Don't listen to them. You'll be fine smoking before your MRI. I've had too many and am quite experienced with them, although mine have always been on my knees. But smoking will affect nothing with the MRI. They could give you one with contrast which is just some dye in your bloodstream.
I know how stressful MRI's can be for your first time. You're cold, uncomfortable, and nervous. You can't move and most of the time you are watching a crappy movie or listening to the radio. It can get very claustrophobic (sp??) if you are like that and can actually be a little scary because you don't know if the prognosis is gonna be positive or negative. I've personally never blazed before any of mine but if you think it'll make the process a little more bearable or even fun then go for it! I'd just reccomend not getting too high because it is a hospital or doctor's office and they can probably detect somethings up. All the best.
