CT Angiogram

hi all
have to go for one of these on friday on both my "upper limbs" as the letter says.
And I'm completely freaking out that i have to have an injection (X-ray dye) during this as I'm convinced something will go wrong or i will have a reaction to it because it states in the letter that they ask you to remain in the hospital 15-30mins to see if you will have a delayed reaction to the dye, which is putting me in a state of panic and don't know if i'll even be able to make to the hospital.

i should ad that about 10 years ago i was injected with a dye for a kidney X-ray,
would this be the same stuff or different?

somebody please put my mind at rest!!