Image restoration in X-ray microscopy: PSF determination and biological applications

We show that digital image processing methods can be applied to enhance the quality of X-ray microscopic images. One application of X-ray microscopy is imaging of biological specimens in their natural aqueous environment. Since X radiation can introduce structural changes in these objects when observing them at room temperature, it is sometimes necessary to take images with short exposure time. The image quality can thus be reduced due to low signal-to-noise ratio (SNR). Digital image processing methods can be applied to reduce image degradation caused by noise. Another example of a digital image processing method we applied to X-ray microscopy images is contrast enhancement of structures near the resolution limit of the microscope. Structures of 20 nm in size, which show weak contrast in the original image, become more clearly visible after restoration. Since image restoration methods are based on the knowledge of the optical transfer function (OTF) or the point spread function (PSF), a handy method for quick PSF determination is presented. An iterative image-restoration method was applied to pictures obtained with the Gottingen transmission X-ray microscope at BESSY. Results of image quality enhancement by this method are shown for images of polytene chromosomes of Chironomus Thummi larvae, test-structures and in situ hybridization on Xist RNA using biotinilated DNA probe.