PERFORMANCE OF FILTERS FOR NOISE-REDUCTION IN MAXILLARY ALVEOLAR BONEIMAGING

Film-grain noise degrades image detail, hinders detection of subtle ra diographic bone changes, and could thwart attempts to use dental radio graphs of alveolar bone to detect osteoporosis. The purpose of this in vestigation was to quantify and compare the performance of various 1- and 2-D spatial and frequency domain fillers in suppressing this noise , Estimates of noise-free bone profiles (scan lines) from each of five maxillary interdental areas were made by superimposing and averaging 16 identically exposed and digitized radiographs, The average mean abs olute error and mean-squared error between the 80 initially noisy imag es and their respective noise-free profiles were calculated to provide an estimate of initial noise, Filter performance was measured as the change in these values after filtering the noisy images. Frequency dom ain analysis revealed that bone signal power spectra dominated at freq uencies less than 2-3 cycles/mm and that some form of low-pass filteri ng would be applicable, The 2-D Butterworth lowpass filter provided th e best performance, removing 57% of the film-grain noise when measured by mean absolute error, and over 80% when measured by mean-squared er ror, Surprisingly, the Lee, L(p) mean, geometric mean, binomial, media n, and simple neighborhood averaging filters offered comparable levels of performance.