Head Jerome

A. Barakos,

MD

#{149} William

Orbit, Skull Contrast-enhanced

Base,

The high signal intensity of fat on Ti-weighted magnetic resonance images has limited the utility of gadopentetate dimeglumine in imaging of the extracranial head and neck. Enhancing lesions may be obscured either by proximity to fat or by chemical misregistration artifact. The authors evaluated the role of a gadolinium-enhanced fat suppression imaging technique in the detection of extracranial head and neck abnormalities in 29 patients. These studies were directly compared with conventional pre- and postcontrast Ti- and T2-weighted SE sequences. In detecting and defining the extent of abnormalities, fat-suppressed images were superior to non-fat-suppressed gadoliniumenhanced Ti-weighted images in the majority of cases (22 of 27 [81%]). Fat-suppressed images were particularly beneficial in the detection of perineural spread of tumor as well as in defining lesions situated within or adjacent to fat-containing areas such as the base of the skull. These findings demonstrate that fat suppression techniques in combination with gadolinium enhancement are of value in extracranial head and neck imaging and should replace conventional postcontrast Tiweighted SE imaging. Index terms: Gadolinium #{149} Head and neck neoplasms, 12.3, 22.3, 26.32, 27.3 #{149} Magnetic resonance (MR), chemical shift #{149} Magnetic resonance (MR), fat suppression #{149} Magnetic resonance (MR), image processing #{149} Meninges, neoplasms. 222.3661 #{149} Orbit, MR studies, 22.1214 #{149} Orbit, neoplasms, 222.3661 #{149} Phanynx, neoplasms, 26.373, 27.373 Radiology

1991;

P. Dillon,

MD

#{149} Wilbert

M. Chew,

Neck

MR Imaging’

gadopentetate dimegluhas proved invaluable in magnetic resonance (MR) imaging of various areas of the body, its use has remained limited in the extnacnanial head and neck. This is because enhancing lesions may be concealed by the high signal intensity of adjacent fat on short repetition time (TR)/ echo time (TE) spin-echo (SE) images (1-4). In addition, fat may obscure abnormal as well as normal strucLTHOUGH

mine

tunes

by

fact

(5-8).

means

ing

techniques

of chemical

Fat suppression have

shift

MR

proved

arti-

imag-

useful

in

several ventional

areas of the body where conMR imaging techniques mesult in limitations owing to fat (1,91 1). This study was designed to determine the utility and role of a fat suppression imaging technique used with contrast material enhancement in the detection of orbital as well as head and neck disease. PATIENTS

AND

as well

During an 8-month period (June 1989 through January 1990), 29 patients with orbital or head and neck disease were evaluated on a i.5-T superconducting MR (Signa;

GE

Medical

Systems,

Milwau-

kee). There were patients, ranging (mean, 52 years). tions are listed in

18 male and 1 1 female in age from 2 to 75 years Tumor types and locathe Table. All diagnoses were histologically proved (surgery, n 16; biopsy, n 8) except for five cases of optic sheath meningioma, in which the diagnosis was based on radiographic findings and clinical follow-up.

Imaging

Patients

were

nique.

studies

compared

with

al pre- and post-gadopentetate mine Ti-weighted (short

TR/TE)

I

From

the

Department

of Neuroradiology,

University

Ave. P0 Box 0628, San Francisco, 10; revision received November

of California,

CA 94143. 16; accepted

San

Francisco

Received July 13, 1990; December 21. Address

dimegluimages

Medical revision reprint

(RF)

were

then

tech-

directly

which

lesion

sequences

conspicuity

and

Lesions of the floor of the mouth, mandible, and base of the tongue were evaluated with use of a neck coil (n 8). Conventional precontrast Ti-weighted im-

ages were

obtained

in the axial

and/or

coronal msec/TE

plane with an SE 600/20 (TR msec) sequence, two excitations, a 20-cm field of view, and a 192 X 256 matnix size. Section thickness was 5 mm, with a 1-mm gap. In several patients (n 3), precontrast fat suppression images

were

also obtained.

dimeglumine weighted

lowed

After

gadopentetate

infusion a conventional sequence was performed,

immediately

obtained identical

in similar

planes

parameters

SE imaging

one excitation).

fat and/or images

and with

as conventional

Ti-weighted images. Patients went conventional precontrast T2-weighted

Tifob-

by a Ti-weighted

suppression sequence in the axial coronal projection. Fat-suppressed

also undermultiecho

(2,800/30,

A section

80,

thickness

of 5

mm with a 2.5-mm intersection gap was used with a matrix size of 192 X 256. Studies of the orbit (n 8) were performed with use of a head coil, allowing evaluation of the apex, chiasm, and optic tracts. Nonenhanced Ti-weighted images

were

obtained

plane. Then, conventional

in the axial and/or fat-suppressed contrast-enhanced

coronal

as well as Ti-

weighted SE images were obtained in the axial and/or coronal planes. All Tiweighted studies were obtained with an sequence

with

two

a 22-cm field of view, and trix size. Section thickness

convention-

use of a

the best depiction of tumor extent. The nasopharynx studies were performed with use of a head coil (n 13).

excitations, X 256 ma-

a 192

was 3 mm,

with a 0.5-mm intersection gap. tional axial multiecho T2-weighted

images ter, 505 Parnassus quested September quests to JAB. RSNA, 1991

greatest

SE

with

radio-frequency

to determine

provided

SE

Ti-weighted

also obtained

These

TR/TE)

fat-suppressed

images were presaturation

SE 600/20 imaged

(long

In addition,

gadolinium-enhanced

were

METHODS

as T2-weighted

sequences.

the

unit

Radiology

PhD

and Pharynx: Fat Suppression

MR

179:191-198

and

(2,800/30,

ConvenSE

80, one excitation)

were

Cenrere-

Abbreviations: spin echo, time.

TE

RF echo

=

radio

time,

frequency, TR

SE

=

repetition

191

also mm used,

obtained. A section thickness with a 2.5-mm intersection with a matrix size of i92

of 5 gap was X 256 and

Head

our

study,

fat

suppression

was

achieved by means of a presaturation technique. The spectral fat peak is first determined individually for each patient before imaging. Subsequently, an RF pulse

is applied,

centered

at the

resonant

frequency of fat (12-i4) (Fig 1). The RF pulse is followed by a homospoil gradient in order to disperse all fat signal. This sequence consisting of a selective RF pulse and a homospoil gradient precedes each section-selective pulse of a multisection Ti-weighted SE sequence. The fat suppression technique is a standard feature on

our

MR

system

(4.0

Advantage

amounting tions

A minor

to the

per

i2-image

curred. This sary to apply

RF pulse In nique

two

loss

or two

acquisition

set,

loss reflects the time the fat-frequency-selective

and homospoil cases,

the

resulted

fat

Orbit(n

in severely

Plexiform neurofibroma Aspergillosis Pharynx (n = 17) Sq uamous cell carcinoma On,- or nasopharynx Tongue Rhabdomyosancoma Adenoid cystic carcinoma Hemangiopericytoma Hemangioma (nasal turbinate)

Malignant Baseofskull(n Squamous Adenoid Chordoma

of the

verified. must

four

#{149} Radiology

the

could

not

study is by no it was thought

objectives

,

I I

Normal

for

report.

not

Ti-

MR imaging demfeatures of anatomy

palate

within surface.

was

signal and The

short

on

conventional

images. On Ti-weighted noted

cause

to contain

of this

fatthe areas

intrinsic

characteristic

unknown but may be glandular or lymphatic this region. With the administration

is

related to the elements in

pentetate normal

of gadodimeglumine, a variety of structures enhanced, includ-

ing

pterygoid

the

well

as the

bopes

the

venous

penineunium tmigeminal

plexus

that nerve.

I I J

T

I J I I

I

Frsqu.ncy

Rngs

CL) Ct) fat

Fet.SatPuIi

SHc.$.190

SHc.s.1180

:

Figure

1. Fat suppression MR imaging was achieved in our study by applying a presaturation RF pulse centered at the resonant frequency of fat. The frequency response of the suppression pulse is presented schematically in a. This RF pulse sequence precedes each section-selective pulse of the multisecconventional

=

Ti-weighted

in b. Since acts

alone,

on

SE

gadopentetate

the

relaxation

removing

the

interfere

with

contrast

saturation,

sel

selection.

sequence

dimeproperties

fat signal enhancement.

of

does Sat

Ti-

precontrast images,

intensity, particularly just deep to the mucosal

Ti relaxation

I I

a.

not

appreciated

J

0ri__H,

glumine

Anatomy

weighted suppressed

I I

as shown

fat

Fat suppression onstmated various

I

Siippm.ko.

water

tech-

of high

(precontrast

I

we

of abnor-

means a rigorous to be suitable

of this

S

be

shortcoming,

margins

H 2#{176} a

tion

weighted, postcontrast Ti-weighted, postcontrast fat-suppression, and T2weighted) different imaging sequences. Images were interpreted by two of the authors (J.A.B., W.P.D.). To accomplish a direct comparison, all sequences for each patient were visually evaluated side by side. A qualitative grading scale was established to rate the conspicuity and cx-

i92

of this

that

2 1 1

RESULTS

In this study we set out to directly cornpare lesion conspicuity and lesion extenthe

Because

study

1

carcinoma

is in-

Analysis

with

imaging

assume

soft

sion

cystic

tent of abnormality: 0 = not visible, i = poorly visualized, 2 = moderately apparent, 3 = very apparent. The signal intensities of normal and abnormal structures were also compared. Images were also evaluated for their depiction of normal anatomy. The nature of head and neck lesions is such that they are frequently sampled at biopsy and then treated with radiation and chemotherapy without being resected en bloc. As a result, in the majority of our cases, surgical specimens were not available to allow for a direct comparison with the imaging study. Thus, the accuracy of tumor extension as predicted on the basis

6 4 2 2 i 1

fibrous histiocytoma 4) cell carcinoma (nasopharynx)

neces-

asymmetric

5 1 1 1

meningioma

gradient. saturation

of Cases

Retinoblastoma

the

sec-

suppression across the image. This rendered the fat-suppressed images difficult to interpret, and thus these two cases were removed from the study group, leaving 27 patients.

Image

No.

8)

=

Optic

in this analysis,

penalty,

of one

in Study

mal signal define pathologic extent. In addition, this limitation precludes determining specificity and sensitivity for the various imaging sequences. Although the subjective assessment method employed

soft-

ware; GE Medical Systems). This sequence is easily applied and accomplished in the same time required for a standard Ti-weighted sequence without the need for additional data manipulation or postprocessing.

Tumors

Location

a 20-cm field of view. In most cases, both fat-suppressed and conventional contrast-enhanced Tiweighted images were each obtained in two planes. These sequences were performed within 30 minutes after intravenous bolus administration of contrast matenial, 0.1 rnmol/kg (gadopentetate dimeglumine [Magnevist], Berlex Imaging, Wayne, NJ). For a given sequence (eg, conventional enhanced Ti-weighted sequence or fat suppression sequence), when assessing the degree of lesion enhancement over the 30-minute period aften contrast material injection, no significant change in lesion intensity was detected. This finding has been noted by other investigators (2) and is useful since it suggests that the time interval between contrast material injection and imaging is not a factor in determining whether one contrast-enhanced imaging sequence is superior to another. In

and Neck

as

enveWith

con-

ventional weighted

contrast-enhanced Tiimages, these enhancing structures are difficult to visualize, since they are adjacent to high-signab-intensity fat. By subtracting the fat signal from the image, these structures were clearly identified (Fig 2). In addition, by suppressing the fat signal, chemical misregistration antifact was eliminated. This allowed visualization of structures that are often obscured by chemical shift antifact on conventional images, such as the maxillary division of the trigeminab nerve as it passes through the foramen rotundum (Fig 3).

April

i991

4). In one patient with an extensive plexiform neurofibmoma, associated dumal enhancement was detected only on the fat-suppressed Furthermore, in a patient spread orbital aspergilbosis,

image. with wideoptic

nerve sheath and was easily detected

dural involvement with fat suppmes-

sion

was

imaging

but

difficult

to ap-

pmeciate on pre- and postcontmast Tior T2-weighted images (Fig 5). In a single case of a large optic meningioma that was focal without pemineumal spread,

pre-

and

postcontrast

Ti-

as

well as T2-weighted images were comparable to fat-suppressed images in the detection of the lesion. OverFigure

2.

(a) Coronal

Ti-weighted

tration. The enhancing difficult to distinguish weighted image. Note After fat suppression, easily

defined.

Note

(600/20)

image

obtained

penineunium of the mandibular from surrounding fat (arrowhead). the uniform suppression of the the perineunium that envelopes the

enhancement

of

the

perineunium

after

contrast

material

adminis-

division of the trigeminal nerve is (b) Coronal fat-suppressed Tisubcutaneous and diploic space fat. the tnigeminal nerve (arrowheads) is but

not

the

normal

nerve

itself.

all, the T2-weighted to be of little value

theme

was

little

images proved in the orbit, since

contrast

differentia-

tion between the diseased area and the surrounding orbital fat. Pharynx (n 15).-In ii of i5 patients (73%) with nasoand oropharyngeab lesions, fat-suppressed im-

ages were superior to pre- and postcontrast Ti-weighted images in delineation of the presence and extent of abnormality. Mean overall conspicuity grading scores for the 15 nasopharyngeal

lesions

were

as follows:

on precontrast Ti-weighted images, 1.1; on postcontmast Ti-weighted images, 1.2; on T2-weighted images, 2.5; and on fat-suppressed images, 2.7. The value of fat suppression imaging was noted in a wide variety of besions,

including

ryngeal

naso-

squamous

and

cell

oropha-

carcinoma

5), squamous cell carcinoma tongue (n = 3), adenoid cystic noma (n = 1), rhabdomyosarcoma =

a.

Figure

3.

(a) Coronal

Ti-weighted

obscuration of the maxillary men rotundum (arrows). (b) allowing better visualization

(600/20)

Chemical shift artifact division of the mandibular nerve as it courses After fat suppression, the chemical shift artifact of the second division of the trigeminal nerve

Tumors

Orbit (n = 8).-In seven of eight cases of orbital disease, contrast-enhanced fat-suppressed images improved visualization of the lesion as compared with preand postcontrast Ti- and T2-weighted images. Mean overall conspicuity grades for the eight orbital lesions were as follows: on precontrast Ti-weighted images, 0.9; on postcontrast Ti-weighted images, 0.9; on T2-weighted images, 0.7; and on contrast-enhanced fat-suppressed images, 2.9. By eliminating orbital fat signal, the enhancement of the optic nerve sheath tumors was markedly apparent against the backdrop of the suppressed fat and the dark optic nerve. In addition, the re-

Volume

179

Number

#{149}

1

b. image.

results in partial through the forais eliminated, (arrows).

moval of fat signal resulted in elimination of chemical shift artifact, furthem improving definition of subtle perineural abnormality. In four of five cases of presumed optic sheath meningioma, the extent of tumor was most conspicuous on the fat-suppressed images. The superior ability of the fat-suppressed images to enable detection of perineural spread of disease was clearly illustrated in three cases (one orbital retinoblastoma and two optic nerve sheath meningiomas) in which the lesions had considerable perineural extension. In these patients the spread of tumor along the optic nerve sheath was detected only on the fat-suppressed images and not on the pre- and postcontrast Ti- or T2-weighted images (Fig

=

1), and

malignant

fibrous

(n of the carci(n

histiocy-

toma (n = 1). In six of these cases some portion of the lesion was sumrounded by fat. As a result, the periphery of the enhancing lesion was difficult to distinguish from adjacent high-signal-intensity fat, making the true extent of tumor difficult to accurateby gauge. In each of these cases the lesions became more conspicuous with use of the fat suppression sequence. Fat suppression also increased besion contrast by adjusting the image dynamic gray scale. The portion of the image with the highest signal intensity, typically fat, is assigned as the brightest pant of the image. Subsequently, all lower signal intensities

are rank removed highest

ordered below. Once from the image, the signal,

which

may

of intermediate brightness, be assigned as the brightest. manner, with the removal dynamic

gray

scale

have

fat is next been

will now In this of fat, the

is expanded

Radiology

over

193

#{149}

p.

‘)L

1_#{149}___

#‘.a

fr-.’

t

..

‘F b.

a.

Figure

Images

4. after

tamed

fat-suppressed pressed pression

of a 47-year-old woman material administration.

contrast

coronal

image

image demonstrates imaging proved

very

the remaining contrast resulted in images that

more

subtle

neural

extension

pnession

of pemineural

of the optic

sheath meningioma perineural extension

division

sheath

(600/20) image ob(b) Corresponding the optic canal (arrow). (C) Axial fat-supthe optic canal (arrowheads). Fat sup-

within through

sig-

In a case

: +/

fat sup-

allowed

AlqS’

detection

spread

dibulan

nerve

extending posteriorly of disease.

along the mantrigeminal both pre- and post-

..;,

(.,‘

of the

nerve.

However,

contrast

Ti-

failed

to reveal

and

T2-weighted

any

Ti-weighted to appreciate.

was also of pen-

carcinoma,

imaging

enhancement

meningioma. (a) Coronal conventional left optic nerve sheath (arrow) is difficult

sensitiv(Fig 6).

of tumor.

cystic

subtle

optic nerve in detecting

in tissue

and increased enhancement

Fat suppression imaging beneficial in the detection of adenoid

left useful

sheath of the

mange. This demonstrated

differences

nal intensities ity to contrast

with a left optic Enhancement

demonstrates the

C.

p.

\.

/

images

pemineumal

abnon-

malities.

In 1 1 of 1 5 pharyngeal

cases,

con-

trast-enhanced fat suppression imaging was superior to conventional postcontmast Ti-weighted images.

However, did not

the fat-suppressed necessarily demonstrate

images pre-

viously occult abnormalities in all instances. In five cases fat-suppressed images demonstrated an abnormality that had been obscured by the administration of gadopentetate dime-

glumine. on

These

the

ages

lesions

precontrast

as defects

were

im-

normal

fat

planes. However, once gadopentetate dimeglumine was administered, these enhancing lesions were masked by the adjacent fat on Ti-weighted images. Despite the visibility of these lesions as negative defects within fat on the Ti-weighted images, in all

cases

the

ter defined normality.

fat-suppressed

images

the full extent In six additional

bet-

of the cases

abthe

enhancing lesion became more conspicuous with fat suppression, and thus lesion detectability and reader level of confidence were improved. In the

194

remaining

Radiology

#{149}

four

cases,

fat-sup-

b.

5

Images

of a 67-year-old man with acute myelogenous leukemia and orbital aspengillosis. (a) Axial contrast-enhanced fat-suppressed image demonstrates optic sheath enhancement (arrowheads) as well as diffuse retroorbital involvement. (b) These findings are difficult to appreciate on the corresponding T2-weighted image (arrowheads).

pressed

and

ed images

postcontrast

were

A specific suppressed ticulanly lesions. hancement

of the

area

in the

extent

of pha-

in which

the

fat-

images did not prove useful was mucosab-based In two cases, marked enof the mucosal surfaces

obscured to the

Ti-weight-

comparable

demonstration ryngeal disease.

visible

Ti-weighted

within

a.

Figure

enhancing mucosa.

of a large that had maxillary

lesions

The

first

extended sinuses

adjacent

case

nasopharyngeal

par-

was

that

carcinoma

throughout and nasal

On both the postcontrast ed images and fat-suppressed

the cavity.

Ti-weightimages,

marked contrast enhancement of the nasal and sinus mucosa made differentiation of tumor from surrounding

normal margin on

mucosa of the

a fat-suppressed,

difficult. tumor was

In fact, the best defined

nonenhanced

image. In this image, the removal of fat, the concomitant expansion of the gray scale, and the increased sensitivity to differences in tissue signal intensities allowed differentiation of the tumor from the surrounding mucosa. The second case was that of a treated rhabdomyosarcoma of the pharynx. In this case the lesion demonstrated mixed contrast enhancement. As a result, following fat suppression, the lesion was obscured by the marked enhancement of the sumrounding mucosa. In 12 of 15 cases (80%), contrast-enhanced fat-suppressed images provided information concerning the detection and extent of tumor that was comparable or superior to that provided by T2-weighted images. This is in contrast to the postcontrast Ti-weighted images, which, when

April

1991

Figure

6.

difficult

Recurrent

nasopharyngeal

to appreciate

nonenhanced

the

site

fat-suppressed

carcinoma

of recurrent

image

in a 26-year-old

tumor

(arrow).

(arrow).

By removing

(b)

woman. The

focus

(a) On the nonenhanced of recurrent

tumor

the high-signal-intensity

and expanded over the remaining contrast range. In this manner tissue signal intensity, as demonstrated in this case. The adjustment enhancing structures such as the brain parenchyma. (c, d) Although postcontrast Ti-weighted image (arrow in c), it is more conspicuous

the

Ti-weighted

is more

fat from

easily

the image,

the dynamic

coronal the

gray

image

it is

corresponding

scale

is adjusted

four

cases,

tumor

extension

into the clivus could be detected means of replacement of normal val fat on the nonenhanced Ti-

,1

weighted

images.

Nevertheless,

by cli-

in

these cases the contrast-enhanced fatsuppressed images still provided supeniom delineation of the full extent

‘I

of the

..

on

fat-suppressed images become more sensitive to subtle differences in of the gray scale also accounts for the increased signal intensity of noncontrast enhancement of the lesion can be detected on the conventional on the fat-suppressed image (arrow in d).

of these

I

(600/20)

visualized

primary

creasing rounding dition,

A’

tumor

as well

lesion contrast with soft-tissue structures. in two of these cases

tected dural enhancement, was not as well appreciated

I’.

preb.

and

weighted

postcontmast

Ti-

as insumIn adwe de-

which on the or T2-

images.

Figure

7. Recurrent nasopharyngeal carcinoma in a 45-year-old man. (a) Postcontnast Tiweighted coronal image. It is difficult to distinguish contrast-enhancing tumor (arrowhead) from clival fat (arrow). (b) Fat-suppressed image demonstrates the extent of enhancing tumon invading the clivus (arrow). Contrast enhancement is present within the right temporal lobe, a finding consistent with postimradiation changes (arrowheads).

compared with the T2-weighted images, demonstrated comparable or superior lesion extent in only four of 15 cases (27%). In six cases, fat-suppressed images confirmed abnormalities detected on T2-weighted images. In these instances, although fat-suppressed images did not enable detection of new disease, the observers’ level of confidence was markedly improved because the findings on T2weighted images could be confirmed. In six additional cases, fat-suppressed images proved superior to T2weighted images in the detection or characterization of disease. In one patient with a carcinoma at the base of the tongue, the lesion did not demonstrate significant signal intensity on the T2-weighted image but did enhance on the fat-suppressed image. In three additional cases the fat-suppressed images provided additional

Volume

179

Number

#{149}

1

Adenopathy Contrast-enhanced fat suppression imaging proved very useful in the evaluation of adenopathy. On fatsuppressed images, enhancing nodes within the fat-filled fascial planes of the neck became readily apparent once the surrounding fat was suppressed. In most cases (four of six),

information concerning lesion architecture. In these cases tumor necrosis and liquefaction not appreciated on the T2-weighted images were demonstrated. In the final three cases, T2weighted images proved more useful in detection of disease. Skull base (n = 4).-Extension of tumom into the skull base was detected in four cases. Theme were two cases of nasopharyngeal carcinoma, one of

adenoid chordoma. trast-enhanced provided

cystic

carcinoma,

In all better

four cases, fat-suppressed delineation

and

one

of

the

conimages of tumor

extension into the clivus than could be appreciated on either the postcontrast Ti- or T2-weighted images. On the conventional postcontrast Tiweighted images it was not possible to distinguish infiltrating enhancing tumor from the high signal intensity of clival fat (Fig 7). However, in three

fat-suppressed images were superior to preand postcontmast Ti-weighted images in the detection of adenopathy. In three of six cases fat-suppressed images were comparable with T2-weighted images in the detection of adenopathy, but in the memaining three cases fat-suppressed images provided additional infommation. In two of these cases the fatsuppressed image demonstrated low signal intensity in the central portion of the node consistent with metastatic adenopathy. One of these necrotic nodes measured less than 1 cm in diameter, and necrosis was not visual-

ized

on either

the

postcontrast

T2-weighted images. In the final case, extracapsular nodal extension

Radiology

Ti-

or of

#{149} 195

tumor, suspected on the basis of T2weighted images, was confirmed on the fat-suppressed image. In contrast to the fat-suppressed images, on the conventional postcontrast Ti-weighted images the enhancing extmacapsulam tumor was obscured by peminodal fat.

Asymmetric

Fat Suppression

A uniform

fat-suppressed

image

was not achieved in all instances. In two cases fat suppression was markedly asymmetric, resulting in an uninterpretable study (Fig 8). In both instances complete fat saturation occumred in the anterior portion of the image, with complete water saturation in the posterior portion of the image. In six of the remaining 27 cases, a mild degree of asymmetric fat suppression was noted. In these instances the degree of asymmetric fat suppression was minimal and did not interfere with interpretation of the study. All instances of incomplete fat suppression occurred in a region of changing body contour, for example, at the junction of the floor of the mouth and the neck. Asymmetric fat suppression did not occur with orbitab or nasopharyngeal imaging; however, susceptibility artifacts at the interface of air and soft tissue (ie, sinuses, airway) were sometimes pronounced.

DISCUSSION Although dimeglumine in MR imaging the body, its the presence intensity of be obscured misregistration ity to distinguish ment

from

tensity

the

use of gadopentetate has proved invaluable in various areas of use is often limited by of fat. The high signal enhancing lesions may either by chemical shift artifact or the inabilcontrast enhance-

adjacent

high-signal-inThis is especially in areas with large

fat (1,3,4).

problematic

amounts of fat such as the extracranial head and neck. Despite these limitations, several investigators have mecently demonstrated advantages of gadopentetate dimeglumine in routine head and neck MR imaging (2,15-18). Preliminary investigations have demonstrated that the shortcomings related to fat may be overcome by means of fat suppression imaging techniques (1). In this study we set out to determine linium-enhanced

fat

the role of gadosuppression im-

aging in the evaluation al head and neck disease. by, we wanted to identify

196

Radiology

#{149}

of extracraniSpecificalhow this

Suppression

a.

Figure

Frequency

Range

b. 8.

Bulk

susceptibility artifact. (a) Axial Ti-weighted (600/20) fat-suppressed MR imfat suppression anteriorly and water saturation posteriorly. When imaging extends over areas of changing volume, such as from the oral cavity to the neck, a minor shift in the frequency of the water and fat peaks may occur. In the anterior portion of the image, the fat-selective RF suppression pulse overlies the fat peak (top spectrum in b), resulting in fat suppression. However, in the posterior portion of the image, water and fat resonate at frequencies that are slightly different from those at the anterior portion of the image. As a result of this frequency shift, the RF saturation pulse now overlies a portion of the water peak, resulting in water suppression posteriorly (bottom spectrum in b). This artifact did not occur in areas of constant imaging volume such as in the orbits or nasopharynx.

age demonstrates

be acquired (21method of fat suppression short-inversion-time inversion-recovery sequence (STIR) (6,25,28). This method of suppressing the signal from fat is based on relaxation time rather than on chemical shift. Without gadopentetate dime-

technique compares with non-fatsuppressed imaging techniques and what its specific uses and indications are. Fat suppression imaging has been used to advantage by numerous investigators in several areas of the body (1,7,9-1 1,19,20). Fat suppression imaging is of particular value in eliminating chemical shift artifact, especially when imaging with meduced bandwidth at high field strength (9). The removal of chemical shift artifact improves boundary delineation, allowing better definition

image

data

23,27).

Another is the

of water-containing tissues such as nerves, since they are no longer obscured by the misregistered signal of fat. Previous authors have described a variety of imaging techniques leading to the production of separate water- and fat-based images (12-14,21-

laxation

25).

at

Dixon

first

described

a simple

chemical shift imaging technique that allows differentiation of the fat and water signals of an MR image (26). This method consists of generating two phase-sensitive images, a conventional (in-phase) image and an image in which fat and water protons are 180#{176} out of phase. Manipulation of these images results in either water-only on fat-only images. The Dixon method and variations of this technique require that two separate

glumine,

found ease,

sets

this

technique

useful particularly

amounts

of

ly, this the use

for fat

has

been

the detection in areas with

(28-34).

method is not of gadopentetate

of dislarge

Unfortunateas reliable dimeglu-

with

substances with short Ti retimes (eg, both fat and areas of gadolinium enhancement) will undergo some degree of nulling (35). The method of fat suppression empboyed in our study consists of a selective presaturation RF pulse technique. The RF pulse is approximately mine,

as

16 msec in duration and is centered the resonant frequency of fat. This is followed by a homospoil or crushem gradient in order to disperse the fat signal (12-14). This sequence consisting of a selective RF pulse and homospoil gradient is applied prior to each section-selective pulse of a conventional Ti-weighted multisection SE sequence. This technique is easily performed as a sequence option (Signa 4.0 software; GE Medical Systems),

lations

and

no

operator

or postprocessing

data

manipu-

need

April

be

1991

performed. loss

The

of one

12-image result

only

or two

set. of the

This time

penalty

is the

images

in a typical

section

penalty

required

is a

to apply

the fat-selective saturation RF pulse and dephasing gradient. In comparing contrast-enhanced fat suppression imaging with both pme- and postcontnast Ti- and T2weighted imaging, we found that the fat-suppressed images provided 5evemal important advantages in the detection of abnormalities. The most significant

advantage

was

that

of im-

proved lesion conspicuity. This phenomenon is a function of an adjustment of the image gray scale, as described earlier. In addition, the elimination of fat signal also serves to reduce the overall image signal intensity, and thus the MR receiver gain is increased, further improving sensitivity to subtle amounts of contrast enhancement. Due to a combination of these factors, contrast-enhanced fat-suppressed images were superior to postcontnast Ti-weighted images in delineation of the presence and extent of abnormality in 22 of our 27 cases (81%). An advantage of fat suppression imaging was noted in the orbit, phamynx, and skull base. The advantages of fat suppression imaging were most striking in cases of perineumal spread of disease. However, in all 22 cases there was some degree of improved lesion conspicuity, with lesion margins and the degmee of deep extension being more cleanly visualized. In evaluation of the orbit, fat suppression imaging proved to be significantly

more

pre- and weighted

advantageous

than

both

Ti- and T2In fact, the latter is now unnecessary for optic nerve sheath lesions, being reserved for evaluation of optic masses. The detection of phamyngeal and skull

proved most

postcontrast imaging.

base

abnormalities

was

by fat suppression of our

cases.

also

im-

imaging

Both

the

neck

in and

the

clivus were ideal for fat-suppresimaging. However, contrast-enhanced fat suppression imaging was not helpful in cases in which the enhancing lesion was located adjacent to a mucosal surface. Here, the contrast enhancement of both the mucosion

sa and

the

lesion

was

so great

that

differentiation between the two was not possible. In these locations, the best tumor-tissue contrast may be achieved with fat suppression imaging without contrast material. In most cases of phamyngeal and skull base abnormality, the lesions were detected with both fat-suppressed Volume

179

Number

#{149}

i

and

T2-weighted

less,

the

images. Nevertheprovided by the fat suppression was helpful in confirming suspected abnormalities and increasing the observers’ level of confidence. Therefore, if gadolinium-enhanced Ti-weighted imaging is to be performed, we prefer to use it in conjunction with fat suppression. In several instances in our study, this provided additional information meganding the presence of tumor or nodal necrosis and subtle dural enhancement not provided by T2weighted imaging. Fat-suppressed imaging may also permit improved visualization of a variety of normal structures that are typically difficult to visualize at conventional Tiweighted imaging due to obscuration either by adjacent fat or by chemical shift artifact, such as the mandibular division of the tnigeminal nerve passing through the foramen motundum, the ptenygoid venous plexus, and the perineurium surrounding the tngeminal nerve. In our study, enhancing adenopathy and extracapsular spread of tumon were more conspicuous on contrast-enhanced

information

fat-suppressed

images

than on pre- and postcontnast Tiweighted images. In general, adenopathy detected on T2-weighted images was equally well visualized on fat-suppressed images. However, in two cases, fat-suppressed images demonstrated

malignant

nodal

ne-

cmosis that was not seen on the T2weighted images. In one case the malignant necrosis was detected in a normal-size node (less than 1.5 cm). The presence of necrosis is a useful finding for suggesting malignant nodal disease (36) and has been a majon reason for the use of computed tomognaphy in the staging of head and neck tumors. The detection of nodal disease is particularly important, since the presence of nodal as well as extranodal spread of tumor is one of the most important determinants of local treatment failure as well as patient survival (37,38). A disadvantage of fat suppression imaging was an artifact that resulted in asymmetric fat suppression. The cause of this artifact is not fully understood but may be a bulk susceptibility phenomenon is thought to arise

(39).

This

These magnetic field inhomogeneities cause water-fat frequency shifts within the imaging volume. As a resuit, the fat-selective RF pulse that ovemlies the fat frequency in one pontion of the imaging volume may overlie the water frequency in a different portion of the imaging volume. The result is fat suppression in one portion of the image with water suppression in another. This artifact did not occur in areas of constant imaging volume such as in the orbits on nasopharynx but was most troublesome in areas of changing tissue volume. This was most noticeable when imaging extended from the oral cavity to the susceptibility

neck.

These patient-specific differences can be

part-

ly corrected by centering on the area of interest when obtaining the waterfat spectrum and appropriately applying the fat suppression RF pulse. In addition, the use of water bags about the anterior neck help create a more uniform imaging volume and thus eliminate or reduce this artifact. In summary, contrast-enhanced fat suppression imaging appears to be a valuable technique for imaging cxtracranial head and neck abnonmalities. Our findings suggest that fatsuppressed images are complimentary to T2-weighted images and will replace conventional postcontrast Tiweighted images. Specifically, fat suppression imaging proved to be of greatest benefit in evaluating perineural as well as extracapsular nodal spread of tumor. In addition, the visualization of nodal necrosis not detected with other imaging sequences may prove invaluable in the staging of head and neck cancer by allowing differentiation between benign reactive and malignant adenopathy. Conventional nonenhanced Ti-weighted images still play a role, as they ensure that lipid-containing lesions are not overlooked on fat-suppressed images. They also serve to confirm that all bright lesions detected with fat suppression are in fact areas of contrast enhancement rather than areas containing a substance that is intrinsically bright on Ti-weighted images, such as proteinaceous fluid or blood. U

artifact

from susceptibility effects of the main magnetic field Bo caused by variations in the shape of the anatomy being studied. Thus, when the volume of imaged tissue changes, there are differences in tissue induction that result in main magnetic field inhomogeneities.

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