Original image, 101 x 101 pixels, cropped from 512 x 512 axial slice of two adjacent rats (top and bottom).  See Rat_polymer.tif for actual file, without JPEG artifacts.
 

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Resampled image with vector overlay of the most salient (weighted path integral of length and gradient magnitude) edges identified with Scale Space Edges.

Contrast of edges (yellow) with background (grayscale) indicates relative normalized gradient at nearest edge point.

Polymer regions are hypointense elongate bands, one at top center and one at bottom center.
 
 

Normalized gradient map at all identified edge points.