How Does Ultrasound Therapy Work?
Ultrasound; sound waves vibrating at frequencies above those that the human ear can hear, greater than 20,000cycles per second (hertz, Hz).
Therapeutic ultrasound is torque transducer which is used for therapeutic (rather than diagnostic) purposes and is usually produced at 1 million cycles per second or 1 megacycle (MHz). Generally US frequency is between 0.7 and 3.3 MHz.
Ultrasound was first researched in Europe and was introduced to the United States in the early 1950`s, since this time, the use of ultrasound has grown, and it remains a very popular modality today.
Mechanism of action
US is produced by a high-frequency alternating current being applied to a crystal made of quartz or a synthetic (i.e. barium titanate or lead zirconate). The current causes the crystal to bend, first in one direction and then the other, as the current alternates. This deformation of the crystal creates what is called piezoelectric energy. The vibration energy is transferred to a metal plate, usually nickel-plated brass, and then the patient’s tissue. The crystal and metal plate are housed in the transducer or sound head of the unit.
It is not transmitted through a vacuum
It is transmitted poorly through gases
It is transmitted best through gas-free liquids and high-density solids
Similar to a radio transmitter.
Has an oscillator that produces electromagnetic wave at high frequencies.
Converts electromagnetic waves into sound waves.
Larger transducer surface is better (7-13cm*), but small enough to maintain skin contact.
Waves are emitted in a cylindrical beam perpendicular to traducer surface. Therefore hold the transducer head parallel to tissue surface.
Never allow the beam to shoot straight into the air without gel or water as this may damage the crystal.
To test crystal: water on the head, turn up the intensity the water should stream.
Air does not conduct well, must use a coupling medium. Use a commercial gels, not mineral oil and some gels will contain substances that can promote healing such as, vit E, aloe.
As US travels through human tissue, it gradually decreases in intensity due to attenuation. Attenuation is the result of absorption, reflection and refraction, with absorption accounting for about half of attenuation.
- Proteins are the major absorbers of US
- Increase protein content increase absorption
- Tissues with high water content decrease absorption (waves pass through)
- Homogenous tissues decrease absorption (fat)
- Heterogeneous tissues, fascia planes increase absorption
e.g. bone\ periosteum, fat \ muscle, diff muscle groups.
- Bone reflects waves, which meet incoming waves: resonance therefore heat production (periosteal burning)
- Metal artefacts reflects waves but absorb heat therefore caution
Conversion of the mechanical energy of US into heat is expressed as an absorption coefficient and is dependent on tissue type and frequency. This coefficient is expressed in decibels/cm and is highest for tissues with the highest collagen content and increase in proportion to the US frequency.
Absorption coefficients in decibels/cm at 1 and 3 MHz
The depth of tissue at which the US intensity is half its initial intensity is frequency and tissue type dependent
Half depths in mm at 1 and 3 MHz
Is the redirection of an incident beam away from a reflecting surface at an angle equal and opposite to the angle of incidence. US waves are reflected at tissue interfaces with most reflection occurring where there is the greatest difference between the acoustic impedance of adjacent tissues.
There is 100% reflection of US at an air-skin interface and only a 0.1% reflection at couple medium- skin interface. In the body most reflection approx 35% occurs at soft tissue-bone interface.
The redirection of a wave at an interface
When refraction occurs, the US wave enters the tissue at one angle and continues through the tissue at a different angle.
The number of compressions-rarefactions cycles per unit of time expressed as (hertz). Increasing the frequency decreases the depth of penetration.
Use 1 MHz for tissue up to 5 cm deep and 3 MHz for tissue 1 to 2 cm deep.
There is no certain way of exactly calculating how much energy is absorbed in any particular tissue, the decision about dosage is a matter of judgement to some extent. This judgement must be based on the known factors governing ultrasound absorption and is based on the following:
Mode of delivery:
- Continuous output
Continuous delivery of US throughout the treatment period
Used sub acute and chronic stages