A new method is proposed for direct determination of bone porosity based on histograms of 3D mu CT scans and for precise definition of the global image segmentation threshold, preserving assessed porosity in the reconstructed binary image of the bone sample. In this method, the normed histogram is considered to be a probability distribution of voxel density (CT number or gray level) in the scan. It is a linear combination of two distributions characterizing the frequency of occurrence of voxels of pore and matrix type with various densities. Volume porosity, in this model, defines the probability of pore voxel occurrence in the whole set of voxels in the scan of the sample. This parameter and the parameters of both probability distributions are determined by an optimization method. The new method was used to determine the porosity and segmentation thresholds for lCT images of two 3D samples of human cancellous bone. The results were compared with those determined by the standard method and Otsu's method. The new method allows the porosity and the image segmentation threshold to be determined even in cases where use of the other methods is questionable or impossible.