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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,. Therefore, it is now recognized as a major therapeutic method in treating musculoskeletal disorders,,. 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.

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.

Ultrasound Therapy

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

References

Ultrasound Therapy

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