Objective: The objective of this study was to define the Laws’ texture signature of magnetic resonance imaged anterior cruciate ligaments and to compare these texture patterns to the collagen structure through second harmonic generation imaging.
Methods: Three knees from Yorkshire pigs were magnetic resonance imaged using the Constructive Interference in Steady State and T₂* sequences and the anterior cruciate ligaments were segmented. Laws’ texture features were then applied to the segmented images, producing four texture maps for each ACL. k-means clustering was used to group voxels with similar texture values together in order to analyze the Laws’ texture maps. The anterior cruciate ligament was divided into three regions of interest (tibial and femoral attachments and the ligament midsubstance) and the texture features were defined for each. Second harmonic generation imaging was used to relate these texture features to the collagen organization found in the ligament. Second harmonic generation parameters, such as preferred collagen alignment, relative area of dark, anisotropic, and isotropic regions, enstrophy, energy, and tortuosity were used to describe the collagen morphometry.
Results: The femoral region of interest was shown to have a strong correspondence to the edge and wave texture features and a low correspondence to the spot texture feature. The collagen in this region had high circular variance (0.342deg²), enstrophy (636.6), and energy values (624.2), and a wide range of tortuosity values, suggesting a relatively disorganized collagen structure. The tibial and middle regions of interest had a high correspondence to the spot texture feature and a low correspondence to the edge texture feature. The collagen in this region had low circular variance (tibial: 0.06deg²; middle: 0.07deg²), enstrophy (tibial: 187.8; middle: 116.6), and energy 11values (tibial: 179.2; middle: 106.5), and a small range of tortuosity values, suggesting highly organized collagen.
Conclusions: The texture signature of the anterior cruciate ligament has been defined and can now be compared to the texture signatures of healing ligaments or grafts, potentially giving researchers and clinicians a quantitative tool to assess the healing stages of the ligament.