A diagnostic method for monitoring changes in a fluid medium in a patient's head. The method includes positioning a transmitter at a first location on or near the patient's head, the transmitter generates and transmits a time-varying magnetic field into a fluid medium in the patient's head responsive to a first signal； positioning a receiver at a second location on or near the patient's head offset from the transmitter, the receiver generates a second signal responsive to a received magnetic field at the receiver； transmitting a time-varying magnetic field into the fluid medium in the patient's head in response to the first signal； receiving the transmitted magnetic field； generating the second signal responsive to the received magnetic field； and determining, a phase shift between the transmitted magnetic field and the received magnetic field for a plurality of frequencies of the transmitted time-varying magnetic field.
