Influence of CBCT-based volumetric distortion and beam hardening artefacts on the assessment of root canal filling quality in isthmus-containing molars

Abstract

Objectives: To evaluate the influence of artefacts in cone beam CT (CBCT) images of filled root canals in isthmus-containing molars.

Methods: 10 teeth presenting canals with an isthmus were instrumented and filled with a thermoplasticised obturation technique. The teeth were scanned using a micro-CT device and two CBCT devices: 3D Accuitomo 170 (ACC) and NewTom VGi evo (NT), with different acquisition protocols: larger and smaller voxel size. Three examiners assessed the CBCT images for: (1) detection of filling voids; (2) assessment of under- or overestimation of the filling material and (3) resemblance of CBCT images to the reference standard. Analyses of Task 1 yielded accuracy, sensitivity and specificity for detection of filling voids. For tasks 2 and 3, statistical analysis was performed using Wilcoxon test. The level of significance was set at p < .05.

Results: For Task 1, ACC showed higher sensitivity, whereas NT presented higher specificity. No significant difference was found between the protocols in ACC, however, for NT, differences between protocols were significant for all diagnostic values. In Task 2, visualisation of the filling was overestimated for NT, while for ACC, underestimation was observed. For Task 3, images with smaller voxel size were more similar to the reference image, for both CBCT devices.

Conclusions: Different artefacts compromise the detection of filling voids on CBCT images of canals in mandibular molars with isthmus. ACC and NT present rather similar diagnostic accuracy, even though artefact expression remains device-specific.

Keywords: Artefacts; Cone-beam computed tomography; Microcomputed tomography; Root canal obturation..

Figure 1.

Representative image of an axial slice of CBCT (on top) showing the root canal in the mesial root divided into five regions for the assessment of presence of filling voids during Task 1. Scheme of a mesial root used for examiner´s calibration (1). Visual analogue scale used to indicate the examiner´s level of confidence (2). Representative set of slices of a sample (below), showing the same tooth on micro-CT and different CBCT devices and protocols, for blind evaluation in tasks 2 and 3. CBCT, cone beam CT.

Figure 2.

Confidence levels for the observers and CBCT protocol, and according to correct or incorrect diagnosis (i.e. when the diagnosis provided by the observer was in agreement or disagreement, respectively, with micro-CT findings). Confidence levels are represented by the relative frequency of cases where the observers reported that they were confident about their diagnosis. CBCT, cone beam CT.

Figure 3.

Bar graphs showing the distribution of the ranking for tasks 2 and 3 for both CBCT devices, in comparison to a micro-CT reference image. p values according to Wilcoxon signed-rank test. CBCT, cone beam CT; Ns, Non-significant.

Figure 4.

Representative CBCT images of a sample acquired with larger voxel protocols demonstrating that the blooming artefact may simulate a root canal perforation, which is not evidenced in the micro-CT image. CBCT, cone beam CT.