Ultrasound therapy is a type of thermotherapy (heat treatment) that helps relieve some types of acute and chronic pain. A console provides an electrical current via a coaxial cable to a hand-held applicator (transducer). The applicator contains a quartz crystal that expands and contracts, producing sound waves, which are then transmitted to the patient’s skin and into the body.

Therapeutic ultrasound is typically delivered at frequencies between 0.8 to 3 megahertz (800 to 3,000 kilohertz). Lower frequency provides deeper penetration, up to about 2 inches (5 centimeters). The sound waves cause molecules in the tissues to vibrate, producing heat and mechanical energy. This allows for deep heating of tissues such as muscles, tendons, ligaments, joint capsules and bone.

Ultrasound therapy is a form of diathermy, the deep heating of tissues. Other types of diathermy include microwave and radiofrequency (short-wave) or high-frequency devices. At a tissue depth of 2 inches, diathermy devices should produce heat between 104 and 114 degrees Fahrenheit (40 to 45.5 degrees centigrade) for a maximum of 20 minutes, according to the U.S. Food and Drug Administration.

There are two approaches when using ultrasound therapy: continuous wave and pulsed wave. In cases where heating is not desired, such as with acute inflammation or with wound care,  intermittent (pulsed) is used, rather than the typical continuous application, which provides mechanical effects such as increased permeability of cell membranes and possibly repair of tissues.

Therapeutic ultrasound is different from diagnostic ultrasound, which uses lower-intensity sound waves to produce images of internal structures rather than to create thermal and mechanical actions. In addition, intense highly focused ultrasound can be used to eliminate unwanted tissue, such as kidney stones, gallstones, benign prostatic hyperplasia and some types of tumors including uterine fibroids and (in some countries) prostate cancer.

The thermal effects of ultrasound therapy have been found to be beneficial in the treatment of musculoskeletal injuries, breaking down scar tissue and helping to stretch tendons. The use of ultrasound as deep heat therapy may also be accompanied by electrical stimulation of the muscle. This results in mechanical pumping, which would promote the removal of products of increased metabolism, and treat muscle spasms and microscopic tissue adhesions.

Therapeutic ultrasound may be used to treat many other conditions as well. It has anti-inflammatory effects that can relieve the pain and stiffness of arthritis and some other inflammatory conditions. It may be used to treat impingement (compression) of nerve roots and various types of neuritis (nerve inflammation) and may be useful in the care of post-traumatic injuries.

However, ultrasound therapy has some risks. Unlike diagnostic ultrasound, it is not used near the abdomen of a pregnant woman because the higher intensity of sound waves may harm a developing fetus. It may also have negative effects on cancerous tissues or areas of bone overgrowth. It is also avoided over bony prominences and, in children, over epiphyseal plates of growing bones. Ultrasound therapy is not advised for patients who cannot perceive pain and heat, such as some patients with diabetic neuropathy.