The invention relates to a magnetic resonance imaging system (100) for determining an approximation (150) of an electric conductivity distribution within a three-dimensional anatomical structure of interest. The determining comprises acquiring a first set of (3-n)-dimensional magnetic resonance data (144), reconstructing a (3-n)-dimensional phase distribution (146) using the (3-n)-dimensional magnetic resonance data (144), calculating a (3-n)-dimensional electric conductivity distribution (148) using spatial derivatives within the (3-n) dimensions and applying to the (3-n)-dimensional electric conductivity distribution (148) a scaling factor compensating for the relative reduction of dimensions by n.