What does scar tissue look like under an ankle mri?

Nice question: The radiologist use multiple different sequences to assess structures on mri. We look for brightness and darkness on the t1, t2, stir, and other sequences. In general scar shows up as "dark" on all sequences. The scar can form into a ball of tissue and exhibit mass effect. This "meniscoid" tissue can even cause impingement. These are difficult processes to evaluate, best reviewed by a ortho rad.

See below: It usually looks thick dense and dark.

Dense area: The area would be dense and disorganized. Compared to a tendon which is organized into linear fibrils, the scar tissue will not follow any particular path.

Thickening: Scar tissue often appears as thickening of tissue structures in area of concern. Frequently this is associated with increased signal intensity on an MRI (whitish signal on t2 images, greyish on t1 in an area where normal structures are dark black in color).