"Even if it is a body cavity [bomb], you still have to have an initiator, you have to have some external device to cause that initiation," he said. "There's got to be something external that you can then initiate the device and that's what the advance imaging technology machine will pick up: Any anomaly outside of the body." [— TSA Administrator John Pistole.]
TSA Administrator John Pistole is full of it or stupid or both!
A person can have both the explosive and the initiator in a cavity or separate cavities and then go to the restroom to mix both together to be blown there or back at the seat or anywhere the passenger may go on the plane.
This certainly would occur to any halfway intelligent would-be terrorists, let alone "al Qaeda," which supposedly thinks about such things nearly nonstop.
There are other ways of doing it too while both remain in the cavity(s), but I'm not going to share how.