What I would do is call him and ask him or nurse what he means by a letter. There's no shame in asking questions. This way you will get the correct answer.
Can't answer that question at all, unless I know of your symptoms?
Not sure about the UK but here in Canada the Dr will send a requisition(name,DOB,symptoms,exam requested) to the designated hospital where u will have the MRI done.After recieving the req the hospital will book you with an appt date & time.They then send you a letter stating what procedure u are having done with a time and dtae.That letter should explain the procedure as well.Again...this is how its done in Canada...hopefully its the same in the UK