
infarction.

TABLE 1. Clinical presentation and neurological studies of patients

Case no. Age (yrs)/sex Symptoms/signs CT MR Angiography

1 83/M “Locked in’” Old L. occipital and Bilateral pontine ND
syndrome cerebellar infarctions infarctions, BAO

2 76/F L. gaze and vertical R. occipital infarction R. occipital, L. ND
gaze paresis, pontine, L. cerebral
rotatory nystagmus, peduncle, and L.
L. hemiparesis, R. cerebellar infarction,
hemisensory deficit, BAO
dysmetria

3 65/M “Locked in’ Normal Bilateral pontine ND
syndrome infarctions, BAO

4 32/F L. gaze and vertical Normal Bilateral cerebellar BAO and
gaze paresis, pontine infarctions, occlusion of
nystagmus skew BAO L. vertebral
deviation, bifacial and
paresis, R. arm subclavian
monoplegia artery

5 6B3/F R. gaze paresis, Suprasellar mass L. pontine infarction, BAO at AICA
bifacial paresis, BAO level
absent gag,
quadriparesis, R.
hemisensory deficit

6 64/F “Locked in’’ Multiple infarction Infarction in both ND
syndrome thalamus, pons, and thalami, pons,

cerebellum

cerebellum, BAO

BAO, basilar artery occlusion; AICA, anterior inferior cerebellar artery.

J Comput Assist Tomogr, Vol. 14, No. 1, 1990
34 L. KNEPPER ET AL.

FIG. 1. Sagittal T1-weighted MR image shows faint isoin-
tense linear structure (arrow) along the course of the basilar
artery in the pontine cistern.

DISCUSSION

The basilar artery has a relatively constant
course, beginning at or slightly below the pon-
tomedullary junction and stretching the length of
the pons, tapering to its termination at the pons-
midbrain junction. Kubick and Adams (8), in their
classic paper, described 18 patients with basilar ar-
tery occlusion including postmortem examination
of the basilar artery and brain stem pathology.
Atherothrombosis was the main etiology of arterial
occlusion in these patients. Atherosclerosis of the

FIG. 3. Lateral digital intraarterial subtraction angiogram
demonstrates basilar and left vertebral artery occlusion.

J Comput Assist Tomogr, Vol. 14, No. 1, 1990

FIG. 2. Axial T2-weighted MR image at the level of the pons
shows absence of basilar artery flow void phenomena and
areas of abnormal signal intensity in the brachium pontis
bilaterally (right) and pontine tegmentum (left).

basilar artery most often leads to occlusion of the
proximal basilar artery near the origin of the ante-
rior inferior cerebellar artery branches (9). Rarely
basilar artery occlusion occurs from propagation of
a vertebral artery ‘‘clot,”’ as it did in one of our
patients. Occlusion may occur suddenly, but most
often the neurologic deficits develop gradually. Pro-
gression of the deficit often continues for 2448 h,
and sometimes as long as 7 days (10). The clinical
course and outcome are most likely related to the
availability and development of collateral circula-
tion. The ‘locked in’? syndrome may occur with
extensive basis pontis infarction, as was seen in
three of our patients. Because of the distressing out-
come, and in an effort to improve management of
these patients, it is essential to confirm a diagnosis
of basilar artery thrombosis as soon as possible af-
ter onset.

FIG. 4. Sagittal T1-weighted MR image shows a linear struc-
ture with a mixed signal intensity along the course of the
basilar artery in the pontine cistern (arrow) and a large area
of increased signal intensity in the suprasellar region.
BASILAR ARTERY OCCLUSION: MR FINDINGS 35

FIG. 5. Axial T2-weighted MR image at the pontine level
shows absence of flow void phenomena of the basilar artery
(a) and areas of hyperintensity in the midpons (b).

In the past, cerebral arteriography has been the
only method to document basilar artery occlusion.
However, this invasive procedure can be risky, as
patients with basilar artery occlusion are often clin-
ically unstable and there is a possibility of further
clot propagation. Transcranial Doppler ultrasound
may detect basilar artery stenosis or occlusion, but
an absent signal may reflect technical problems
(11,12).

Magnetic resonance has been shown to be supe-
rior to CT in imaging normal posterior fossa struc-
tures owing to the absence of linear bone artifact,
good gray-white matter differentiation, multiplanar
image capabilities, good visualization of vascular
structures, and the availability of different pulse se-
quences (1). Magnetic resonance is also superior to
CT scans in the early detection of brain stem and
cerebellar strokes including medullary infarctions
(3-7). Changes may be seen on T2W and T1W im-
ages within 24 h. A dichotomy has been reported
between clinical signs and T2W image abnormali-
ties, suggesting it is difficult to differentiate infarc-
tion from edema early after stroke onset (4). The
earliest abnormality on MR in patients with basilar
artery thrombosis is the absence of flow void on
axial T2W images, which may be seen within 90 min
of onset of symptoms (7). This has also been de-
scribed with vertebral artery occlusion (2). The bas-

gee .
Bi ae ee
FIG. 6. Digital intraarterial subtraction angiography (antero-

posterior view) shows basilar artery occlusion at the level of
the anterior inferior cerebellar artery.

ilar artery thrombus is also seen well on T2W sag-
ittal and coronal images as an area of iso- or hyper-
intensity. A case of basilar artery occlusion with
MR and angiographic findings similar to those in
our cases has been reported by Baker and Carr (13).

One potential problem in obtaining MR in pa-
tients with basilar artery occlusion is the current
length of the procedure. These patients often have
progressive neurological and respiratory deficits,
and it is difficult to monitor these parameters during
the scanning process. Research in this area has
been encouraging, and most likely shorter imaging
time and monitoring equipment will become avail-
able in the near future, eliminating this concern.

In conclusion, MR is an excellent noninvasive
method to image basilar artery occlusion with brain
stem infarction and is clearly superior to CT. Ab-
normalities may be seen within 24 h of symptom
onset. Arteriography revealing basilar artery occlu-
sion correlates with abnormal prepontine signals on
TIW sagittal and coronal images. Magnetic reso-
nance, if performed early in patients with suspected
basilar artery occlusion, may avoid the need for ar-
teriography and make earlier therapeutic interven-
tion possible.

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J Comput Assist Tomogr, Vol. 14, No. 1, 1990