Determination of effective atomic numbers for tissue- and water-equivalent materials

it is convenient to describe a material composed of several elements by their effective atomic number, Z_eff. However, Z_eff is not a single number and should be understood as a parameter that depends on energy and that represents the processes of photon interaction with matter. As such, the effective atomic number can be employed in the study of materials radiologically equivalent to human tissue and water. In the present work, five samples radiologically equivalent to water, most abundant substance in the human body, and three samples radiologically equivalent to breast tissue, two being similar to the material that composes the BR12 phantom and one similar to the material that composes the CIRS 70/30 phantom, had their effective atomic numbers determined by direct method and Auto-Z_eff software. The samples were compared within the radiodiagnostic energy range (5-150 keV for water simulators and 5-35 keV for breast tissue simulators) with the results for their respective reference materials. Among the five water-equivalent samples, three presented a discrepancy of 8% for the direct method and about 10% for the Auto-Z_eff. Among the samples equivalent to breast tissue, the CIRS 70/30 simulating sample 47 and the BR12 simulating sample 17 presented the best results with a mean relative difference of less than 23%. Both methods presented variations in the results of effective atomic number in the region of interest, but these differences do not affect the comparative analysis highlighting the possibility of using both methods on comparative analysis of equivalence of tissue- and water-equivalent materials. (author)