I'm an MRI radiographer, a lot of people are claustrophobic so firstly don't worry you are not the only person who feels this way. Most patients I've scanned who are claustrophobic always apologise and say they are being silly, please don't feel like that we understand you can't control it.
Let the radiographers know how you are feeling when you arrive and if you have any questions they will be more than happy to answer them and try to put you at ease.
The diazepam shouldn't make you feel anything other than relaxed, some people don't feel any difference but they do help them to manage the scan.
Whatever body part is being scanned needs to be in the middle of the tube, so where your head will be will be dependant on how tall you are but even a shorter person the head would be near the entrance to the scanner if your tall your head might not even go in. You can go in feet first for a hip scan, if it's not already set up that way don't be afraid to ask.
They will put a piece of equipment over your hips this is called a coil and is like an aerial it picks up signal from the scanner to make the pictures. This just rests on top and doesn't hurt or do anything else.
Every hospital has different rules with regards to what you can wear during the scan. Some places have a blanket rule that u must wear a gown for any type of scan this is for safety reasons. However the hospitals I have worked in would allow you to go in wearing anything metal free. So leggings or jogging bottoms would be fine. If you have a bra without any clips or wires I'd wear that too as u may be asked to remove it.
You can also take someone in with you while you have the scan, as long as they answer the questions on the screening form (you may have been sent one with your appointment) and there is no safety contraindications on their part. They can stand right next to you while they do the scan and even hold your hand.
If you find your head does go in the scanner and you feel panicky ask the radiographers to take away the pillow and tilt your head backwards and you will be able to see out.
It is very noisey when the scanner takes the pictures, I often describe it as being in the middle of the roadworks when they are drilling the road. But nothing is going to touch you or hurt you and once your in position if the table moves at all it will be millimetres so you can be assured that you won't suddenly go all the way through the scanner or anything.
You will be given ear protection for the noise (as will anyone accompanying you during the scan) either ear plugs or headphones (sometimes both) and you can have music so if you have a favourite CD take it with you.
The scan should take about 20 minutes, it's very sensitive to movement so if you happen to move at all during the scan (it's very hard to keep still for 20 minutes) the radiographers might have to repeat some pictures, so the stiller you can keep the quicker it will go.
Oh also some people like to bring a sleep mask and put it on as they get onto the bed so they can't see what's going on.
If you ask the radiographers they will speak to you during the scan and as others have said you will be given a buzzer which u can use to stop the scan.
Wow this was a longer post than I expected. I hope everything goes well for you, and if it doesn't go to plan please don't be hard on yourself xxx