● Part I : Science and Technology NONLINEAR IMAGING

It was Lord Rayleigh who first described the nonlinear radial motion of a gas bubble in an acoustic field. His theory, subsequently elaborated by a series of investigators, predicted that a bubble, driven into nonlinear resonant oscillation by an incident sound field, gives rise to a scattered signal that contains higher harmonics of the incident frequency (Fig. 1). This observation led, nearly 100 years later, to the development of a method that forms an image from the components of the echoes around the second harmonic of the transmitted frequency. These first harmonic images were made as part of an attempt to improve sensitivity to microbubble contrast agents in very small vessels (Burns et al. 1996). They have, however, stimulated fresh investigation of nonlinear acoustic phenomena in tissue and an interest in nonlinear imaging methods, which is sure to be sustained into the new millennium.