Backscatter dose perturbation at high atomic number materials with a 2.8MV linac X-ray beam was studied using an absorbed dose distribution image generated by a microphotometer/microcomputer system. The absorbed dose distribution image clealy demonstrated the dose enhancement by backscatter electrons at the interface of high atomic number materials. A specially-made ready pack film (SR-IC metal pack film), in which single emulsion film (SR-IC: KONICA) and a high Z metal plate were packed, was used to measure the dose at the metal interface, and electron attenuation in a very thin layer (less than 50mg/cm²) was investigated. The dose distribution image obtained using XV-2 film was estimated to be that at a depth of 32mg/cm² depth from the metal surface. The enhanced dose ratio (R_ed=(D₁-D₀)/D₀×100(%); where D₁ is the absorbed dose at the metal interface and D₀ is the absorbed dose in homogeneous soft tissue) was highly correlated with the square root of the mean atomic number (Z^*) of backscatterer calculated by Z^*=∑w_iZ_i, (where w_i is the percent of weight of Z_i). The linear regression equation obtained for XV-2 film was R_ed=9.11√Z^*-23.3 (r=0.999) and that for SR-IC metal pack film was R_ed=16.91√Z^*-45.4 (r=0.999). These equations were used to calculate the 32mg/cm² depth dose and the exact surface dose for any mixture or alloy. Backscatter electron attenuation was investigated using XV-2 film and paraffin wax in order to estimate the optimum shielding thickness for protecting the oral mucosa. R_ed exponentially decreased as the thickness of the wax increased. The attenuation coefficients (μ_se) were evaluated corresponding to the mean atomic number of backscatterer. The optimum shielding thickness (S₀) corresponding to the mean atomic number was calculated using the equation R_ed(S₀)=R_ed(0)·exp(-μ_se(Z^*)·S₀), where R_ed (0) and R_ed (S₀) is the enhanced dose ratio with and without shielding materials, respectively. When permissible the enhanced dose ratio (R_ed (S₀)) is 2 or 5%, and the corresponding optimum shielding thickness can be calculated by above equation by substituting appropriate values for R_ed (0) and μ_se (Z^*).

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This study was conducted to evaluate the diagnostic ability of the images of RVG-S (Trophy Radiologie, Vincennces, France) in comparison with those of Ektaspeed film (Kodak, Rochester, NY, USA) obtained from patients. Both a film packet and an RVG sensor were simultaneously exposed under exactly the same geometry. The exposure time required for RVG was only 29% of that needed for film.
Twenty-nine sets of images were prepared for the evaluation. Each set consisted of an image on the RVG's monitor, a hard copy through the RVG's printer, and a film. Three evaluators independently assessed the diagnostic ability using 18 items of observation. Of These, 7 items were related to anatomical landmarks: dentin-enamel junction, pulp horn, root canal, lamina dura, periodontal space, cortical bone on the alveolar crest and trabecular pattern of alveolar bone. Eleven items were related to pathological radiographic findings: occlusal caries, interproximal caries, dental calculus, horizontal bone resorption, vertical bone resorption, periapical bone resorption, size of the periapical bone resorption, widening of the periodontal space, osteosclerosis, disttnce between the guttapercha point tip and the roentgenographic root apex, and the quality of condensation of guttapercha. All images were evaluated for each item by a scoring method or by measuring apical radiolucent areas in millimeters.
The diagnostic ability of the film was superior to that of the RVG monitor's image in 4 out of 7 of the anatomical items: dentin-enamel junction, root canal, periodontal space and trabecular bone pattern. The images of the RVG's prints were significantly inferior to those of the films (p<0.01) in the anatomical structures except for cortical bone on the alveolar crest. In the images of the RVG monitor, osteosclerosis was apt to be judged falsely positive, and the size of the periapical bone resorption to be judged smaller than in the films. The images of the RVG monitor were comparable to those of the films in the other nine pathological items (p>0.05). It is concluded that the RVG image can be safely used for clinical purposes with suitable selection criteria.

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To clarify the clinical significance of morphologic bony changes in temporomandibular joints (TMJs) with closed lock, we investigated the incidence of these changes in 82 closed lock patients (4 males and 78 females) using axial X-ray computed tomography, and evaluated their relations to the clinical findings.
The results were as follow.
1) In the 114 joints with displaced discs without reduction, 61 joints (53.5%) exhibited bony changes in the condylar head, but there were few changes in the contralateral control sides without displaced discs or with reducible discs.
2) The frequency of such changes at the lateral and anterior parts of the condylar head was high. Regarding the kind of bony changes of the condylar head, absorptive changes were found most frequently (57.4%), followed by deformed changes (22.9%) and hyperplastic changes (19.7%).
3) The patients with bony changes in the condylar head had a long closed lock period, a high incidence of joint crepitus, and large mobility of the condyle.
4) There were no relations between the other clinical findings (such as the age of patients, degree of pain and maximum mouth opening range) and the morphologic bony changes of the condylar head.
These results suggest that the presence of morphologic bony changes in TMJs with closed lock may not be correlated well with TMJ pain and dysfunction in patients with closed lock.

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In this study, 12 patients diagnosed with benign cementoblastoma by radiographical and histopathological examination are reviewed. These patients were seen at Osaka Dental University in the Department of Oral Radiology in the 22-year period from April 1972 to December 1993. All tumors involved the mandible. Ten tumors occurred in the molar region and one tumor in the premolar region. Only one tumor occurred in deciduous dentition. There were 5 males and 7 females. The age of the patients at the time of diagnosis ranged from 7 to 47 years, with an average age of 26.0 years. The tumors were classified by radiopaque pattern into two types: homogeneous (type I, n=5) and mottled (type II, n=7). Almost all tumors were well defined and the central radiopaque area was commonly surrounded by a radiolucent zone of uniform width; only one tumor did not show this radiolucent zone. Condensation of the bone occurred in 11 of 12 tumors. Six of 11 tumors developed wide perifocal osteosclerosis of the bone, which was considered to be severe Type II. One patient showed a relapse approximately 18 months after surgical excision of benign cementoblastoma, and follow-up observation by radiographical examination is advisable for approximately 3-5 years after surgery.

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Calcifying odontogenic cyst (COC) is a rare benign lesion of the jaw. It is difficult to diagnose because of many variations in radiographic features. In many previous studies, COC has been reported to occur as a monostotic lesion, and reports of multiple occurrence have been scarce.
The present study reports a case of COC which occurred bilaterally in the molar region of the mandible. The pathological features of the left and right lesions were nearly identical, but the two lesions differed radiographically in the border of the lesion, the state of root resorption, and the relationship between the lesion and the impacted teeth. Only one earlier case of bilateral COC could be found in our review of literature, and there were some similarities in the pathologic and radiographic features of these two cases.

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We report herein the diagnostic imaging of a patient with von Recklinghausen's disease with malignant schwannoma in the mandible. The patient was a 19-year-old male with a complaint of swelling in the region of the right mandibular angle. The radiographs showed severe bone destruction in the region of right mandibular angle. There was an area of moth-eaten appearance and the margin of the defect was irregular.
Preoperative diagnostic imaging was done using computed tomography and magnetic resonance imaging (MRI). On the T1-weighted MRI images, the density of the mass was homogeneous and the signal intensity was slightly greater than that of the surrounding muscles. On the T2-weighted MRI images, the signal intensity of the mass was high, with an area of super-high intensity in the central portion. The images following Gd-DTPA administration showed significant enhancement with low signal intensity in the central portion. In the present case, MRI offered the most definite information concerning tumor extension and the relationship between the tumor and surrounding tissue.

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