1. A computed tomography (CT) imaging apparatus, comprising: an X-ray source configured to emit X-rays; a plurality of photon-counting detectors (PCDs) arranged in a circular ring between the X-ray source and a CT detector; and processing circuitry configured to cause the X-ray source and the CT detector to perform a helical scan of an object to obtain projection data; extract, from the obtained projection data, a plurality of sets of projection data corresponding to a plurality of X-ray tube positions of the helical scan; and combine the plurality of sets of extracted projection data to obtain a scanogram.