Abstract
Purpose
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.
Method
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
Where:
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.
Results
The patella tendon had an AI=0.6453. The Anterior Cruciate Ligament (ACL) had an AI=0.2732. The meniscus had an AI=0.1847.
Discussion
The most aligned knee structure is the patella tendon where the collagen fibres align with the skeleton to transmit forces through bones and muscles. This structure had the AI closest to 1. The ACL had the second highest AI and is composed of two fibre bundles aligned diagonally across the knee. The meniscus acts as a shock absorber and is made up of vertical, radial and circumferential fibres which disperse forces more equally. The complexity of the meniscal structure resulted in the lowest AI.
To date, this technique has only been performed with healthy tissue; the AI may become closer to zero if there is damage disrupting the collagen fibre alignment. The AI can further our understanding of collagen orientation distribution and could be used as a quantitative, non-invasive measure of structural health.
