Physical measurements of heavy metal filter performance.

Contrast, relative tube loading, and relative entrance exposure have been measured for a number of heavy metal filters (Gd, Ho, Yb, and W) at several filter thicknesses, tube voltages (70-100 kV), and phantom thicknesses (1.5-2.5 cm of aluminum). The rare-earth filters consistently gave higher contrast (relative to water) for air, CaCl2, iodine, and oil than the standard 2 mm of added aluminum, at a given tube voltage. It was found that the best filter choices gave a constant product of relative exposure and relative tube loading.