Apparatus for minimally invasively measuring concentrations of constituents contained within a biological tissue structure includes a microwave energy source arranged generate a range of microwave frequencies, a first antenna coupled to the microwave energy source and arranged to transmit at least a portion of the microwave energy into the tissue structure, a second antenna arranged to receive at least a portion of the microwave energy transmitted through the tissue structure, a signal processor arranged to determine the resonant frequency of the received microwave energy, and a data processor arranged to provide an output of the concentration of constituents within the biological tissue structure according to the determined resonant frequency.
