Table 1.

Absorption coefficients for 70 keV x rays and estimate of absorption of materials in HU. ABS is acrylonitrile butadiene styrene copolymer, and is reported to be the principle material in LEGO pieces. The attenuation coefficient of air is neglected in the estimate of the Hounsfield Units. The experiment is a maximum value taken reported by the medical CT. Air is −1000 HU and bone is typically + 1000 HU

Material	Attenuation coeff.3 (cm2/g)	Density Attenuation (g/cm3)	(cm) length	Est. HU	Exp. HU
Water	0.193	1.00	5.18	0 (by def.)
ABS	0.180	1.05	5.29	−20	+18
PTFE	0.173	2.2	2.63	970	+905
Fused silica	0.195	2.2	2.33	1331	+2684