1. A method of performing spectral reconstruction for a region of interest (ROI) of an object, the method comprising: designating the ROI within the object, the ROI being located within a scan field of view (FOV) of a combined third- and fourth-generation computed tomography (CT) scanner, the CT scanner including a plurality of fixed photon-counting detectors (PCDs), and an X-ray source that rotates in a trajectory about the object in synchronization with an energy-integrating detector; determining, for each PCD of the plurality of PCDs, as a function of a position of the X-ray source along the trajectory, an on/off timing schedule of the PCD, based on a size and location of the designated ROI; performing a scan of the object by rotating the X-ray source and the energy-integrating detector to obtain a first data set from the energy-integrating detector and a second data set from the plurality of PCDs, wherein the step of performing the scan includes turning each PCD on and off according to the determined on/off timing schedule of the PCD; and performing spectral reconstruction within the ROI using the first and second data sets to obtain spectral images of the ROI.