Early 2-dimensional images of the heart and mitral valve, from (A) Hertz 1964¹¹⁴ and (B) Hertz 1967,¹¹⁵ both obtained with a mirror system and recorded on 16 mm film at 7 frames per second; and (C) from Roelandt (1980; page 33)¹¹⁹ recorded about 1971 and described as ‘Stop-frame from the first two-dimensional ultrasonic image obtained from a patient with severe pericardial effusion; aHW = anterior heart wall.’ In this display, the transducer is along the left side of the image, and the echo-free space to the left of the arrow was interpreted as pericardial fluid in front of the heart.