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Ultrasound Therapy; Types, Frequency, Medical Uses

Ultrasound Therapy is one of the most widely used physical modalities in the clinical practice of rehabilitation. In particular, therapeutic ultrasound in rehabilitation has a number of uses including the treatment of musculoskeletal disorders such as pain, muscle spasm, joint contracture, and tissue injury^[rx^,^rx]. Therefore, it is now recognized as a major therapeutic method in treating musculoskeletal disorders^,^[rx^,^rx]. Essential treatment parameters for therapeutic ultrasound include frequency, intensity, duty cycle, treatment time, and treatment area. The frequency for therapeutic ultrasound ranges from 1 to 3 MHz, with 3 MHz used specifically for the treatment of superficial tissues, and 1 MHz is applied to treat deeper tissues^[rx].

Ultrasound therapy is a popular method of treatment for chiropractors, physical therapists, and other medical care providers. It involves transferring sound waves with frequencies greater than the human sound spectrum (above 20 kilohertz) into a patient. The energy that is transferred with the sound waves can be used to treat focused, isolated areas of tissue to help with relieving pain in affected areas of the body and to speed the recovery process for injured muscle or other tissues.

Types of Ultrasound Therapy

There are two main types of ultrasound therapy: thermal and mechanical. Both use sound waves generated by a transducer head (which looks a bit like a microphone) to penetrate soft tissues. The difference between the two types of ultrasound therapy is the rate at which the sound waves penetrate the tissues.

Thermal ultrasound therapy – Thermal ultrasound therapy uses a more continuous transmission of sound waves. The sound waves cause microscopic vibrations in the deep tissue molecules, increasing heat and friction. The warming effect encourages healing in the soft tissues by increasing the metabolism at the level of the tissue cells.
Mechanical ultrasound therapy – Mechanical ultrasound therapy uses pulses of sound waves to penetrate tissues. While this still has a minor warming effect on the tissues, it also causes expansion and contraction in the tiny gas bubbles of the soft tissues. This helps to decrease the inflammatory response, reducing tissue swelling and thus decreasing pain.

Frequency

The number of times a particle experiences a complete compression/rarefaction cycle in 1 second. Typically 1 or 3 MHz.

Wavelength

The distance between two equivalent points on the waveform in the particular medium. In an ‘average tissue’ the wavelength @ 1MHz would be 1.5mm and @ 3 MHz would be 0.5 mm.

Velocity

The velocity at which the wave (disturbance) travels through the medium. In a saline solution, the velocity of the US is approximately 1500 msec-1 compared with approximately 350 msec-1 in the air (sound waves can travel more rapidly in a more dense medium). The velocity of the US in most tissues is thought to be similar to that in saline.
These three factors are related but are not constant for all types of tissue. Average figures are most commonly used to represent the passage of US in the tissues. Typical US frequencies from therapeutic equipment are 1 and 3 MHz though some machines produce additional frequencies (e.g. 0.75 and 1.5 MHz) and the ‘Longwave’ ultrasound devices operate at several 10’s of kHz (typically 40-50,000Hz – a much lower frequency than ‘traditional US’ but still beyond human hearing range.
The wavelength of Ultrasound Therapy – 0.15cm
The frequency of Ultrasound Therapy – 1mHz or 3mHz

The intensity of Ultrasound Therapy

Low intensity – 0.3watt/cm²
Medium intensity – 0.3-1.2watt/cm²
High intensity – 1.2-3watt/cm²

Medical Uses of Ultrasound Therapy

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 elastic 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.
Cleaning teeth in dental hygiene.
Focused ultrasound sources may be used for cataract treatment by phacoemulsification.
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).

Enhanced drug uptake using acoustic targeted drug delivery(ATDD).

Delivering chemotherapy to brain cancer cells and various drugs to other tissues is called acoustic targeted drug delivery (ATDD).^[rx] 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.^[rx]^[rx]
Ultrasound has been used to trigger the release of anti-cancer drugs from delivery vectors including liposomes, polymeric microspheres and self-assembled polymeric.^[rx]
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 Liposuction assisted by ultrasound.

A listing of FDA approved modes for ultrasound therapy.

Therapy Method	Therapeutic Outcome	Bioeffect Mechanism	Device Characteristics			General Reference
Therapy Method	Therapeutic Outcome	Bioeffect Mechanism	Applicator	Frequency	Delivery	General Reference
Unfocused beam	tissue warming	heating	portable hand-held	1–3 MHZ	continuous or repeated bursts	[rx]
Hyperthermia	cancer therapy	regional heating	multi-element applicator	1–3.4 MHz	1 hour	[rx]
HIFU	uterine fibroid ablation	thermal lesion	computer directed	0.5–2 MHZ	long bursts	[rx]
HIFU	glaucoma relief	permeabilization	fixed probe with waterbath	4.6 MHZ	1–3 s	[rx]
HIFU	laproscopic tissue ablation	thermal lesion	hand-held	4 MHz	long bursts	[rx]
HIFU	laparoscopic or open surgery	thermal lesion	hand-held	3.8–6.4 MHz	long bursts	[rx]
Focused ultrasound	skin tissue tightening	thermal lesion	hand-held, imaging and treatment	4.4–7.5 MHz	20–50 ms bursts	[rx]
Extracorporeal Lithotripsy	kidney stone comminution	mechanical stress; cavitation	mainframe with image guidance	~150 kHz	shockwaves	[rx]
Intracorporeal lithotripsy	kidney stone comminution	mechanical stress; cavitation	Percutaneous probes	25 kHz	continuous	[rx]
Extracorporeal shockwave therapy	plantar fasciitis epicondylitis	unknown	mainframe with the applicator head	~150 kHz	shockwaves	[rx]
Phacoemulsification	lens removal	vibration; cavitation	generator with probe	40 kHz	continuous	[rx]
US-assisted liposuction	adipose tissue removal	fat liquefaction; cavitation	generator with probe	20–30 kHz	continuous	[rx]
Tissue cutting and vessel sealing	laparoscopic or open surgery	thermal lesion, vibration	hand-held	55.5 kHz	continuous	[rx]
The intravascular US	thrombus dissolution	unknown; gas body activation	intravascular catheter	2.2 MHZ	continuous	[rx]
Skin permeabilization	transdermal drug delivery	unknown	hand held	55 kHz	continuous	[rx]
Low intensity pulsed US	bone fracture healing	unknown	attached transducer	1.5 MHz	pulsed, long duration	[rx]