Abstract: Accurate interpretation of digitized X-ray mammograms has been limited by the lack of a specialized image acquisition system. We have developed a novel image acquisition system based on an area scanning scientific grade CCD array. The distinguishing features of our system are: (1) fast method of digitizing mammograms and (2) high spatial and photometric resolution. The system is capable of acquiring 6 frames per second where each frame consists of over 1.3 million pixels digitized to 10 bits per pixel, 8 of which are displayed at any one time on a high resolution monitor. The fixed pattern and the random noise (of optical and electronic origin) are minimized using background subtraction and signal averaging techniques. The resulting image is equal to or better than that obtained by a drum laser-scanning microdensitometer. In order to restore the image from the degrading effects of the system blur and noise, Wiener filtering is used. The modulation transfer function of the system is measured using a bar pattern test object and also the classical knife edge technique. In one filter implementation the ensemble power spectrum of the mammograms is estimated from the degraded images. The noise is assumed to be independent from the signal and its power spectrum is estimated from selected smooth regions of the noisy and blurred images. The spectral energy of the relatively low contrast soft tissues images on a mammogram are generally concentrated near zero frequency. In an alternative implementation the noise to signal power ratio is assumed constant. The results show a marked improvement in detectability of smallest particles of microcalcifications when judged by a human observer. The impact of our image restoration in clinical detection of microcalcifications by a radiologist will be tested.!12
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