 TRIAL OBJECTIVES AND PURPOSE Specific Aim 1: To study the biophysical properties of the white and gray matter in critical cortical and subcortical regions using magnetization transfer in patients with type 2 diabetes and MDD, non-depressed diabetic controls, patients with unipolar depression without diabetes and nondiabetic healthy controls. The investigators are interested in the separate and cumulative effects of both diabetes and depression on Magnetization Transfer Ratios MTR in different brain regions. 