A new approach to generating MRI contrast by solving the magnetic field to susceptibility source inverse problem is presented to address the quantification difficulties associated with traditional T1/T2 relaxation and susceptibility weighted T2* methods. The forward problem from source to field is reviewed. Its inverse field to source problem is ill posed. Accurate solutions are found by conditioning the data acquisition or regularizing the solution. Preclinical and clinical applications using this magnetic source MRI are discussed for quantitative mapping magnetic biomarkers such as contrast agents in molecular MRI and iron deposits in diseases.