John G. Lynn et al. proposed the idea that ultrasound could be intensely focused to produce extreme heat and non-invasively destroy targeted diseased tissue within the body. They were able to produce lesions deep in bovine liver without damaging surrounding tissue.
J. G. Lynn, R. L. Zwemer, A. J. Chick and A. F. Miller: J. Gen. Physiol. 26 (1942) 179.