Radiation barriers

Radiation barriers

R

ad&ion barriers are used in dentistry to cnsurc that the rate of esposurc of the dentist and others will not vxceecl recommended limits. Primary barriers protect against the useful beam, whereas secondary harriers are those exposed only to leakage and scattered radiation. When computing the thickness of a radiation barrier, the maximum permissible exposure (P) of an occupat,ionally exposed person, such as a dentist or his x-ray technician, may be consitlcrcd to bc 0.1 r per week. For nonoccupationally cxposcd persons, such as the clentist’s other helpers, paticnt,s in the reception room, neighbors, and others, the cxposar~ should not exceed 0.01 r per week. The x-ray exposure that a person incurs while hc receives dental or medical t rcatmtnt is not ilss(~ssc~(Iagainst t I\(’ afol~(~lncrltio:lc~tl limits. PRIMARY

RADIATION

BARRIERS

701

702

Pig. radiation

Ric~h II 1~7s

1. Plan of a dental office in which a wall of thickness barrier for the protection of a person at position A.

OS., O.M. & O.P. May, 1968

“X”

is used as a primary

FG/. 2. Attenuation in concrete of x-rays produced by potentials of 50 to 100 KVP. Total filtrations of 1.0, 14 and 2 mm. of aluminum wore used with 50, 70, and 100 KVP, respectively. Density of the concrete was 2.35 grams per cubic centimeter. (Redrawn from Trout et al. : Radiology 72: 62-67, 1959.)

Radintion

bnrriers

703

nate of this graph is the factor K, which is determined by Formula I, and the abscissa is the desired thickness (“X”) of the radiation barrier shown in Fig. 1. Occupancy

factor

If a person oc,cupies position A (Fig. 1) for 40 or more hours per week, an occupancy factor (T) of 1 is applicable, which means 100 per cent occupancy. Occupation of position A -for a small part of the time or only occasionally would permit occupancy factors of l/4 and l/16, respectively, to bc usrd in the computation of primary radiation barriers. The maximum weekly exposure permitted 1 P at’ position A is P x - or - r/week. Therefore, a person may occupy a position T T in spa,ce where the exposure rate is grcatcr than the maximum permissible exposure per week, provided his occupancy of that position is proportionately reduced. The occupancy factor (T) is a dimensionless number. Distance

The distance between position A and the x-ray target is “d” meters. The maximum weekly exposure permitted at position A is P/T r/week. By applying the inverse-square law, the maximum weekly exposure permitted at 1 meter from the target is 4

x $

or Pd’/T.

If d is measured in feet, it must be divided Work

by 3.28 to convert feet to meters.

load

The weekly work load ( W ) is the product of the t,ube current (Ma.) and the number of minutes the x-ray machine is act,i\-atctl per week. The number of films exposed per week multiplied by the average time of exposure per film provides the length of time, in seconds, t.hat the x-ra.,v machine is in operation each week. This length of time must bc (ii\-itled by 60, to convert seconds into minutes, for the determination of the work load in milliampere-minutes per week. Although this t~stimat(~ probably will be too high, let us assume that the average dentist IMPS one box of films i 150 exposures) per week. If these are fast films with an avc:ragc t3posur(~ time of 0.25 srconct or slow films with an average cq)osure time ot’ I .5 seconds. thr lt~ngth ot’ time the x-ray macbint3 is nctivwt,etl per ~4i wolii!i r:irt~(’ frorrl ST..? to times of r~xposurc~ apylivd 10 il ~l~ntal s-ray machinc~ ~!p<~rate(\ at 10 31;~. wonltl indicate act,ual pork loads of 6.25 to 37.5 nlillia~n~)ere-min1~t~cs per week. When planning primary radiation barriers, it would be advisable to anticipate that the usage of an s-ray machine may incrrasc somewhat in the fnturr: therefore some increase ovw the present work 1~~1 should be used in the computations. The maximunt possible ~sagc of the s-ray eyuipmmtj is dettbrmincd by a human limitation-the rate at which film packets ran bc positioned and exposed in the patient’s mouth. One esposurc every 30 swonds would be a very fast a.versge pace when the time spent by the operator in washing his hands a.nd in 225

Si~~0ilcls.

TitWC

total

O.S., ox. c O.P. Ma?-, 1968

qutting the patients in ad out of the chair is also included in this average time. A rate of one film esposecl t~ery 30 seconds for 40 hours indicates that 4,800 films corrceivably c~ould hc exposetl in one week. If these were slow films, which nla~- rc(luirc 1.,5 sccontls for the avcragc~ csposnrc with a tube current of IO Ma., tl10 \vorli 1o:rtl WOlllCl be as follows: 4,X00 exp./week x 1.5 sec./exlh x 10 11a. -__-= 1,200 Ma.-min./week GO sec./rnin.

\‘Vith fast films, which may require only 0.25 second for the average exposure, the work load would be as follows : 1,800 exp./week

x 0.25 sec./exp.

x 10 Na.

GO sec./min.

Z= 200 Ma.-min./week

Some dental x-ray machines operate on a 5 per cent duty cycle, which means that they should not be activatecl, on the average, for more than 3 seconds per minute. This limitation is necessary to prevent damage to the x-ray tube through overheating. This mechanical limitation of 1.5 seconds of use per 30 seconds is identical to the previously postulated maximum rate of usage with slow films. The mechanical limitation is not approached when high-speed films are employed. Use factor

During the x-ray examination of a patient’s teeth, the useful x-ray beam is tlircctrd in many different vertical and horizontal clirections, with only a small fraction of the total number of exposures occurring while the useful beam is directed toward position A in Fig. 1. This fraction is known as the use factor (U), which is a dimensionless number. The actual work load that is directed at position A would be as follows: TV x U Ma.-min./week

It has been suggestcdl that when complete data regarding use factors are not, available for the computation of primary protective barriers for dental offices, t,he factors to use should be l/3 for the four walls and l/l6 for the floor and ceiling. However, a recent study 2 has indicated that the use factor should be l/16 for the floor and three exposed walls and 0 for the ceiling and wall which the patient faces. This wall and the ceiling must be treated as secondary barriers. SECONDARY

RADIATION

BARRIERS

All particles of matter that arc exposed by the useful x-ray beam become SOII~CCSof secondary radiation which scatters in all directions. Thus, the filter, the pointed plastic (*one, aiuZ the il~l*acliiltetl portion of the patient momentarily becomc sour~s of secondary radiation each time the x-ray machine is activated. ,Since the walls, floor, and ceiling are all exposed by this secondary radiation, a use factor of I is required in all computations of secondary radiation barriers. IImdbook 76 states that ” . . . the amount of 90” scattered radiation is approximately 0.1 percent of that incident, nnnn the a~n++~~~~ ‘J’+n, : -.Y&c sf K

JO00 times greater may bc allowed for scattcrcd radiation than for that of the useful beam .” Thrreforr, Formula 1I for the SWIFT I~~ltation of sccondar~ barriers is as follows :

For il given set of conditions, Ihe rcyuircvl thickness for the secontlwr- ril(liaOf K with l+‘OITlll1lil 11 t ion barrier can be tletermincd by calcnlatin, 0' the ViLlllP and then reading, froirl Fig. 2, the thickness of a corlcretc l);trYirr corrrspontling to the value determined for K. COMPUTING

RADIATION

BARRIER

THICKNESSES

IIow many inches of concrete or its qnivalent woul~l bc neeessaq- in the side mall of the dental office shown in Fig. 8 to protect a nonoccupationally exposed person at point A from excemive exposure to primary radiation? Five solutions to this problem are given in Table I. The first example applies when slow films and a short cone (8 inch source-film distance) are used with the x-ray machine operated at 70 KVI’. The previously described maximum 1,200 Ma.-min. -for low-speed films is assumed. Values for possible work load of week the other terms in Formula T plus its solut,ion arc likcwisc listed in Table I. Fig. 2 indicates that a primary barrier of 2.3 incahcs of concrete or it,s equivalent is JJNV3SSary at, 70 KVP when K = 0.0012. In the second example, the short CWJIP is replaced by one that is 8 inches \OJJger. This change doubles thr previous sourvca-film distance, and the times of thv work load becomes tspOsnJ~c aIT invrCas(vl l)\- il factor of 4. Thrrrforc, -l x 1,200 Ma.-min. -----. 7’11~10ng~r (*OI~P11~Jlgt~l ~L;IS who m0v~t1 the radi;ltion sourc’(: week S inches farther from I)osition -1; IINIW, Ihc distanccl ((1,) I~vv~mes 10 feet i:

O.S., O.M. & O.P. May, 1968

Table

I II I

/ Tttbe

/~smnple 1 2 3 4 5

/

TO~~OQP

(gyp) 70 70 100 100 100

Length of ame Short Long Short Long Lone

Source-film

wo~~;~m~nIw) . .

distance

(inches)

1: 1: 16

Film speed Slow Slow Slow Slow Fast

week 1,200 4 x 1,200 l/3 x 1,200 4 x l/3 x 1,200 4 x l/3 x 200

inches, or 10.67 feet. A thicker wall, 3.1 inches of concrete or its equivalent, would be needed to achieve adequate protection under the conditions of this example. The third example is the same as the first, with the exception that the tube voltage is increased to 100 KVP. At this higher voltage, the x-ra.y tube emits t,hree times as much radiation per second; therefore, the times of exposure need be only one third as long as those used with 70 KVP. The work load then beMa.-min. comes I$,$ x 1,200 week . A primary barrier of 3.3 inches of concrete or its equivalent is required this example.

when radiation

generated with

100 KVP is employed in

The fourth example is similar to the second in that a long cone is used; in addition, however, the higher voltage of example 3 is also used. The work load Ma.-min. then becomes 4 x 1/3 x 1,200 and the distance from the source to position week A becomes 10.67 feet. Under these new conditions, the thickness of the primary barrier must be 4.5 inches of concrete or its equivalent. The fifth and last example is identical to the fourth except that the previously described maximum work load for high-speed film has been used. Thus, the work Ma.-min. load with the long cone and high voltage becomes 4 x 1/3 x 200 week . A concrete wall 2.8 inches thick will be adequate when high-speed film is utilized. Would a 4 inch brick wall suffice as a primary radiation barrier for the person at position A in Fig. l? The concrete equivalent of most common building materials can be found in the same manner as is indica,ted here for brick: Thickness of brick wall x density of brick = Thickness of concrete equivalent to brick x density of concrete. 4 inches x 1.9 grams per cubic centimeter = Concrete equivalent x 2.35 grams per cubic centimeter. 4 x 1.9 Concrete equivalent to 4 inches of brick = 235 = 3.24 inches. Therefore, a 4 inch brick wall, which is equivalent to 3.24 inches of concrete, would be adequate as a primary protective barrier in examples I, II, and V but

Volume 25 Number 5

Radiation

( Distance (cl) f Yom source (feet) 10 10.67 10 10.67 10.67

Permissible exposure (P) (r/week) 0.01 0.01 0.01 0.01 0.01

not in examples III in each example.

USC factor (17) l/16 l/16 l/l6 l/16 l/16

1

Occupnncy

factor

(T) 1 1 1 ;

bnrriers

E 0.0012 0.0003 0.0037 0.0011 0.0063

707

l’hickness of primary barrier (inches of conCrete) 2.3 3.1 3.3 4.5 2.8

and IV, even though the snme number of films were exposed

SUMMARY

Five examples have been given of the computations involved in determining the proper thicknesses of primary radiation barriers. The total number of films exposed was the same in each cxa.mple, but the time of exposure and hence the work loads were different because various combinations of film speed, kilovoltage, and target-film distances were employed. It was found that the use of higher-speed films permitted the thickness of the primary radiation barrier to be decreased, while increases in either the tube voltage or the target-film distance required t,hicker primary radiation barriers. X brick wall, which afforded adequate protection against primary radiation exposure when the x-ray machine was operated at 70 KVP with either the short or the long cone, was no longer adequate when the same number of exposures was made at 100 KVP, regardless of the cone length. The required thickness of primary radiation barriers provc~l to be clcpendent upon the target-film distance, tube volt,age, and film speed. REFERENCES

1. National Bureau of Standards: Medical X-ray Protection up to Three Xillion Volts, Handbook 76, Washington, D. C., 1961, Government Printing Office. 2. Richards, A. G.: The “Use ‘Factor” in Radiation Bnrric,r Design, C&AI. S,rw;., ORAL MEL rP ORAL PATII. 23: 745-750, 1967.