I actually felt a strong sensation when I had a high resolution brain MRI for research, and I rather enjoyed it. It switched on and off a few times during the scan, and it felt a bit like having a back massage, or significant mechanical vibrations in my back, or that feeling like gentle electric currents during some therapies, except for switching on and off abruptly.
I asked about this after the scan because I had been told you don't feel anything. Surely it wasn't just my imagination, from the noises? Was it from machine vibrations? I didn't have any metal in my body except amalgam fillings, and they said those wouldn't affect it. And, if I could feel something, perhaps it wasn't as harmless as they made out.
They explained after, some people feel a stimulation of their peripheral nervous system when the RF is on, from the tens of kW of microwave energy beamed through the body. For a few people this sensation is too much, even painful, and they have to stop which is one reason for the patient having the mechanical alert button. But most people don't feel anything at all from the MRI, just psychological feelings associated with the strange noises and confinemnt.
They said it's a peripheral nerve stimulation sensation, a kind of phantom feeling, rather than a physical effect on the body being sensed by the nerves. Don't ask me why I felt it in my back given it was a head and neck scan.
I enjoyed how it felt when I didn't know what it was, as it felt like it might loosen up my back a bit. I was a bit disappointed to not feel anything the next time I had a head MRI, for a medical reason (thankfully nothing found). The research scan had twice the field strength of the medical scan, and presumably different RF settings. Perhaps that made the difference.