My daughter has had a couple of SPECTs. They probably do it differently for a twelve year old, but because Daphne is little (23 months old -- 18 months at the time of her first SPECT) she had to be put under general anesthesia so that she wouldn't move during the study.  I don't think they need to do that with an older child or adult.

I think they just start an IV, and inject a radioactive isotope and the diamox through the IV, and from there on out, the study is very much like an MRI.

The only pain would be from the IV going in, and possibly a mild burning sensation as the dye goes in through the IV.

Since I haven't gone through it myself, and my daughter is not able to tell me about it, I can't give you all the details... hopefully some others will join in with more personal experiences 

Good luck!
Jenny