Image Distortion and Spatial Resolution of a Commercially Available CBCT

Objectives: To evaluate images produced by a commercially available CBCT for measurement and spatial resolution (i.e. the ability to separate two objects that are in close proximity in the image) for all settings and in all dimensions. Methods: A custom phantom (CIC-Phantom) containing 0.3 mm diameter chromium metal markers approximately 5 mm apart in three planes of space was developed for analyzing distortion and measurement accuracy. This phantom was scanned in the CBCT machine (i-CAT – Imaging Sciences International Inc. – Hatfield, PA) using all twelve of the commercially available settings. The distance between the markers was measured three times on the 3D images using a DICOM viewer and was also measured three times directly on the phantom with a fine tipped digital caliper. A line pair phantom (C-Phantom) was utilized to evaluate spatial resolution. Thirty judges analyzed images and assigned a resolution from 0.2mm to 1.6mm according to the separation of the line pairs. Results: Intra-operator reliability showed intraclass correlations of 0.892 for CBCT measurements and 0.902 for direct measurements. There were no statistically significant differences among 3D images for any of the image settings, in any of the three dimensions, or in images divided by thirds in terms of measurement accuracy. Comparison of the CBCT measurements to the direct digital caliper measurements showed a statistically significant difference (p < 0.01). The absolute difference was <0.1 mm and can be considered not clinically significant for most applications. Reliability of the 30 judges showed an intraclass correlation of 0.78. The highest spatial resolution measured was 0.86mm for a 10 seconds scan, and the lowest resolution was 0.62 for the 40 second scan. Spatial resolution was overall lower at faster scan times and larger voxel sizes. Conclusions: The CBCT tested showed no significant distortion, good accuracy and spatial resolution on all tested settings.