Effects of niobium filtration and constant potential on image quality in dental radiography. 2. Objective assessment.

The effects of niobium filtration and constant potential on image quality were explored by asking observers to identify numbers of circular areas of small density differences in zones of high and low density. Successful identification was related negatively to half-value layer. This relationship was particularly strong in the case of self-rectified units without additional niobium filtration and the constant potential units (r2 = 0.994, P < 0.0005). Constant potential had no effect other than in relation to half-value layer. Additional niobium filtration detracted from successful identification, but slightly less than would be predicted by the half-value layer. Whether this difference is significant or relevant has not been established. The findings of this study, in conjunction with our previous investigations, have failed to demonstrate any clear indications for preferring constant potential supply over self-rectification or for using niobium as an additional filter material. Half-value layer has been shown to be a reliable indicator of the effects of beam quality on emulsion speed, attenuation in water, and image quality. It appears that the methods employed to change the beam quality have no specific effects.