Accuracy of gingival measurement using CBCT: the critical impact of FOV and current

In implant and periodontal surgery, the accurate determination of the gingival phenotype is a major clinical parameter. While CBCT has established itself for its three-dimensional imaging capabilities at a reduced radiation dose, this exposure optimisation is not without consequence. By reducing the current intensity (mA) or modifying the field of view (FOV), the practitioner faces increased noise and contrast degradation, potentially compromising the reliability of thin tissue measurements.

The objective of this in vitro study was to evaluate the combined impact of the FOV and tube intensity on the measurement accuracy of different simulated gingival thicknesses (1, 2 and 3 mm) on a dry mandible. The authors sought to identify whether variations in acquisition protocols induce significant measurement errors compared to a physical reference standard.

The researchers tested the hypothesis that the choice of FOV (50 x 100 mm vs 165 x 180 mm) and current (4 mA vs 10 mA) directly influences the accuracy of linear measurements. For the clinician, the challenge is to determine whether a 'low-dose' protocol or a large field maintains sufficient reliability for tissue diagnosis, or whether it induces a systematic overestimation of gingival thickness.

Methodology: An in vitro experimental protocol

This in vitro experimental study used a dry mandible as an anatomical model to evaluate the accuracy of gingival thickness measurements by CBCT. The gingiva was simulated by applying wax (pink baseplate wax) to different mandibular regions, calibrated to three distinct thicknesses: 1 mm, 2 mm and 3 mm.

The protocol was structured around two comparative measurement stages:

• Establishment of the reference (Gold Standard): Two radiologists performed direct physical measurements of the wax using a periodontal probe. To ensure reproducibility, each measurement was taken twice, perpendicular to the bone surface, with a one-week interval between sessions.
• CBCT acquisitions: The model was scanned by systematically varying two critical parameters to observe their impact on resolution:
  • The field of view (FOV): 50 × 100 mm (S+) and 165 × 180 mm (XL).
  • Tube intensity (current): 4 mA and 10 mA.

A total of 168 radiographic measurements were collected, including the buccal and lingual aspects of the mandible. Accuracy was evaluated by statistical analysis comparing the imaging data with the initial physical measurements, allowing the identification of significant overestimations or underestimations according to the exposure parameters.

Measurement accuracy: marked variability depending on the FOV and intensity

The analysis of the 168 measurements performed on a dry mandible reveals that the accuracy of CBCT in assessing simulated gingival thickness (1, 2 and 3 mm wax) is closely linked to the field of view (FOV) and current intensity (mA). The results show an overall tendency towards overestimation, particularly marked for thin tissues.

Analysis by thickness increments

• 1 mm thickness: This is where errors are most frequent. Only the large field of view (FOV 165x180 mm) at 10 mA yielded measurements statistically comparable to the gold standard. All other configurations, including the small field of view (S+), generated significantly higher values, both buccally and lingually.
• Thickness of 2 mm: Most settings proved reliable. Significant overestimations were only noted for the 50x100 mm FOV at 4 mA (buccal side) and at 10 mA (lingual side).
• 3 mm thickness: The small field (FOV 50x100 mm) performed particularly well. Conversely, the large field (165x180 mm) induced significant overestimations at 10 mA buccally and at 4 mA lingually.

Impact of current (mA) and image noise

The study highlights that using a 10 mA current offers better overall reliability than 4 mA, regardless of the chosen FOV. Reducing the intensity to 4 mA, although decreasing radiation exposure, degrades image quality by increasing noise and reducing contrast resolution, which distorts the detection of the boundaries between bone and simulated soft tissues.

In summary, the FOV XL (165x180 mm) proves versatile for thicknesses of 1 to 3 mm, whereas the FOV S+ (50x100 mm) should be reserved for thick tissues (3 mm), at the risk of significantly overestimating thin phenotypes.

Diagnostic precision: the dilemma of FOV and intensity

This in vitro study highlights a technical paradox: the small field of view (FOV 50x100 mm), although often preferred for its theoretical spatial resolution, proves to be the least reliable for measuring thin tissues of 1 mm. Conversely, the XL FOV (165x180 mm) combined with an intensity of 10 mA offers superior precision for the assessment of thin phenotypes. For the practitioner, this means that reducing the field does not automatically guarantee a better periodontal reading.

The impact of current intensity (mA) is the determining factor here. The results show that 10 mA consistently outperforms 4 mA in terms of fidelity to the actual measurements (gold standard). In clinical practice, whilst dose reduction is commendable in accordance with the ALARA principle, it generates image noise here, leading to an overestimation of gingival thickness. Such a bias can compromise implant planning or mucogingival surgeries, where the boundary between a thin and thick phenotype is a matter of less than a millimetre.

The limitations of this study lie in its experimental nature on a dry mandible. The use of wax (pink baseplate wax) does not perfectly replicate the density of living tissues nor the impact of vascularisation on X-ray attenuation. Nevertheless, the 168 measurements performed provide solid evidence: to secure your pre-surgical measurements on a patient with a thin phenotype, the 10 mA setting is preferable to 4 mA, regardless of the chosen FOV.

In practice, for the practitioner:

• Optimise the intensity: Always favour a 10 mA current over 4 mA for your periodontal analyses; reducing the dose to 4 mA generates image noise leading to clinically significant measurement errors.
• Target the FOV according to the phenotype: To assess a thin phenotype (≈ 1 mm), use a large FOV (165x180 mm) at 10 mA. Avoid small FOVs (50x100 mm) which may lead you to overestimate the available tissue thickness.
• Thick tissue measurements: The small field (FOV 50x100 mm) is really only recommended for the evaluation of thick tissues (3 mm), where its accuracy matches that of the gold standard.

Technical glossary of the study

CBCT (Cone-Beam Computed Tomography): Three-dimensional imaging system of the maxillofacial skeleton using an external scanner to produce reliable linear measurements with a lower radiation dose and cost than a conventional CT scanner.

FOV (Field of View): Field of view delimiting the scanned anatomical area. The study compares a FOV of 50 x 100 mm (S+) to a FOV of 165 x 180 mm (XL) to determine their impact on the accuracy of soft tissue measurements.

Tube current (Current intensity): Exposure setting parameter expressed in milliamperes (mA). The study evaluates the influence of a 4 mA versus 10 mA current on the resolution, contrast and noise of the resulting image.

Gold standard: Physical reference value obtained by manual measurement of calibrated wax thicknesses (1, 2 and 3 mm) using a periodontal probe, serving as a point of comparison for radiographic measurements.

Pink baseplate wax: Material used in this in vitro model to simulate gingival thickness on a dry mandible, allowing precise standardisation of experimental measurements.

Noise: Image degradation phenomenon that intensifies when the radiation dose is reduced, potentially affecting the accuracy of detecting the limits of gingival thickness.

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