MRI on viagra?

[Asphyxia]

I have mri very soon and because im switching doctors so often there are different suggestions from each.
Other docor suggested to do the mri with an erection but never mentioned how i ought to get an erection
Got the general doctor to prescribe me some viagra to take before the mri but he said he doesnt know if thats the method the urologist wanted to induce an erection with.
I know people here used injection to have an erection for mri but im pretty sure i wont be injected as there wont be any doctors there that do that.
So should i take viagra? If yes then how long before? Never took viagra so i dont know anything.