The present invention relates to an apparatus for determining a dielectric function in an object. The apparatus comprises one transmit antenna (42) for transmitting microwave radiation through said object, and one receive antenna (43) for receiving the transmitted microwave radiation, one ultrasound transmitter for emitting ultrasound radiation through said object to generate a density variation in the object, means to analyse the microwave radiation transmitted through the density variation to determine the acousto-electric interaction δ in the object, and means to calculate the dielectric function in the object from the acousto-electric interaction. The invention also relates to a method for determining the dielectric function in an object.