PET and CT on a single detector... What I was saying works for CT/SPECT, but you're right it would never work for CT/PET. Still, there is at least one idea for doing PET/CT, see for example:
CT Acquisition Using PET Detectors and Electronics
Berard, P.; Pepin, C.M.; Rouleau, D.; Cadorette, J.; Lecomte, R
IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 3, JUNE 2005
So yes, by whirling around what I was really referring to was SPECT, not PET. You got me there. I didn't think that one through all the way. Though, you can still combine PET and CT into a single detector by detecting X-rays on a 360 degree ring.
Since I was curious I started looking up all kinds of things:
SPECT/CT have been combined into a single detector (what I was really referring to in my other post).
A single CdZnTe detector for simultaneous CT/SPECT imaging
William C. Barber, Koji Iwata, Bruce H. Hasegawa, Paul R. Bennett, Len J. Cirignano and Kanai S. Shah
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Volume 505, Issues 1-2 , 1 June 2003, Pages 595-598
The Hawkeye has two separate detectors.
That being said, there's other tricks people are using to combine modalities like PET and MR too. For example:
Development of a combined microPET-MR system.
Lucas AJ, Hawkes RC, Ansorge RE, Williams GB, Nutt RE, Clark JC, Fryer TD, Carpenter TA.
Technol Cancer Res Treat. 2006 Aug;5(4):337-41.
These guys stuck the detectors in the magnet. One of many possibilities.