The scan may be to see if she is emptying her bladder completely when she wees. If they ask her to go in for an ultrasound with a full bladder that will be what they are looking for.
My 12 year old daughter has the same problem. Her GP keeps prescribing antibiotics, but I think that sets up a cycle of recurring problems, as all the good bacteria are killed off, creating an unbalanced system.
The last time UTI started, I gave her large doses of cranberry extract (Cysticlean is the name of the brand we used) and probiotics to replace the lost good bacteria. She didn't need antibiotics, as the infection cleared up! Eating yoghurt and drinking cranberry juice drink DOES NOT do the trick. We tried that, with no success, but the (fairly expensive, from a good health food store) concentrated extracts really worked.
Also sugar is not good for a child about to come down with a UTI. If you can give her a really healthy diet and talk to a knowledgeable person at a reputable health food store, you may be able to break the cycle and avoid all the antibiotics.
I am not a fan of pills and potions, and don't have any faith in quackery, such as homeopathy. This treatment really worked for us. Of course, if your daughter has some kind of problem which is shown up on the scan she may need a different additional treatment or surgery.