As the others have said, its quite noisy, also ensure your as comfortable and as relaxed as possible so that you can keep still without getting edgy, I spent almost half an hour in the tube. I've had a couple of them, one on my brain and one on my knee where they did not stick me in all the way but the worst part on that one was they gave me some headphones to listen to music too and drown out the noise, but the music was bloody awfull and there was nothing I could do about it. It would have been an ideal advert for WKD
And as it has been mentioned you get well checked for metallic objects, but even so you can easily forget things, as I was going into the tube I had a sudden panic, I had left my great big copper bracelet on, made from a lightning strip which I had hammered round my wrist and no one had noticed, but as it was non magnetic they were not bothered about it.