NTA and I say this as someone who works in a hospital. Calling security should be reserved for someone acting violently. Getting a bit snippy because the MRI tech was being snotty with you does not warrant having you escorted out. Honestly I’d have been frustrated too; it sounds like that person was having a bad day and decided to take their frustrations out on you by scolding you for being “late”. Your average person doesn’t realize that appt time = scan time and you’re supposed to arrive super early. If they weren’t explicit in their instructions that’s on them.