This is from the "THE HISTORY OF ECHOCARDIOGRAPHY INGE EDLER* and KJELL LINDSTRO" in Ultrasound in Med. & Biol., Vol. 30, No. 12, pp. 1565 - 1644, 2004:

........... It is of interest to note that Keidel, in the introduction to his paper (Keidel 1947) in a general discussion of the possibility of using ultrasound for medical diagnostics, also mentioned the pulse-echo method as proposed by Gohr and Wedekind (1940). He pointed out a number of difficulties, including the very short pulse echo times involved. For a maximal body diameter of 30 cm, the resulting delay time would be 0.0004 s and for a tissue at 3-cm depth 0.00004 s, very short time intervals at that period of history, but not impossible to measure. Furthermore, the reflection interfaces in the human body are far more complicated than the simple seawater¡Vseabottom interface for the normal sonar method. Finally, the need to use high-frequency ultrasound to generate narrow beams diminishes the penetrating power of the sound. In 1939, Pohlman (1939) had reported a halfvalue distance in fat and muscles of 4.9 cm at 800-kHz ultrasound. For a sound wave that propagates through the thorax and back, it would imply that the initial acoustic energy had been reduced to about 2 x 10 (-4). For ultrasound waves with a frequency of 2400 kHz and a corresponding half-value distance of 1.5 cm, the same figure would be 1x 10 (-12). His conclusion was that the im- mense technical problems compared with the crude diagnostic possibilities hardly seemed worthwhile ........... .

Both Dussik and Keidel eliminated the possibility of using reflected ultrasound for theoretical reasons; therefore, they never performed any practical experiments with reflected ultrasound........., .......... Inspired by the visualisation problem solved by radar, many scientists supposed that reflected ultrasound could be used to visualise structures or internal organs in the human body. In the early 1950s, different groups in the USA, Europe and Japan, independent of each other, began such investigations in different fields and with different purposes. Shortly after World War II, the USA was leading in the field of advanced electronics, and the experiments started there.................

This is from: "Physics and engineering: milestones in medicine' by P. N. T. Wells in : Medical Engineering & Physics 23 (2001) 147- 15:

" ...... Perhaps this contempt is because of familiarity, for the history of medical ultrasound extends over more than 50 years and has been characterised by numerous incremental advances and not a few setbacks [11]. The first attempts were based on the assumption that it would be possible to demonstrate tissue masses by their different attenuations. Thus, the method would have been analogous to that used in traditional X-radiology. In radiology, the entire transmitted beam can be recorded photographically or by an image converter. No detector of comparable performance existed for ultrasound and so, in order to test the visualisation of the internal structures of the skull, the patient¡¦s head was immersed ¡X as far as possible! ¡X in water and an ultrasonic beam was transmitted through the head to be scanned pointby- point by a detector on the other side. The work was carried out by the Austrian brothers Dussik in 1947. The patterns thus recorded appeared to represent intracerebral structures, particularly the ventricles. Encouraged by these results, the influential Americans Hueter and Bolt concluded that ¡§A preliminary evaluation indicates that the echo-reflection method is considerably less promising than the transmission method for general ventriculography, mainly because of the small amount of reflection at the interface between the tissue and the ventricular fluid¡¨. It was not long before this rash statement was refuted! Thus, within a year, Guttner and his colleagues pointed out that the attenuation owing to the ventricles is small compared with that of the brain and the skull, and that the skull distorts the ultrasonic beam. It was merely fortuitous that the skull has a transmission pattern that resembles the shape and position of the normal ventricles; indeed, similar results could be obtained whether the skull contained a brain or not.

The earliest work on pulse-echo ultrasound was reported by Ludwig and Struthers, who succeeded in detecting gallstones. They used an ultrasonic flaw detector, a device developed during the war, independently by Firestone in the USA and by Sproule in the UK, for the detection of cracks and laminations in metals. Sproule and his colleagues worked in the steel mills at Newport in South Wales. John Wild, an expatriate Englishman working in Minneapolis with Jack Reid, also obtained encouraging results and extended the method to two-dimensional imaging. However, it was probably Douglass Howry, an intern at Denver General Hospital in 1947, who was really the first to demonstrate the potential of two-dimensional pulse-echo ultrasonic imaging, publishing his first and remarkable pictures in late 1950....... "

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