Photon counting detectors are sparsely placed at predetermined positions in the fourth-generation geometry around an object to be scanned in spectral Computer Tomography (CT). Optionally, integrating detectors are placed between the two adjacent ones of the sparsely placed photon counting detectors in the fourth-generation geometry. Furthermore, the integrating detectors are placed in the third-generation in combination to the sparsely placed photon counting detectors at predetermined positions in the fourth-generation geometry.
