Therapeutic ultrasound is a method used by physical and occupational therapists to promote tissue healing and reduce pain. This treatment uses high-frequency sound waves, which are well above the range of human hearing, to generate a mechanical effect within the body. This is a distinct treatment modality and not the diagnostic imaging method used to view a fetus or internal organs.
The Physics Behind Therapeutic Ultrasound
The technology relies on the “reverse piezoelectric effect” to produce the high-frequency sound waves. Inside the handheld applicator, known as the transducer head, there are specialized crystals, such as quartz or lead-zirconium-titanate. When an alternating electrical current is applied to these crystals, they rapidly expand and contract, generating mechanical vibrations that become ultrasonic waves.
These therapeutic waves typically operate within the range of 0.8 to 3.3 megahertz (MHz). The chosen frequency determines the depth of penetration into the body’s tissues. A lower frequency, such as 1 MHz, is generally used for treating deeper structures located 3 to 5 centimeters below the skin. Conversely, a higher frequency, like 3 MHz, is utilized for more superficial injuries that absorb the energy closer to the surface.
The energy delivery is controlled by selecting one of two primary settings: continuous or pulsed. In a continuous setting, the sound waves are delivered constantly, which results in a sustained energy transfer. A pulsed setting, however, delivers the sound waves in short bursts with quiet periods in between, which alters the resulting biological effect.
Therapeutic Effects on Biological Tissue
The mechanical energy from the sound waves interacts with the tissue in two fundamental ways, categorized as thermal and non-thermal effects. The thermal effect is generated when the sound waves are delivered continuously and the tissue absorbs the mechanical energy. This absorption causes friction among the molecules, which raises the localized temperature deep within the target area.
Heating the tissue to a temperature between 40 and 45 degrees Celsius can increase blood flow and metabolism in the area. This localized deep heating is often used to prepare collagen-rich tissues, such as tendons and joint capsules, for stretching and to help reduce muscle guarding.
The non-thermal, or mechanical, effects are primarily achieved using the pulsed wave setting, which minimizes the overall heat generated. This mechanical vibration causes two main phenomena: stable cavitation and acoustic streaming. Stable cavitation involves the oscillation of microscopic gas bubbles that naturally exist in the tissue fluids.
This oscillation facilitates the movement of ions at the cellular level and increases the permeability of the cell membrane. Acoustic streaming describes the unidirectional movement of fluid along the boundaries of the cell membranes, caused by the mechanical pressure waves. This fluid movement and the changes in cellular permeability stimulate a faster repair process, enhancing the body’s natural inflammatory and proliferative phases of healing.
What to Expect During an Ultrasound Therapy Session
A typical therapeutic ultrasound session is a straightforward process performed by a licensed professional. The therapist will first apply a layer of conductive gel to the skin over the area being treated. This gel is necessary because sound waves do not travel efficiently through air, and the medium ensures the energy is properly transferred from the transducer head into the tissue.
The therapist then presses the transducer head firmly against the skin and moves it continuously in small, overlapping circles over the treatment area. Constant movement of the soundhead is a safety measure that prevents the energy from concentrating in one spot, which could otherwise cause excessive localized heating. The duration of the treatment is usually brief, often lasting between five and ten minutes.
The sensation during the treatment can vary depending on the settings used; if a non-thermal setting is applied, the patient may feel nothing at all. If a thermal setting is used, a gentle, deep warmth may be felt, but patients should immediately report any feeling of discomfort or burning to the therapist. Therapeutic ultrasound is not applied over certain areas, including the abdomen of a pregnant woman, over a pacemaker, or over actively growing bone plates in children, to minimize potential risks.