A method for measuring the CT numbers of an in vivo tissues includes the steps of calculating a histogram of a region including the tissue, locating a leading edge of a peak within the histogram, substracting CT numbers assumed to result from background and intermixing of tissues, fitting the background-substracted peak to a gaussian distribution, and calculating the mode of the resulting peak. Whe the CT numbers of two different tissues having know densities are measured according to the method of the invention, the CT numbers may be used to obtain a standard for analyzing tissues of unknow density. The method can be implemented by means of a CT scanner, a computer, and appropriate software.