Clifford

R. Jack,

Jr, MD

Brain and Measurement

C

Cerebrospinal Fluid with MR Imaging’

neurologic diseases are characterized pathologically by global or regional cerebral atrophy. Foremost among these diseases are Alzheimer disease (dementia of the Alzheimer type [DAT]) (1-4) and focal epilepsy (5,6). The role of cross-sectional anatomic imaging in the clinical management of dementia, epilepsy, and certain psychiatric disorders has been primarily to exdude space-occupying or macroscopic intracranial lesions. However, many investigators have attempted to develop criteria for detection of global or regional cerebral atrophy by which the above conditions could be positively diagnosed. To date, most such studies have used computed tomography (CT) and have been directed toward developing techniques that would provide positive (vs purely exclusionary) diagnostic information in DAT. This is not surprising, considering that DAT is the leading cause of dementia in the elderly (7-10). As yet, no peripheral biochemical marker exists for this condition, and, consequently, improvement in diagnosis is needed. A diagnosis of definite DAT can be made only histologically, by means of brain biopsy or autopsy. A diagnosis of probable DAT can be made by means of clinical evaluERTAIN

ation, but there have been variable reports of accuracy (60%-90%) with respect to subsequent autopsy results (the higher accuracy figures were derived from serial evaluations of patients over several years) (7-1 1). The clinical diag-

nosis

remains

terms:

rebrospinat

Brain, MR fluid,

#{149} Dementia,

10.83

nance

image

(MR),

Radiology

I

the

Mayo

MR studies,

137.1214,

#{149} Editorials

#{149} Magnetic

137.91

reso-

processing

Department Clinic,

MN 55905. October

#{149} Ce-

10.1214

1991; 178:22-24

From

ogy,

studies

of

200

Received

First

Diagnostic St.

October

3. Address

reprint

SW,

Radiol-

2, 1990; accepted requests

to the

au-

thor. ©

RSNA,

1991

See also the articles 122), at (pp

22

Tanna 109-114)

et at (pp in

this

by Kohn 123-130), issue.

et at (pp and

Rusinek

115et

difficult

in

early stages of this condition. Efforts have been made with CT to identify patients with DAT both on the basis of qualitative perceptual rating and quantitative measurement of cerebrospinal fluid (CSF) spaces (12-18). Some atrophy occurs normally with ag-

ing,

and

a fairly

wide

range

of age-re-

lated atrophy exists in cognitively normal elderly persons. For this reason, and because atrophy is a continuous (vs a discreet) phenomenon, much more effort has been devoted to developing quantitative criteria of global cerebral atrophy by which normal elderly mdividuals might be separated from those with DAT. Because beam-hardening artifact can obscure cortical margins, thus making direct measurements of

the brain

difficult,

investigators

using

CT have quantified the size of CSF spaces as an indirect measure of cerebral atrophy. Technical approaches to quantifying cerebral atrophy with CT have evolved from linear to single-section planimetric to multisection planimetric (volumetric) measures of the ventricular and

CSF spaces

(12-14).

A semi-

automated segmentation scheme is used for single and multisection planimetric measurements. Typically, this involves manually outlining regions of interest (eg, the ventricle). All pixels within these regions are then automatically counted by a computerized systern, and that number is multiplied by a known constant to give the area or volume of the region of interest. Alternatively, a threshold range can be established that would automatically segment the region of interest into CSF and brain compartments on the basis of pixel intensity. While studies have con-

sistently

Rochester,

particularly

the

the sulcal

Index

Volume:

shown

that

the mean

region

of interest values of DAT patients differ from those of age-matched controls, enough overlap has been present that semiautomated techniques with use of CT have not had clinically useful predictive power in individual cases. This

lack

of specificity

automated

attaining

has prevented

quantitation

with

widespread

clinical

semiCT from

applica-

tion. To my knowledge, the first report of semiautomated measurement of brain volume with use of MR imaging appeared in 1988 (19). Many of the technical barriers inherent with CT-based

brain

volume

measurement

are not

present with MR imaging. One of the advantages of MR is that there is no bone artifact, which results in more precise definition of the CSF-brain boundary, thus permitting direct measurement of brain volume (vs indirect CSF space measurement with CT). Another advantage is the multiplanar imaging capability of MR, which permits direct measurement of structures such as the hippocampal formation, the orientation of which is ideally suited to

oblique coronal imaging. MR imaging also provides superior CSF-brain and gray matter-white Several validation

ing brain

matter contrast. studies of measur-

(or CSF[20])

volume

with

use

of MR imaging have appeared in the literature to date (20-24). All report reasonably high accuracy and low interobserver variability. Original work documenting the clinical utility of MR imaging in brain quantitation has already been published. The diseases studied were epilepsy (25), DAT (26), schizophrenia (27), and amnesia (28). These original clinical investigators have for the most part measured specific brain structures (eg, the temporal lobe or hippocampal formation) rather

than

the entire

brain.

schemes in these studies based on either tracing

and

Segmentation have been or both tracing

thresholding.

This issue of Radiology contains three articles (29-31) that describe two new approaches to MR-based brain and CSF volume measurement, as well as the application of these techniques to the study of DAT and aging. Two of these articles are from the same institution (29,30) and describe a segmentation technique based on bifeature pixel clas-

sification

of spin-echo

MR images.

Val-

idation of the technique is reported by Kohn et a! (29), and a clinical study concerning DAT and aging is reported by Tanna et al (30). This bifeature space

segmentation

technique

(29)

involves

acquisition

of a single,

double

spin-

echo

sequence.

pulse

intracranial

pixels

CSF compartments ferent relaxation two tissues. The

significant tation

Segmentation

into

is based properties technique

improvement

based

threshold

on

intensity present

brain

over

when

segmen-

single

regional

variation (shading in an image.

Rusinek technique

or

on the difof these represents a

a (simple)

value

of

either

signal artifact)

is

et al (31) describe a novel that involves sequential ac-

quisition of two inversion-recovery pulse sequences. One of these Sequences is designed to maximize CSF

signal

(ie, inversion

crossover

point

time

halfway

and white matter) signed to maximize matter

contrast.

construction ing these

two

set at the between

gray

and the other is degray matter-white Real

(vs

modulus)

re-

contents gray

and

white

that rors

they are able to compensate caused by partial volume

matter-and

ing of different

have

tissue

types.

shown for averag-

They

er-

also

measured gray matter volume loss and have shown this parameter to be the most sensitive measure for separating

patients

with

DAT

from

control

sub-

jects. This differentiation was most effective when regional volumetric measurements of the temporal lobes, or a “central area,” were analyzed. Both groups of investigators are to be commended for outstanding work. Key elements of the work by these groups include (a) carefully done studies vali-

dating

the accuracy

ment

technique

toms

of known

of a new

with

respect

volume,

and

measureto phan-

low

inter-

observer measurement variability; (b) clinical studies comparing normalized brain and CSF volumes in patients with DAT with those of control subjects; (c) correlation of MR-based volumetric data with functional data (ob-

tamed with positron emission phy) in the same subjects; and tions herent

to specific technical in the use of MR

shading

artifact

tomogra(d) solu-

problems volumetry,

(29), and

partial

in-

vol-

ume averaging (31). In addition to the intrinsic diagnostic merit of the use of brain and CSF volume to separate patients with DAT from control subjects, both groups of investigators pointed out another important use for volume measurement. An accurate estimate of brain volume (by means of MR imaging) should greatly improve estimates of global and regional cerebral glucose metabolism made with use of positron emission to-

mography. had

been

Volume

In the past, contaminated

178

#{149} Number

these

Autopsy

av-

proof

patients

with

DAT

jects,

or, like

ume

measurement,

from

control

semiautomated

4.

clinical

8.

techniques,

then

the efficacy

of each

a

For example,

several

tive

of having

DAT.

10.

11.

12.

Acknowledgments:

The

Katzman

13.

L. Maxwell

Hillier

for

L. Baker,

15.

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Joachim

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