The medical applications of ultrasonics

Ultrasonic techniques are complementary to other physical methods used in surgery, therapy and diagnosis. Most medical applications of ultrasound are based on the properties of longitudinal waves in the frequency range 1-15 MHz. The relevant plane-wave equations are given, and the methods of production, the form and the measurement of the ultrasonic field are discussed. Ultrasonic waves travel at similar velocities (about 1500 m s⁻¹) in most biological tissues, and are absorbed at a rate of about 1 dB cm⁻¹ MHz⁻¹. Absorption occurs chiefly due to relaxation processes. It leads to thermal effects in biological systems. Mechanical effects, such as streaming and cavitation, are also important in certain situations, particularly at low frequencies. Highly focused ultrasound is used in neurosurgery; it is the only method for producing trackless damage deep in the brain. In vestibular surgery, ultrasonic irradiation is used routinely for the treatment of Ménière's disease; it can alleviate the symptoms without damage to the hearing. Therapeutic applications include the treatment in physiotherapy of various soft-tissue ailments, and the production of aerosols for inhalation. The examination of soft tissues is possible both by pulse-echo and continuous-wave techniques. Ultrasonic diagnosis provides information about the position and extent of characteristic impedance discontinuities; this information cannot be obtained directly by any other method. One- and two-dimensional displays, and time-position wave forms, are produced by pulse-echo techniques. Continuous-wave techniques include those based on the Doppler frequency shift of ultrasound reflected by moving structures.