Estimating photon interaction coefficients from single energy x-ray CT

Single energy x-ray analysis is explored in the context of computed tomography (CT), whereby Hounsfield numbers (HN) are used to estimate electron density N(e) and parameters that describe composition. We examine measurements with tissue substitute materials and theoretical HN for a broad range of tissues. Results are combined with parametric models for the x-ray linear attenuation coefficient μ and energy absorption coefficient μ(en) to predict values at energies 10 keV to 20 MeV. At photon energies employed for CT, the fractional contribution to μ from composition is 0.1-0.4 for soft tissues to bone respectively, and is responsible for strong correlations between HN and N(e). The atomic density of tissues excluding lung is near constant allowing the models to be re-expressed as a function of N(e) alone. The transformed model is subjected to propagation of error analysis and results are presented as the ratio of uncertainties for μ or μ(en) to those for N(e). For soft tissues to bone the ratios are as follows: at photon energies 20-100 keV the ratio is 5.0-2.0, at intermediate energies it is unity and increases above 4 MeV to reach 1.5-2.0 at 20 MeV. Results are discussed in the context of attenuation correction and dosimetry calculations for the same range of photon energies.