Physical evaluation of a new technique for X-ray dose reduction: measurement of signal-to-noise ratio and modulation transfer function in an animal model.

Aim of this study was to record objective changes in image quality of optically re-exposed, radiation-reduced X-ray images in comparison to a normally exposed reference image in an animal model. Under investigation is the question if optical re-exposure of conventional, radiation-reduced X-ray images partially or even fully compensates the loss of information caused by underexposure. Dose-reduced, underexposed images were prepared by reducing the mAs product to 50% with constant anode voltage. Reproduction of the image was performed with a 52% decrease in the radiation dose. Comparing different re-exposure times, the optimal time was found to be 60s. These underexposed X-rays were then optically re-exposed for a defined period of time before development. In all X-ray images of the animal model, different osseous structures were defined as regions of interest (ROI) for evaluation of the objective changes in image quality. The density curves were plotted with the two-beamed densitometer. The contrast transfer factors as the function of local frequency were determined from this, which served as the basis for calculating the modulation transfer factor. To establish if X-ray sensitisation by optical re-exposure leads to a change in the sensitometric gradation, the sensitometric curves were determined using a standardized aluminum scale and thermal luminescence dosimetry. In the comparison the lowest correlation with the standard technique film (X-ray 1) was seen in the purely dose-reduced X-ray. In the range of 1.6-3.4Lp/mm, both SNR curves have an identical course. Despite a 52% dose reduction in the re-exposed image, both densitometry curves of the conventional and re-exposed X-ray show an almost identical distribution of the transmittance levels. In conclusion film sensitisation provides a technically simple and inexpensive procedure, which is easily integrated into previous film development processes and considerably reduces the patient radiation exposure as well as clearly improving the image quality.