Bruce: CAT and CT are the same; PET is different! Based on my measurements in the standard acrylic CT dose phantom and our typical exposure factors for a spinal CT, the absorbed dose at the center of the phantom would be 2600 mrad and 1 cm below the surface 5600 mrad. I didn't go to Boston either. I don' know where the first version of this note went but I got a message that it was undeliverable. Best Wishes, Ken