Impact of angle on photothermal radiometry and modulated luminescence (PTR/LUM) value

To evaluate the impact of scanning angles to detect/quantify non-cavitated caries by photothermal-radiometry and modulated-luminescence (PTR/LUM, Canary System) and to evaluate the association of PTR/LUM value with lesion depth (LD), including sound tissue thickness under the lesion (ST).

Thirty human extracted premolars were selected based on micro-computed tomography [μ-CT: sound (n=12), lesions into outer-half of enamel (n=6), lesions into inner-half of enamel (n=6), lesions into outer one-third of dentine (n=6)]. Each tooth sample was scanned 90° directly contacted to the center of non-cavitated lesion or sound smooth surface, and tilted 10° and 20° in four directions: buccal/lingual/occlusal/cervical. The procedure was repeated 48 h later. Lesion depth and ST [ST=5000 µm (maximum PTR/LUM scanning depth)-LD] were measured at the same scanning direction on μ-CT images. Sensitivity, specificity, area under the Receiver Operating Characteristic curve (AUC), and intraclass correlation coefficients (ICC) for different scanning angles were calculated. Sensitivity was further evaluated based on lesion extensions. Relationships between PTR/LUM value and lesion depth, and between PTR/LUM value and LD/ST-Ratio were evaluated.

PTR/LUM value showed significant differences among scanning angles. Overall sensitivity (78%-89%), specificity (66%-87%), AUC (0.86-0.92) and ICC (0.89-0.99), sensitivity based on lesion extensions presented no significant differences among angles. PTR/LUM value showed moderate correlations (0.56-0.74) with deepest lesion depth and LD/ST-Ratios.

The scanning angle within 20° increments might impact PTR/LUM value statistically; however, it did not affect PTR/LUM detection performance. PTR/LUM values were positively correlated with non-cavitated lesion depth, and not affected by sound tissue thickness under the lesion.

Clinically, it is challenging to measure/scan at the same location and same angle longitudinally, however, it is important to standardize these parameters. Scanning within 20° deviation from perpendicular did not affect detection performance of PTR/LUM, and PTR/LUM value showed positive moderate correlation with caries depth.