Figure 2. (a) CT noise figure vs photon incidence on the detector (KLP_det). The noise figure data are plotted separately for each phantom for CT_a (—) and CT_b ( x --- x ). The theoretical relation between KLP_det and CT noise figure (Equation 7) (—) was plotted at a level where it coincided with the data of the best performing CT system. Error bars representing the maximum and minimum values obtained from six scans are smaller than the spatial extent of the data symbols and are therefore not shown. The horizontal separation of the plots for CT_a and CT_b indicates the difference in the number of photons required by the two systems to achieve the same noise figure level. CT_a yielded erroneous attenuation values at the four lowest levels of KLP_det (see Results). Vertical dashed lines show the values of photon incidence required for the noise level specified for tissue volume determination with CT (30 HU). (b) When the attenuation of 1 cm water was added to the attenuation of the phantoms that were positioned on the tabletop (to account for the attenuation in the tabletop), the curves of these phantoms merged with the curves of the other phantoms (which were attached to the phantom holder) and formed one distinct line for both CT_a (—) and CT_b ( x --- x ).