1. A method performed by a computer system having one or more processors and memory storing one or more programs for execution by the one or more processors, the method comprising: accessing multiple images of a biological structure, wherein the multiple images of the biological structure include one or more of: a T1 MRI image, a T2 MRI image, and a proton density MRI image; generating a combined image based on the multiple images of the biological structure, said combined image having a plurality of elements each corresponding to a respective pixel or voxel in the T1 MRI image, T2 MRI image, and proton density MRI image; calculating electrical property values from pixel or voxel data of said combined image, the respective electrical property values comprising respective tissue resistivity values, wherein determining the respective electrical property values includes determining respective tissue resistivity values in accordance with an equation: 
 R ( v )= K (1− v ) E +D, where R is a respective tissue resistivity value, K is a multiplier value, v is a respective normalized numeric value representing an intensity of a respective pixel or voxel of the combined image, E is an exponent, and D is a tissue density value; generating an electrical property map of the calculated electrical property values for the biological structure; and displaying the electrical property map on a display device of the computer system.