- Email: [email protected]
The “use factor” in radiation barrier design
Oral roentgenology American
Arthur
Academy
of Oral
Roentgenology
H. Wuehrmann, Editor
T:he “use factor” in radiation barrier design A. CT.Richards, ill.&‘.,* Ann Arbor, Mich.
W
hen dental radiographic examinations are performed, the primary x-ray beam is successively directed in many directions about the patient’s head in both a vertical and a horizontal sense, with only a fraction of the total number of exposures occurring while the beam is directed in a given direction. This fraction of the total radiation work load is known as the “use factor.” It has been suggested’ that when complete data regarding use factors are not available for the planning of barriers against primary radiation exposure in the dental office, the use factor should be 1/4 for the four walls and 1/1e for the floor and ceiling. The present study was designed to provide a more realistic assessment of the use factors to be used in the planning of primary radiation barriers for dentistry. In order to learn where, and at what frequency, the x-ray beam actually exposes the walls, floor, and ceiling during periapical x-ray examinations, it was necessary to construct a scale model of a dental office and x-ray machine. The ratio of 1 inch to 1 foot was chosen for the construction of a dental-office model 8 inches wide, 10 inches long, and 8 inches high. Under suitable safe-light conditions, the walls, floor, and ceiling of the room were “wallpapered” with photographic enlarging paper. A simulated x-ray machine was placed within the room (Fig. 1). The simulated x-ray machine consisted of a miniature light bulb which provided a point source of light. The bulb was mounted in a cylindrical brass tube, the end of which was covered with a black photographic negative displaying a small clear dot surrounded by a clear circular band (Fig. 2). The diameter of this band was carefully chosen to provide the scaled-down diameter and the s.ame divergence of the light beam as an x-ray beam. The transmitted images of t:he dot and circle corresponded, respectively, to the central ray and the circumference of the simulated x-ray beam. Means were provided so that the light bulb could be moved along the length of the brass tube to simulate both long- and short-cone techniques. The brass “Member,
American
Academy
of Oral
Roentgenology.
745
Fig. 1. Three walls and the floor of a wale model of optical device which casts a circular light beam pattern on as a dental x-ray machine would expose the same areas in of the model room is laid flat for this illustration. The machine duplicates the vertical ant1 horizontal directions periapical x-ray examinations.
Fig.
“x-ray”
3. Photographic beam on the
negative used enlarging-paper-covered
in
a (l~~nt:ll offic~b a I’(’ .sIIo\\ II wit It an the four cxposc~l surfatars, exactly a dental office. The left-hand wall optical device 01‘ sirrrrtlatetl x-ray of the brarr~ whicsh are usr~l in
the simulated x-ray machine floor and walls of the scale
to cast images of model dental office.
Volume Number
23 6
“Use
fmtor”
in radiation
barrier
&sign
747
tube was supported in such a manner that the direction of its long axis could be varied in both vertical and horizontal planes, thus providing the vertical and horizontal angulation capabilities for the simulated x-ray machine. Suitable vertical and horizontal angulation scales and indicators were provided. Provisions were also made for raising the entire assembly to duplicate the raising or lowering of the patient in the dental chair (Fig. 3). It was assumed that when periapical x-ray examinations are performed, the central ray of the x-ray beam passes through a fixed point located at the level of the occlusal plane and in the center of the patient’s mouth. This assumption is a useful approximation, for it provides a definitive description of the location of the patient and the x-ray machine in the model of the dental office. In this study, the patient was located 5 feet from the wall behind him and 31/2 feet from the wall at his left side. The occlusal plane was located 31/2 feet from the floor. In Table I are listed the vertical and horizontal directions of the beam used for a fourteen-film short-cone (7 inch source-skin distance) periapical x-ray examination of a patient. The vertical angles are measured relative to the horizon, and the horizontal angles are measured relative to the median sagittal plane of the patient. At 7 inches from the light bulb, the beam was 3 inches in diameter. In Table II are listed the vertical and horizontal directions of the beam used for an eighteen-film long-cone (14 inch source-skin distance) periapical x-ray examination of a patient. The diameter of the light beam was 3 inches when measured 14 inches from the lamp. Parallel film positioning is assumed with the long-cone technique, and angular positioning with the short-cone technique.
Fig. contains angulation
J. View of the simulated x-ray machine looking down the photographic negative shown in Fig. 2 and the scale is shown at the base of the device.
the cylindrical light source.
portion which The horizontal
Table I. Angles used in fourteen-film Region
short-cone periapical
Ifo,‘izo7ztaztmgle (degrees)
dlaxilla Central incisors Cuspid and lateral incisor Premolars Molars
0 45 65 85
Mandible Central incisors Cuspid and lateral incisor Premolars Molars
0 45 65 85
Table II. Angles used for eighteen-film Region
SUP\-ey
long-cone periapical
surrey
Horizontal ample [degrees)
Nascilla Central incisors Lateral incisor Cuspid Premolars Molars
0 20 45 65 $5
Xandible Central incisors Lateral incisor Cuspid Premolars Molars
0 20 45 65 85
After the simulated x-ray machine was aimed in the proper vertical and horizontal directions to duplicate the examination of a given oral region, the light bulb was turned on briefly to cast the image of the circle and dot somewhere on the “wallpapered” walls, floor, or ceiling of the miniature dental office. This procedure was repeated until all the motions that an x-ray head makes during a complete periapical examination were duplicated. The photographic wallpaper was removed from the six interior surfaces of the miniature dental office and then processed. The results are shown in Figs. 4 and 5. The wall facing the patient and the ceiling were not exposed during either survey; therefore, these two surfaces have been omitted in Figs. 4 and 5. DISCUSSiON
Figs. 4 and 5 indicate that no one area on the floor or walls is exposed to one-fourth of the work load but, instead, to some far smaller fraction of the total. In the past, the attenuation of the x-ray beam by the patient’s head has always been ignored in x-ray barrier computations, even though the head may have attenuated 90 per cent or more of the primary beam. This omission is simply an added factor of safety. Even if an edge of the primary beam entirely misses the patient’s head, it can now be shown that successive misses
750
OS., O.M. & OP.
Richards
June, 1967
would not expose a given point on the floor or three walls more than once in a given survey. The low frequency of exposure of any given point on the floor or three exposed walls of the office, plus the attenuation of the beam by the patient’s head, justifies a recommendation that a use factor of l/l6 is reasonable for use in comput.ing the primary radiation barrier requirements for the floor and the three exposed walls of the dental office. Since the primary beam is not directed at the ceiling or the wall facing the pat’ient, a use factor of zero would apply to these two surfaces. However, these surfaces are exposed along with the others to secondary radiation and therefore must be considered as secondary radiation barriers. In secondary radiation barrier computations, the use factor for all surfaces in a room is always 1, because secondary radiation radiates in all directions and reaches all surfaces. SUMMARY
By using photographic methods in conjunction with a scale model of a dental office and a simulated x-ra.y machine which transmits visible light images of the x-ray beam, it has been possible to record the size and location of the floor and wall areas within a dental office which are exposed by the primary beam during complete periapical x-ray examinations performed with both long- and short-cone techniques. These results indicate the use factor should be 1/16 for the floor and three exposed walls, and “0” for the ceiling and the remaining wall which faces the patient. The use factor for secondary radiation barrier computations remains unchanged at 1. REFERENCE
1. National Bureau of Standards: Medical X-ray Protection Handbook 76, Washington, D. C., 1961, Government Printing
up to Three O&e.
Million
Volts,
Arthur
Academy
of Oral
Roentgenology
H. Wuehrmann, Editor
T:he “use factor” in radiation barrier design A. CT.Richards, ill.&‘.,* Ann Arbor, Mich.
W
hen dental radiographic examinations are performed, the primary x-ray beam is successively directed in many directions about the patient’s head in both a vertical and a horizontal sense, with only a fraction of the total number of exposures occurring while the beam is directed in a given direction. This fraction of the total radiation work load is known as the “use factor.” It has been suggested’ that when complete data regarding use factors are not available for the planning of barriers against primary radiation exposure in the dental office, the use factor should be 1/4 for the four walls and 1/1e for the floor and ceiling. The present study was designed to provide a more realistic assessment of the use factors to be used in the planning of primary radiation barriers for dentistry. In order to learn where, and at what frequency, the x-ray beam actually exposes the walls, floor, and ceiling during periapical x-ray examinations, it was necessary to construct a scale model of a dental office and x-ray machine. The ratio of 1 inch to 1 foot was chosen for the construction of a dental-office model 8 inches wide, 10 inches long, and 8 inches high. Under suitable safe-light conditions, the walls, floor, and ceiling of the room were “wallpapered” with photographic enlarging paper. A simulated x-ray machine was placed within the room (Fig. 1). The simulated x-ray machine consisted of a miniature light bulb which provided a point source of light. The bulb was mounted in a cylindrical brass tube, the end of which was covered with a black photographic negative displaying a small clear dot surrounded by a clear circular band (Fig. 2). The diameter of this band was carefully chosen to provide the scaled-down diameter and the s.ame divergence of the light beam as an x-ray beam. The transmitted images of t:he dot and circle corresponded, respectively, to the central ray and the circumference of the simulated x-ray beam. Means were provided so that the light bulb could be moved along the length of the brass tube to simulate both long- and short-cone techniques. The brass “Member,
American
Academy
of Oral
Roentgenology.
745
Fig. 1. Three walls and the floor of a wale model of optical device which casts a circular light beam pattern on as a dental x-ray machine would expose the same areas in of the model room is laid flat for this illustration. The machine duplicates the vertical ant1 horizontal directions periapical x-ray examinations.
Fig.
“x-ray”
3. Photographic beam on the
negative used enlarging-paper-covered
in
a (l~~nt:ll offic~b a I’(’ .sIIo\\ II wit It an the four cxposc~l surfatars, exactly a dental office. The left-hand wall optical device 01‘ sirrrrtlatetl x-ray of the brarr~ whicsh are usr~l in
the simulated x-ray machine floor and walls of the scale
to cast images of model dental office.
Volume Number
23 6
“Use
fmtor”
in radiation
barrier
&sign
747
tube was supported in such a manner that the direction of its long axis could be varied in both vertical and horizontal planes, thus providing the vertical and horizontal angulation capabilities for the simulated x-ray machine. Suitable vertical and horizontal angulation scales and indicators were provided. Provisions were also made for raising the entire assembly to duplicate the raising or lowering of the patient in the dental chair (Fig. 3). It was assumed that when periapical x-ray examinations are performed, the central ray of the x-ray beam passes through a fixed point located at the level of the occlusal plane and in the center of the patient’s mouth. This assumption is a useful approximation, for it provides a definitive description of the location of the patient and the x-ray machine in the model of the dental office. In this study, the patient was located 5 feet from the wall behind him and 31/2 feet from the wall at his left side. The occlusal plane was located 31/2 feet from the floor. In Table I are listed the vertical and horizontal directions of the beam used for a fourteen-film short-cone (7 inch source-skin distance) periapical x-ray examination of a patient. The vertical angles are measured relative to the horizon, and the horizontal angles are measured relative to the median sagittal plane of the patient. At 7 inches from the light bulb, the beam was 3 inches in diameter. In Table II are listed the vertical and horizontal directions of the beam used for an eighteen-film long-cone (14 inch source-skin distance) periapical x-ray examination of a patient. The diameter of the light beam was 3 inches when measured 14 inches from the lamp. Parallel film positioning is assumed with the long-cone technique, and angular positioning with the short-cone technique.
Fig. contains angulation
J. View of the simulated x-ray machine looking down the photographic negative shown in Fig. 2 and the scale is shown at the base of the device.
the cylindrical light source.
portion which The horizontal
Table I. Angles used in fourteen-film Region
short-cone periapical
Ifo,‘izo7ztaztmgle (degrees)
dlaxilla Central incisors Cuspid and lateral incisor Premolars Molars
0 45 65 85
Mandible Central incisors Cuspid and lateral incisor Premolars Molars
0 45 65 85
Table II. Angles used for eighteen-film Region
SUP\-ey
long-cone periapical
surrey
Horizontal ample [degrees)
Nascilla Central incisors Lateral incisor Cuspid Premolars Molars
0 20 45 65 $5
Xandible Central incisors Lateral incisor Cuspid Premolars Molars
0 20 45 65 85
After the simulated x-ray machine was aimed in the proper vertical and horizontal directions to duplicate the examination of a given oral region, the light bulb was turned on briefly to cast the image of the circle and dot somewhere on the “wallpapered” walls, floor, or ceiling of the miniature dental office. This procedure was repeated until all the motions that an x-ray head makes during a complete periapical examination were duplicated. The photographic wallpaper was removed from the six interior surfaces of the miniature dental office and then processed. The results are shown in Figs. 4 and 5. The wall facing the patient and the ceiling were not exposed during either survey; therefore, these two surfaces have been omitted in Figs. 4 and 5. DISCUSSiON
Figs. 4 and 5 indicate that no one area on the floor or walls is exposed to one-fourth of the work load but, instead, to some far smaller fraction of the total. In the past, the attenuation of the x-ray beam by the patient’s head has always been ignored in x-ray barrier computations, even though the head may have attenuated 90 per cent or more of the primary beam. This omission is simply an added factor of safety. Even if an edge of the primary beam entirely misses the patient’s head, it can now be shown that successive misses
750
OS., O.M. & OP.
Richards
June, 1967
would not expose a given point on the floor or three walls more than once in a given survey. The low frequency of exposure of any given point on the floor or three exposed walls of the office, plus the attenuation of the beam by the patient’s head, justifies a recommendation that a use factor of l/l6 is reasonable for use in comput.ing the primary radiation barrier requirements for the floor and the three exposed walls of the dental office. Since the primary beam is not directed at the ceiling or the wall facing the pat’ient, a use factor of zero would apply to these two surfaces. However, these surfaces are exposed along with the others to secondary radiation and therefore must be considered as secondary radiation barriers. In secondary radiation barrier computations, the use factor for all surfaces in a room is always 1, because secondary radiation radiates in all directions and reaches all surfaces. SUMMARY
By using photographic methods in conjunction with a scale model of a dental office and a simulated x-ra.y machine which transmits visible light images of the x-ray beam, it has been possible to record the size and location of the floor and wall areas within a dental office which are exposed by the primary beam during complete periapical x-ray examinations performed with both long- and short-cone techniques. These results indicate the use factor should be 1/16 for the floor and three exposed walls, and “0” for the ceiling and the remaining wall which faces the patient. The use factor for secondary radiation barrier computations remains unchanged at 1. REFERENCE
1. National Bureau of Standards: Medical X-ray Protection Handbook 76, Washington, D. C., 1961, Government Printing
up to Three O&e.
Million
Volts,