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Experiments were performed to establish the source of the echo produced when a needle was placed through the lumen in a specially modified ultrasonic transducer. These transducers had been used successfully to monitor the removal of fluid from various compartments of the body, whether they be cystic masses or fluid within such areas as the thorax, pericardium, or abdomen. The laboratory experiments conducted showed conclusively that the echoes recorded in clinical practice were produced at the needle tip-fluid interfaces. The distance from the transducer surface to the echo produced was equal to the actual length of the needle. A secondary echo was observed which was produced by transmission of the ultrasonic beam through the metal of the needle. However, this was of minor importance since it was much weaker in strength and was not recorded in clinical practice.