Shin splints and stress fractures fall at opposite poles of the same spectrum. MRI can help differentiate the two.
Shin splints present on T1-weighted images as intermediate signal within the deep subcutaneous tissues overlying the medial tibial cortex, intermediate signal within the cortex, or eccentric hypointense marrow. STIR images may show periosteal edema in direct contact with medial tibial cortex with anteromedial to posteromedial extension to the origin of soleus. The marrow may be hyperintense anteriorly or posteromedially, and there may be intermediate-to-hyperintense focus within the anterior tibial cortex.
Stress fractures, on the other hand, may show interruption of the cortex posteromedially, as well as a wide high signal in the marrow on STIR images.
Delayed images on bone scan will show activity along the posteromedial border of the tibia (i.e., soleus muscle origin). Perfusion and blood pool images will be normal.
The T1-weighted image here shows edema along the subcutaneous tissues anteromedial to the tibia. We went kind of overboard with the STIR, but you should be able to make out the edema-like signal corresponding to the T1 findings. No marrow edema was seen and no fracture could be identified.
See Fredericson Grading of Tibial Stress Reaction. Usually the findings at the ends of the spectrum are clear. The middle range can be muddled sometimes.
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