Therapeutic ultrasound refers generally to any type of procedure that uses ultrasound for therapeutic benefit. This includes HIFU, lithotripsy, targeted ultrasound drug delivery, trans-dermal ultrasound drug delivery, ultrasound hemostasis, cancer therapy, and ultrasound assisted thrombolysis  It may use focused ultrasound (FUS) or unfocused ultrasound.
Ultrasound is a method of stimulating the tissue beneath the skin's surface using very high frequency sound waves, between 800,000 Hz and 2,000,000 Hz, which cannot be heard by humans.
The first large scale application of ultrasound was around World War II. Sonar systems were being built and used to navigate submarines. It was realized that the high intensity ultrasound waves that they were using were heating and killing fish. This led to research in tissue heating and healing effects. Since the 1940s, ultrasound has been used by physical and occupational therapists for therapeutic effects.
Physical therapy 
Ultrasound is applied using a transducer or applicator that is in direct contact with the patient's skin. Gel is used on all surfaces of the head to reduce friction and assist transmission of the ultrasonic waves. Therapeutic ultrasound in physical therapy is alternating compression and rarefaction of sound waves with a frequency of >20,000 cycles/second. Therapeutic ultrasound frequency used is 0.7 to 3.3 MHz. Maximum energy absorption in soft tissue occurs from 2 to 5 cm. Intensity decreases as the waves penetrate deeper. They are absorbed primarily by connective tissue: ligaments, tendons, and fascia (and also by scar tissue).
Conditions which ultrasound may be used for treatment include the follow examples: Ligament Sprains, Muscle Strains, Tendonitis, Joint Inflammation, Plantar fasciitis, Metatarsalgia, Facet Irritation, Impingement syndrome, Bursitis, Rheumatoid arthritis, Osteoarthritis, and Scar Tissue Adhesion.
Benefits of ultrasound 
There are three primary benefits to ultrasound. The first is the speeding up of the healing process from the increase in blood flow in the treated area. The second is the decrease in pain from the reduction of swelling and edema. The third is the gentle massage of muscles tendons and/ or ligaments in the treated area because no strain is added and any scar tissue is softened. These three benefits are achieved by two main effects of therapeutic ultrasound.The two types of effects are: thermal and non thermal effects. Thermal effects are due to the absorption of the sound waves. Non thermal effects are from cavitation, microstreaming and acoustic streaming.
Cavitational effects result from the vibration of the tissue causing microscopic air bubbles to form, which transmit the vibrations in a way that directly stimulates cell membranes. This physical stimulation appears to enhance the cell-repair effects of the inflammatory response. Effectiveness of therapeutic ultrasound for pain, musculoskeletal injuries, and soft tissue lesions remains questionable.
Biomedical applications 
Relatively high power ultrasound can break up stony deposits or tissue, accelerate the effect of drugs in a targeted area, assist in the measurement of the eleastic properties of tissue, and can be used to sort cells or small particles for research.
- Focused high-energy ultrasound pulses can be used to break calculi such as kidney stones and gallstones into fragments small enough to be passed from the body without undue difficulty, a process known as lithotripsy.
- Ultrasound has been used to trigger the release of anti-cancer drugs from delivery vectors including liposomes, polymeric microspheres and self-assembled polymeric.
- Ultrasound can ablate tumors or other tissue non-invasively. This is accomplished using a technique known as High Intensity Focused Ultrasound (HIFU), also called focused ultrasound surgery (FUS surgery). This procedure uses generally lower frequencies than medical diagnostic ultrasound (250–2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by Magnetic Resonance Imaging (MRI); the combination is then referred to as Magnetic resonance-guided focused ultrasound (MRgFUS).
- Delivering chemotherapy to brain cancer cells and various drugs to other tissues is called acoustic targeted drug delivery (ATDD). These procedures generally use high frequency ultrasound (1–10 MHz) and a range of intensities (0–20 W/cm2). The acoustic energy is focused on the tissue of interest to agitate its matrix and make it more permeable for therapeutic drugs.
- Using to generate cellular effects in soft tissue. This particular application has fallen out of favor as research has shown a lack of efficacy and a lack of scientific basis for proposed biophysical effects. Ultrasound has been used in cancer treatment.
- Cleaning teeth in dental hygiene.
- Low intensity pulsed ultrasound is used for therapeutic tooth and bone regeneration. Researchers have successfully used ultrasound to regenerate dental material.
- Focused ultrasound sources may be used for cataract treatment by phacoemulsification.
- Additional physiological effects of low-intensity ultrasound have recently been discovered, e.g. the ability to stimulate bone-growth and its potential to disrupt the blood–brain barrier for drug delivery.
- Application of focused ultrasound in conjunction with microbubbles has been shown to enable non-invasive delivery of epirubicin across the blood–brain barrier in mouse models.
- Ultrasound is essential to the procedures of ultrasound-guided sclerotherapy and endovenous laser treatment for the non-surgical treatment of varicose veins.
- Ultrasound-assisted lipectomy is lipectomy assisted by ultrasound. Liposuction can also be assisted by ultrasound.
- Doppler ultrasound is being tested for use in aiding tissue plasminogen activator treatment in stroke sufferers in the procedure called ultrasound-enhanced systemic thrombolysis.
- Ultrasound can also be used for elastography. This can be useful in medical diagnoses, as elasticity can discern healthy from unhealthy tissue for specific organs/growths. In some cases unhealthy tissue may have a lower system Q, meaning that the system acts more like a large heavy spring as compared to higher values of system Q (healthy tissue) that respond to higher forcing frequencies. Ultrasonic elastography is different from conventional ultrasound, as a transceiver (pair) and a transmitter are used instead of only a transceiver. One transducer acts as both the transmitter and receiver to image the region of interest over time. The extra transmitter is a very low frequency transmitter, and perturbs the system so the unhealthy tissue oscillates at a low frequency and the healthy tissue does not. The transceiver, which operates at a high frequency (typically MHz) then measures the displacement of the unhealthy tissue (oscillating at a much lower frequency). The movement of the slowly oscillating tissue is used to determine the elasticity of the material, which can then be used to distinguish healthy tissue from the unhealthy tissue.
- Ultrasound has been shown to act synergistically with antibiotics in killing bacteria.
- Ultrasound has been postulated to allow thicker eukaryotic cell tissue cultures by promoting nutrient penetration.
- Ultrasound in the low MHz range in the form of standing waves is an emerging tool for contactless separation, concentration and manipulation of microparticles and biological cells, a method referred to as acoustophoresis. The basis is the acoustic radiation force, a non-linear effect which causes particles to be attracted to either the nodes or anti-nodes of the standing wave depending on the acoustic contrast factor, which is a function of the sound velocities and densities of the particle and of the medium in which the particle is immersed.
- Ultrasound laboratory research based on clinically diagnostic systems is a popular way of making use of a real-time, lower cost (in comparison to MRI and CT) imaging modality for study of biomedical applications and image processing techniques. The ultrasound research interface is a tool that bridges the gap between useful laboratory equipment and a clinical device, and can be used to collect raw data for external or real-time analysis using special algorithms and protocols.
See also 
- Steven Mo, Constantin-C Coussios, Len Seymour & Robert Carlisle (2012). "Ultrasound-Enhanced Drug Delivery for Cancer". Expert Opinion on Drug Delivery 9 (12): 1525. doi:10.1517/17425247.2012.739603.
- Therapeutic Ultrasound: A Promising Future in Clinical Medicine
- Woo, Joseph. "A short History of the development of Ultrasound in Obstetrics and Gynecology". esource Discovery Network, University of Oxford. Retrieved March 12, 2012.
- Watson, T. (2006). "Therapeutic Ultrasound". (see here for a pdf version with the author and date information)
- Wilkin, H. D., et al. (2004). Influence of Therapeutic Ultrasound on Skeletal Muscle Regeneration Following Blunt Contusion. International Journal of Sports Medicine, 25, 73-77.
- A Review of Therapeutic Ultrasound: Effectiveness Studies
- Lewis Jr., George K.; Olbricht, Willam L.; Lewis, George (2008). Acoustic enhanced Evans blue dye perfusion in neurological tissues. Proceedings of Meetings on Acoustics 2 (1). p. 020001. doi:10.1121/1.2890703.
- Lewis, George K.; Olbricht, William (2007). "A phantom feasibility study of acoustic enhanced drug perfusion in neurological tissue". A phantom feasibility study of acoustic enhanced drug delivery to neurological tissue. p. 67. doi:10.1109/LSSA.2007.4400886. ISBN 978-1-4244-1812-1.
- "Acoustics and brain cancer".
- Valma J Robertson, Kerry G Baker (2001). "A Review of Therapeutic Ultrasound: Effectiveness Studies". Physical Therapy 81 (7): 1339–50. PMID 11444997.
- Kerry G Baker; Robertson, VJ; Duck, FA (2001). "A Review of Therapeutic Ultrasound: Biophysical Effects". Physical Therapy 81 (7): 1351–8. PMID 11444998.
- Toothsome research may hold key to repairing dental disasters – ExpressNews – University of Alberta. Expressnews.ualberta.ca. Retrieved on 2011-11-13.
- Carmen, JC; Roeder, BL; Nelson, JL; Beckstead, BL; Runyan, CM; Schaalje, GB; Robison, RA; Pitt, WG (2004). "Ultrasonically enhanced vancomycin activity against Staphylococcus epidermidis biofilms in vivo". Journal of biomaterials applications 18 (4): 237–45. doi:10.1177/0885328204040540. PMC 1361255. PMID 15070512.
- Pitt WG, Ross SA (2003). "Ultrasound increases the rate of bacterial cell growth". Biotechnol Prog. 19 (3): 1038–44. doi:10.1021/bp0340685. PMC 1361254. PMID 12790676.
- Watson, T. (2006). "Therapeutic Ultrasound".[dead link] (see here[dead link] for a pdf version with the author and date information)
- International Society for Therapeutic Ultrasound[dead link]
- Ultrasound Use in Physiotherapy