Therapeutic ultrasound is presented in this section rather than in the thermal agents section for two reasons: (1) it is an acoustical modality that uses mechanical energy rather than an electromagnetic or infrared modality and (2) it is a deep-heating agent. In addition, ultrasound is capable of producing mechanical, nonthermal effects in addition to its thermal effects. Also refer to the basic physiological responses to heat described in Chapter 5.
• Therapeutic ultrasound is a deep-penetrating agent that produces changes in tissue through thermal and nonthermal (mechanical) mechanisms. Unlike most other physical agents, ultrasound is not a part of the electromagnetic spectrum •, but uses acoustical energy (Box 7-1). Depending on the frequency and wavelength of the energy, ultrasound is used for diagnostic imaging (0.5 to 50 mW/cm2), therapeutic deep tissue heating (1 to 3 W/cm2), or tissue destruction (0.2 to 100 W/cm2). This chapter focuses on the thermal and nonthermal effects of therapeutic ultrasound.
Box 7-1. ACOUSTICAL ENERGY
Frequency is the number of times the wave passes per second; wavelength is the distance between the start of the wave to the end of the wave (one complete cycle). As shown above, frequency and wavelength are inversely proportional: the higher the frequency, the shorter the wavelength (and vice versa). Amplitude is the maximum distance from the baseline to the peak of the wave.
Acoustical energy is transmitted differently from electromagnetic energy. Electromagnetic radiation involves the transmission of individual energy particles that do not require a transmission medium. The sun emits light particles that travel unhindered through the vacuum of space. Unlike sound waves it does not require a physical medium.
To be transmitted, acoustical energy (sound waves) requires a physical medium, such as air. Mechanical vibrations form waves in the medium that transmit acoustical energy. Therefore, the transmission of acoustical energy is impossible in the vacuum of space. If you yell at a person across the street, your voice creates waves in the air. These waves travel through the air and are received by the other person's ear.
Unlike audible sound, ultrasound's high frequency requires a medium that is denser than air to be transmitted. Water is the ideal media. The most effective ultrasound transmission media have a high water density.
In a uniform environment, sound waves travel at a constant speed. These waves have three properties: wavelength, frequency, and amplitude. Wavelength and frequency are inversely related. Referring to the above figure, the top has a longer wavelength and a lower frequency than the bottom. Frequencies are expressed in Hertz (Hz). A larger Hz number means a higher frequency. In the figure, fewer wavelengths that are 2 meters long at a frequency of 1 Hz will travel past a specific point in 1 second than sound with a wavelength of 1 meter and a frequency of 2 Hz. Amplitude is simply how loud or intense a ...