Noise influence on low contrast image correction for soft X-ray projection microscopy

Soft X-ray projection microscopy has been developed for high magnified imaging of hydrated biological specimens because water window region is available. The projection microscopy is a simple optical layout and has advantages over other types of microscopes particularly for biological specimens because of its wide viewing area, easy zooming function and easy extension to CT. However the projection image is blurred by the diffraction of X-rays, resulting in the deterioration of the spatial resolution. In this study, the blurred images have been corrected by an iteration procedure, i.e., Fresnel and inverse Fresnel transformations are repeated. The correction was found to be not effective for every image, especially for images with low contrast. A contrast enhancement method prior to the iteration procedure was installed to make the iteration procedure more effective, but it was not enough yet due to the influence of background noise. We evaluated dependency between the background noise level and iteration effect in the cases with or without the contrast enhancement prior to the iteration procedure by simulation. We also demonstrated upper limits of the background noises which chromosome images are effectively corrected by the iteration procedure.