tomographic images of a body are constructed by placing a plurality of surface electrodes at spaced intervals on the body , causing currents to flow in the body , and measuring the potential between pairs of electrodes , calculating the potential in each case on the assumption that the body consists of one uniform medium , plotting the isopotentials corresponding to the calculated results to create a uniform image of the body , obtaining the ratio between the measured potential and the calculated potential in each case , and modifying the image in accordance with the respective ratios by increasing the impedance along an isopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity . the calculations of potentials and the obtaining of ratios are carried out using a computer and the plotting of the isopotentials is carried out by a visual display unit and / or a print - out unit run off the computer . an additional method is provided for indicating a change of state in the body . this method includes the step of determining ratios between initial and subsequent measured actual potentials between electrodes .