Joseph

E. Heiserman,

MD,

Intracranial Evaluation Time-of-Flight

PhD

Burton

P. Drayer,

#{149}

MD

R

studies

ECENT

documented of two- and time-of-flight mag-

resonance

(MR)

the evaluation carotid artery

Cerebral

blood

vessels,

or obstruction, 17.721 #{149} Magnetic (MR), three-dimensional, 17.7214 resonance (MR), vascular studies,

Radiology

stenosis resonance

Magnetic

#{149}

17.7214

185:667-673

1992;

I From

the

Barrow

Neurological

Institute,

St

print v

requests

RSNA,

to B.P.D.

1992

angiography

potential

lateral

to the

blood

supply

distributions

of the

evaluation toms

the

include

sympof

or infarction. METHODS

Twenty-nine patients, all of whom had focal neurologic deficits of varying duration, were studied to exclude vascular stenosis or occlusion. All patients underwent MR imaging of the brain, three-dimensional

time-of-flight

MR

of the intracranial vessels, tional angiography. The tween MR and conventional varied from 0 to 29 days The study group included female patients, with an years (range, 4-78 years). The MR examinations Medical dimensional,

were

performed

(Signa;

Milwaukee).

Fourier

angiography

and conventime interval beangiography (mean, 5 days). 13 male and 16 average age of 56

system

Systems,

GE

The

transform,

three-

time-of-

flight MR angiographic images were obtained with a gradient-echo pulse sequence with “spoiling” of residual transverse magnetization (5) known as spoiled gradient-recalled acquisition in the steady

angle used. used, partition

(6). This a repetition

sequence time

was implemented of 50 msec and

a flip

of 20#{176}. No presaturation pulses were Typically, a 256 x 256 matrix was with a field of view of 20 cm and a thickness of 0.7 mm, resulting in

a 0.78 x 0.78 x 0.7-mm

voxel.

were

thickness 13 minutes

of 4.5 cm in an imaging time of 40 seconds. Six of 29 examina-

(21%)

were

acquired

within

Sixty-four

partitions

tions

acquired

192 matrix and a 19-cm field of view, with a corresponding decrease in imaging time. All examinations were obtained with first tion-

with

moment flow compensation and frequency-encoding

a section

a 256

x

on secaxes. The

echo time varied during the course of the study from 10 msec to 4.9 msec due to technical improvements: 16 of 29 (55%) were

obtained

msec, two (7%) (38%) at 10 msec.

The

evaluation

MD

Occlusion:

ischemia

AND

K. Fram,

4.7

the intracranial arteries. In this article, we explore the usefulness of threedimensional time-of-flight MR angiography of the intracranial vasculature in the assessment of patients with symptoms of cerebral ischemia

MATERIALS

Evan

#{149}

images

full

with

cerebral

ideally

for colvascular

brain,

of the patient

suggesting

would

in

of narrowing of the bifurcation (1-4). Ow-

ing to the extensive

state with

Joseph’s Hospital and Medical Center, 350 W Thomas Rd, Phoenix, AZ 85013. Received April 13, 1992; revision requested May 28; revision received July 7; accepted July 13. Address re-

and

efficacy

three-dimensional netic

PhD

have

the clinical

on a 1.5-T imaging terms:

Keller,

Vascular Stenosis with Three-dimensional MR Angiography’

To assess the usefulness of magnetic resonance (MR) angiography in the characterization of intracranial arterial stenosis and occlusion, a threedimensional time-of-flight method was compared with conventional angiography in 214 vessels in 29 patients. Studies were independently interpreted by two neuroradiologists who scored each vessel as normal, narrowed, or occluded. Overall, 97% of normal vessels and 100% of occlusions were correctly graded. Sixtyone percent of stenoses were graded correctly; the remainder were graded as normal. The portions of the intracranial vessels near the skull base and especially the paracavernous and supraclinoid segments of the internal carotid arteries were areas of frequent over- and underestimation of stenosis due to the presence of dephasing artifacts. In patients with stenosis or occlusion, MR angiography also provided information regarding the presence of collateral flow in the circle of Willis. When used in conjunclion with MR imaging of the brain and MR angiography of the extracranial carotid arteries, intracranial MR angiography allows a more complete evaluation of the patient with symptoms of cerebral ischemia or infarction. Index

PaulJ.

#{149}

this

64 axial

way

were

source

with

echo

time

at 5.3 msec,

and

images

obtained

postprocessed

by

of

11 in

means

of

a maximum-intensity-pixel projection algorithm (7) to produce multiple projeclions rotated about the section axis. The anterior and posterior circulations were processed separately, with regions of interest that excluded orbital and scalp fat. In each case, 10 projection images at 18#{176} increments were obtained, as well as one axial “collapse” projection image. Selective conventional angiography of the carotid and vertebral artery distnbutions was performed in all cases. A combination of magnification screen-film and digital-subtraction images (512 x 512 ma-

trix) were

obtained.

Oblique

views

and

views with cross compression were not obtained routinely and were not included in this study. The MR and conventional angiograms for each patient were reviewed retrospeclively and independently at separate reading sessions by two neuroradiologists (B.P.D., J.E.H.). The MR angiograms included the projection images and the source images for each patient. For the conventional angiographic studies, anteroposterior and lateral images from the selective carotid and vertebral injections were supplied. The readers were blinded to all clinical information. A standardized scoring scheme was employed with three grades: normal, narrowed (50% diameter narrowing or greater), and occluded. Six vessels were scored bilaterally: internal carotid artery, the horizontal (A1) and postcommunicating (A2) segments of the anterior cerebral artery, the horizontal (M1) and distal (M2) middle cerebral artery, and the posterior cerebral artery. The yessels in the circle of Willis were evaluated for variants such as hypoplasia and fetal origin of the posterior cerebral artery. From these data, tables were constructed that compared the scoring of a

667

Figure ample image.

1. Normal MR angiograms. (a) Exof source image. (b) Axial projection (c, d) Anteroposterior-projection im-

ages segmented from data for posterior circulation, respectively. time = 4.9 msec in a-d.

anterior and Echo

given vessel by reader and by technique. For purposes of this study, left- and rightside vessels were considered equivalent, and results were pooled. The percentage agreement and the Spearman rank correlation coefficient T5 (8) were calculated to compare the results for each reader by technique and also to compare the readers for each technique. Part of the observed agreement between readers and tech-

niques due this

in measurements

of this kind

is

to chance alone. To take account of effect, the K statistic (9), which mea-

sures

agreement

beyond

that

expected

on

the basis of chance, was also computed. To examine the effect of echo time on the accuracy of MR angiography, the subgroup of examinations obtained with echo time of 4.9 msec was analyzed separately and findings were compared with those obtained with echo time of 10 msec, by means of the correlation coefficient and K

a.

b.

statistic.

RESULTS All MR and conventional angiowere judged acceptable by both readers, although the quality one MR angiogram was considered grams

suboptimal

and

small

communicating scored on

due

to motion

vessels

such

this

of

artifact,

as the

arteries examination.

were

C.

posterior not A normal

MR angiogram is illustrated in Figure 1. The paracavernous and supraclinoid segments of the internal carotid arteries basilar

are

are visualized, as is the distal artery. The ophthalmic arteries

identified

occasionally,

although

fat signal intensity arising from the orbits on these Ti-weighted images limits visualization of vessels in this

area. The superior are usually seen. tions tenor

cerebellar The proximal

of the anterior, cerebral arteries

variable

portions

ments

of these

The

middle, and posare well seen;

of the vessels

A1 segment

arteries por-

distal are

seg-

visualized.

of the anterior

ce-

rebral artery was absent or hypoplastic in 16 of 58 (28%) of both examinalions (Fig 2). Of the 56 posterior communicating arteries scored, 43

(77%)

were

grams and angiograms. the posterior

identified

on MR angio-

39 (70%) on conventional About half (23 of 43) of communicating arteries seen at MR angiography were present only on the source images and not on the projection images.

668

Radiology

#{149}

d.

A total of eight fetal origins of the posterior cerebral artery, with a hypoplastic or aplastic P1 segment of the ipsilateral posterior cerebral artery, were observed at both MR and conventional angiography. Duplication

of a superior present

in two

cerebellar of 29 cases

artery (7%)

was and

was

apparent

in

both

ventional angiograms. cies of these common

served

in this study

MR

and

con-

The frequenvariants ob-

are comparable

to

the

accepted values (iO). A total of 214 vessels were assessed for acquired narrowing or occlusion by means of both projection and

December

1992

the

internal

carotid

arteries

and

the

middle cerebral arteries, and scoring for these vessels for each of the readers is summarized in Tables 2 and 3. For this case, the four correlation coefficients

two (P

between

readers

basis

C.

Figure 2. Hypoplastic A1 segment of the right anterior cerebral artery as an example mal variant of the circle of Willis. (a, b) Conventional right and left carotid angiograms (c) MR angiogram in the anteroposterior projection (echo time = 4.9 msec).

of a norand

incorrectly

Evaluation at Conventional Angiography Reader

by Technique Evaluation

Normal

for Each

at MR Angiography Narrowed

Ocduded

collateral

ages was

1 188

9

0

2

9

0

Occluded Reader 2 Normal Narrowed Occluded

0

0

6

filling

this

186 9 0

5 8 0

was

Good

correlation

is apparent,

Spearman rank correlation cients rs of .73 and .65 and

Volume

185

Number

#{149}

3

with coeffi-

P value

less

middle,

Findings angiograms

0 0 6 and

posterior

cerebral

was

present

artery

by the posteartery. In one

clearly

in the

of the

apparent

on

the

were

than .005 for readers 1 and 2, respectively. Overall agreement in findings at MR and conventional angiography was seen in 203 of 214 (95%) vessels for reader 1 (K .71) and in 200 (93%) for reader 2 (K .64). Most of the abnormalities were observed to involve

sured

with

of MR

with

echo

compared

ings in those obtained of 10 msec. Improved arteries.

pathway cases.

subgroup

obtained

conventional of internal carotid and anterior, distal branches are not scored.

source images (in the case of MR angiography). The overall agreement in scoring between techniques for each reader is summarized in Table 1.

were

as occluded.

(Fig 4). No collateral identified in these

of 4.9 msec

are numbers occluded vessels,

readers

MR projection angiograms; in the other, this collateral pathway was apparent only on the source images. In two cases of middle cerebral artery occlusion, infarction was present and was apparent on the source im-

Reader

Normal Narrowed

Note-Numbers For proximally

the

graded

middle cerebral rior communicating

1 of 214 Vessels

data,

In four cases in which an occluded internal carotid artery was present, flow was present in the affected anterior and middle cerebral arteries, and the pattern of collateral filling was evaluated (Fig 3). In all four cases, collateral filling was present of the antenor cerebral artery via the anterior communicating artery. In two cases,

case,

of Evaluation

of these

in 103 of 114 (90%) MR angiograms (K .64) and in 102 (89%) conventional angiograms (K .62). In both cases, no occlusion was incorrectly graded as normal or narrowed, and no normal or narrowed vessel was

Comparison

the

to 1.0

in agreement

b.

Table

0.71

rowing was both over- and underestimated at MR angiography compared with conventional angiography in a number of cases. The agreement between readers for each technique for the internal carotid and middle cerebral arteries is summarized in Table 4, with correlation coefficients rs of .74 and .86 and P value less than .005 for both MR and conventional angiography. On the

4.

a.

for

from

.005). The six occlusions present identified by both readers in all Most normal vessels were coridentified; however, vessel nar-