An apparatus for applying alternating current between at least four first electrode elements positioned on a first side of a body and at least four second electrode elements positioned on a second side of the body, the apparatus comprising an AC signal generator, a plurality of switches and a controller configured to control the plurality of switches, the AC signal generator being configured to generate an AC output signal from a first output and a second output, characterized by:, the plurality of switches comprising at least four first switches, each of which is configured to selectively apply the first output to a respective one of the first electrode elements depending on a state of a respective control signal, at least four second switches, each of which is configured to selectively apply the second output to a respective one of the second electrode elements depending on a state of a respective control signal, and at least one third switch, each of which is configured to selectively apply the second output to a respective one of the first electrode elements depending on a state of a respective control signal; and the controller being configured to control the plurality of first switches, the plurality of second switches and the at least one third switch so that, in a first mode, the controller issues control signals that cause a majority of the first switches to apply the first output to corresponding first electrode elements and that cause a majority of the second switches to apply the second output to corresponding second electrode elements, and, in a second mode, the controller issues control signals that cause the first switches to sequentially apply the first output to respective first electrode elements in turn while the at least one third switch applies the second output to at least one respective first electrode element, and sequentially receive impedance measurements corresponding to respective combinations of the first electrode elements. the plurality of switches comprising at least four first switches, each of which is configured to selectively apply the first output to a respective one of the first electrode elements depending on a state of a respective control signal, at least four second switches, each of which is configured to selectively apply the second output to a respective one of the second electrode elements depending on a state of a respective control signal, and at least one third switch, each of which is configured to selectively apply the second output to a respective one of the first electrode elements depending on a state of a respective control signal; and the controller being configured to control the plurality of first switches, the plurality of second switches and the at least one third switch so that, in a first mode, the controller issues control signals that cause a majority of the first switches to apply the first output to corresponding first electrode elements and that cause a majority of the second switches to apply the second output to corresponding second electrode elements, and, in a second mode, the controller issues control signals that cause the first switches to sequentially apply the first output to respective first electrode elements in turn while the at least one third switch applies the second output to at least one respective first electrode element, and sequentially receive impedance measurements corresponding to respective combinations of the first electrode elements.