In magnetic resonance-electrical impedance tomography, magnetic flux density due to current injection is the measured quantity. Different conductivity distributions create different magnetic flux density distributions. Distinguishability for MR-EIT is defined using this fact. The definition is general and valid for 2D as well as 3D structures of any shape. It is not always possible to find an analytic expression for distinguishability. However, when a 2D cylindrical body with concentric inhomogeneity is considered, the distinguishability can be analytically formulated. For this structure, distinguishable regions defined by potential based and magnetic flux density based distinguishabilities are compared. Magnetic flux density is found to provide better distinguishability than surface potential case. This distinguishability definition also puts the main criterion for current optimization problems for MR-EIT.