clare,
If a dentist can see the cavitat from the panorex x-ray then they can do surgury on it. Unfortunatly many cavitats are not picked up by the "normal' dentist by looking at your panorex. The cavitat uses a special machine much like an Ultrasound.
You can read about it on Dr. Shanklands site. He explains it much better than I can.
Hope this helps.

Veg