Pain has subjective parts but it is not merely subjective. You can measure how much effect ibuprofen or tylenol has, even with someone knocked out. You can see pain on an mri, even with someone knocked out.
Maybe I'm misunderstanding context, but my cat may need an MRI and I've learned it's standard in all veterinary medicine to use anesthesia for scans in order to keep the patients immobile.
Yeah, but they aren't trying to watch the effect of a drug happening. Anesthesia is accomplished with drugs. You can't give a person 3 different drugs, then expect a brain scan to tell you how one works in isolation.
Also, anesthesia would ruin whatever you hoped to see on an MRI.