A field measurement algorithm and measuring circuitry in an implantable stimulator, and an field modelling algorithm operable in an external device, are used to determine an electric field in a patient's tissue. The field measuring algorithm provides at least one test current between two electrodes, and a plurality of voltage differentials are measured at different combinations of the electrodes. The voltage differential data is telemetered to the field modelling algorithm which determines directional resistance at different locations in the patient's tissue. The field modelling algorithm can then use a stimulation program selected for the patient and the determined directional resistances to determine voltages in the patient's tissue at various locations, which in turn can be used to model a more-accurate electric field in the tissue, and preferably to render an electric field image for display in a graphical user interface of the external device.