How can an MRI show a fracture and possible osteomyelitis without an injury?

Pathological bone. Fractures that occur without an injury are called pathologic fractures and are related to some bone disease such as the infection you mention.
Shows inflamation. Inflamation and fractures can be seen on an mri. Only a culture will let you know if it is an actual infection / osteomyelitis.
MRI's don't know! Mri stands for magnetic resonance imagery. Mri works by using a large magnetic force to excite hydrogen molecules. When the molecules.