Is it safe to take just 1. 10mg tablet for anxiety before having a MRI? it was prescribed by my Rhe?

depends on what it is
benzodiazepines are the most common prescribed for any MRI or CT scan although not required many people have a phobia
about those exams which is normal
again depending on what it is 10mg might be a bit to much unless its diazepam which i assume that is what you got since you say 10mg which isn't to strong but i would go with alprazolam since it kicks in fast and leaves fast doubt you wanna be loopy all day which you will be with diazepam

It is not uncommon for the doctor to prescribe 10mg Valium before an MRI especially if you are the least bit claustrophobic.