OBJECTIVES In a multiplanar reconstruction CT program (CT/MPR), the altered gantry angle corresponding to the patient's position during scanning for multiplanar reformatted CT may cause distortion of the image. The aim of this study was to quantitatively assess the distortion in reformatted central panoramic and cross-sectional images owing to the change of gantry angle. METHODS A resin block model with four cylindrical holes and a human dry mandible were used in high resolution conventional CT scanning. A MPR software package was used for reformatted panoramic and cross-sectional images. The block and the gantry were equally inclined at 0 degrees, 15 degrees and 30 degrees. The relationship between the positional difference and depth of the hole as determined in the reformatted image, and the actual depth was analysed with respect to the gantry angle. The actual depths (H) were calculated by use of the measured depths (h) and the inclined angles: H=hxcos beta. The positional differences of the hole bases (W) were also calculated by use of the measured depths and the inclined angles: W=hxsin beta or Hxtan beta. RESULTS Increasing the gantry angle, increased the depth and inclination of the holes in the reformatted central panoramic images. Inclined angle (beta) was the same as gantry angle. CONCLUSION When the gantry angle is not at 0 degrees, certain CT/MPR programs may distort the reformatted image. If distortion occurs, the corrected position and usable length of pre-implant sites can be calculated by use of the above formula.