A radiation detector (24) includes a two-dimensional array of upper scintillators (30τ) which is disposed facing an x-ray source (14) to convert lower energy radiation events into visible light and transmit higher energy radiation. A two-dimensional array of lower scintillators (30B) is disposed adjacent the upper scintillators (30τ) distally from the x-ray source (14) to convert the transmitted higher energy radiation into visible light. Upper and lower photodetectors (38τ, 30B) are optically coupled to the respective upper and lower scintillators (30τ,30B) at an inner side (60) of the scintillators (30τ,30B). An optical element (100) is optically coupled with the upper scintillators (30τ) to collect and channel the light from the upper scintillators (30τ) into corresponding upper photodetectors (38τ).