Development of an optical coherence tomography image analysis method to characterise cellular structure of kiwifruit

Optical coherence tomography (OCT) is a non-destructive imaging method that enables acquisition of three-dimensional (3D) images of sub-surface structures of semi-transparent and turbid objects. This technology is a potential tool to provide new information about the structural nature of horticultural products. This work used kiwifruit (Actinidia deliciosa × chinensis, 'G14') as a case study to develop an appropriate OCT image capture and analysis method, to characterise microstructural properties of the large parenchyma cells such as: total volume, surface area, maximum length and equivalent diameter. An automated image processing protocol was established using Avizo^® 7.1. Raw images were pre-treated using smoothing filters by averaging a small box of voxels. Image segmentation was carried out by automated interactive threshold binarisation, which identifies black voids (large cells) from the background (the rest of the outer pericarp tissue) based on two manually-selected grey level threshold values. Connected cells were separated by applying a watershed algorithm and then displayed using a cyclic colour-map so that objects in close proximity were labelled in a different colour. Screening of mislabelled objects was conducted by selecting specified ranges of microstructural properties of the cells, which had been previously identified, based on manual selection of cells using a 'wrapping' method. A final 'closing' process was applied to fill up small voids within cells as well as to smoothen cells boundaries. The resulting methodology enabled non-destructive characterisation of cellular structure immediately beneath the epidermis of kiwifruit.