Hi Trail: I don't know if your insurance would let you repeat the test, but if so, I would do that. Unfortunately QCT and DEXA, give out varying t-scores which are dependant on age, so it's hard to say how accurate it is. Check your daughters age against the score list (link below) and you'll see how the younger you are the *less* it overestimates. QCT's do emit a large amount of radiation, so some Drs don't like them for that reason alone. If you want more info on it, do a search on QCT or quantitative cat scan and see what you find.
Below is the link from the Univ of WA, explaining the differences in scores between the two. I had a QCT done twice, so when I have my first DEXA I won't know if the differing scores are from treatment or equipment, which is really frustrating. Unfortunately neither of these test are fool proof, which would explain why different machines/technicians give different results.
DEXA's are still considered the gold standard in bmd, but if they could improve the QCT, by eliminating the problems of reproducibility, radiation, and age matching, it could be a really good test, but they have a long way
Some people disagree with this reasoning so you'll have to decide for yourself.
Good luck, I hope your daughter gets a good score, and I think it was brilliant of you to decide to have her tested; if she were to have a low bmd you've just provided her with years of prevention management and possibly early treatment.
Scroll up to the Para on QCT, and check out the embedded link on age/scores on the following page.