They may or may not inject you with contrast medium (which makes your mouth taste a little odd)
you lie on a table which goes inside the MRI machine (and you with it). It is quite noisy and may feel a little claustrophobic.
It works by making all the molecules of your body with iron in them (so the blood cells for starters) 'wobble' a little. the machinery picks up the 'wobble' of the iron atoms and converts that to a picture.
This is quite a nice site that explains the experience - mri
And Martianbishop - sorry but it actually uses the hydrogen ions (not enough iron to generate the picture - so says my DH) (because he doesn't read things on mumsnet, oh no not ever) (because apparently reading over my shoulder doesn't count!)
To be very specific its the protons - the nuclei of the hydrogen. Magnet makes them line up, and then a small radio pulse knocks them off that alignment. As they wobble back to alignment they give off their own radio signal - the length and frequency of that is determined by how 'fixed' the water is.
Im going next week too huffpuff. Its my 2nd. You lie on a table which slides inside the machine. I dont remember the noise but apparently its noisy. I wouldnt open my eyes the whole way through because I was scared of what Id see inside the machine. They can talk to you and they can hear you talk to them. Its not so bad honest, there are far worst things to have done
it is very claustrophobic, you really can't move at all, but it's okay. you just have to go to a happy place in your head, and often if you take a cd with you the staff will play it into the scanner to help you with that. it's a bit like being placed in a clanging, banging radiator tube.