Collagen-rich structures of the knee are prone to damage through acute injury or chronic “wear and tear”. Collagen becomes more disorganised in degenerative tissue e.g. osteoarthritis. An alignment index (AI) used to analyse orientation distribution of collagen-rich structures is presented. A healthy caprine knee was scanned in a Siemens Verio 3T Scanner. The caprine knee was rotated and scanned in nine directions to the main magnetic field B0. A 3D PD SPACE sequence with isotropic 1×1×1mm voxels (TR1300ms, TE13ms, FOV256mm,) was optimised to allow for a greater angle-sensitive contrast. For each collagen-rich voxel the orientation vector is computed using Szeverenyi and Bydder's method. Each orientation vector reflects the net effect of all the fibres comprised within a voxel. The assembly of all unit vectors represents the fibre orientation map. Alignment Index (AI) in any direction is defined as a ratio of the fraction of orientations within 20° (solid angle) centred in that direction to the same fraction in a random (flat) case. In addition, AI is normalised in such a way that AI=0 indicates isotropic collagen alignment. Increasing AI values indicate increasingly aligned structures: AI=1 indicates that all collagen fibres are orientated within the cone of 20° centred at the selected direction. AI = (nM - nRnd)/(nTotal - nRnd) if nM >= nRnd AI = 0 if nM < nRnd
nM is a number of reconstructed orientations that are within a cone of 20° centred in selected direction nRnd is a number of random orientations within a cone of 20° around selected direction nTotal is a number of collagen reach voxels By computing AI for a regular gridded orientation space we are able to visualise change of AI on a hemisphere facilitating understanding of the collagen fibre orientation distribution.Purpose
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