Non-invasive, accurate assessment of anterior cruciate ligament (ACL) injury where disruption to baseline collagen structure is subtle and gross morphological changes are absent remains challenging. Although magnetic resonance imaging (MRI) is routinely used for ACL evaluation, conventional sequences lack sensitivity to capture collagen fibre anisotropy and typically rely on negative contrast, such as adjacent fluid, rather than direct visualisation of the disrupted collagen structure[1-3]. 

Magic angle directional imaging (MADI) is a novel MRI technique that acquires images at multiple orientations to the main magnetic field. This exploits orientation-dependent signal changes in ordered collagen tissues (the magic angle effect) and infer fibre orientation within 1mm³ voxels[5,6]. This enables the generation of three-dimensional (3D) vector fields depicting collagen fibre organisation.

MADI derived vector fields may be visualised as raw vector fields or glyphs, which can be visually complex to interpret. Alternatively, streamline tractography may be utilised providing an intuitive display of collagen fibre pathways. However, this inherently qualitative method may obscure local structural changes in collagen alignment due to averaging effects of tractography algorithms and limitation of display methods[7,8]. This limits sensitivity to subtle organisational disruption relevant to injury assessment.

To address the challenges of visualising MADI data we sought to develop and apply a quantitative 3D scalar metric derived from MADI vector fields to enhance 3D visualisation of ACL collagen structure beyond current visualisations. Specifically, we focused on quantifying fibre coherence (FC) defined as the degree to which collagen fibres within adjacent 1mm³ vectors were aligned in a local uniform orientation. Adjacent vectors with more marked deviation from local uniform alignment, representing reduced FC, were hypothesised to reflect disruption of native collagen structure potentially associated with injury[9]. This was supported by published studies denoting the loss of uniform alignment in damaged ACLs using second harmonic generation microscopy [10,11].

Finally, by generating 3D colour-maps of FC and overlaying them onto existing tractography representations, this proof-of-concept exploratory study aimed to create hybrid visualisations capable of highlighting localised mesoscopic patterns of collagen disruption that were not appreciable from tractography alone. This approach has potential to improve subtle ACL injury detection, with particular relevance to radiological diagnosis and surgical assessment of ligament integrity and management planning.