A method of detecting liquid in animal tissue is disclosed. The method comprises providing an electromagnetic radiation generator, and applying electromagnetic radiation to a tissue region using the electromagnetic radiation generator. The characteristics of the electromagnetic radiation are selected so as to cause differential heating in a tissue region based on liquid content in the tissue region, differential expansion of matter in the tissue region, and thereby generation of ultrasound signals that vary according to the liquid content. The method also comprises providing an ultrasound detector, detecting the ultrasound signals using the ultrasound detector, and using the detected ultrasound signals to produce an indication of concentration of liquid in the tissue region and a determination as to whether a change in liquid concentration in the tissue region has occurred.