1. A method of obtaining the level of expression of glutaminase in a tumor using MR imaging, comprising: applying to a suspected tumor a Chemical Exchange Saturation Transfer (CEST) saturation pulse train at a range of frequency offsets around a frequency of exchangeable protons of glutamate as well as the same frequency offsets on the opposite of the water resonance; MR imaging a slice of the body containing the suspected tumor to which the CEST saturation pulse has been applied to obtain a first glutamate CEST MRI map of the area of the tumor; injecting L-glutamine or a glutaminase blocker into the tumor; after injection of the L-glutamine or glutaminase blocker into the tumor, MR imaging a slice of the body containing the suspected tumor to which the CEST saturation pulse has been applied to obtain a second glutamate CEST MRI map of the area of the tumor; determining a CEST contrast image by taking a ratio of a difference image obtained by subtraction of saturation images obtained during said MR imaging steps with saturation applied at both positive and negative frequencies with respect to water resonance and an image with no saturation; and displaying the CEST contrast image as an indication of the level of expression of glutaminase in the tumor.