Fig 5.

Sample photon-counting detector images of low- and high-energy bins of a 70-year-old woman (section thickness, 2 mm; increment, 2 mm; window center, 45 HU; window width, 80 HU). A, Axial PCD image reconstructed from all detected photons (22–120 keV) at the level of the basal ganglia. B, Axial PCD image reconstructed from a low-energy bin image (22–52 keV) at the same level as A. C, Axial PCD image reconstructed from the high-energy bin image (52–120 keV) at the same level as A. The image noise for both the low- and high-energy bins is higher than that of the PCD image reconstructed from all detected photons because each bin contains only a subset of all detected photons. The low-energy bins provide good gray matter–white matter differentiation but are susceptible to beam-hardening, best seen as an artifactual increase in attenuation of the cortical GM and the subarachnoid space (arrows in B). The high-energy photons are less susceptible to beam-hardening but have poorer GM–WM differentiation. The image reconstructed from all photons is a trade-off between the good GM–WM differentiation of the low-energy image and the lower beam-hardening artifacts of the high-energy images.