An another way of research concerns medical applications. The coherent - Compton ratio imaging technique allows to measure the effective atomic number in such a way to give dosage additional information from computed tomography. This technique is interesting, for examples, to search specifics elements in brain or assess mineral concentration in trabecular bone. Thus, we are investigating the simultaneous use of Rayleigh - Compton ratio tomography and standard transmission computed tomography. For this experiment, we use monochromatic synchrotron radiation with a 16000 channels analyser connected to a cooled germanium detector. The coherent and Compton photons are discriminated by their energy and their ratio is proportional to the effective atomic number.