X-ray energies for effective atomic number determination.

The measurement of the effective atomic number of a piece of material in vivo can be achieved using computed tomography. The precision of measurement of this parameter depends on the precision of measurement of the X-ray absorption coefficient at two energies and the separation of these energies. With the assumption of a fixed photon flux, it is shown that two optimum energies exist for the measurement of effective atomic number. The analysis indicates that if energies of 40 keV and 80 keV are employed, a precision of at least 1 part in 400 in the measurement of effective atomic number may be achieved.