A student revising for an exam writes: "A Hounsfield unit is basically a density:…

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Life at Thermal Energy, Ch Images from attenuation and from decay

Problem

A student revising for an exam writes: "A Hounsfield unit is basically a density: cortical bone reads about +1930 HU+1930\,\mathrm{HU}, so it must be about 19.319.3 times denser than water." At 60 keV60\,\mathrm{keV}, water has μw=0.206 cm−1\mu_{\rm w} = 0.206\,\mathrm{cm^{-1}} and cortical bone has μ=0.604 cm−1\mu = 0.604\,\mathrm{cm^{-1}} (Chapter 6); cortical bone's mass density is about 1.8×103 kg m−31.8\times10^{3}\,\mathrm{kg\,m^{-3}}, against water's 1.0×103 kg m−31.0\times10^{3}\,\mathrm{kg\,m^{-3}}. ((a)) Compute the HU value for cortical bone and check the quoted number. ((b)) Compute the ratio of bone's density to water's density, and compare it with the ratio of their μ\mu values. ((c)) Explain why the two ratios differ, and what an HU value actually tracks.

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