Sentence:  Sonar can be used to measure the thickness of the heart by placing small crystal transducers at opposite sides of the heart or blood vessel and exciting one with some pulsed ultrasonic <WSD>energy</WSD> . 
Meaning of the word marked with <WSD> tags:  (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs