Why are injections required before doing a bone scan to check for a stress fracture?

Only one: Only one inflection is needed. That is the injection of tc-mdp, the tracer taken up by he bones and detected by the nuclear medicine camera. This tracer shows active bone turnover and is focally "hot" in sites of fracture.

Blood to skeleton: Bone scan involves intravenous injection of radio tracer usually technetium 99m mdp which accumulates in skeleton. Scan may involve early phase(5-10 min) and late phase(2-3 hr) imaging. Increased uptake is seen in fractures, tumors, infection, etc. Decreased activity is seen in metal artifacts, aggressive tumor/infection, & loss of blood supply to bones. Bone scan very sensitive but not specific.

Radiotracer injectio: A bone scan or bone scintigraphy is obtained few hours after the injection of a radiotracer ( usually 99mtc-mdp). The distribution of this radiotracer in the various bones will be detected by a specific type of camera named gamma camera ( to detect the gamma rays from 99mtechnetium). This is the contrary of a bone x-ray where external radiation source is used.