Techn ica l R e p o r t

Absorbed Doses Reduced by the Use of Rare-Earth

Intensifying Screens in Rotational Panoramic

Radiography with Constant Potential Generator

Yoshihiko H A Y A K A W A , B.Sc., M.Sc., Nor io K O B A Y A S H I , R.T.,

Yuji KOUSUGE, R.T., Hisao F U J I M O R I , R.T.,

Kouichi H A R A D A , D.D.S. and Kinya K U R O Y A N A G I , D.D.S., Ph.D.

Department of Oral and Maxillofacial Radiology, Tokyo Dental CoIlege, Chiba, Japan

(Received : March 1, 1993, Revision received : May 7, 1993, Accepted : May 14, 1993)

Key Words : Radiography,dental ; Radiography,panoramic ; Radiation dosage ;Screen-film

Absorbed doses modified by the use of rare-earth intensifying screens were measured with rotational

panoramic radiography. Equipments with the constant potential, Veraview (J.Morita) and PM 2002 CC

(Planmeca) was utilized. The optimum exposure settings at each tube voltage were determined by

radiography of a Rando | phantom. Absorbed doses to parotid and thyroid glands were measured under

the optimum exposure settings by using the Rando | phantom and thermoluminescence dosimeters.

The sensitivity of the rare-earth intesifying screen system increased as the tube voltage became

higher. Absorbed doses decreased steeply as the tube voltage became higher. The highly sensitive

system of the LANEX REGULAR and T-Mat G (Eastman Kodak) reduced the thyroid gland dose to a

half at 70 kV in comparison with the regular system of the PX-III (Kasei Optonics) and X-Omat S

(Eastman Kodak). This high sensitivity, however, made it impossible to set the optimum tube current

at above 70 kV. The another rare-earth system, the TRIMAX 16 and TRIMAX XUD (3M) had a lower

sensitivity than the regular system of the PX-III and X-Omat S at below 80 kV. But they had equal

sensitivity at the highest tube voltage of 80 kV.

I n t r o d u c t i o n

T h e use of r a r e - e a r t h in tens i fy ing

screens and o r thochromat i c films is an

effect ive method for dose reduct ion in

ro ta t iona l panoramic rad iography ~-1~ How-

Oral Radiol. Vol,9 No.1 1993(49~55)

ever, some problems have been ra ised on this

application.

The sensi t iv i ty of ra re -ea r th intensifying

screens falls down at low x- ray energy 2'1').

Keur 2) repor ted tha t its sensi t ivi ty at 60 kVp

49

was lower than t ha t of CaWO~ intensifying

screens and regu la r - type films. Therefore ,

r a r e -ea r th screens should be used with h igher

tube vo l tage for dose reduct ion.

High sens i t iv i ty requi res a reduced x - r a y

output. However , the e q u i p m e n t used by

Keur 2> or Hur lbu r t and Coggins 4) had a fixed

tube current which made it imposs ib le to

reduce x - r ay output when the higher x - r a y

energy was used .The mer i t of high sens i t iv i ty

to the higher x - r a y energy 12) could not be put

to good use. Tynda l l and W a s h b u r n 5) or

Ponce et al. 9''~ a t t e m p t e d to resolve this

p rob lem by using a th ick added filter. Ponce

et a12 ) reduced absorbed doses by 60 percent

when a thin foil of r a r e - e a r t h sc reen or alumi-

num of 4 m m th ickness was added. The

n a r r o w beam width which was ob ta ined by

the thinner sli t is the o ther me thod for output

reduct ion ~,7~. T h i s me thod s imul taneous ly

p roduced a th i cke r image layer .

Fur ther , the a l t e r a t i on of i m a g e qua l i ty

is also a p rob lem TM. But in observer perfor-

mance examina t i ons a'4'~'8-1~ resu l ted tha t the

r a r e - ea r t h screen produced the d iagnos t ic

i m a g e qual i ty compa t ib l e wi th the CAW04

screens. McDavid et al. 8> desc r ibed tha t the

high sens i t iv i ty of the imag ing sys t em did not

d i r ec t ly re la ted to the low sharpness .

On the o ther hand, the use of a cons t an t

po ten t ia l of x - r a y gene ra to r was thought to

be the other m e t h o d for dose reduction13L

This m a k e s i t poss ib le to genera te a h igher

energy of x-ray.

The purpose of this s tudy was to ob ta in

the op t imum e x p o s u r e se t t ings when rare-

e a r t h i n t e n s i f y i n g s c r e e n s and o r tho -

ch roma t i c films were used for ro t a t i ona l

p a n o r a m i c r a d i o g r a p h y with cons t an t poten-

tial. The m e a s u r e m e n t of absorbed doses to

both pa ro t id and thy ro id glands was perfor-

med. I t was inves t iga ted how the abso rbed

dose changed by depending on the exposu re

se t t ings and wha t ex t en t of dose reduct ion

was caused by using r a r e ea r th screens.

Materials and Methods

X-ray equipment, intensifying screen and film

T w o x - r ay mach ines for ro t a t iona l pano-

r amic r ad iography , Verav iew (J. Mor i t a

Corp., Kyoto, Japan) and PM 2002 CC (Plan-

meca Oy, Hels inki , Finland) were used. The

specif icat ions a r e l is ted in T a b l e 1.

T w o types of r a r e - ea r th intensifying

screens and o r t h o c h r o m a t i c films combina-

tion, such as the L A N E X R E G U L A R screen

and T - M a t G film (Eas tman K o d a k Company,

Rochester , N e w York, U.S.A.), and the

Table 1 Specifications for Veraview and PM 2002 CC

X-ray generator Tube voltage

Tube current

Total filtration Exposure time Focal spot Focus-film distance

Veraview PM 2002 CC

constant potential 60-80 kV

(5 kV intervals) 5-10 mA

(1 mA intervals) 2.1 mmA1

15 sec 0.5 x 0.5 mm

510 mm

constant potential 60-80 kV

(2 kV intervals) 4-12 mA

(1 mA intervals) 2.5 mmAl 3.1-18 sec*

0.6 x 0.6 mm 480 mm

*Exposure time is set at 18 sec in this study.

50

TRIMAX 16 screen and T R I M A X XUD film

( 3M company, Germany) were used. These

rare-earth intensifying screens were com-

posed of Gd202S:Tb. The combination of PX-

III (Kasei Optonix Corp., Kanagawa, Japan)

and X-Omat S film (Eastman Kodak) as a

CaWO4 intensifying screen and regular-type

film was used. All films were processed by an

automatic image processor New RN (Fuji

Medical System Corp., Tokyo, Japan) at the

processing t ime of 210 sec and a temperature

of 30~ according to the manufacturer ' s

recommended method.

Exposure settings An operator can choose the settings of

both the tube voltage and current for the

optimum exposure. The optimum tube cur-

rent at each tube voltage was determined by

radiography of a Rando | anthropomorphic

phantom (Alderson Research Laboratories,

Stamford, CN, USA). A Rando | phantom

which was sectioned horizontally at 2.54 em

intervals was used. Sections 6 and 7 corre-

spond to the upper and lower jaws. The four

radiographs obtained by different tube cur-

rents in turn were examined by four

observers, two oral radiologists and two

radiological technologists. The best radio-

graph in four with every tube voltage was

determined by them.

Dosimetry Absorbed doses to the parotid and thy-

roid glands were measured with the optimum

exposure settings of tube voltage and current.

The absorbed doses to the parotid gland and

thyroid gland were measured by the use of

fifty thermoluminescence dosimeters (TLD-

100, Harshaw Chemical Co.,Solon, OH, USA)

in the form of LiF rods 1 • 1• 6 mm.

Since the method of thermolumineseent

dosimetry which was utilized in this study

was the same as was described in a previous

article 14~, the method in detail is abbreviated

in this article. Four TLDs were placed at the

appropriate sites within the sections between

6 and 10. Two were placed in both sides of

section 6 at the location of the parotid gland,

and two were used for the thyroid gland in

section 10. Each dosimeter received ten x-ray

exposures. Each measurement was repeated

three times. The results of the absorbed dose

per single exposure are presented.

Results

The selected opt imum exposure settings

with every tube voltage are indicated in

Figure 1. Observers more than three in four

agreed with all opt imum exposure settings.

The alteration in the sensitivity made a

difference in the optimum setting. The opti-

mum tube current decreased as the tube

voltage increased at all three combinations

examined.

The absorbed doses to the parotid and

thyroid glaads at each optimum setting are

indicated in Figures 2 and 3. Absorbed doses

obtained from using the orthochromatic sys-

tems decreased steeply as the tube voltage

became higher. The highly sensitive ortho-

chromatic system of the LANEX REGULAR

intensifying screen and T-Mat G film reduced

the thyroid gland dose to a half at 70 kV in

comparison with the regular system of the

PX-III intensifying screen and the X-Omat S

film. This high sensitivity, however, made it

impossible to set the optimum tube current at

the tube voltage above 70 kV. The sensitivity

of the or thochromat ic sys tem of the

T R I M A X 16 intensi fying screen and

TRIMAX XUD film was lower than the sys-

tem of the PX-III and X-Omat S at below 80

kV. But, they had equal sensitivity at the

highest tube voltage of 80 kV.

5/

Fig. 1 Opt imum e x p o s u r e set t ing of tube cur- rent with every tribe voltage. (A : Ver- aview, B : PM 2002 ~ 10

CO) ~ 9 �9 : LANEX REGU- --

c 8 LAR and T-Mat G �9 : T R I M A X 16 and k 7

D T RI M AX XUD C) 6 �9 : PX-III and X-

_o 5 O r n a t S

I--

A

Fig. ~ Parot id gland dose A (mean+_S.D.) at each

o p t i m u m e x p o s u r e 1600 set t ing wi th every tube voltage. >, (A : Veraview, B : PM (.9 2002 CC) ~ 1 2 0 0 �9 : LANEX REGU- r LAR and T-Mat G o

�9 : T R I M A X 16 and -o 8 0 0 T R I M A X XUD e-

ra �9 : PX-[II and X- Omat S -o

�9 . = 400 s s

Fig. 3 Thyroid gland dose (mean_+S.D.) at each o p t i m u m e x p o s u r e 40 set t ing wi th every ~., tube voltage. C~ (A : Veraview, B : PM :::1.

v 30 2002 CC) r �9 : LANEX REGU- tn

o LAR and T Mat G

�9 : T R I M A X 16 and "o 2 0 c T R I M A X XUD r

�9 : PX-III and X- Omat S "o

�9 1o (-

I.-.-

0

1 I I I I 6O 70 8O

12 11 lC 9 8' 71

f ,1 I I, I . I I I I I I I 60 70 80

Tube Voltage (kV)

B

1600

1200

800

400

I I I . 60 70

I 0 " I i I I I I I I I I I 80 60 70 80

Tube Voltage ( kV )

I I I I I 6 0 70 8 0

40

30

20

10

0 ,, I I I t I I 1 I I I I 60 70 80

52 Tube Voltage ( kV )

Discuss ion

Optimum exposure settings and absorbed

doses

The optimum exposure settings were

determined as the tube current with every

tube voltage. The high sensitivity of rare-

earth intensifying screens and ortho-

chromatic films need a reduced x-ray output.

The higher sensitivity of the LANEX REGU-

LAR screen and T-Mat G film made it impos-

sible to have an adequate image quality

above 70 kV, because of the excess x-ray

output even if the lowest tube current was

used. The merit of the high sensitivity to the

higher x-ray energy 12) could not be put to

good use. Accordingly, the reduction of x-ray

output by some methods, such as the setting

of lower tube current, use of narrower slit

width 1'7~ or installation of thicker filter 5,9,1~

was needed.

Parotid and thyroid glands doses when

using rare-earth intensifying screens and

orthochromatic films decreased steeply as the

tube voltage became higher. On the other

hand, doses from PX-III CaWO4 intensifying

screens and X-Omat S regular-type films

showed to be nearly constant, although the

tube voltage changed. The rare-earth screen

is more sensitive to the higher x-ray energy in

comparison with the CaWO4 screen, because

the high energy x-ray above the k-shell

absorption energy of rare-earth metal is

absorbed efficiently by the rare-earth screen.

The use of the high energy x-ray with the

rare-earth screen is effective for dose reduc-

tion. The constant potential also generate a

highly effective energy of x-ray. The combi-

nation of rare-earth screens and constant

potential generator result in dose reduction.

Parotid gland and thyroid gland doses

Absorbed doses measured in this study

are compared with previous studies 1~-23).

Some researchers gave attention to absorbed

doses to the parotid and thyroid glands which

had stochastic radiological risks.

The parotid gland doses were much

lower than the doses reported by previous

studies which were shown in Table 2. The

parotid gland doses measured in this study

were distributed in the range of 0.4-1.6 mGy

which are thought to be comparatively low.

This decrease of the parotid gland doses was

caused by the difference with the position of

the rotational axis of the x-ray beam. The

rotational axis at the start and end of projec-

tion in the PM 2002 CC x-ray machine were

17 cm apart from each other and located

outside and behind the mandibular ramus 24).

Namely, the position of the lateral rotational

axis where the absorbed dose increased was

not close to the parotid gland in this equip-

ment.

The thyroid gland doses which were

reported previously were shown in Table 3.

The thyroid gland doses measured in this

study were distributed in the range of 0.01-

0.04 mGy which are similar to previously

reported absorbed doses.

When rare-earth intensifying screens

were used and the tube voltage became

higher, the parotid gland dose showed a stee-

Table 2 The parotid gland doses in the literatures and in this study

Authors Parotid gland dose (mGy)

Stenstr6m et al. TM

Stenstrtim et al. 21) Manson-Hing and Greer ~6) Wall et al. ~71 Bartolotta et aI. ~9) Nilsson et al. TM

Underhill et al. 22) this study

0.30-1.42 0.26-0.36 0.07-1.9 0.45-2.8 0.84-4.17 1.0-3.2 0.1-1.2 0.4-1.6

53

Table 3 The thyroid doses in the literatures and in this study

Authors Thyroid gland dose (mGy)

Antoku et al. TM

Manson-Hing and Greer 1~ WalIet al. In

Stenstr0m et al. TM

Bartolotta et al. TM

Nilsson et al. TM

Stenstr0m et al. TM

Underhill et al. TM

Kassebaum et al. TM

this study

0.01-2.69

0.04-0.51

0.01-0.30

0.03-0.09

0.13-0.37

0.01-0.09

0.03

0.03-0.06

0.05

0.01-0.04

per d e c r e a s e t h a n the t h y r o i d g land . T h i s

was caused by the s c a t t e r i n g r a d i a t i o n w h i c h

was e f f ic ien t ly de l ive red by the h ighe r t ube

vo l t age . T h e r a t e of s c a t t e r i n g r a d i a t i o n

which i n c r e a s e d the abso rbed doses was sup-

posed to be h igher in the t h y r o i d g l a n d t h a n in

the p a r o t i d gland.

S t o c h a s t i c r i sk f r o m r a d i a t i o n e x p o s u r e

a r e in p r o p o r t i o n to e f fec t ive dose. E f f e c t i v e

dose is g iven by the s u m of t he p r o d u c t of t h e

dose e q u i v a l e n t to each t i s sue and i ts we igh t -

ing f ac to r . A b s o r b e d doses to t he t h y r o i d

g l a n d and s a l i v a r y g lands inc lud ing p a r o t i d

g l a n d occup ied the m o s t of e f f e c t i v e dose

w i t h r o t a t i o n a l p a n o r a m i c radiography25,2%

C o n c l u s i o n

W e r e a c h e d the fo l l owing conc lu s ions "

1 . T h e sens i t iv i ty of the r a r e - e a r t h in tes i fy-

ing s c r een s y s t e m irwin:eased as t he t u b e

v o l t a g e b e c a m e higher. A b s o r b e d doses

to the pa ro t i d and t h y r o i d g l ands de-

c r e a s e d s t eep ly as the t u b e v o l t a g e

b e c a m e higher .

2 . T h e h igh sens i t ive s y s t e m of t he L A N E X

R E G U L A R and T - M a t G r e d u c e d the

t h y r o i d g l and dose to a ha l f a t 70 k V in

c o m p a r i s o n wi th the r e g u l a r s y s t e m of

t he PX-III and X - O m a t S . T h i s h igh

sens i t iv i ty , h o w e v e r , m a d e i t imposs ib le

to set t he o p t i m u m tube c u r r e n t a t a b o v e

70kV.

�9 T h e r a r e - e a r t h s y s t e m of t he T R I M A X

16 and T R I M A X X U D had the l o w e r

s ens i t i v i t y t h a n t h e r e g u l a r s y s t e m of the

PX-III and X - O m a t S a t b e l o w 80 kV.

But t h e y had the equa l s ens i t i v i t y a t the

h ighes t t ube v o l t a g e 80 kV.

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Tokyo Dental College i-2-2 Masago, Mihama-ku, Chiba 261, Japan

55