The dual energy x-ray CT scan data are acquired at two energy levels.  For example, the tube is set at the low and high energy levels of 80 kV and 120 kV. Dual source CT-scanners is equipped with two X-ray sources, and each runs at a different energy level for generating the two data sets.  On the other hand, in a sandwich detector, the upper layer records the low energy data while the lower layer records the high energy data.  To use the dual energy data for material separation, the projection data undergo preconstruction decomposition. 
More generally, spectral information is obtained at more than two energy levels for certain x-ray CT scanners.  For example, a predetermined number of N energy thresholds is determined according to an average material thickness or the in-air scan, and the basis material thickness is calculated directly based on the N sets of measurement data.  In this regard, all detector units and projection views share the same threshold settings.  In reality, it is desirable to alter the threshold level between certain views as the spectral changes. 
As the spectra change during the scan, the photon counting bins should also accordingly change to maintain a low noise level in the acquired data.  Although it may be theoretically possible to dynamically change the thresholds between views, it is technically challenging due to the very short duration of a CT scan.  As limited by the current photon counting detector technology, the threshold values may not be properly adapted in the short duration between the views as used in one exemplary rate at 1800 views/0.5 seconds.  In general, because read out electronics as well as the detectors have a finite response time and a dead time, the threshold varying implementation under the above requirements is limited given the currently available technology. 
Due to the highly non-uniform object composition or thickness, the use of bow-tie filter is also difficult.  A bow-tie filter faces difficulty in matching itself with patient geometry, different detector units and views.  Furthermore, different units and views are subject to significantly varying attenuations and incident x-ray spectra on detectors. 
Consequently, the current use of a universally constant threshold results in undesirable noise balance in a substantially large number of the acquired data sets.  The unbalanced noise in the acquired data sets potentially causes severe artifacts in the spectral images.  With additional energy bins, the photon numbers in low and or high energy bins should be better balanced. 
For these and other reasons, the above described prior art technique remains desired in substantially improving noise balance in the images reconstructed from acquired data including the spectral information.