Acoustic tomography: Promise versus reality

Imaging with acoustic waves has made great advances in recent decades. In opposing limits of wavelength, acoustics have played a major role in geophysical applications on the one hand and in medical ultrasound imaging on the other. In contrast to X-rays, acoustic waves interact strongly with materials through which they propagate, through processes such as refraction, reflection and diffraction. The interactions can be very strong in heterogeneous media such as human tissue. Tomographic reconstructions of acoustic data therefore require much more sophisticated modeling of acoustic wave propagation often involving highly non-linear inversions. These factors have impeded progress in this otherwise promising methodology. The advancement of computing power and the rise of high-throughput data acquisition hardware have made acoustic tomography (AT) feasible in recent years. The objective of this paper is to relate these developments to practical applications of AT, particularly in the area of medical imaging. Today, a number of laboratory groups are collecting data with AT prototypes and some projects have become commercial ventures. This paper reviews the status of AT imaging, particularly in the area of breast cancer detection, where some of the most recent advances have taken place. It is shown that parallel developments in AT methodologies have given rise to exciting new possibilities for acoustic tomography, at all wavelengths, with potential applications in areas as diverse as seismic exploration, non-destructive testing and cancer detection.