Therapeutic ultrasound is the use of high-frequency sound waves beyond the range of human hearing to treat medical problems. It should be distinguished from diagnostic ultrasound, in which ultrasound is used in a medical context to identify health problems by producing images of the inside of the body rather than as a treatment in itself. Therapeutic ultrasound is frequently used in physical therapy to speed healing and reduce pain in patients with injuries to muscles, bones, or connective tissues. It can also be used for other medical purposes, such as destroying tumors, breaking up blockages inside the body such as kidney stones and blood clots, and assisting in the administration of drugs.
Ultrasound is defined as sound at a frequency higher than the upper limit of what the human ear is able to detect, which is about 20 kilohertz (kHz) in a healthy adult. Ultrasound produced by medical equipment can have far higher frequencies, reaching into the thousands of kilohertz. When vibrations at ultrasonic frequencies enter the patient’s body, their energy is absorbed or redirected by the patient’s tissues. The chief advantage of ultrasound is that it can noninvasively penetrate deep into the body and cause effects inside the patient without the need for surgery.
This affects the body in two ways, thermal and nonthermal. The thermal effects of ultrasound occur as kinetic energy from the ultrasound waves is absorbed by the patient’s tissues, generating heat. The nonthermal effects are mechanical effects caused by the ultrasound waves in the medium they pass through. These involve causing the rapid expansion and contraction of microscopic gas bubbles in the fluids of the body in and around cells, a process called cavitation, and the redirection of fluid as it flows around these bubbles, called acoustic streaming.
A common application of therapeutic ultrasound is in physical therapy for people with injuries. This is especially common for treating patients with injuries to connective tissues such as tendons and ligaments, although it can be used for muscular and skeletal injuries as well. The heat generated as the vibrations are absorbed stops muscle spasms and increases blood flow to the affected area, reducing pain and promoting healing. As the ultrasound waves penetrate into the body and produce heat inside it, their thermal effects can reach deeper than heat applied to the surface of the body, as with a heating pad, and so can be used to treat deeper tissues that would otherwise be inaccessible. Cavitation can also reduce inflammation and swelling.
Therapeutic ultrasound can be used to destroy both cancerous and benign tumors. High-intensity focused ultrasound (HIFU) is a form of therapeutic ultrasound in which intense ultrasonic waves are carefully focused on a small area, causing a rapid rise in temperature in the affected tissue that kills or weakens the tumor’s cells. This is used in the treatment of uterine fibroids, a type of noncancerous tumor that grows on or in the uterus, and for some cancers, most notably prostate cancer. Cancer treatment with HIFU often occurs in combination with other treatments, such as chemotherapy, which becomes more effective if the cancerous cells are already weakened or damaged by heat. Research with HIFU is ongoing, and the technology is currently being studied for its potential therapeutic value treating various types of cancer.
Ultrasound can be used to assist in the effectiveness of drugs. A technique commonly called acoustic targeted drug delivery does this by applying a series of ultrasound pulses in specific patterns to the part of the body the drugs are intended to affect. This allows molecules to be more easily transported in or through the targeted area, which is useful because it makes it easier to target drugs to a specific area of the body, increasing effectiveness and decreasing the amount of the drug needed. This is especially desirable for treatments with drugs that have significance negative side effects, such as chemotherapy, and so much of the research done in this area is focused on the treatment of cancer. This usually involves ultrasound waves with a lower power level than those produced in high-intensity focused ultrasound, though HIFU is being researched for possible use assisting drug delivery as well.