Estimation of Effective Dose from Limited Cone Beam X-ray CT Examination

Abstract

The limited cone beam X-ray CT (Ortho-CT) was developed on the basis of multi-functional panoramic apparatus, SCANORA (Soredex Co. Helsinki Finland). The imaging intensifier (I.I.) was built in this apparatus as a X-ray detection device instead of X-ray film. The signal provided from I.I. was converted from analog into digital by an analog-digital converter and image reconstitution was done as a three. directional image of the dimensions 3.8cm of width, 3.0cm height and 3.8 cm depth with the personal computer. The “3DX Multi image micro CT” (3DX) was developed along similar lines by MORITA Co., Ltd. (Kyoto, JAPAN).
In this study, the stochastic effect on organ and tissue caused by examinations using Ortho-CT and 3DX was measured. The effective dose was estimated according to the recommendation of ICRP60 and was compared with those of panoramic radiography and computed tomography. The irradiation conditions were as follows: 85kV, 10mA with the filtration of 3mmAl and added 1mmCu for Ortho-CT, and 80 kV, 2 mA and the filtration of 3.1mmAl for 3DX. The measurement of organ and tissue dose was performed using an anthropomorphic Rando woman phantom (Alderson Reseach Laboratories Co., Stanford, CN), as well as by using two different type of thermoluminescent dosimeter (TLD); Panasonic UD-170A (BeO) and UD-110S (CaSO₄: Tm). The UD-170A was for dose measurment of the inner useful X-ray beams, while the UD-110S was for outer beams. The measured organ and tissue were those recommended with ICRP60 (gonad, breast, bone marrow, lung, thyroid gland, esophagus, stomach, colon, liver, bladder, skin, brain, thymus, adrenal, kidney, spleen, pancreas, upper large intestine, uterus, eyes and major salivary gland). The imaging by Ortho-CT was made in the left maxillary 1st molar, left mandibular 1st molar and temporomandibular joint. 3DX measurement was made in the maxillary incisor region and middle ear regions other than the regions mentioned above.
The skin dose measurement with Ortho-CT showed that mean values were within avariation of 750-615μSv. The effective dose for the mandible, maxilla and TMJ with Ortho-CT was 13.2μSv-23.0μSv. The resultant effective dose for the maxillary incisor, maxillary 1st molar, mandibular 1st molar, TMJ and middle ear regions with 3DX were 7.4μSv, 6.3μSv, 11.7μSv, 9.3μSv and 14.2μSv, respectively. It was found that the effective dose values were lower when compared with those in CT or panoramic radiography. Furthermore since three-directional images and three-dimensional images can be obtained easily, we suggest that these system will be useful for diagnostic imaging in dentistry.