+This is quite helpful when comparing a phantom to a reconstruction.
+
+The three distance measures are:
+
+\begin{twocollist}
+\twocolitem{\textbf{$d$}}{The normalized root mean squared distance measure.}
+\twocolitem{\textbf{$r$}}{The normalized mean absolute distance measure.}
+\twocolitem{\textbf{$e$}}{The worst case distance measure over a \latexonly{$2\times2$}\latexignore{\emph{2 x 2}} pixel area.}
+\end{twocollist}