Beam alignment and image distortion assessment of dental cone beam computed tomography (CBCT) system

Cone-beam computed tomography (CBCT) technique has been widely used in dental imaging as it offers different image views such as panoramic, cephalometric, tomographic, and three dimensions (3D). This study aimed to assess image distortion due to beam misalignment for CBCT system using phantom study. A ball phantom was scanned with default exposure setting for standard panoramic imaging (68 kV, 8 mA and patient size of M) using Planmeca ProMax CBCT unit. Three different measurements were performed on the phantom images such as distance measurement between the centre of the middle ball and rear middle ball, distance measured from the centre of middle ball to tenth ball (left and right), and measurement of both vertical and horizontal distance of any four balls. From the findings, most of measured distance between middle ball and rear middle ball were below the acceptable level, but two measurements exceeded the reference value by 6% and 75%. Similar with the measured distances of the tenth balls, two measurements exceeded the acceptable level by 3% and 28%. The vertical and horizontal distance of 4 balls were slightly differ from standard value with overall magnification of 1.06 and percentage error of 6%. The beam alignment and patient positioning should be accurately evaluated to avoid image distortion and misdiagnosis in dental CBCT images.