Table 1.

Pertinent properties of inhomogeneity layer materials

Material	Physical density $\mathit{(}g/c{m}^3\mathit{)}$	Electron density (relative to water)	$CT\mathrm{\#}$ (HU)	Thickness of material used (cm)
Air	$1.2\times {10}^{-3}$	$1.1\times {10}^{-3}$	$-1000$	2, 4, 6, 8, 10
Lung‐equivalent material	0.27	0.265	$-750$	2, 4, 6, 8, 10
Foam PVC	0.61	0.56	$-430$	2, 4, 6, 8, 10
Adipose‐tissue equivalent	0.92	0.93	$-100$	2, 4, 6, 8, 10
PVC	1.40	1.29	545	1.3, 2.6, 3.9
Aluminum	2.70	2.34	2200	1.3, 2.6, 3.9

PVC = Polyvinylchloride.