An imaging system for both PET and CT imaging, comprising: a gantry that defines an examination region; a plurality of radiation detector systems disposed around the examination region; and at least one x-ray transmission radiation source ( 20, 20a, 20b, 20n); wherein each radiation detector system comprises: a first detector layer including a group of CT detectors ( 72, 74) having a first cross-section dimension and being adapted to convert incident radiation from the x-ray transmission source into CT data and emission radiation into PET data; and at least one second detector layer of detectors ( 62, 64) disposed such that the first detector layer is interposed between the examination region and the second detector layer and having a second cross-section dimension and being adapted to convert emission radiation into PET data; at least one reconstruction engine (40,46) for reconstructing the CT data into a CT image and the PET data into a PET image; wherein for CT imaging the imaging system is adapted to acquire CT data from the first detector layer; characterised in that for PET imaging the imaging system is adapted to acquire PET data from both the second detector layer and the first detector layer, wherein the outputs of the CT detectors ( 72, 74) of the group of CT detectors ( 72, 74) are electronically coupled together and the group of CT detectors ( 72, 74) is operated as a single PET detector. a gantry that defines an examination region; a plurality of radiation detector systems disposed around the examination region; and at least one x-ray transmission radiation source ( 20, 20a, 20b, 20n); wherein each radiation detector system comprises: a first detector layer including a group of CT detectors ( 72, 74) having a first cross-section dimension and being adapted to convert incident radiation from the x-ray transmission source into CT data and emission radiation into PET data; and at least one second detector layer of detectors ( 62, 64) disposed such that the first detector layer is interposed between the examination region and the second detector layer and having a second cross-section dimension and being adapted to convert emission radiation into PET data; at least one reconstruction engine (40,46) for reconstructing the CT data into a CT image and the PET data into a PET image; wherein for CT imaging the imaging system is adapted to acquire CT data from the first detector layer; characterised in that for PET imaging the imaging system is adapted to acquire PET data from both the second detector layer and the first detector layer, wherein the outputs of the CT detectors ( 72, 74) of the group of CT detectors ( 72, 74) are electronically coupled together and the group of CT detectors ( 72, 74) is operated as a single PET detector. a first detector layer including a group of CT detectors ( 72, 74) having a first cross-section dimension and being adapted to convert incident radiation from the x-ray transmission source into CT data and emission radiation into PET data; and at least one second detector layer of detectors ( 62, 64) disposed such that the first detector layer is interposed between the examination region and the second detector layer and having a second cross-section dimension and being adapted to convert emission radiation into PET data; at least one reconstruction engine (40,46) for reconstructing the CT data into a CT image and the PET data into a PET image; wherein for CT imaging the imaging system is adapted to acquire CT data from the first detector layer; characterised in that for PET imaging the imaging system is adapted to acquire PET data from both the second detector layer and the first detector layer, wherein the outputs of the CT detectors ( 72, 74) of the group of CT detectors ( 72, 74) are electronically coupled together and the group of CT detectors ( 72, 74) is operated as a single PET detector.