1. A computer system for generating an estimated volume of activation (VOA), the system comprising: a computer processor configured to: for each of a plurality of neural elements that are in a vicinity of a leadwire: determine a respective threshold activation value; and determine whether a user-input stimulation value to be applied to the leadwire meets the respective threshold activation value of the respective neural element; and generate a surface structure corresponding to those of the neural elements whose respective threshold activation values have been determined to be met by the user-input stimulation value.