Image contrast using high frequency and half-wave rectified dental x-ray generators.

Abstract

When high frequency (constant potential) x-ray units are operated at the same kV as conventional half-wave rectified (single pulse) generators, image contrast decreases. The contrast can be restored by lowering the kV. The decrease needed to achieve the same contrast as with a conventional 65 kVp unit is shown to be 5–8kV. Contrast was evaluated using an ivory phantom with a continuously varying thickness of 0–9.6 mm and determined both experimentally and theoretically. The theoretical calculations were based on energy spectra measured for both types of generator by means of a Compton spectrometer. The theoretical results are in excellent agreement with the experiments demonstrating that knowing the complete energy spectrum of primary photons, time-consuming measurements can be replaced with calculations. A consequence of this study is that current Swedish national regulations as to the minimum permitted kilovoltage in intra-oral dental radiography may have to be adjusted.