I'm not fond of tight spaces either, not sure if I'm actually claustrophobic or not. But I had to have an MRI done on my hand a couple years ago leading up to my surgery. I was nervous but I quickly found that the horrible noise was so distracting and loud that I didn't care about the tight space LOL.
They sometimes give you headphones to listen to music, but that does absolutely nothing to drown out the sound of the actual machine. Seriously, after 45 minutes of that you'll be more concerned about the headache you've gotten then the tight space.
Since you are having the MRI done on your knee, I imagine they won't need to place you fully in the tube ... but I'm not sure. Like others said, ask for an Open MRI if possible. If not, all I can say is close your eyes and focus on how irritating the sound is and that might help. It sure helped me LOL. Good luck with everything!
