Volumetric accuracy of cone-beam computed tomography

Abstract

Purpose: This study was performed to investigate the influence of object shape and distance from the center of the image on the volumetric accuracy of cone-beam computed tomography (CBCT) scans, according to different parameters of tube voltage and current.

Materials and methods: Four geometric objects (cylinder, cube, pyramid, and hexagon) with predefined dimensions were fabricated. The objects consisted of Teflon-perfluoroalkoxy embedded in a hydrocolloid matrix (Dupli-Coe-Loid TM; GC America Inc., Alsip, IL, USA), encased in an acrylic resin cylinder assembly. An Alphard Vega Dental CT system (Asahi Roentgen Ind. Co., Ltd, Kyoto, Japan) was used to acquire CBCT images. OnDemand 3D (CyberMed Inc., Seoul, Korea) software was used for object segmentation and image analysis. The accuracy was expressed by the volume error (VE). The VE was calculated under 3 different exposure settings. The measured volumes of the objects were compared to the true volumes for statistical analysis.

Results: The mean VE ranged from -4.47% to 2.35%. There was no significant relationship between an object's shape and the VE. A significant correlation was found between the distance of the object to the center of the image and the VE. Tube voltage affected the volume measurements and the VE, but tube current did not.

Conclusion: The evaluated CBCT device provided satisfactory volume measurements. To assess volume measurements, it might be sufficient to use serial scans with a high resolution, but a low dose. This information may provide useful guidance for assessing volume measurements.

Keywords: Cone-Beam Computed Tomography; Imaging, Three-Dimensional; Phantom.

Fig. 1. Four geometric objects with predefined dimensions made of Teflon-perfluoroalkoxy material were fabricated using a highly precise computer numerical-control milling machine. The volumes of the cylinder, pyramid, cube, and hexagon blocks ranged between 0.48 mL and 1 mL.

Fig. 2. Four geometric reference objects were placed in the cylinder assembly and scanned in the cone-beam computed tomography device.

Fig. 3. The alignment of the objects in the polymethyl methacrylate cylinder. The center-to-center distances of the objects were 30 mm.

Fig. 4. Axial cone-beam computed tomography images show the geometric objects, with cylinder, pyramid, cube, and hexagon shapes.

Fig. 5. Segmented phantom. Three-dimensional segmentation allows measurement of the volume of each scanned object.

Fig. 6. Correlation analysis of the volume measurements of cone-beam computed tomography (CBCT) imaging data. Each diamond (♦) indicates the volumetric deviation of a crossing (y-axis) and the distance of this crossing from the center of the CBCT imaging data (x-axis).