A partially-masked electrode includes a conductive material and an insulated coating having an outer surface. The insulated coating defines a contoured opening that exposes or reveals an area of the conductive material, wherein the contoured opening has an upper perimeter at the outer surface of the insulated coating. When the upper perimeter of the insulated surface coating is placed in contact with a tissue of interest, wherein the tissue of interest is proximate a blood pool, the insulated coating creates a seal between the blood pool and the contoured opening so that no blood in the blood pool can contact the conductive material. This seal reduces or eliminates the reception of far field effects in the blood pool by the electrode, making it easier to locate and diagnose unhealthy tissue.