Epidermal thickness assessed by optical coherence tomography and routine histology: preliminary results of method comparison

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Abstract
Optical coherence tomography (OCT) is a promising non-invasive imaging technique for studying the epidermis and upper dermis in vivo. As proposed previously by Welzel et al. (J Am Acad Dermatol 1997; 37: 958-963), distance measurements between the entrance peak and the second peak of the A-scan seem to correspond to epidermal thickness (ET). However, there is a lack of systematic studies comparing OCT with histology. Sixteen healthy subjects were included in this pilot study. OCT assessments were conducted on the upper back. To determine ET by OCT, the distance between the entrance peak and the second peak of the A-scan was calculated (ET-OCT). After OCT measurement a 4 mm punch biopsy was taken on each subject from the same site previously assessed. The maximum thickness of the epidermis (ET-Histo) was determined microscopically using routine histological slices (formalin-paraffin technique, haematoxylin-eosin staining). Correlation and agreement between the two methods were assessed by means of the Pearson correlation procedure and Bland-Altman plots, respectively. ET-Histo was 79.4 +/- 21.9 microm, including a stratum corneum thickness of 20 +/- 12.1 microm. OCT measurements resulted in an ET of 106 +/- 15.4 microm. No correlation between ET-Histo and ET-OCT was observed (r = 0.29, P = 0.27). There was a considerable lack of agreement between histology and OCT measurements as expressed in a high bias of 26.63 microm [95% confidence interval (CI) 14.49-38.77]. Furthermore, the lower 95% limits of agreement were -18.03 microm (95% CI -37.11 to 1.05) and the upper 95% limits of agreement were 71.28 microm (95% CI 52.2-90.36) indicating that ET-OCT may be 71.28 microm above or 18.03 microm below ET-Histo. Our preliminary data suggest that the above-described OCT algorithm is probably not a valid measure for the evaluation of ET. The second peak of the A-scan seems not to correspond to the dermo-epidermal junction zone, but rather to fibrous structures in the upper dermis. Nevertheless, further systematic comparison studies between OCT and histology are warranted, using different OCT algorithms for ET determination and cryopreparation, which has a higher reliability than the formalin-paraffin technique.

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