Ultrasound (US) has been used by various health-care specialists for over 60 years. Biological effects in tissues exposed to US were first reported by Wood and Loomis in 1927.1 They demonstrated lysis of red blood cells and hindrance of mobility in mice following exposure to high-frequency (300 kHz), high-intensity sound waves. The application of US for medical treatment was introduced in Germany in the late 1930s, and in the United States in the late 1940s.2,3 The purpose of this chapter is to describe the physical properties and biological effects of ultrasound and how they can be used to enhance wound healing.
Physics and Generation of Ultrasound–Piezoelectric Effect
Ultrasound is produced by vibration of piezoelectric transducer disks that are located in the treatment handpiece behind a metal plate or probe that serves to transmit the acoustic energy to the patient's tissues. The ceramic disks are crystals such as barium titanate or lead zirconate titanate that are capable of transducing electrical energy into sound energy or vice versa. There are two types of piezoelectric effect: direct and indirect (also called reverse).4 (Fig. 28.1) The direct piezoelectric effect generates an electric voltage across the disk when it is mechanically compressed. If the crystal is expanded instead of compressed, a voltage of opposite polarity is generated. If acoustic energy from a sound wave at a given frequency impinges upon a crystal, it will expand and contract at the same frequency and will cause an oscillating voltage to be generated across the crystal face. The direct piezoelectric effect is used to convert US into an electrical signal that duplicates the sound pattern.
Direct and indirect (reverse) piezoelectric effects. (A) The direct piezoelectric effect. Crystals having piezoelectric properties produce positive and negative electrical charges when they are mechanically compressed or expanded. (B) The reverse (indirect) piezoelectric effect. These same crystals expand and contract or vibrate at the same frequency as that of an alternating voltage that is applied to them. (Used by permission of: Starky C: Ultrasound. In: Starky C (ed): Therapeutic Modalities, ed. 2. Philadelphia, F.A. Davis, 1993, p 272.)
The reverse piezoelectric effect is the contraction or expansion of a crystal when an alternating current voltage is applied across its surface. A change in the polarity of the applied voltage causes a contracted crystal to expand and vice versa. Thus, a piezoelectric crystal can be used to generate US at any desired lower (eg, 20-40 kHz) or higher (1-3 MHz) sound wave frequency.
Ultrasound is generated when an alternating voltage at any frequency makes the crystal vibrate (repetitively expand and contract) at the frequency of the electrical oscillation.
US is nonionizing radiation and therefore does ...