Medical B-mode scanners operating under conditions typically encountered during clinical work produce ultrasonic wave fields that undergo nonlinear distortion. In general, the resulting harmonic beams are narrower and have lower sidelobe levels than the fundamental beam, making them ideal for imaging purposes. This work demonstrates the feasibility of nonlinear harmonic imaging in medical scanners using a simple broadband imaging arrangement in water. The ultrasonic system comprises a 2.25-MHz circular transducer with a diameter of 38 mm, a membrane hydrophone, also with a diameter of 38 mm, and a polymer lens with a focal length of 262 mm. These components are arranged coaxially giving an imaging geometry similar to that used in many commercial B-scanners, but with a receiver bandwidth sufficient to record the first four harmonics. A series of continuous wave and pulse-echo measurements are performed on a wire phantom to give 1-D transverse pressure profiles and 2-D B-mode images, respectively. The reflected beamwidths wn decrease as wn/W1 = 1/n0.78, where n is the harmonic number, and the reflected sidelobe levels fall off quickly with increasing n. In imaging terms, these effects correspond to a large improvement in lateral resolution and signal-to-clutter ratio for the higher harmonics.

The basic physical characteristics of ultrasound waves are reviewed in terms of the typical displacements, velocities, accelerations and pressures generated in various fluid media as a function of frequency. The effects on wave propagation of interfaces are considered, and the way in which waves are reflected, transmitted and mode converted at interfaces introduced. Then the nonlinear propagation of high amplitude ultrasound is explained, and its consequences, including the generation of harmonic frequencies and enhanced attenuation, considered. The absorption of ultrasonic waves and the resulting heat deposition in absorbing media are described together with factors determining the resulting temperature rises obtained. In the case of tissue these include conduction and perfusion. The characteristics of cavitation in fluid media are also briefly covered. Finally, secondary nonlinear physical effects are described. These include radiation forces on interfaces and streaming in fluids.

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This paper considers some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. In the second part of the paper a simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.

The propagation of sound through a one-dimensional periodic array of water and perspex plates is studied theoretically and experimentally. It is shown that the passbands and stop bands of a scatterer with a finite number of layers correspond to the bands and bandgaps of an infinite "sonic bandgap crystal." The transmission coefficient of various finite structures is computed and measured as a function of frequency. The analogy with the electronic bandstructure of crystals, and the photonic bandstructure of macroscopic periodic dielectric structures, is found to be a close one. It is shown that the position and width of passbands can easily be engineered. Results are included for a finite "crystal" with a vacancy defect, in which a narrow passband appears in each of the stop bands.