An apparatus for using magnetic induction spectroscopy, MIS, to determine a measure of the fluid content of a tissue sample of a subject, the apparatus comprising: - a first excitation coil that is to be placed near to the tissue sample for inducing a current in the tissue sample; - a receiver coil that is to be placed near to the tissue sample so that the first excitation coil and the current in the tissue sample induce a current in the receiver coil, wherein the first excitation coil and the receiver coil are arranged in the same orientation with respect to each other; - a reference coil; - a second excitation coil that is arranged close to the reference coil and that is for inducing a current in the reference coil, wherein the reference coil and the second excitation coil are arranged in the same orientation with respect to each other and are arranged such that the coupling between the second excitation coil and the reference coil matches the coupling between the first excitation coil and the receiver coil; wherein the receiver coil is connected to the reference coil such that the current induced in the receiver coil by the first excitation coil and the current in the tissue sample flow in the opposite direction to the current induced in the reference coil by the second excitation coil; and - a control unit that is configured to: - apply an alternating current to the first excitation coil and the second excitation coil; - obtain a measure of the current induced in the tissue sample using the output of the interconnected receiver coil and reference coil and a first signal, the first signal representing the alternating current applied to the first excitation coil and the second excitation coil; and - determine a measure of the fluid content of the tissue sample from the measure of the current induced in the tissue sample. - a first excitation coil that is to be placed near to the tissue sample for inducing a current in the tissue sample; - a receiver coil that is to be placed near to the tissue sample so that the first excitation coil and the current in the tissue sample induce a current in the receiver coil, wherein the first excitation coil and the receiver coil are arranged in the same orientation with respect to each other; - a reference coil; - a second excitation coil that is arranged close to the reference coil and that is for inducing a current in the reference coil, wherein the reference coil and the second excitation coil are arranged in the same orientation with respect to each other and are arranged such that the coupling between the second excitation coil and the reference coil matches the coupling between the first excitation coil and the receiver coil; wherein the receiver coil is connected to the reference coil such that the current induced in the receiver coil by the first excitation coil and the current in the tissue sample flow in the opposite direction to the current induced in the reference coil by the second excitation coil; and - a control unit that is configured to: - apply an alternating current to the first excitation coil and the second excitation coil; - obtain a measure of the current induced in the tissue sample using the output of the interconnected receiver coil and reference coil and a first signal, the first signal representing the alternating current applied to the first excitation coil and the second excitation coil; and - determine a measure of the fluid content of the tissue sample from the measure of the current induced in the tissue sample. - apply an alternating current to the first excitation coil and the second excitation coil; - obtain a measure of the current induced in the tissue sample using the output of the interconnected receiver coil and reference coil and a first signal, the first signal representing the alternating current applied to the first excitation coil and the second excitation coil; and - determine a measure of the fluid content of the tissue sample from the measure of the current induced in the tissue sample.