Dear Susie ,
We can only tell you from our experiences what we think and advise you - it is always up to you to make the final decision. MRI can show fistulas and big polyps and such. Rectal polyps sometimes run in families and should never be taken lightly. They usually need some time to grow , though. Why don't you ask your CRS why he thinks you need one and what is he looking for , as Lecia suggested ?
Botox can have many side-effects , including death (as was written in the release form I had to sign before the procedure ,and I did not proceed with it for that reason) . But, for some people it was helpful , at least for a time being, and nobody here had extremely bad side-effects - soreness, and pain and some mild gas incontinence were the most common complaints.
Botox was covered by my insurance, but I guess it also depends of a type of the insurance one has
Best of luck !