Routinely carrying a concealed handgun can create considerable difficulties when a person unexpectedly ends up somewhere they can't do so. Leaving it in a car is a significant risk for theft (and stolen guns get used for bad things). Leaving it with medical staff isn't great. Leaving it unattended outside an exam room presents obvious dangers to anyone who might happen upon it.

It's hard to find yourself in an MRI room without adequate warning that anything magnetic will become a projectile though. I think most places give the patient both written and verbal warnings, and the article here says this patient was warned about metal and specifically about guns.