This is a story about M.R.I.’s, those amazing scans that can show tissue injury and bone damage, inflammation and fluid accumulation. Except when they can’t and you think they can.

I found out about magnetic resonance imaging tests when I injured my forefoot running. All of a sudden, halfway through a run, my foot hurt so much that I had to stop.

But an M.R.I. at a local radiology center found nothing wrong.

That, of course, was what I wanted to hear. So I spent five days waiting for it to feel better, taking the anti-inflammatory drugs ibuprofen and naproxen, using an elliptical cross-trainer, and riding my road bike with its clipless pedals that attach themselves to my bicycling shoes. By then, my foot hurt so much I had to walk on my heel. I was beginning to doubt that scan: it was hard to believe nothing was wrong. So I went to the Hospital for Special Surgery in New York for a second opinion from Dr. John G. Kennedy, an orthopedist who specializes in sports-related lower-limb injuries. And there I had another M.R.I.

It showed a serious stress fracture, a hairline crack in a metatarsal bone in my forefoot. It was so serious, in fact, that Dr. Kennedy warned that I risked surgery if I continued activities like cycling and the elliptical cross-trainer, which make such injuries worse. And I had to stop taking anti-inflammatory drugs, since they impede bone healing.