The flood-field uniformity index has been investigated as a function of the total number of counts and the image matrix size. The analysis showed the dependence of the NEMA integral uniformity index on the counting statistics. A linear model to determine the noise-free component of the uniformity index that was applied successfully to the experimental data is presented. In addition, the heterogeneity of the integral uniformity index IU presented as a function of image matrix size m fits well to the fractal equation IU(m)/IU(1) = mD-1 with the fractal dimension D = 1.34. This result shows that the uniformity index could be handled as a fractal quantity.