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Absorbed doseswith intraoral radiography Function of various technical parameters Yoshihiko Hayakawa, BSc, MSC,~Hisao Fujimori.,b and Kinya Kuroyanagi, DDS, PhD,C Chiba, Yapan DEPARTMENT OF ORAL AND MAXILLOFACIAL RADIOLOGY, TOKYO DENTAL COLLEGE Both the maxillary incisor and mandibular molar regions of a Rando phantom were radiographed using specific technical parameters. The parotid gland and thyroid gland doses were measured using thermoluminescence dosimetry. Tube voltage, total filtration, mAs, collimation, and cone length were varied while keeping the other factors constant. Increased tube voltage or total filtration generally resulted in a slightly increased absorbed dose. Absorbed dose was decreased as the beam was more tightly collimated or the cone length was increased. (ORAL SURG ORAL MED ORAL PATHOL 1993;76:519-24) Various technical parameters are possible for in- traoral radiography. The operator can usually choose the settings for exposure time, tube voltage, and tube current in addition to selecting collimation and cone length. The optimum settings for each examination may be difficult to determine. The absorbed dose with intraoral radiography should be minimized by choosmg the proper setting for each parameter that will still provide optimum image quality. Therefore it is important to know how both the absorbed doseand the image quality vary at each possible technique setting. Many articles have already been published con- cerning the absorbed radiation dosages during in- traoral radiography. 1-36 These have included reports about the dose effectsof various technical parameters, including tube voltage,* total filtration,1-3, 5,‘& 24 tar- get-to-film distance,7 collimation,$ vertical align- ment of the x-ray beam, 11, 17, 29 the number of radio- graphs, 2o the t.ypeof receptor,3,lo and the presence or absenceof a barrier.24> 25 The method of -measurement and anatomic struc- tures considered varied in these reports. Researchers have reported tissue and organ doses,§ dosedistribu- tion in the head and neck regionA\ and integral dose.3> 16, 21, 22, 32 Doseevaluation criteriavary greatly. Moreover, only total absorbed doses were reported in most of the published reports. Absorbed doses for the aAssistant Professor. bRadiologic Technologist, Chief. cProfessor and Chairman. Copyright @ 1993 by Mosby-Year Book, Inc. 0030-4220/93/$1.00 -!- .lO 7/16/47044 *References 1, 2, 7, 10, 12, 13, 16-18, 26. tReferences 2, 3, 9, 12, 13, 16-18, 21. $-References 3, 8, 9, 11, 19, 20, 23, 29, 31. SReferences 2-5, 7, 13, 14, 16, 20, 22, 23, 25, 29, 32, 34-36. 111, 9-13, 15, 17-19, 23, 24, 26, 27, 31, 33. specific tissues have been described only in a small number of articles.203 28, 36 The occurrence of nonstochastic effectsis probably not a consideration with low-doseexposures, however, the incidence of stochastic effects increases linearly with the absorbed dose.In the head and neck region, the salivary gland, thyroid gland, and red bone mar- row particularly are at a potentially increased risk of malignant transformation. After irradiation, dose measurements are therefore useful for estimating risk in these organs and tissues. This article describes the absorbed doses in the thyroid and salivary glands for various techniques used in intraoral radiography. MATERIAL AND METHODS X-ray equipment The Coronis 20 (Asahi Roentogen Corp., Kyoto, Japan) was used in this study. Its specifications are shown in Table I. Tube voltage, tube current, and ex- posure time can be varied. Two conesof 20 and 30 cm are supplied for setting the distance from the x-ray tube to the cone tip. The exposedarea in either case is collimated by a lead ring to 28.3 cm2 (6 cm in di- ameter) at the cone tip. The internal diameters of the lead rings are 43 mm for the 20 cm cone and 29 mm for the 30 cm one. In addition there are aluminum filter disks of 0.5 mm thickness, two of which are usu- ally installed. The inherent filtration is 1 mm Al equivalent. A stabilizer is also provided to maintain the primary voltage. Exposure settings Absorbed doses were measuredunder 135 different combinations of five technical parameters for in- traoral radiographic examinations in both the maxil- lary incisor and the right mandibular molar regions. The absorbed dose was measured under selected combinations of tube voltage, total filtration, distance 519

Absorbed doses with intraoral radiography: Function of various technical parameters

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Page 1: Absorbed doses with intraoral radiography: Function of various technical parameters

Absorbed doses with intraoral radiography Function of various technical parameters

Yoshihiko Hayakawa, BSc, MSC,~ Hisao Fujimori.,b and Kinya Kuroyanagi, DDS, PhD,C Chiba, Yapan DEPARTMENT OF ORAL AND MAXILLOFACIAL RADIOLOGY, TOKYO DENTAL COLLEGE

Both the maxillary incisor and mandibular molar regions of a Rando phantom were radiographed using specific technical parameters. The parotid gland and thyroid gland doses were measured using thermoluminescence dosimetry. Tube voltage, total filtration, mAs, collimation, and cone length were varied while keeping the other factors constant. Increased tube voltage or total filtration generally resulted in a slightly increased absorbed dose. Absorbed dose was decreased as the beam was more tightly collimated or the cone length was increased. (ORAL SURG ORAL MED ORAL PATHOL 1993;76:519-24)

Various technical parameters are possible for in- traoral radiography. The operator can usually choose the settings for exposure time, tube voltage, and tube current in addition to selecting collimation and cone length. The optimum settings for each examination may be difficult to determine.

The absorbed dose with intraoral radiography should be minimized by choosmg the proper setting for each parameter that will still provide optimum image quality. Therefore it is important to know how both the absorbed dose and the image quality vary at each possible technique setting.

Many articles have already been published con- cerning the absorbed radiation dosages during in- traoral radiography. 1-36 These have included reports about the dose effects of various technical parameters, including tube voltage,* total filtration,1-3, 5, ‘& 24 tar- get-to-film distance,7 collimation,$ vertical align- ment of the x-ray beam, 11, 17, 29 the number of radio- graphs, 2o the t.ype of receptor, 3, lo and the presence or absence of a barrier.24> 25

The method of -measurement and anatomic struc- tures considered varied in these reports. Researchers have reported tissue and organ doses,§ dose distribu- tion in the head and neck regionA\ and integral dose.3> 16, 21, 22, 32 Dose evaluation criteriavary greatly. Moreover, only total absorbed doses were reported in most of the published reports. Absorbed doses for the

aAssistant Professor. bRadiologic Technologist, Chief. cProfessor and Chairman. Copyright @ 1993 by Mosby-Year Book, Inc. 0030-4220/93/$1.00 -!- .lO 7/16/47044 *References 1, 2, 7, 10, 12, 13, 16-18, 26. tReferences 2, 3, 9, 12, 13, 16-18, 21. $-References 3, 8, 9, 11, 19, 20, 23, 29, 31. SReferences 2-5, 7, 13, 14, 16, 20, 22, 23, 25, 29, 32, 34-36. 111, 9-13, 15, 17-19, 23, 24, 26, 27, 31, 33.

specific tissues have been described only in a small number of articles.203 28, 36

The occurrence of nonstochastic effects is probably not a consideration with low-dose exposures, however, the incidence of stochastic effects increases linearly with the absorbed dose. In the head and neck region, the salivary gland, thyroid gland, and red bone mar- row particularly are at a potentially increased risk of malignant transformation. After irradiation, dose measurements are therefore useful for estimating risk in these organs and tissues.

This article describes the absorbed doses in the thyroid and salivary glands for various techniques used in intraoral radiography.

MATERIAL AND METHODS X-ray equipment

The Coronis 20 (Asahi Roentogen Corp., Kyoto, Japan) was used in this study. Its specifications are shown in Table I. Tube voltage, tube current, and ex- posure time can be varied. Two cones of 20 and 30 cm are supplied for setting the distance from the x-ray tube to the cone tip. The exposed area in either case is collimated by a lead ring to 28.3 cm2 (6 cm in di- ameter) at the cone tip. The internal diameters of the lead rings are 43 mm for the 20 cm cone and 29 mm for the 30 cm one. In addition there are aluminum filter disks of 0.5 mm thickness, two of which are usu- ally installed. The inherent filtration is 1 mm Al equivalent. A stabilizer is also provided to maintain the primary voltage.

Exposure settings Absorbed doses were measured under 135 different

combinations of five technical parameters for in- traoral radiographic examinations in both the maxil- lary incisor and the right mandibular molar regions.

The absorbed dose was measured under selected combinations of tube voltage, total filtration, distance

519

Page 2: Absorbed doses with intraoral radiography: Function of various technical parameters

520 Hayakawa, Fujimori, and Kuroyanagi ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY October 1993

’ 50 60 70 80 90 I I I I I I

’ 50 60 70 80 90

A kVp El kVp

Fig. 1. Product of tube current and exposure time selected to obtain adequate optical density on Ultraspeed Dental X-ray Film for various technical parameter settings. A, Periapical radiography of maxillary incisors. B, Periapical radiography of mandibular molars. (Total filtration: ____ 2, ________ 2.5, _____-__ 3 mm Al

eq., distance from x-ray tube to cone tip: 0 20, A 30, q 40 cm.)

Table 1. Specifications for the Coronis 20

Type X-ray tube Rectification Tube voltage

Tube current

Exposure time

Total filtration Focal spot size Collimation

DFW-20 D-088 Full-rectified 50 - 90 kVp (9 steps at intervals of 5 kVp) 2-20mA (6 steps al various intervals) 0.01 - 5.0 set (23 steps at various intervals) 2 mmA1 equivalent 0.8 X 0.8 mm 60 mm 4 at the tip of cone

from the x-ray tube to the cone tip, exposed area (di- ameter at the cone tip), and the product of tube cur- rent and exposure time (mAs). The tube voltage was varied between 50 and 90 kVp in 10 kVp steps. The total filtration was set at 2, 2.5, and 3 mm aluminum equivalence by adding aluminum disks. The distance from the x-ray tube to the cone tip was set at 20, 30, or 40 cm. A lead-lined plastic cone of 10 cm in length and 6 cm in internal diameter was installed on the long (30 cm) cone to measure doses at 40 cm. The exposed area was varied with the use of lead rings of different internal diameters as collimators. The diameter at the cone tip was either 5, 5.5, or 6 cm.

Lastly, the mAs to produce adequate optical den- sity on Ultraspeed Dental X-ray Film (Eastman Kodak Company, Rochester, N.Y.) was determined when an aluminum step-wedge varying in thickness

from 1 to 15 mm was radiographed for each tube voltage, total filtration, and distance setting (Fig. I). The initial mAs value was selected using the exposure conditions of 70 kVp and 2 mm Al equivalence. The optical densities of each step-wedge (located 10 cm from the cone tip) was measured with a PDA 15 (Sakura Medical Co., Tokyo, Japan). MAs values for various other kVp and filtration settings were deter- mined to maintain the same optical densities as at this first setting (70 kVp and 2 mm Al). When a change in step-wedge contrast was caused by the difference in kVp, the mAs value was set to achieve a similar op- tical density at aluminum thickness of 6 to 10 mm.

Phantom The absorbed doses were measured with the use of

a Rando phantom (Alderson Research Laboratories, Inc., Stamford, Corm.) that had soft tissues made of an isocyanate rubber equivalent to human soft tissues in atomic number and density. The phantom was constructed of axial sections at a thickness of 2.54 cm that had 5 mm diameter holes for insertion of thermoluminescence dosimeters (TLDs).

Dosimetry The method of the dosimetry was similar to that

described in an earlier article.37 The parotid and thyroid gland doses were measured

by using 100 TLDs, TLD-LOO (Harshaw Chemical Co., Solon, Ohio). These were LiF rod-shaped do- simeters of 1 X 1 X 6 mm. The TLD-100s were cho-

Page 3: Absorbed doses with intraoral radiography: Function of various technical parameters

ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY Volume 76. Number 4

Hayakawa, Fujimori, and Kuroyanagi 527

a,

::

“, 20 OA~oHo

0 0 z O-0 ~0-0

o 0- 0

i A A-A-A 2 A A-A A-A-A- A-A

-A- A A-

10

I 2 mmAl 2.5 mm Al 3mmAl

OLl ’ ” ’ ’ ’ ’ ’ 50 60 70 80 90 50 60 70 8w

A kVp

2 ; 60-. V

$ 5o-A- A A

A A-A-A A-A A A-A-A A

0

: 40- $ b 30-- 03 2 o-o-o- Q-0 o--o-o--o--.o o- 0 -0-o 0

20-

2mmAl 2.5 mm Al 3mm Al

’ 50 60 70 80 90 50 60 70 80 90 50 60 70 80 90

B kVp

Fig. 2. Average absorbed dose for the parotid gland (a) and thyroid gland (0) as a function of tube voltage and total filtration. A, Periapical radiography of maxillary kcisors. B, Periapical radiography of mandibular molars.

sen as they have high sensitivity for the x-ray energy range used in oral radiology and little energy depen- dence because of a low effective atomic number.38, 39 The TLDs were exposed to uniform irradiation at 70kVp with the use of a KXO-15 x-ray generator (Toshiba Medical Corp., Tokyo, Japan), and those not reading within 10% of the mean were discarded from the experiment.

Thermoluminescent intensity was measured with a TLD-2000A detector and TLD-2000B integral pi- coammeter (Harshaw Chemical Co., Solon, Ohio). The Radocon III type 550 ionizing chamber and 550-3 probe type (Victoreen Inc., Cleveland, Ohio) were used to calibrate the light output of the TLDs. The KXO- 15 generator was used to determine the re-

Table II. Average absorbed doses (y Gy) and standard deviation for the 135 combinations of the five technical parameters

Region exposed 1 Parotid gland / Thyroid gland

Maxillary incisor Mandibular molar

12.9 (53.2) 20.3 (i 14.7) 53.3 (I 19.4) 24.7 (+ 5.8)

Average (S. D.)

lationship between exposure and thermoluminescent intensity at each voltage. 4o The exposure was con- verted to absorbed dose by using a factor of 35 Gy . C-‘.41

All TLDs were annealed at 400” C for 1 hour,

Page 4: Absorbed doses with intraoral radiography: Function of various technical parameters

522 Hayakawa, Fujimori, and Kuroyanqi ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY October I993

Diameter of the beam at cone t i P (cm)

I I I I I I I I I 5 5.5 6 5 5.5 6 5 5.5 6

I3 Diameter of the beam at cone tip (cm)

Fig. 3. Average absorbed dose for the parotid gland (A) and thyroid gland (0) as a function of exposed area and distance from the x-ray tube to the cone tip. A, Periapical radiography of maxillary incisors. B, Periapi- cal radiography of mandibular molars.

cooled to room temperature and heated again at 100” C for 2 hours. 42 After irradiation, each dosim- eter was heated on a planchet in a TLD reader oven operating at the photomultiplier’s voltage at 550 volts up to 240” C at intervals of 5.8’ C/set. The integral thermoluminescence was calculated for the range from 125“ C to 240” C.“O Nz flow was maintained throughout the system. Ten TLDs were used to determine the background radiation To measure doses within the right parotid and thyroid glands, four TLDs were placed in these locations.3s Each dosim- eter received five x-ray exposures, and each measure- ment was repeated three times. The results were cal- culated as the mean absorbed dose per single expo- sure.

RESULTS Absorbed doses for various tube voltages and total

filtrations are shown in Fig. 2. Doses were averaged at each voltage and filtration value for the various settings of the other parameters. With increased tube voltage, the thyroid gland and parotid gland doses slightly increased for radiography of the maxillary incisors and the parotid gland dose also slightly increased for radiography of the mandibular molars, In contrast, the thyroid gland dose slightly decreased for radiography of the mandibular molars. Increased total filtration resulted in a slight increase in the ab- sorbed dose.

The dependence of absorbed dose on both distance and collimation is shown in Fig. 3. The doses were av-

Page 5: Absorbed doses with intraoral radiography: Function of various technical parameters

ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY Hayakawa, Fujimori, and Kuroyanagi 523 Volume 76, Number 4

eraged for each diameter and distance value for the decreased and the filtration became thicker. An different settings of the other parameters. A smaller increase in total filtration clearly decreased the diameter and a greater distance always resulted in a skin exposure. Nevertheless, filtration did not affect decrease in absorbed dose. Geometry has the greatest the amount of scattered radiation measured in this effect on these absorbed doses. study.

Variations in the average absorbed doses and their standard deviations among the 135 combinations of the five parameters are shown in ‘Table II. Both the thyroid gland and parotid gland doses were greater for exposures of the mandibular molars than for expo- sures of the maxillary incisors.

DISCUSSION It is well known that organ doses in the maxillofa-

cial region may vary more than two-fold for the same intraoral radiographic procedure depending upon the technique used.*

Stochastic risk from radiation exposure are in pro- portion to effective dose. Effective dose is given by the sum of the product of the dose equivalent to each tis- sue and its weighting factor. Absorbed doses to the thyroid gland and salivary gland including parotid gland occur in effective doses with intraoral radiog- raphy.43, 44 The limitation of beam expansion by means of long distance and tight collimation contrib- uted reductions in the effective dose.

CONCLUSIONS

The thyroid gland receives a higher dose with radi- ography of maxillary incisors whereas the parotid gland is more at risk during radiography of mandib- ular molars.20, 34 The absorbed dose to the parotid gland is high during intraoral radiographic examina- tion of the molar region. On the other hand, the ab- sorbed dose to the thyroid gland is high during exam- ination of the m,axillary incisors, canines, and maxil- lary and mandibular molars. This study confirms the previous finding by others.

We: reached the following conclusions: Restriction of the size of the exposed area by col- limation was the most important parameter to af- fect absorbed dose. Increased distance from the tube to the cone tip had an appreciable effect in the reduction of the absorbed dose by effectively narrowing the volume of tissue exposed.

Characteristics of the primary x-ray beam are de- pendent on various technical parameters. Collimation and distance change the geometric conditions. Tube voltage and total Mtration affect x-ray quality. The product of tube current and exposure time also regu- lates the absorbed dose.

1.

2.

3.

4.

Analysis of the relationship between absorbed dose and the technical parameters revealed that the geom- etry of the exposed area was the most critical param- eter to affect th.e dose. A 30% reduction in the exposed area, accomplished by reducing the diameter from 6 to 5 em, reduced t.he absorbed dose by as much as 67%. The distance from the x-ray tube to the cone tip also had a large effect on absorbed dose. These results were consistent for both regions exposed and both an- atomic sites measured and also are consistent with previous rep0rts.t

Conclusions 1 and 2 were consistent for both regions exposed and both anatomic sites measured. The greatest exposure changes were caused in the organs closest to the primary x-ray beam for the technical parameters studied. Changes in kVp had little influence on the ab- sorbed doses in the studied tissues. An increase in the tube voltage and total filtration resulted in a slight increase in absorbed doses except in the thy- roid gland during radiography of the mandibular molars.

REFERENCES

Changes in the tube voltage and total filtration had little influence on absorbed dose. Increasing the tube voltage and total filtration resulted in a slight increase in absorbed dose except in the thyroid gland during radiography of the mandibular molars.

The radiation output decreased as the tube voltage

1.

2.

3.

4.

5.

6.

7. *References 8-13, 16-20, 24, 26-31, 33, 34. TReferences 2, 3, 8, 9, 11-13, 16-21, 23, 29, 31.

Baily NA. Patient exposure to ionizing radiation in dental ra- diography. Radiology 1957;69:42-5. Richards AG. Roentgen-ray doses in dental roentgenography. J Am Dent Assoc 1958;56:351-68. Ardran GM, Crooks HE. Observations on the dose from den- tal x-ray procedures with a note on radiography of the nasal bones. Br J Radio1 1959;32:572-83. Bjdrngard B, Hollender L, Lindahl B, Sonesson A. Radiation doses in oral radiography: I-Measurements of doses to gonads and certain parts of head and neck during full mouth roent- genography. Odontol Rev 1959;10:355-66. Bjarngard B, Hollender L, Lindahl B, Sonesson A. Radiation doses in oral radiography: II-The influence of technical fac- tors on the dose to the patient in full mouth radiography. Od- onto1 Rev 1960;11:100-12. Goepp RA, Standjord NW, Hodges PC, Moseley RD Jr. The reduction of unnecessary x-ray exposure during intraoral ex- amination. ORAL SURG ORAL MED ORAL PATHOL 1963; 16:39-45. Richards AG, Webber RL. Dental x-ray exposure of sites within the head and neck. ORAL SURG ORAL MED ORAL PATHOL 1964;18:752-6.

Page 6: Absorbed doses with intraoral radiography: Function of various technical parameters

524 Hayakawa, Fujimori, and Kuroyanagi ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY October I993

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

Winkler KG. Influence of rectanguiar collimation and in- 29. Stenstrom B, Rehnmark-Larsson S, Julin P, Richter S. Radi- traoral shielding on radiation dose in dental radiography. J Am ation shielding in dental radiography. Swed Dent J 1983;7:85- Dent Assoc 1968;77:95-101. 91. Weissman DD, Sobkowski FJ. Comparative thermolumines- cent dosimetry of intraoral periapical radiography. ORAL SURG ORAL MED ORAL PATHOL 1970;29:376-86. Bushong SC, Galbreath JC, Garvis R, Merritt E. Reduction of patient exposure during dental radiography. Health Phys 1971;21:281-6. Weissman DD. Comparative absorbed doses in intraoral peri- apical radiography. J S Calif Dent Assoc 1971;39:886-92. Ice RD, Updegrave WJ, Bogucki EI. Influence of dental radiographic cones on radiation exposure. J Am Dent Assoc !971;83:1297-302, Greer DF. Determination and analysis of absorbed doses resulting from various intraoral radiographic techniques. ORAL SURG QRAL MED ORAL PATHOL 1972;34: 146-62. Sitzmann F. Measurement of x-ray exposure of ocular lens in dento-maxillo-facial radiography. Dentomaxillofac Radio1 1973;2:88-92. Jerman AC, Kinsley EL, Morris CR. Absorbed radiation from panoramic plus bite wing exposures vs full mouth periapical plus bite wing exposures. J Am Dent Assoc 1973;86:420-3. Lee W. Comparative radiation doses in dental radiography. ORAL SURG ORAL MED ORAL PATHOL 1974;31:962-8. Alcox RW, Jameson WR. Patient exposures from intraoral radiographic examinations. J Am Dent Assoc 1974:88:568-79. Frey-NW, Wuehrmann .4H. Radiation dosimetry and in- traoral radiographic techniques: II-Internal and external dose measurements. ORAL SURC ORAL MED ORAL PATHOL 1974;38:639-52. Lilienthal B, Rak D, Wang J. Minimizing radiation exposure in dental radiology. Aust Dent J 1975:1-6. Antoku S, Kihara T, Russell WJ, Beach DR. Doses to critical organs from dental radiography. ORAL SLXG ORAL MED ORAL PATHOL 1976;41:251-60. Weiander U, Wickman G. The relationship between the inte- gral dose and the focus to object distance in dental radiology. Dentomaxillofacial Radio1 1977;6:83-7. Bengtsson G. Maxillofacial aspects of radiation protection fo- cused on recent research regarding critical organs. Dentomax- illofacial Radio1 1978;7:5-14. White SC, Rose TC. Absorbed bone marrow dose in certain dental radiographic techniques. J Am Dent Assoc 1979: 98:553-8. Yiilek GG, Soydan E, Ugur Z. Reduction of patient exposure during dental radiography. Health Phys 1979;36:17-20. Whitcher BL, Gratt BM, Sickles EA. Leaded shields for thy- roid dose reduction in intraoral dental radiography. ORAL SURG ORAL MED ORAL PATHOL 1979;48:567-70. Wall BF, Fisher ES, Paynter R, Hudson A, Bird PD. Doses to patients from pantomographic and conventional dental radi- ography. Br J Radio1 1979;52:727-34. McKlveen JW. X-ray exposures to dental patients. Health Phys 1980;39:211-7. Rehnmark-Larsson S, Stenstrom B, Julin P, Richter S. Radi- ation absorbed doses at radiographic examination of third mo- lars. Swed Dent J 1982;6:189-201.

30. Bankvall G, Engstrdm H, Engstrijm C, Hollender L. Absorbed doses in the craniofacial region during various radiographic and radiotherapeutic procedures. Dentomaxillofacial Radio1 1985;14:19-25.

31. Stenstriim B, Henrikson CO, Holm B, Richter S. Absorbed doses from intraoral radiography with special emphasis on collimator dimensions. Swed Dent J 1986;10:59-71.

32. Stenstrom B, Henrikson CO, Karlsson L, Sarby B. Energy imparted from intraoral radiography. Swed Dent J 1986; 10:125-36.

33. Kircos LT, Angin LL, Lorton L. Order of magnitude dose re- duction in intraoral radiography. J Am Dent Assoc 1987; 114:344-7.

34. Miles DA. Absorbed x-ray doses to critical organs of the head and neck. Dentomaxillofacial Radio1 1987:16: 17-21.

35. Underhill TE, Chiivarquer I, Kimura K, et al. Radiobiologic risk estimation from dental radiology: Part l-absorbed doses to critical organs. ORAL SURG ORAL MED ORAL PATWOL 1988;66:11 l-20.

36. Brand JW, Kuba RK, Aeppli DM, Johnson JC. Radiation do- simetry in specific area radiography. ORAL SLJRG ORAL MED ORAL PATHOL 1989;67:347-53.

37. Hayakawa Y, Sakoh T, Fujimori H, Kuroyanagi K. Dose re- duction to the thyroid gland in intraoral source radiography. Dentomaxillofacial Radio1 1993;22:21-4.

38. Altonen M, Heikkil’d M, Mattila K. A comparative study of radiation doses received during examinations with the panto- mograph, orthopantomograph, Panorex, Status-X, and con- ventional roentgen apparatus. Proc Finn Dent Sot 1974;70:67- 74.

39. Morgan TJ, Brateman L. The energy and directional response of Harshow TLD-100 thermoluminescent dosimeters in the diagnostic x-ray energy range. Health Phys 1977;33:339-42.

40. Horowitz YS. The theoretical and microdosimetric basis of thermoluminescence and applications to dosimetry. Phys Med Biol 198 1;26:765-824.

41. International Commission on Radiation Units and Measure- ments. Measurement of absorbed dose in a phantom irradiated by a single beam of X or gamma rays. Report 23, Washington DC: ICRU 1973.

42. Dhar A, DeWerd LA, Stoebe TG. EfTects of annealing and cooling processes on thermoluminescence of LiF (TLD-100). Health Phys 1973;25:427-33,.

43. Wall BF. Kendall GM. Collective doses and risks from dental radiology in Great Britain. Br J Radio1 1983;56:51 l-6.

44. Stenstriim B, Henrikson CO, Karlsson L, Sarby B. Effective dose equivalent from intraoral radiography. Swed Dent J 1986;10:95-101.

Reprint requests: Yoshihiko Hayakawa Department of Oral and Maxillofacial Radiology Tokyo Dental College l-2-2 Masago, Mihama Chiba, 261 Japan