Acoustics and wave physics in modern applications of ultrasound in therapy

Ultrasound methods and devices have become widespread in modern medicine for diagnostics and therapy. Therapeutic applications are increasingly common, including tumor ablation, kidney stone fragmentation, targeted drug delivery, physiotherapy, and essential tremor. The success of ultrasound methods has relied not only on engineers and physicians but also on many years of scientific research in wave physics and acoustics. From an acoustics point of view, medical ultrasound utilizes inaudible high-frequency waves that propagate in water-like media. In the context of wave physics, such ultrasound waves are short-wavelength elastic waves that can be effectively focused, reflected, and refracted. Ultrasound is able to propagate over significant distances and thus reach deep parts of the human body. Many general properties of waves, in particular their ability to carry energy and momentum, are used in medical ultrasound for remote thermal or mechanical destruction of tissue, as well as for providing a pushing effect as a result of the emergence of a radiation force. Key features of the wave field include aberration and refraction when passing through inhomogeneous tissue layers, the diffraction limit of energy localization during focusing, reflection from interfaces, and scattering by inhomogeneities and inclusions.