Work is done by sound waves when the pressure of the wave acts against the resistance to deformation of tissue. Ultrasonic energy is required to produce an echo. Measures of energy relate to the energy directed toward tissue, difficult to measure energy absorbed.
Power is a measure of the rate of the work being done by an ultrasonic field.
Power is measured in watts. Power describes the potential for the field to interact with the medium in which it is propagating. It is the rate at which energy is transformed from one form into another. It is the major determinant of heat production by the ultrasound beam. Intensity is both space and time dependent.