Development of a calcium phantom for dental cone beam CT

Objectives: A dental cone beam CT (CBCT) has been developed and widely used in a dental clinic for diagnosis or prognosis in a dental implant treatment. However, a dental CBCT does not have CT values and a bone density can not be accurately measured. Therefore, the purpose of this study was to develop a new reference phantom for dental CBCT, which contains the same amount of calcium density as cancellous bone for measuring the accurate cancellous bone density. Methods: A reference phantom is based on a base resin composed of urethane dimethacrylate and triethylene glycol dimethacrylate, hydroxyapatite (Hoya, Tokyo, Japan) for radiopaque filler, and poly (methyl methacrylate) for dispersant. The phantom has 2.0 mm thickness and calcium density of 100, 200 and 300 mg/cm³. Three kinds of calcium density were determined from cancellous bone density and that contained in a conventional CT phantom for diagnosis of osteoporosis. The reference phantom with an aluminum step was taken X-ray by periapical film for calculating an aluminum equivalent thickness. After processing, the radiographs were digitized to tagged image file format (TIFF) images by film scanner. The images were analyzed using image analyzing software(Image J ver.1.44, National Institutes of Health, USA) and the aluminum equivalent thickness was determined. The homogeneity of each calcium density was also evaluated. Results: All phantoms had homogeneous calcium density. An aluminum equivalent thickness ± s.d. of the reference phantom containing calcium density of 100, 200 and 300 mg/cm³ were 0.29 ± 0.08, 0.57 ± 0.04, and 0.80 ± 0.03, respectively. Conclusion: A new developed reference phantom contains the same amount of calcium density as cancellous bone and the accurate cancellous bone density can be measured using the phantom for dental CBCT.