The areas within the lesion that are low signal on all pulse sequences most likely represent what type of tissue?
Fibrous tissue. If it were tumor necrosis it should be more central within the mass not peripheral, and if it were hemorrhage/blood it would show susceptibility artifact on gradient echo. This can be ruled out by looking at the scout images. We know it's not calcium from the x-ray. This leaves only fibrous tissue as a primary consideration.
If this is a fibrous lesion as we've established, why is it likely not fibrosarcoma or malignant fibrous histiocytoma?
These would be highly cellular, so even the low/low signal areas would be expected to enhance. In this lesion they stay dark, suggesting it is benign, even though it appears locally aggressive.
There is a large mass arising deep to the extensor digitorum brevis which invades the 3rd metatarsal diaphysis. It shows heterogeneous signal characteristics on STIR, with high signal areas interspersed with smooth amorphous areas of low STIR and T1 signal. These low/low signal areas are not central in location but more peripheral, especially laterally, and show no blooming on the gradient echo scout images. There is avid contrast enhancement throughout the lesion, with the exception of the areas that were consistently low on all other pulse sequences.