Variation of ACTA scanner numbers with the physical properties of the scanned material

An experiment has been carried out in which an ACTA computerized axial-transverse tomographic scanner was used in a series of scans on a polystyrene phantom. Cavities in the phantom contained distilled water and various concentrations of acetic acid or ferric nitrate in aqueous solution. It is shown that the ACTA numbers generated for the low-Z acetic acid solution are proportional to the electron density of the solution, but that such is not the case for the higher-Z ferric nitrate, where photoelectric absorption is significant. The correlation of scanner numbers with electron density rather than with mass density is discussed and, as an illustrative example, the electron densities of whole blood and blood cells are calculated and the relative value is compared with the relative mass densities of the materials.