CT number
= CT number
{c}
{title2=$H=1000(\mu-\mu_w)/\mu_w$}
A dimensionless relative attenuation value calibrated so that water is zero and air is approximately $-1000$. It depends on the <computed tomography> spectrum and material composition. It is not directly a <linear attenuation coefficient> at the $511\,\mathrm{keV}$ energy used in <positron emission tomography>.