Why was the Ultrasound scan invented?

It evolved out of defense-oriented electronics devices such as Radar and more specifically Sonar, which uses sound waves. Sonic altimeters operating on pulses of Compressed Air were in use in l934 by what was to become the Luftwaffe and they were installed on commercial Zeppelins such as the Graf Zeppelin and HIndenberg. A sort of variant called the sonicaltimeter ( an early use of run-on language now so common) was made and used on Blimps by the US Navy and was adapted to submarine detection during the war. so it goes.