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Usually, the inauguration of clinical echocardiography is attributed to Edler and Hertz who published their first paper in 1954,1 followed by Japanese authors in 1956.2 It is less well-known that there was a first description on using ultrasound for investigating the heart by Wolf-Dieter Keidel already a decade earlier3 ( Figure 1 ). His first investigations were conducted at the Physiologic Institute of the University of Erlangen, Germany, which had become prominent scientifically for its research and mathematical definition of human hearing and its work on audible sound. Apparently, the institute also became involved in the study of inaudible ‘sound’.Figure 1 Prof. Dr Wolf-Dieter Keidel.In 1949 Keidel presented3 and in 1950 he published4 about the ‘acoustic heart shadow’ and its correlations to heart volumes and function. The title of the work was ‘A Method to Register the Volume Changes in the Human Heart’. He tried to analyse the energy change of ultrasound transmitted through the thorax, analogous to radiographic examinations. Thus, his primary application of cardiac ultrasound was still based on transmitted sound waves, but he was nevertheless able to generate the first interpretable information about cardiac filling and physiology. Interestingly, this was accomplished by using very low ultrasound frequencies at 57.5 kHz.Figure 2 demonstrates Keidel's ‘Ultraschallkardiogram’,3 an ultrasound cardiogram recorded at the third intercostal space and compared with the ultrasound cardiogram at the apex. The cardiogram was generated by a magnetorestrictive …

The inaugurator of transmitted echocardiography: Prof. Dr Wolf-Dieter Keidel