An embodiment of the invention is to make possible a non-invasive grading of a tumor based on parameters determined from a frequency distribution (histogram) of values in a map representing cerebral blood volume (CBV) or cellular metabolism in the tumor. The method is especially applicable to brain tumors such as gliomas where histological grading is difficult. The invention provides a precise and consistent grading since it relies on values selected from the whole tumor (not just from hot spots) since it takes the diversity or heterogeneity of the vascularization into account by analyzing the frequency distribution (not just a mean value) and since it involves and allows for a more automated procedure wherein any subjective contributions from human operators is not critical to the resulting grading. CBV maps may be obtained by perfusion imaging using MRI or CT scanning. Cellular metabolism maps may be obtained from a glucose metabolism map obtained by positron emission tomography (PET).
