Scattered radiation absorbers in intraoral radiography

Scattered radiation absorbers in intraoral radiography A. H. Shawkat, B.D.S., M&L,* SCHOOL

OF DENTAL

MEDICINE,

Philadelphia, UNIVERSITY

Pa.

OF PENNSYLVANIA

In a study of scattered radiation absorbers, lead foil was used as a substitute for the stationary grid. There was a gradual increase in contrast as the thickness of lead foil was increased. A thickness of 0.08 to 0.10 mm. lead provided as much improvement in contrast as the stationary grid. The improvement in contrast was more pronounced at a lower KVP. However, the use of lead foil required an increased exposure in order to maintain an acceptable density level.

I

t is known that a significant portion of the x-radiation that reaches the film is composed of secondary radiation from the structures that intervene between the source of the primary beam and the film. The linear portion of that beam contributes largely to the diagnostic quality of the image, while the scattered part adds to what is referred to as scattered radiation fog.l The amount of fog and its deleterious effect on radiographic contrast have been investigated in previous studies.** 3 The effect of scattered radiation can be reduced by the introduction of a grid between the irradiated structures and the film. The grid is a device which consists of alternating strips of lead and radiolucent material arranged in a plane in front of the f&n in such a manner that the straight portion of the beam is transmitted relatively freely, whereas scattered radiation from the structures under examination is highly attenuated.2 The use of the grid is well known in radiology ; however, the applicability of a grid in intraoral radiography has not heretofore been investigated. The purpose of this study was to examine the feasibility and value of the This investigation was supported by General Research Support Grant FR 5 Sol RR 05337 from the General Research Support Branch, Division of Research Facilities and Resources, National Institutes of Health. *Ad&ant Professor of Radiology; Active Member of the American Academy of Dental Radiology.

439

440

Silawkat

Oral March,

120

c

.5

I

Added Pig.

1. Attenuation

8urg. 1971

curves

for

half-value

1.5

Filter, layers

2

2.5

3

(mm Al) (total

filter

=

2.5 mm.

aluminum).

use of a stationary grid or a suitable substitute with periapical and occlusal films, as well as the effect of the introduction of such a medium on radiation exposure to both the film and the patient. MATERIALS AND METHODS A standard self-rectified dental x-ray unit (G.E. Model 100) was used. It had an open-ended plastic cone that provided an g-inch target-film distance. The unit may be operated up to 100 KVP, using 10 or 15 Ma. and a total filtration equivalent to 2.5 mm, aluminum. In this study 65, 75, and 90 KVP and 10 Ma. were used to conduct the various experiments. Half-value layers (HVL’s), measured for the three KVP’s, using aluminum disks and a Victoreen 25r ionization chamber, were 1.8, 2.0, and 2.6 mm. aluminum, respectively (Fig. 1). For exposure rates, readings were adjusted after calculating the correction factors of the r meter for each HVL (Table I).

Volume Number

Table

Scattered

31 3

I. Exposure rates and HVL’s EVP 65 75 90

HVL

(mm. 1.8 2”::

Al)

radiation

absorbers

441

(10 Ma., 2.5 mm. aluminum total filter) Correction factor

r meter reading per minute

Corrected reading per minute

66 84 116

0.90 0.89 0.87

59.4 74.8 101.0

Stationary grids the size of occlusal and periapical film packets were prepared.* Each grid had a focal distance of 8 to 16 inches, eighty lead strips per inch, and a grid ratio of 5:l (the grid ratio is referred to as the ratio of the height of the lead strips to the distance between them). The grid absorbs a considerable amount of scattered radiation and thus improves the radiographic contrast of the image. However, the use of such a grid with periapical and occlusal films was disappointing (Fig. 2)) as superimposition of grid lines on anatomic structures made interpretation difficult. Therefore, further use of stationary grids in the study was abandoned, and it was decided to investigate lead foil as a scattered radiation absorber. The question was whether it would be possible to achieve any improvement in image contrast by intervening a definite thickness of lead between the film and the structures to be examined. Various thicknesses of lead foil (0.02, 0.04, 0.06, 0.08, and 0.10 mm.) were cut and placed in front of periapical film packets (Kodak Ultra-Speed). In order to make exposures under conditions simulating tissue examination and also facilitate the measurement of film density that was due only to secondary radiation, an apparatus devised for previous studies was used.2$3 This consisted of a l-inch water phantom in which a circular disk of lead, l/s inch thick, could be suspended to shield part of the film from the primary beam. Darkness of the film under the lead disk would be due to its exposure by secondary radiation. The remainder of the film was exposed to both primary and secondary radiation, except for a second portion where another piece of lead was in contact with the film to shield it completely from any radiation. Darkness of this part provided base or control density (Fig. 3). A number of exposures were made for each thickness of the lead foil under conditions shown in Table I in order to determine the exposure time that was necessary to provide a background density of 2, which is considered the density that is adequate for interpretation with conventional illuminators. Processing was carried out in fresh solution in total darkness, using Kodak Rapid developing solution. Densityi- was then measured on each film at three different sites (Fig. 3) : A. At the area shielded by the lead disk from primary rays ; darkening of this area would produce a density due to secondary radiation that was scattered in the water phantom and designated Ds. B. At the site where the film was directly exposed to both primary and secondary radiation (background density), designated De+*. *From Liebel-Flarsheim, Cincinnati, Ohio. tMeasured by photodensitometer from Photovolt

Corp.,

Model

No.

501A,

New

York,

N. Y.

442

Shawkat

Oral March,

Fig.

2. Occlusal

and

periapical

Fio. secondary

3. Sample radiation

radiograph (D.), (B)

radiographs

showing

of the lead disk background density

effect

of grid

for measurement (D,,,), and (C)

lines

Surg. 1971

on images.

of (A) density base or control

due to density.

C. At the portion which was completely shielded from any radiation and provided base or control density. To evaluate contrast differences among exposed films, the following formula was used to obtain a quantitative value for radiographic contrast* :

c ma =

EP x 100, Es + E,

Volume 31 Number 3

.I0

.20

.30 40

Scattered

radiation

.60 .80 1.0

1.5 2.0

absorbers

443

Exposure (sec.) Pig. 4. Characteristic mm. aluminum).

curves for various thicknesses of lead foil at 05 KVP

(HVL

= 1.8

where C& is radiographic contrast, E, is exposure that is due to primary radiation, and E, + E, is exposure that is due to primary and scattered radiation. In order to obtain exposure values necessary for the calculation of radiographic contrast, background densities (DB+P) of different films were plotted against their corresponding exposure times on semilogarithmic paper, as shown in Fig. 4. From these graphs, exposures for a background density of 2 were traced to obtain E, + E,. Exposures that were due to primary radiation (Ep) were traced after subtracting D, from Dscp. After the exposures that were needed to produce a background density of 2 had been determined, a series of exposures was made for the mandibular right premolar-molar area of a manikin. * Dose in air was calculated from r meter measurements (Table I), and doses at the film were measured by placing rods of LiF TLDi between the film and the lingual surface of the teeth.5 The purpose of such measurements was to determine the amount of radiation that was absorbed by the tissues and the various thicknesses of lead foil. RESULTSAND DISCUSSION

The calculated contrast values for different films that were exposed under varying conditions are shown in Table II. From this table, the following can be inferred : 1. There was a gradual increase in contrast value with increases in lead foil thickness. Fig. 5 shows a graphic representation of the relation between foil thickness and contrast value which illustrates this pattern. Clinically, Fig. 6 shows three radiographs of the manikin that were exposed at 65 KVP (HVL = 1.8 mm. aluminum). Such a pattern was consistent for all three HVL’s that were *DXTTR-II from Alderson Research Laboratories, Inc., Stamford, tFrom Harshaw Chemical Company, Cleveland, Ohio.

Conn.

444

Shawkat

Oral March,

Surg. 1971

Table II. Contrast values for the different thicknesses of lead foil with corresponding radiation exposures at a background density of 2.0 Dose Radiographic

Exposwe

time

(seconds)

At 65 EVP

(HVL

0.30 0.35 0.60 0.80 1.0 1.50

At 75 KVP

0.00 0.02 0.04 0.06 0.08 0.10

(HVL

0.20 0.25 0.35 0.60 0.80 1.00

At 90 KVP

Lead foil thickness (mm.) = 1.8 mm. Al)

(HVL

70

In air by ionisation chamber (mr)

To jlilm by LiF (m rad)

;: 81 i:

297 347 594 792 990 1485

117 137 214 312 39rJ 585

70 71 74 78 80 83

249 311 436 747 997 1245

126 158 221 378 504 630

64 65 68 5:

235 336 420 588 672 1008

117 166 208 291 332 498

= 3.0 mm. Al) 0.00 0.02 0.04 0.06 0.08 0.10

0.14 0.20 0.25 0.35 0.40 0.60

eontrast (per cent)

= 3.6 mm. Al) 0.00 0.02 0.04 0.06 0.08 0.10

78

used in the study. It is noteworthy that the thickness of lead foil on the back of Ultra-Speed film packets is 0.06 mm. and that by sandwiching the film between such thicknesses the contrast could be improved by about 10 per cent. Unreported data showed that 0.08 to 0.10 mm. lead provided as much improvement in contrast as the grid described earlier. 2. When contrast values among the three HVL’s were compared, it was evident that contrast decreases with increases in HVL, especially between 1.8 and 2.6 mm. aluminum. 3. As far as radiation exposure to the patient was concerned, it increased as the foil thickness increased to maintain a constant density. As the HVL increased, more radiation penetrated the tissues and lead foil, and thus less exposure was required. About 70 per cent of the directed beam was absorbed by tissues and different thieknesses of lead at 1.8 mm. aluminum HVL and about 50 per cent at 2.0 and 2.6 mm. aluminum. CONCLUSION

The use of the stationary grid, which produced grid lines and made interpretation difficult, does not appear to have a place in intraoral radiography, even though it is currently being used in several extraoral projections. The main function of the grid is to absorb scattered radiation and thus enhance the contrast of the image. This function can be achieved by using proper thicknesses of lead foil. The improvement in contrast, however, required an increased exposure in order to maintain an acceptable density level. In this respect, one must consider

Volume Number

31 3

Scattered

.Ol

.02

.03

.04 Foil

Fig. 5. Contrast

versus

lead foil

.05 Thickness

thickness

A of a manikin Fig. 6. Radiographs foil, (B) with 0.04 mm., and (C) with

.06

.O?

radiation

.08

.09

absorbers

445

.I0

(mm,)

(+ 1 per cent).

C

B to illustrate 0.08 mm.

differences

in contrast:

(A)

without

lead

the value of enhancing the diagnostic yield of the film at the expense of increasing the exposure to the patient. Subject contrast is an important factor to be considered. If the area to be examined possesses a high subject contrast, as manifested by its density and thickness, the use of lead foil might not cause a marked and clinically noticeable increase in radiographic contrast. On the other hand, in areas that have a low subject contrast, the use of lead foil will be more effective in improving the radiographic contrast. Low subject contrast is expected in areas with dense sclerosed bone or edematous structures. The lead foil can absorb most of the scatter that is emitted from these areas, and thus the improvement in contrast is clinically evident. SUMMARY

In this study scattered radiation absorbers were examined to determine their effect on the contrast of the image. The stationary grid was examined first but, because of superimposition of the grid lines on the anatomic structures, it was decided to use lead foil as a substitute. There was a gradual increase in contrast as the thickness of lead foil was increased. A thickness of 0.08 to 0.10 mm. lead provided as much improvement in contrast as the stationary grid. The improve-

446

Oral Surg. March, 1971

#hawkat

ment in the contrast was more pronounced at a lower KVP. However, the use of lead foil required an increased exposure in order to maintain an acceptable density level. The author wishes to acknowledge the valuable assistance of Dr. John Hale, Professor of Radiologic Physics at the University of Pennsylvania. REFERENCES

1. Glasser, O., Quimby, E. H., Taylor, L. S., Weatherby, J. L., and Morgan, R. H.: Physical Foundations of Radiology, ed. 3, New York, 1961, Paul B. Hoeber, Inc., pp. 161-164. 2. Shawkat, A. H. : A Quantitative Study of Scattered Radiation Fog on Low- and High-Speed Dental Roentgenograms, Oaa~ Suao. 20: 42-55, 1965. 3. Shawkat, A. H.: Effect of Radiation Fog on the Radiographic Contrast of Dental Films, ORAL Swo. 27: 177-183, 1969. 4. Morgan, R. H.: An Analysis of the Physical Factors Controlling the Diagnostic Quality of Roentgen Images, Amer. J. Roentgen. 54: 128, 1945. Dosimetry, 5. Cameron, J. R., Suntharalingam, N., and Kenney, G. N.: Thermoluminescent Madison, Wis., 1968, University of Wisconsin Press, pp. 30-30. Reprint

requests

to:

A. H. Shawkat, B.D.S., M.Sc. School of Dental Medicine University of Pennsylvania 4001 Spruc.e St. Philadelphia, Pa. 19104