Low Frequency Filtering Of Digital Radiographic Images

To display the wide range of attenuation values in a computed radiographic image, mathematical processing to suppress low frequency information is often used. This increases the apparent visibility of low contrast structures and permits the simultaneous display of these structures in all areas of the image. Low frequency suppression is performed by subtracting from each pixel in the image a fraction of the weighted mean of a number of surrounding pixels. This produces a degree of edge enhancement which results in an image similar in appearance to that of a Xeroradiograph. The fraction of the weighted mean and the number of surrounding pixels to be used in the filtering process can be varied, thus producing varying degrees of edge enhancement and low frequency suppression. We have investigated the effect of different low frequency filtering parameters on the recognition of low density lung nodules. Digital radiographs of an anthropomorphic phantom which contained lesions ranging in size from 3 to 20 mm were obtained using the GE SV 8800 system. Different filters were applied and images were obtained on film. The detectability of the lesions using the different filter parameters was determined.