I'm a runner who was recently diagnosed with a stress fracture in my tibia. As dumb as it may sound, I can't seem to get my head wrapped around what a stress fracture actually is. Is it a hairline crack? Is it fractured internally (and this is why is doesn't show on the X-ray)? What am I actually seeing on the MRI? Which leads to my next point of confusion, why does it show apparently so clearly on the MRI but is completely invisible on the X-ray?
An early stress fracture is tiny by nature and thus hard to spot using imaging.
X-ray imaging relies on the absorption of X-ray photons by different tissues in the body. Planar X-ray imaging accumulates absorption along the path of the X-ray beam as it passes through the body. The difference formed by such a tiny fracture along the path is simply not large enough to form a visible contrast.
MR imaging reflects proton density and thus "the amount of liquid" in a certain body part. Moreover, MR is a three-dimensional modality and your image shows a cut plane (at the posterior tibia) through the volume, precisely a multi-planar reformat.
Having (a) the possibility to look inside the bone and (b) enhancing liquids, the MR scan can show contrast for the subtle edema that forms along the stress fracture.
