Detection Thresholds for Root Canal Visibility on High-Resolution CBCT: A Micro-CT Validation Study

Cone-beam computed tomography (CBCT) is widely used to assess root canal morphology, yet practical size thresholds for canal visibility have not been established. This study determined the relationship between canal diameter and the probability of confident detection on high-resolution CBCT using micro-computed tomography (micro-CT) for reference.

Sixty-one extracted human teeth (251 canal cross-sections, including 9 with no visible lumen) were imaged with both imaging modalities. True canal diameters measured on micro-CT were spatially matched to corresponding CBCT cross-sections using anatomical landmarks. Three calibrated observers rated canal visibility on a 5-point scale; scores ≥4 indicated confident detection. Interobserver reliability was quantified using Krippendorff's α and quadratic-weighted κ. A binomial logistic regression modeled detection probability as a function of diameter. Receiver operating characteristic (ROC) analysis assessed overall discriminative accuracy.

Interobserver agreement was excellent (α = 0.87; κ = 0.78-0.91). The probability of confident canal detection increased sharply with canal diameter. Model estimates indicated a 70% probability (D₇₀) at a canal diameter of approximately 265 μm, an 80% probability (D₈₀) at 299 μm, and a 90% probability (D₉₀) at 350 μm. ROC analysis demonstrated excellent discriminative performance (area under the curve = 0.89), confirming strong accuracy of CBCT visibility scores for identifying canals present on micro-CT.

On 80-μm-voxel CBCT, consistent canal visibility emerges at approximately 300 μm. Canals measuring 200 to 300 μm are often visible, whereas those smaller than about 200 μm are frequently missed. The detection thresholds identified in this study may help clinicians anticipate when a canal is likely still patent but radiographically undetectable.