It is not dangerous. Mine generally last between 30 and 45 minutes. If you are claustrophobic you will want to let the doctors know before hand so they can medicate you. You are asked to hold your breath several times. You must remain still. I generally end up dosing off to be honest. But it can be quite loud in there, and you can ask for headphones to listen to music.
It's actually super easy as long as you are not claustrophobic.