Comparison of chi-square and join-count methods for evaluating digital image data

The significance of each bit of the pixel in digital image data from various radiological modalities is tested to determine the contrast resolution. Two statistical methods, join-count statistic and chi-square goodness of fit test, are used to perform the test. Join-count statistic is used to measure the spatial coherence among pixels, while the chi-square test is used to determine if the bit data are randomly distributed. A residual image is formed by subtracting an original image from its smoothed version. The contrast revolution is determined by applying both statistics on each bit plane of the residual image starting from the least significant bit up to the bit plane whose statistic does not show a random pattern. Images from three digital modalities, computerized tomography, magnetic resonance, and computed radiography, are used to evaluate the gray-level dynamic range. Both methods are easy to implement and fast to perform