An apparatus and processing method for assessing the electrical potential of cells in a living organism using a high-density sensor array having a density of at least about 9 electrodes per square inch for measuring the potential of the static and quasi-static electromagnetic fields of the group of cells proximal to each electrode, where the array can be placed overlying at least a portion of a surface of a living organism with substantially all of the electrodes electrically contactable with such surface, and the surface overlying a site containing cells of clinical interest.