A PET system for generating a PET image of an object, the PET system comprising: - a PET data acquisition apparatus with: - a first detector with first detector elements for detecting photons emitted by a radiation source located within the object and for generating first detection events being indicative of the detected photons and detection times assigned to the first detection events, which first detector is an outer detector ring and the first detector elements are outer detector elements, wherein the outer detector ring surrounds an examination region, - a second detector with second detector elements for detecting photons emitted by the radiation source and for generating second detection events being indicative of the detected photons and detection times assigned to the second detection events, which second detector is an inner detector and the second detector elements are inner detector elements, wherein the inner detector is adapted for detecting the photons inside the outer detector ring, - a reconstruction apparatus for reconstructing a PET image of the object, the reconstruction apparatus being adapted to reconstruct the PET image based on detection events and detection times assigned to the detection events generated by the PET data acquisition apparatus, wherein the reconstruction apparatus comprises: - a position providing unit for providing the position of the second detector relative to the first detector; - a timing window determination unit for providing a first-second timing window by providing a first-second upper threshold, wherein the first-second timing window is provided based on the position of the second detector relative to the first detector, - a detection event pairs generation unit for generating first-second detection event pairs based on the provided first-second timing window, wherein a first-second detection event pair comprises a first detection event and a second detection event, to which detection times have been assigned whose difference is equal to or smaller than the first-second upper threshold, and - a reconstruction unit for reconstructing the PET image based on the generated first-second detection event pairs. - a PET data acquisition apparatus with: - a first detector with first detector elements for detecting photons emitted by a radiation source located within the object and for generating first detection events being indicative of the detected photons and detection times assigned to the first detection events, which first detector is an outer detector ring and the first detector elements are outer detector elements, wherein the outer detector ring surrounds an examination region, - a second detector with second detector elements for detecting photons emitted by the radiation source and for generating second detection events being indicative of the detected photons and detection times assigned to the second detection events, which second detector is an inner detector and the second detector elements are inner detector elements, wherein the inner detector is adapted for detecting the photons inside the outer detector ring, - a reconstruction apparatus for reconstructing a PET image of the object, the reconstruction apparatus being adapted to reconstruct the PET image based on detection events and detection times assigned to the detection events generated by the PET data acquisition apparatus, wherein the reconstruction apparatus comprises: - a position providing unit for providing the position of the second detector relative to the first detector; - a timing window determination unit for providing a first-second timing window by providing a first-second upper threshold, wherein the first-second timing window is provided based on the position of the second detector relative to the first detector, - a detection event pairs generation unit for generating first-second detection event pairs based on the provided first-second timing window, wherein a first-second detection event pair comprises a first detection event and a second detection event, to which detection times have been assigned whose difference is equal to or smaller than the first-second upper threshold, and - a reconstruction unit for reconstructing the PET image based on the generated first-second detection event pairs. - a timing window determination unit for providing a first-second timing window by providing a first-second upper threshold, wherein the first-second timing window is provided based on the position of the second detector relative to the first detector, - a detection event pairs generation unit for generating first-second detection event pairs based on the provided first-second timing window, wherein a first-second detection event pair comprises a first detection event and a second detection event, to which detection times have been assigned whose difference is equal to or smaller than the first-second upper threshold, and - a reconstruction unit for reconstructing the PET image based on the generated first-second detection event pairs.