Automatic Evaluation of Collagen Fiber Directions from Polarized Light Microscopy Images

Mechanical properties of the arterial wall depend largely on orientation and density of collagen fiber bundles. Several methods have been developed for observation of collagen orientation and density; the most frequently applied collagen-specific manual approach is based on polarized light (PL). However, it is very time consuming and the results are operator dependent. We have proposed a new automated method for evaluation of collagen fiber direction from two-dimensional polarized light microscopy images (2D PLM). The algorithm has been verified against artificial images and validated against manual measurements. Finally the collagen content has been estimated. The proposed algorithm was capable of estimating orientation of some 35 k points in 15 min when applied to aortic tissue and over 500 k points in 35 min for Achilles tendon. The average angular disagreement between each operator and the algorithm was -9.3 +/- 8.6 degrees and -3.8 +/- 8.6 degrees in the case of aortic tissue and -1.6 +/- 6.4 degrees and 2.6 +/- 7.8 degrees for Achilles tendon. Estimated mean collagen content was 30.3 +/- 5.8% and 94.3 +/- 2.7% for aortic media and Achilles tendon, respectively. The proposed automated approach is operator independent and several orders faster than manual measurements and therefore has the potential to replace manual measurements of collagen orientation via PLM.