What is attenuation?

"The relationship between the CT number of a structure and its physical density is defined by the equation Q = CT + 1000 (Q in mg/cm3, CT in HU (Hounsfield Units)). This equation holds true as long as the effective atomic number of the substance (i.e. its atomic composition) in the displayed voxel is similar to that of water. This applies with reasonable accuracy to fluids, soft tissues, and less to fat but not to iodinated contrast media of calcified structures. For this reason, the CT number of a soft-tissue structure is often used interchangeably with "density"." (Prokop and Galanski, 2003)

2003 Prokop, M., and M. Galanski. Spiral and Multislice Computed Tomography of the Body, A. J. van der Molen, and C. M. Schaefer-Prokop (eds.), Thieme, New York, pp. 1-1090.