Is it on the surface of the heel that contacts the ground or is it up the back of the heel? On the bottom of the heel it would most likely be plantar fasciitis and if it it up say a 1/2" or so it would most likely be achilles tendonitis. I have treated many hundreds of cases of heel pain. In fact heel pain is most common complaint I have treated for people with foot pathologies. I have never seen a patient who had an MRI for heel pain unless it was an elite athlete where a stress fracture was suspected.