Ultrasound waves are acoustic vibrations of high frequencies above ranges of 17,000 to 20,000Hz that are inaudible to humans. Ultrasound treatment uses frequencies of 0.5~5MHz, and mostly around 1MHz to treat various diseases. Ultrasound phoresis is a method that increases transdermal penetration of drug substances with the ultrasound energy.
To briefly discuss the history of ultrasound, in 1880, Pierre Curie and Paul Jacque Curie first described the piezoelectric effect where applying pressure to quartz generates electricity. Langevin, a friend of Curie, developed a method to generate ultrasound using the piezoelectric effect in 1910 and first recorded the biological effects of ultrasound after observing the death of fish that was exposed to the ultrasound beams in 1917. Based on the theoretical research of Langevin, American physicists Wood and Loomis published a study on the biological effects of the ultrasound energy, and many other studies followed suite. In 1928, Woeber believed ultrasond to be effective in osteosclerosis and in 1934, Nakahara and Kobayashi et al. observed that intracutaneous adenocarcinoma developed in mouse skin exposed to ultrasound irradiation. In 1944, Horvath J published a book on the medical application of ultrasound and ultrasound started to be extensively studied in Europe. The medical use of ultrasound began mainly in Germany in the 1930s and was introduced in the US in the 1940s. In 1952, the American Council of Physical Medicine & Rehabilitation officially recognized ultrasound as physical therapy, which established ultrasound as a major modality of thermal therapy today.
Classification of sound waves
Much like laser, sound waves are categorized into infrasound, acoustic, and ultrasound waves according to the frequency (See <Image 20-2>). Animals such as dogs and cats have greater range of audible sound waves. Bats can see in the dark by detecting sound waves over 150KHz. On the other hand, human hearing is limited to the range of 16-20Hz up to 17,000-20,000Hz. Personal and age differences also affect the hearing ability, with a young person hearing up to 18,000Hz and an elderly person 12,000Hz. The roars of an earthquake are usually below the lowest hearing range of humans and are called infrasound. Sound waves above the highest range of audible frequencies are called ultrasound.
Types of ultrasound used in medicine
Ultrasound is widely used in various fields including medicine, industry and military, etc. Medical ultrasound is used in diagnosis, treatment and surgery, etc. Physical therapy ultrasound is used for soft tissue recovery with thermal effect. Diagnostic ultrasound is used for imaging and ablative ultrasound is used in surgery and removal of tumor tissues, etc. Therapeutic ultrasound frequencies range from 0.5 to 5.0MHz. Most devices have fixed frequencies of 0.75MHz, 0.87MHz, 1.0MHz, 1.5MHz, or 3.0MHz. The frequency can be selected from several options but the range of 0.8~1MHz is the most frequently used.
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Biophysics of ultrasound
When ultrasound energy is applied to living tissues, the molecules of the tissues absorb the ultrasound energy to convert to thermal energy. Therefore, absorption of ultrasound energy can selectively raise the temperature of the tissue. The absorption of ultrasound energy is affected by the frequency, tissue properties such as density, viscosity and acoustic impedance, fat and water content, as well as incidence angle, reflection, scattering and refraction, etc.
1) Tissue properties
Absorption of ultrasound energy increases with higher content of structure protein. Therefore, ultrasound is more easily absorbed into tissues with high collagen content such as the bone or joint capsule, etc. Muscle with higher protein content absorbs ultrasound 2-3 times better than fat. The bone absorbs ultrasound 10 times more than soft tissues. Protein of neural tissues is sensitive to ultrasound, as shown in <Table 20-1>.
2) Frequency
Absorption of ultrasound energy doubles with higher frequency and shorter wavelength. The amount of ultrasound absorption increases in proportion to frequency. In other words, 3MHz ultrasound has absorption coefficient in muscle three times higher than 1MHz ultrasound.
3) Viscosity of the medium
Absorption of ultrasound energy in liquid is influenced by viscosity and thermal conductivity. In water with the absorption coefficient of 0.0022, absorption of ultrasound is 500~1000 times lower than in air, which makes water an excellent medium in ultrasound therapy.
4) Acoustic impedance of tissue
As electrical resistance is called impedance, acoustic impedance refers to resistance to sound. The propagation velocity of sound waves is affected by the medium density and increases with higher elasticity. Acoustic impedance (z) is the product of medium density (ρ, 105g/cm3) multiplied by conduction velocity (c, m/s) as shown in <Formula 20-1>. Therefore, the propagation of ultrasound waves is influenced by the acoustic impedance of the medium.
Formula 20-1. Z = ρ x c
