Calibration Methods for Cone-beam Computed Tomography Gray Values: A Scoping Review and Critical Analysis

This review aimed to identify and critically analyze calibration methods used to improve the reliability of cone-beam computed tomography (CBCT)-derived gray values (GVs) for radiographic density (RD) assessment in dentistry.

Unlike multidetector computed tomography (MDCT), which uses standardized Hounsfield units (HUs), CBCT systems lack a universal standard for GV accuracy, limiting the reliability and comparability of GV-based measurements. No comprehensive review has systematically evaluated CBCT GV calibration methods.

A PubMed search conducted on July 25, 2024, and updated on January 25, 2025, identified 2,242 articles. After applying predefined inclusion and exclusion criteria, 590 studies underwent full-text assessment, of which 473 were eligible. These studies were classified into four groups: Use of calibrated GV (group I; n = 52), assessment of GV validity (group II; n = 58), evaluation of factors influencing GV (group III; n = 19), and use of GV as a direct measurement tool without calibration (group IV; n = 341). Eight calibration approaches were identified from group I studies: (1) Relative density comparison between two regions, (2) correlation with another imaging modality, (3) use of calibration materials or liquids, (4) use of a single reference material, (5) application of conversion equations from previous studies, (6) conversion to an arbitrary scale, (7) specially designed calibration tools or techniques, and (8) combinations of methods. The most frequently used method was relative density comparison.

Eight distinct CBCT GV calibration methods were identified. Among them, relative density comparisons, use of calibration materials, and correlation with alternative imaging modalities appear to provide the most reliable results. The use of uncalibrated GV measurements raises significant concerns regarding validity.

The direct use of CBCT GVs as quantitative measurements is unreliable, particularly for applications requiring absolute density assessment, such as osteoporosis screening or inter-patient bone density comparison. Calibrated GVs may provide clinically useful information in selected scenarios, including implant site evaluation, localized bone quality assessment, and longitudinal monitoring, especially when relative density-based approaches or calibration materials are used under standardized imaging conditions. How to cite this article: Barngkgei I, Atiq T. Calibration Methods for Cone-beam Computed Tomography Gray Values: A Scoping Review and Critical Analysis. J Contemp Dent Pract 2026;27(2):209-222.