Radiation dose in temporomandibular joint zonography

Radiation dose in temporomandibular joint zonography M. E. Coucke, DDS,’ R. R. Bourgoignie, PhD,b L. R. Dermaut, PhD,a K. A. Bourgoignie, MD,c and R. J. Jacobs, PhD,d Ghent, Belgium UNIVERSITAIR

ZIEKENHUIS

AND GHENT

STATE

UNIVERSITY

Temporomandibular joint morphology and function can be Thermoluminescent dosimetry was applied to evaluate the phantom eye, thyroid, pituitary, and parotid, and the dose when the TMJ program of the Zonarc panoramic x-ray unit reference to similar radiographic techniques. (ORAL SURC ORAL MED ORAL PATHOL 1991;71:756-62)

T

he image quality of radiographs of the temporomandibular joints (TMJs) by plain film radiography is often limited because of clouding effects caused by interference of superimposing bony structures. Combining priniciples of tomography and scanography, the Zonarc system developed by Palomex (Palomex Instrumentation Corp., Helsinki, Finland) represents a substantial advance in imaging capabilities over conventional panoramic units. It is a panoramic x-ray unit specifically designed to provide tomographic images of preselected curved anatomic structures in the head and neck. Together with cassette movement, microprocessor-controlled circular and linear movements of the tube produce detailed images, with manual adjustment for image depth and layer thickness. The features of the Zonarc system and its usefulness in the diagnosis, treatment, and follow-up of traumatic events in the mandibular and maxillofacial regions were discussed by Hartman et al.’ Differences in beam geometry and in scanning motion resulting from the instrument settings for the preselected program evidently affect the amount and the distribution of the dose absorbed by the patient being examined. The radiation dose resulting from panoramic zonography of the TMJ has not yet been evaluated. The purpose of the present investigation was to map the skin dose distribution to the head and neck, and to es-

aDepartment of Orthodontics. bPhysics Laboratory, Faculty of Medicine. CMedical Student. dRadiation Protection Service. 7/16/23285 756

evaluated by panoramic zonography. radiation dose to predetermined sites on a distribution on the skin of the head and neck was used. Findings are discussed with

timate the radiation dose to specific sites representing the lens of the eye, the thyroid gland, the parotid gland, and the pituitary gland when the Zonarc imaging technique for the TMJ is used. MATERIAL

Zonographic

AND METHODS

equipment

The Zonarc unit we evaluated has a Siemens Bi (150/ 12/50 R) x-ray tube with a rotating anode and 0.3 mm focus. The curved metal film cassette has a radius of 165 mm. The primary slit diaphragm is 4 x 60 mm, and the corresponding secondary slit diaphragm on the film holder is 6 X 130 mm. The screens are Siemens special screens (blue-sensitive; Siemens AG, Munich, Germany) and the films are Agfa Gevaert Dentus RPS (Agfa Gevaert, Leverkusen, Germany). The Zonarc equipment is shown in Fig. 1. For the TMJ investigation the head should be tilted far forward to avoid ghost shadows from the cervical aspect of the spine and from the opposite temporal bone. In this program the section is cylindric with two different centers of rotation, as represented by 0 and 0’ in Fig. 2. The tube rotates from ear to ear behind the skull, but exposure is interrupted during switching from one center of rotation to the other. The stippled areas, Land L’ , represent the image layers. With standard positioning the image layer is located as in Fig. 2, A. When a dorsal shift is applied, the centers of rotation and the image layers are displaced dorsally, as shown in Fig. 2, B. For the present dose evaluations, only the dorsal 15 mm shift position was considered. The tube was operated at 77 kVp and L25 mA with a total filtration of 3.2 mm aluminum. The

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2'b

Fig. 2. Image layer locations for TMJ Standard position; 2h, dorsal shift position.

Fig. 1. A, Zonarc equipment with phantom. 1, Stretcher equipped with three-dimensional positioning device; 2, x-ray tube with beam collimator; 3, film cassette; 4, secondary collimator and safety switch; 5, phantom head in TMJ cephalostat. B, Detail of phantom head in cephalostat.

exposure time was 4.5 seconds for each joint. Fig. 3 shows a TMJ zonogram. Dosimeters

and ancillary

equipment

All preliminary work on the dosimeters was done at the X-Ray Calibration Service Facility of the Standard Dosimetry Laboratory (Ghent State University). Phantom. The upper part of a standard averageman radiation analog dosimetry system (RANDO) phantom2 (model No. 154, Alderson Research Laboratories Inc., Stamford, Conn.) was used for all exposures involving radiation dosimetry. The phantom is made of synthetic tissue-equivalent material forming natural body contours and contains a male skeleton, with airway cavities that represent the nasopharynx, the oropharynx, and the trachea. The phantom was transected horizontally into slices or sections 2.5 cm thick that contain predrilled holes for insertion of thermoluminescent dosimeters (TLDs). Sections were registered to each other for rapid

Fig.

757

3. TMJ zonogram

program.

Za,

of patient.

alignment in assembly by means of plastic pins. Twelve consecutively numbered sections were used (0 to 1 l), with the top cranial slice being 0 and section 11 representing the upper thorax region. Choice of TLDs. Extruded lithium fluoride/magnesium/titanium (LiF:MgTi, TLD system 700, 2.64 gm/cm3) and calcium fluoride/dysprosium (CaF2: Dy, TLD system 200, 3.18 gm/cm3) (Harshaw Chemical Co., Solon, Ohio) ribbons or chips (3.1 X 3.1 x 0.89 mm) were used to measure the absorbed dose. (For the considered application, TLD-700 has the same thermoluminescent characteristics as TLD system 100 but was used because it was available.) The thermoluminescent properties of these dosimetric materials are well documented.3 For dose measurements at the energy level of dental procedures, LiF should be used rather than CaF2, because of its near tissue equivalence and its practically energy-independent thermoluminescent response. However, in the range of low x-ray qualities, CaF2 shows a much higher sensitivity than LiF, so it was used to trace the contours of the skin zones where

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Fig. 4. A, Radiographic profile patterns of radiation distribution between phantom sectionsdefining extent of skin areas and tissue zones where high dose values are to be expected. B, Diagrams of reduced top views of phantom sections 3, 4, 5, and 6, indicating measured dose values (micrograys per examination) to sites halfway down 2.5 cm sections.

exposure and absorbed doses were greatest and for rapid comparative purposes. Dosimeter reader. The dosimeter reader (Harshaw TLD system 4000) integrates the thermoluminescent signal over selected regions of interest in the temperature scale and displays the thermoluminescent profile, the glow curve, on a liquid crystal display. A dry nitrogen gas flow was used to prevent chemoluminescence. The characteristics of the glow curves and the amount of light output depend on the thermal treatment of the thermoluminescent material.4, 5 Both types of TLDs were subjected to the same heating cycle: a preirradiation anneal at 400” C for 1 hour, followed by 20 hours at 100” C. After exposure the readout cycle consisted of preheating at 100” C for 60 seconds, followed by a linear increase to 300” C at a rate of 6” C per second. The light-acquiring time was 40 seconds. With this procedure we found that the main dosimetry peak for TLD-700 was well resolved, so the light output was integrated from 160” to 240” C. Although such limitation of the integrating region of the glow curve reduced the numeric value of the response of the thermoluminescent material, it fa-

vored the signal/noise ratio, because the main contribution to the background (second reading or reading of unexposed dosimeters) comes from the persistent glow peaks greater than 240” C. Furthermore, the influence of fading effects on the low temperature peaks was avoided. Under these circumstances the ratio of the thermoluminescent response of TLD-700 to that of TLD-200 was about 60% of the value reported by Weissman et a1.6 For TLD-200 the glow curve was integrated over the entire temperature range. Calibration procedure. The x-ray exposure unit was a constant-potential Philips MG 420 system, modified for dosimetry purposes, coupled to an MCN 42 1 ceramic tube. Exposure was accomplished by operating a mechanical shutter on the horizontal beam. A transmission chamber linked to a current integrator was used to monitor the beam against a secondary standard Baldwin-Farmer probe dosimeter (model 2570; Nuclear Enterprises Ltd., Reading, U.K.). To simulate the Zonarc radiation conditions, the Baldwin-Farmer probe was positioned against the wall of a water phantom at a distance of 150 cm from the focus. Appropriate conversion and correction factors

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were taken into account for air pressure and temperature. For calibration the Baldwin-Farmer probe was replaced by a flat array of TLDs. Electronic equilibrium was achieved by the TLD holder. The TLDs were exposed to 20.1 mGy at 77 kVp with 3.2 mm aluminum filtration. The two sets of TLDs were calibrated as batches: dosimeters that read more than 2 SD from the mean were rejected. The same annealing and calibration procedure was repeated several times until consistently close readings (SD 13%) resulted for batches of about 100 ribbons. An individual reading was then assumed to be representative of the batch. Finally, each batch was divided into groups of 20 ribbons and the calibration procedure was repeated for other radiation qualities, ranging from 2 to 4 mm aluminum half value layer (HVL). In the range of 2 to 3 mm aluminum HVL, both calibration factors increased by & 8% but remained nearly constant at heavier filtration. We also found that in the HVL region from 3 to 4 mm aluminum, the relative response for parallel to perpendicular orientation was -+92% for TLD-700 and 273% for TLD-200. All these findings are in agreement with the results of Morgan and Brateman’ for TLD-100. In the present investigation, values for the calibration factors were adopted as measured at 77 kVp with 3.2 mm aluminum filtration. The overall inaccuracy of the TLD calibration factors was estimated to be +- lo%, including the statistical variation of the thermoluminescent output. Differences in beam geometry between the Zonarc exposure and the calibration exposure may result in larger uncertainties in the dose values.

Table 1. Measured doses to specific sites

MEASUREMENTS Location of the dosimeters phantom

For skin dose measurements, sets of four LiF ribbons (TLD-700), placed adjacent in pouches, were used. Absorbed doses to the eye lenses were estimated from sets of sealed ribbons on the lids of the closed eyes. Sets of three ribbons were also placed halfway down some of the predrilled holes of the phantom sections. A number of sealed ribbons remained unexposed to define the background radiation. For the final investigation, radiation exposure was repeated 36 times. The film was developed after the first exposure and was used to check the positioning of the phantom. The average readout of ribbons exposed in the same location was considered as the readout measurement value. After the averaged background reading was subtracted and divided by the number of exposures, values were converted to dose values by applying the calibration factor. For the ribbons placed internally, dose values were corrected for beam orientation. Correction factors for specific target material attenuation were applied, with data published by Hubbell9 Resulting values are skin dose or tissue dose, expressed in mGy or pGy per single exposure.

and positioning

of

Reference positioning of TLDs was obtained by attaching small self-adhesive labels to the phantom. Although the Zonarc provides aligning light beams, repeated dose measurements (with TLD-200) and zonograms of well-defined anatomic structures indicated that repositioning of the phantom was difficult. For rotational panoramic radiography Preece* remarked that literature references to patient dose seldom agree, because small differences in dosimeter location significantly affect the recorded dose. In our case improved accuracy in positioning was obtained with a cephalostat fixed to the unit’s stretcher. With this mechanical device the procedure by three-dimensional positioning of the table was found to be reliable. Identification

of exposed

areas

Profiles of the radiation exposure distribution were recorded on film. Paper-wrapped radiographic films (Agfa Gevaert Dentus RP5) were contoured to the

Site

Dose

(pGy/exam)

Skin Entry zone Exit zone Dorsal midline Frontal midline Thyroid

gland

Eye lens Parotid gland Pituitary

gland

3000-4000
outlines of the phantom sections and sandwiched between them. The reference number of the profile film is that of the inferior phantom section. The same basic dose distribution pattern was observable on the profile films 3, 4, 5, and 6, whereas no exposure was assessed on the others. Some of the produced profile radiographs are presented in Fig. 4, A. Although these radiographs were not intended for dose estimation, the centers of rotation appeared to represent the highest radiographic densities. In addition, TLD-200 ribbons were used to define the boundaries of the exposed zones on the skin, Single ribbons were put in black, identifiable polyethylene pouches (5 mg/cm*) and stuck to the skull, covering a large part of it in a grid pattern. Also sets of 15 or more ribbons, packaged adjacent in strips were used. In each run three exposure cycles were sufficient to obtain adequate signals even in the areas not exposed to the primary beam. Setup for dose measurements

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Coucke et al.

Fig. 5. Diagrams of reduced top views of sections 9, 10, and I 1, indicating measured doses (micrograys per examination). Selected sites were within expected location of thyroid gland.

RESULTS

The radiographs of Fig. 4, A, indicated that structures such as the lens of the eye and the thyroid were safely out of the region scanned by the primary beam (primary field) and also out of the exit region. The doses to these structures could therefore result only from scattered radiation. Results are summarized in Table I. Numeric values for skin dose and internal dose to specific sites of the head are presented in Fig. 4, R, and Fig. 5. Positioning of the head. The asymmetric features of the phantom skull caused it to be slightly tilted sideways when fixed in the cephalostat. The radiographs, presented in Fig. 4, A, indicate an eccentric and tilted position of the skull. Furthermore, from Fig. 2, copied from the manufacturer’s operating instructions, the exposure pattern itself does not seem to be symmetric. Asymmetry in the skin dose distribution was therefore not unexpected. The numeric values, presented in Fig. 4, B, showed that the differences in skin dose caused by asymmetry were much larger than the statistical variations of the readings from individual TLDs within the batch at each site. This implies that although the observed maximal dose to the skin was only 4 mGy and the surface areas exposed to the primary beam were only about 2 X 30 cm2, wrong positioning of the patient should lead to a shift in the skin dose distribution and also to other dose values to important internal sites, such as the pituitary gland. A similar remark was made by Bankvall and Hakanssen’O about ocular pneumoplethysmography

of a child (“even a slight eccentric positioning of the patient in the apparatus or a slight facial asymmetry could cause great variations in the skin dose.“). Skin dose distribution. From the beam trajectory implied in Fig. 2 and from the radiographs shown in Fig. 4, A, and from the measurements with TLD-200, we deduced that the limitation of the exposure to regions of sections 3,4,5, and 6 was due to beam height collimation and interruption of exposure. Only the areas ranging from about the midear to about 4 cm behind the ears were exposed to the primary beam, subtending angles of less than 45 degrees when viewed from the rotation centers. These approximately rectangular-shaped zones lie obliquely because of the inclination of the head. The highest doses to the skin were located in areas covering sections 4, 5, and 6, at the level of the ears. Maximal skin doses of 4 mGy were found on section 6, under the earlobes. Dorsally, skin doses decreased to about 3 mGy near the edges of the zones exposed to the primary beam. Posterior to these zones the TMJ program provided interruption of the exposure, and doses dropped off rapidly in the next centimeter by a factor of 40 to 50. They further decreased exponentially to less than 20 pGy at the dorsal midline. In the exit zones, skin doses were reduced to values of about 0.1 mGy, also decreasing ventrally. For the frontal unexposed zone, skin dose values were very small, with about 5 pGy on the frontal midline. Similar distribution patterns were found for sections 4 and 5, but dose values were clearly inferior to those on section 6. The largest skin dose to section 7 was about 40 pGy, below the ear. To section 3 it was about 120 rGy behind the ear. Thyroid gland. Fig. 5 shows reduced top views of sections 9, 10, and 11. Dose values halfway down the sections are indicated for specified sites. Doses range from about 3 to I3 pGy. Eyes. The absorbed dose to the eye lenses was estimated from the mean reading of sets of four sealed TLD-700 ribbons placed on the lids of the closed eyes. We estimated a dose of 3.6 1Gy to the right eye and 5.0 pGy to the left eye. Parotid gland. Two extra holes were drilled in section 6 to evaluate the representative dose to the parotid glands. The location of the holes was not symmetric. As indicated in Fig. 5, the left hole was about 2 cm posterior to the right one and was nearer to the skin surface. We found 207 pGy at the right location and 85 pGy at the left. Pituitary gland. The site representing the pituitary gland was considered to contain brain tissue. The measured dose was 130 /*Gy. Other internal locations. Internal doses were all less than the maximal skin dose of 4 mGy. As the

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II. Radiation doses to some phantom sites for different radiographic projections and present values with TMJ zonography

Table

Radiation Point of entrance

Transcranial*

(11 cm2)

Transcranialt Transorbital* (38 cm2) Sigmoid notch* (Parma) Short cone (7.5 cm2) No cone (55 cm*) Lateral tomography$ Grid No grid Zonography$ (2 X 30 cm2)

70 90

dose

(pGy/exposure)

Pituiiary

Parotid

Eye

Thyroid

-

-

71 115 45-177 23

21 26 O-34 (exit side, one device) 102

70

8500 6000 1760-2050 1700

70 90 70

3400 2520 9900

-

-

39 77 182

901 1300 4600

77 75

3070 1560 3000-4000

3.6-5.0

3-13

265 136 130

82 120 85-200

1700

*Reference I I. General Electric 1000 x-ray machine (General Electric, Columbia, Md.); 10 mA. tReference 12. Gendex 900 dental x-ray unit (Gendex Corp., Franklin Park, Ill.), 124nch positioning fReference I I, Quint Sectograph (Denar Corp., Anaheim, Calif.). $Present study.

scanning velocity of the rotating beam decreases near the rotation centers, relatively high doses could be expected in the region enclosing these centers. However, this effect seemed well compensated for by beam attenuation. It was estimated that doses did not exceed 2.5 mGy in this region. DISCUSSION

For comparative purposes, with sole reference to resulting doses, we consulted the article by Brooks and Lanzetta,” who conducted a comprehensive study involving four different TMJ radiographic techniques (transcranial, transorbital, sigmoid notch, lateral tomography). These authors used Kodak X-Omat films together with Kodak Regular intensifying screens. They mention that the exposure to the thyroid was detectable but too low to be measured accurately. The same held for the dose to the eyes, except for the case of the transorbital projection. Some results are presented in Table II. For the transcranial technique the beam diameter at the point of entry was 3.8 cm and the skin dose varied from 6.0 to 8.5 mGy. The dose to the pituitary gland ranged from 7 1 to 115 pGy, and to the parotid from 21 to 26 pGy. For the transorbital technique, the collimated beam diameter was 7 cm at entrance. The skin dose at entry (1.12 to 1.70 mGy) and the dose to the pituitary gland (23 to 25 pGy) were smaller than for the transcranial case. However, the parotid dose was larger (102 to 199 rGy), and the eye received 1.12 to 1.70 mGy. In the case of the Parma projection (sigmoid notch) a reduction in dose by at least a factor of three was recorded, caused by beam collimation. With a short cone the skin dose at entry varied from 2.52 to 3.40

cone.

mGy. The dose to the pituitary gland ranged from 39 to 77 pGy, and the dose to the parotid from 0.9 to 1.3 mGy. It should be reminded that with the use of TMJ zonography both joints are radiographed in a single exposure. In 1986 Saini et al.‘* evaluated the radiation dose distribution to the head and neck for the transcranial projection technique with four commercially available TMJ positioners. The positioners had collimators of various dimensions, curtailing the size of the x-ray beam to include only the area of interest. Saini et al. used the Kodak OL film-Kodak Lanex Regular screen combination (Eastman Kodak Co., Rochester, N.Y .). Some of their results are included in Table II. The skin dose at the point of entrance was approximately one third the dose reported by Brooks and Lanzetta,” whereas with zonography we found a maximal dose to the skin of about 4 mGy close to the borderline of the exposed skin area. Saini et al. reported a skin dose at the beam exit side varying from 25 to 32 pGy, whereas we measured a maximum value of about 100 pGy. Saini et al. attributed their favorable results to the use of narrow collimation and a fast screen-film combination. Although our present objective was to study the dose distribution resulting from the Zonarc TMJ program, interest in the dose distribution from the maxillofacial program (Zonarc-MF) was stimulated by the article by Paukku et a1.i3 These authors compared dosimetric results with the use of panoramic radiography, plain film radiography, and linear tomography of the facial bones. They reported a skin dose value of only 0.8 mGy at the center of the beam entry zone.

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Using the MF program in a preliminary examination, we found skin doses along the midline in the exposed area ranging from about 0.6 to 1.2 mGy. However, we also found two sharply defined peaks in the dose distribution with a maximum of about 1.7 mGy located near the borderline of the exposed skin area (+ ears). A similar situation was found for the dental program (Zonarc-DENT). Here, the proportion of peak values to back midline values was even more pronounced (0.4/3.3 mGy). For the dose to the eyes and the thyroid, our preliminary results confirm the findings of Paukku et al. l3 Further research on the MF and DENT programs is presently being conducted. In terms of radiation protection, a principal advantage of the Zonarc TMJ program compared with other techniques for TMJ imaging stems from the geometric limitation of the scanning beam, the interruption of the exposure at the back of the head, and the imaging of both joints in a single exposure.

ORAL SCIRC;ORAL MED ORAL PATHOI. Jtme 1991

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6. 7. 8. 9. 10.

1I. 12. 13.

REFERENCES 1. Hartman LC, Wolfgang L, Hall RE, DelBalso A. The application of panoramic zonography to the diagnosis of maxillofacial fractures. ORAL SURG ORAL MED ORAL PATHOL 1989: 67:214-9. 2. MacKinlay AF, ed. Thermoluminescence dosimetry. Medical physics handbooks; vol 5. Bristol: Adam Hilger, 1981:83. 3. Horowitz YS. Thermoluminescence and thermoluminescent dosimetry; vol 1. Boca Raton, Fla: CRC Press, 1984:91-107, 141-5. 4. Driscoll CM, Barthe JR, Oberhofer M, Busuoli G, Hickman

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C. Annealing procedures for commonly used radiothermoluminescent materials. Radiat Protection DOS 1986;14:17-32. Bos AJ. Mechanisms of thermoluminescence production in LiF:Mg.Ti (TLD-100). Presented at Symposium on Thermoluminescence Dosimetry. Bilthoven, The Netherlands, March 1988. Weissman DD, Lonhurst CE, Cheng SC. CaFz:Dy dosimetry of 80 kVp x-rays. Health Phys 1972;22:487-90. Morgan TJ, Brateman L. The energy and directional response of Harshaw TLD-100 thermoluminescent dosimeters in the diagnostic x-ray energy range. Health Phys 1977;33:339-42. Preece JW. Principles and practice of panoramic radiology. In: Langland GE, Langlais RP, Morris CR. Biologic effects of panoramic radiography. Philadelphia: WB Saunders, 1982:57. Hubbell JH. Photon mass attenuation and energy-absorption coefficients from 1 keV to 20 MeV. Int J Appl Radiat Isot 1982;33:1269-90. Bankvall G, Hakanssen H. Radiation-absorbed doses and energy imparted from panoramic tomography, cephalometric radiography, and occlusal film radiography in children. ORAL SURG ORAL MED ORAL PATHOL 1982;53:532-40. Brooks SL, Lanzetta ML. Absorbed doses from temporomandibular joint radiography. ORAL SURG ORAL MED ORAL PATHOL 1985;59:647-52. Saini TS, Fischer WG, Verbin RS. Absorbed radiation doses in transcranial temporomandibular joint radiography. J Prosthet Dent 1986;55:62 l-4. Paukku P, Gothlin J, Totterman S, Servomaa A, Hillikainen D. Radiation doses during panoramic zonography, linear tomography and plain film radiography of maxillofaciat skeleton: a comparative study. Eur J Radio1 1983;3:239-41.

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L. R. Dermaut, PhD Kliniek voor Tand-, Mond- en Kaakziekten Afd. Orthodontie Universitair Ziekenhuis De Pintelaan 185 B-9000 Ghent, Belgium

ERRATUM In the article “Electronic System for Digital Acquisition of Rotational Panoramic Radiographs” by McDavid et al., which appeared in the April 1991 issue (ORAL SURG ORAL MED ORAL PATHOL 1991;71:499-502), Figs. 5 and 6 on page 501 were inadvertently transposed.