Longitudinal Optical Coherence Tomography Backscatter Intensity Mapping of Enamel Demineralization

Objectives: The objective of this study was to map Optical Coherence Tomography (OCT) backscatter intensity patterns longitudinally against the histological changes of demineralised enamel using Scanning Electron Microscope.

Methods: 4x4 mm²windows were made on the crowns of six bovine incisors and immersed in a lactic acid buffer solution for 6, 8, 12, 24 and 48 hours. At each time point, one sample was removed for SEM scanning whilst the others were rinsed, air-dried and 3D OCT scans (axial resolution of 4.4 microns) were performed of the exposed window. Cross-sectional SEM scanning were performed at the middle of the window at 1, 5 and 20k x-magnification. For each sample, 10 B-scans were extracted from the middle 2x2 mm²of the exposed window and post-processed with MS Excel and Visual Basic 6.0. An algorithm was developed to identify edge pixels for surface determination and alignment. The range of backscatter intensity in the top 100 microns were determined, divided into six intervals and colour coded. The pattern of these intensity ranges was mapped and compared with the 1k magnification SEM images.
Results: All time point demonstrated two distinct layers under 1k x-magnification SEM images, with the depth of the top layer being 13.5±2.2mm, 19.5±3.0mm, 48.6±3.9mm, 53.3±7.7mm and 58.4±6.8mm with increasing time interval. 20k x-magnification shows the top layer to be a combination of inter-prismatic demineralisation and re-deposition of hydroxyapatite crystals. The bottom layer consists predominantly of inter-prismatic demineralisation. The OCT intensity maps similiarly demonstrates two distinct layers. The top layer is populated predominantly by the third-range of intensities and its depth increased with time interval (15.26±1.2mm, 17.95±0.7mm, 34.84±1.5mm, 35.55±0.6mm, 42.38±1.2mm). Number of pixels in the highest two intensity ranges also increased with time interval.

Conclusions: Optical Coherence Tomography (OCT) backscatter intensity patterns correspond with the longitudinal histological changes of demineralised enamel.