A method of non-invasively monitoring the respiration of a patient comprises: transmitting ultrasound into the body toward an internal structure of the patient's body, the internal structure being one of the liver, the spleen or a kidney； selecting a depth range； measuring the phase of ultrasound echo signals from the internal structure at multiple points along the depth range for at least a first and a second echo signal, the first and second echo signals being received at different times； detecting the motion of the internal structure within the patient's abdomen by reference to differences in the measured phase between the first and the second echo signals； and thereby monitoring the respiration of the patient by associating movement of the internal structure with movement caused by respiration
