Researchers hope a CT scan will help them understand why a child mummy has an unusually large head. The mummy will be scanned Wednesday, fittingly enough, at Children's Hospital of Pittsburgh, in collaboration with experts at the University of Pittsburgh Medical Center. Researchers hope to learn from the CT scan the child's sex and the reason for the unusually large cranium, which was revealed in previous X-rays. The mummy dates back to Egypt's Ptolemaic Dynasty, said Sandra Olsen, curator of anthropology. The child lived sometime between 380 B.C. and 250 B.C., and has been on display at the Carnegie Museum of Natural History in Pittsburgh for 17 years.