A radiation detector (16) having a first detector layer (24) and a second detector layer (26) encircles an examination region (14). Detectors of the first layer include scintillators (72) and light detectors (74), such as avalanche photodiodes. The detectors of the second detector layer (26) include scintillators (62) and optical detectors (64). The scintillators (72) of the first layer have a smaller cross-section than the scintillators (62) of the second layers. A group, e.g., nine, of the first layer scintillators (72) overlay each second group scintillator (62). In a CT mode, detectors of the first layer detect transmission radiation to generate a CT image with a relatively high resolution and the detectors of the second layer detect PET or SPECT radiation to generate nuclear data for reconstruction into a lower resolution emission image. Because the detectors of the first and second layers are aligned, the transmission and emission images are inherently aligned.
