1. A computer implemented method for grading malignancy of a tumor, comprising receiving electronic data comprising a map of values defining perfusion, blood volume, or cellular metabolism in a tumor of a subject; storing in an electronic memory, a plurality of value intervals, and counting with an electronic processor the number of values within selected regions of the map that are within each of a plurality of defined value intervals to determine a frequency distribution of values in the selected regions; assessing a tumor malignancy based on the frequency distribution, where a more heterogeneous distribution corresponds to a higher malignancy; and transmitting tumor assessment data based, at least in part, on the assessment of the tumor malignancy to an output device.