Neuroradiology Torizuka, MD Hidekazu Saitoh, MD Junji Konishi, MD

#{149} Katsumi

Tatsuo

High-Resolution A Modified High-resolution phy

(CT)

ticularly parallel

radiation lenses.

hires

H

tomogra-

temporal

bone,

exposure The authors

the radiation visualization use

to the paevaluated

of scanning

along

the

raphy

computed

(CT)

bone is widely of inflammatory

radiologists, with scans of 45 healthy

duced

by choosing

CT the

and

for comparison.

The change of the baseline from the orbitomeatal line to a line parallel to the hard palate decreased the radiation dose to the lens from 12.7 cGy to 0.274 cGy and improved visualization of the stapes superstructure and the tympanic portion of the facial nerve canal, although visualization of the incus

body,

incudostapedial

joint,

lateral semicircular canal, and oval window was of equal quality. Therefore, the authors recommend a new baseline

parallel

to the

hard

for use at high-resolution temporal bone. Index

terms:

and

the

‘

are

directly

radiation, can

exposed

and

to

the doses

be substantively

re-

a scanning

proto-

palate parallel

was substituted for the baseline to the orbitomeatal line (a line

from the outer canthus the external auditory

optimal

situation

changing ning both exposure

of the meatus).

would

eye to The

be that

provement

in visualization

temporal obtained.

bone

In the

of the

structures

present

study,

would

the

be

Temporal

evaluated

before

and

the

of axial

high-resolution

plane

after

MATERIALS

radiation

changing CT

From the City Hospital, 15,

1991;

Department of Radiology, Kyoto, Japan. Received

revision

sion received ary 3. Address

requested January

reprint

27,

September 1992;

requests

accepted

Kyoto August 19; reviFebru-

to T.T., Depart-

ment of Nuclear Medicine, Kyoto University Hospital, 54 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto, 606 Japan. © RSNA, 1992 See also the article by Yeoman et al (pp 113116) in this issue.

has

METHODS

CT of the temporal

High-resolution

bone

AND

been

performed

in our

institu-

tion since 1988, with both axial and coronal scanning. For the 1st year, a baseline for axial scanning was established parallel to the orbitomeatal line, similar to the plane used at routine head CT, with 2.0mm-thick sections at 2.0-mm intervals. In 1989, a baseline parallel to the hard palate was

used

instead

to reduce

the

radiation

exposure to the lens; this plane continues to be used. Since 1990, 1.0-mm-thick sections vals tions

have been acquired for detailed examination. were performed on

RX; Yokogawa,

its window

Newnan,

extended

at 1.5-mm interAll examinathe same scanner

Ga),

to 4,000

HU.

Other technical factors included 120 kVp, 130 mA, and 4.0-second scanning time. The high-resolution CT scans were evaluated of 45 patients who had temporal bones

that

were

diagnosed

as being

nor-

mal. Fifteen of the patients were examined with the baseline parallel to the orbitomeatal line, and the other 30 patients were examined with the baseline parallel to the hard

palate.

According

to the

baseline

and

the thickness and interval of the sections, the 45 patients were divided into three different groups (groups A-C) (Table 1) (Fig 1). For measuring the radiation dose to the lens, chips

five thermoluminescent (beryllium oxide;

Osaka,

Japan)

were

the region of the lent skull phantom cornea and three

placed

region

were

scanned

three

different

dosimetry National,

directly

over

cornea of a tissue-equiva(two chips on the right on the left, each placed

a different

sections

UD17OA,

of the

cornea).

with

methods

use

Six

of the

(groups

A-C),

and

the mean radiation doses to the five chips were measured for each method. In group A, partial volume irradiation of the chips may

result

in underestimation

of the

ation doses from scanning that irradiates the chips. Therefore,

1992; 184: 109-111

I

MD

RT

(Quantex

over

by

the method of axial scanreduction in the radiation to the patient’s lens and im-

dose was measured by scanning a skull phantom, and visualization of the temporal bone structures was

#{149} Radiations,

personnel

eyes

primary

(i-il). of the

scanning. Radiology

Tanaka,

Nakayama,

with

col that avoids direct irradiation. Thus, since 1989 in our institution, to reduce the radiation exposure to the lens, a baseline parallel to the hard

CT of the

Ear, CT, 21.1211

exposure to patients bone, CT, 12.121

palate

tomog-

temporal

toidectomy or tympanoplasty At axial scanning, the lenses

lenses

high-resolution patients,

of the

such as chronic otitis media or cholesteatoma, and in the evaluation of the middle ear cavity following mas-

to the

performed

#{149} Fumiko

Bone:

used in the diagnosis middle ear diseases,

patient’s

was

MD #{149} Yoshiki

Temporal

orbitomeatal line and on a line parallel to the hard palate. Evaluation of visualization was performed by five

x2 test

Satoh, RT

Ogura,

IGH-RESOLUTION

par-

dose to the lens and the of temporal bone struc-

with

#{149} Akio

MD

Baseline’

axial scanning on a baseline to the orbitomeatal line, pro-

duces tient’s

#{149} Yoshikazu

MD

Okuno,

CT ofthe

computed

of the

Hayakawa,

#{149} Yoshishige

in group

A are

accurately

than

likely

to be

those

measured

in groups

Thermoluminescent

performed with ter (National). Visualization temporal

a model

crura),

incudostapedial

panic lateral

dosimetry

was

dosime-

of six structures

superstructure

the

assessed

in the

the

incus

(the

head

joint,

portion of the facial semicircular canal,

window-was

body,

and

the

following

on

a scale

of 1-3

criteria:

score

tym-

nerve canal, and the oval by

five

the

radiolo-

gists (T.T., K.H., FT., H.S., Y.O.) and scored

C,

on a of the

UD5O2B

bone-including

the stapes

less B and

in which scanning was performed plane to avoid direct irradiation lens.

radi-

directly the doses

according

was to the

1 (excellent)-the

entire image of the structure was completely visualized; score 2 (average)-the entire image of the structure was incompletely visualized or only part of the structure was visualized; and score 3 (poor)-no image of the structure was visualized.

Visualization

of the

temporal

bone

109

structures

was

groups

with

compared

x2

the

among

(P

test


A

Incudostapedialjomt

B = A B > A B = A B = A

Facial nerve Lateral

Oval

*

canal

window

Note.-Images scans

canal*

semicircular

A

Group

C

A=C A A A A A

B

vs

Group

were of equal quality (=) or the first scan was better poorer visualization after the baseline was changed. portion of the facial nerve canaL

showed Tympanic

Group

vs

= > = > than

C

B=C B = B= B B > B =

C C C C C

=

the

scan

second

5.

C C C C C

6. 7.

( > ). No

8.

visualization

were

ble

2). The

site

of involvement

(10). facial

incus

relatively body

high

(Ta-

is a common

in cholesteatoma

Both the tympanic nerve canal and

portion the lateral

of the semi-

circular canal are main areas of focus in advanced cholesteatoma (1,2,7,8). A prosthesis is inserted into the oval window in synthetic ossicular replacement (3,4). Results of the x2 test concerning visualization of the temporal bone structures are shown in Table 3. Visualization of the incus body and the incudostapedial among the

three

visualization the

four

was

change

structures,

canal, suggest

scanning

equal

for

including

produced

184

#{149} Number

temporal

ing to their sure of 3.51

mographic

at high-

the highest expoC/kg to the cornea

plane.

to the

cellent

use

On

tomographic

to the gel

hard

concluded

15#{176} to-

the other

hand,

and

offered

of even

bone plane

palate. that

the

structures. is nearly

Chakeres both

direct

9.

10. 11.

12.

information of this

planes at 0#{176}, 30#{176}, in less radiation

lens

delineation

stapes

bone

report, x i0

use of tomographic 70#{176}, and 105#{176} resulted

the

better

phantom

increase in diagnostic demonstrated with

est temporal

These in the

a skull

CT with use of various toplanes in relation to the baseline (i4). Accord-

was measured for sections at a 15#{176} tomographic plane above the anthropologic baseline, which is very close to the orbitomeatal line. There was no

other

and Spiegel evalexposure to the

1

using

the

and oval window. that the change baseline

by

resolution mographic anthropologic

exposure after

tympanic portion of canal, lateral semicir-

visualization of the structures. In 1983, Chakeres uated the radiation

Volume

or better

in baseline

superstructure, the facial nerve

cular data

joint was equal groups. However,

lens

13.

exsmall-

The

14.

0#{176}

parallel

and

radiation reduced of the

the

decrease

interval

of the

exposure to to approxioriginal dose by

U

References

3.

Table 3 Comparison Temporal

and

and

the change in the baseline. We therefore recommend the baseline parallel to the hard palate for use at high-resolution CT of the temporal bone.

1

Incudostapedialjoint Facial nerve canal*

Note-Score * Tympanic

Group

GroupA

thickness

sections. The the lens was mately 1/50th

Bone

Structure

Stapes

for Each Score for the Temporal

(Total Value for Each Score)

in baseline

in the

phy.

Spie-

axial

(0#{176} or 30#{176}) and coronal (70#{176} or 105#{176}) sections are necessary and complementary for the most sensitive evaluation of the temporal bone. In the present study, not only a marked reduction in radiation exposure to the lens but also better visualization of the temporal bone structures were obtained through the

Johnson OW, Voorhees RU, Lufkin RB, Hanafee W, Canalis R. Cholesteatomas of the temporal bone: role of computed tomography. Radiology 1983; 148:733-737. Virapongse C, Rothman SLG, Sasaki C, Kier EL. The role of high resolution cornputed tomography in evaluating disease of the middle ear. J Comput Assist Tomogr 1982; 6:711-720. Swartz JO, Berger AS, Zwillenberg 5, Granoff OW, Popky GU. Synthetic ossicular replacement: normal and abnormal CT appearance. Radiology 1987; 163:766-768. Swartz JO, Lansman AK, Berger AS, et al. Stapes prosthesis: evaluation with CT. Radiology 1986; 158:179-182. Tono T, Miyanaga 5, Morimitsu T, Matsumoto I. Computed tornographic evaluation of middle ear aeration following intact canal wall tympanoplasty. Auris Nasus Larynx 1987; 14:123-130. Johnson OW. CT of the postsurgical ear. Radiol Clin North Am 1984; 22:67-75. Jackler RK, Dillon WP, Schindler RA. Computed tomography in suppurative ear disease: a correlation of surgical and radiographic findings. Laryngoscope 1984; 94: 746-752. Voorhees RU, Johnson OW, Uufkin RB, Hanafee W, Canalis R. High resolution CT scanning for detection of cholesteatoma and complications in the postoperative ear. Laryngoscope 1983; 93:589-595. Swartz JD, Goodman RS, Russell KB, Ladenheim SE, Wolfson RJ. High-resolution computed tomography of the middle ear and mastoid. Radiology 1983; 148:461-464. Swartz JD. Imaging of the temporal bone: a text/atlas. New York: Thieme, 1986. Shaffer KA, Haughton VM, Wilson CR. High resolution computed tomography of the temporal bone. Radiology 1980; 134: 409-414. Vignaud J, Laval-Jeantet M. Anatomy of the temporal bone and the ear. In: Fischgold H, ed. The ear: diagnostic imaging. Paris: Masson, 1986; 8-26. Isono M, Murata K, Ohta F, Yoshida A, Ishida 0. High resolution computed tomography of auditory ossicles. Acta Radio! 1990; 31:27-31. Chakeres DW, Spiegel PK. A systematic technique for comprehensive evaluation of the temporal bone by computed tomogra-

15.

16.

Brogan

Radiology M, Chakeres

1983;

146:97-106. DW.

Computed

to-

mography and magnetic resonance imaging of the normal anatomy of the temporal bone. Semin Ultrasound CT MR 1989; 10: 178-194. Chakeres DW. CT of ear structures: a taibred approach. Radiol Clin North Am 1984; 22:3-14.

Radiology

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