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Figure 2
Calculations for the required number of incident photons per pixel [\bar{n}_{\rm pixel}] for SNR = 5 imaging at δr = 20 nm spatial resolution. These calculations are for 2D imaging of copper features in silicon (a) to represent an integrated circuit and for imaging protein features adjacent to ice with over- and underlying layers (Fig. 1[link]) of 70% water/30% ice as tissue (b) to represent a biological specimen. These figures show contour lines for log10(x), so that x = 7 refers to a contour of [\bar{n}_{\rm pixel} = 10^{7}]; the underlaid grayscale image also displays x. Also shown as a white dashed line is the 1/e attenuation length μ−1(E) of the background material (either silicon or tissue) as a function of photon energy [equation (4)[link]]. (a) shows the effect of the Si K absorption edge at 1.84 keV, while (b) shows the `water window' between the carbon (0.29 keV) and oxygen (0.54 keV) K absorption edges.

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