Effects of niobium filtration and constant potential on the attenuation of dental X-ray beams by water.

Abstract

Two half-wave, self-rectified dental X-ray units and two constant potential units were compared with respect to the attenuation of the X-ray beams by water. The effect of additional niobium filtration on the half-wave beams was also explored. Exposures were standardized to give the same values at a depth of 2 cm. The constant potential units gave values of relative exposure which fell within the range of exposures produced by conventional half-wave, self-rectified units. The addition of niobium as a filter material resulted in a reduction of surface exposure of between 10% and 12%, depending on the beam quality. The more penetrating beams resulted in relatively smaller exposure at the surface and gave relatively greater exposure at deeper levels. The addition of niobium resulted in an increase in relative exposure of 30.6% at 70 kVp and 10.7% at 90 kVp at a depth of 6 cm. There was a linear relationship between relative exposure and half-value layer, both at the surface and at a depth of 6 cm. Regression analysis yielded r2 values of 0.925 at the surface and 0.919 at 6 cm (P less than 0.0005). This study has failed to show differences in attenuation due to niobium filtration and constant potential, other than might be expected from half-value layer.