Therapeutic Ultrasound in Physiotherapy

Ultrasound is a mechanical pressure wave or mechanical vibration resonating at a high frequency beyond the upper limit of human hearing. The physiotherapist has been using it Since the 1940s. One of the greatest proposed benefits of ultrasound is that it increases the healing process of certain soft tissue injuries. This therapy is used for reducing pain, muscle spasms, and joint contractures. 

Meaning of Ultrasound

  • Ultra means excessive or extreme.
  • Sound means something that can you hear.
  • Sound is the periodic mechanical disturbance of one elastic medium.
  • The normal human sound range is from 16Hz to something 20KHz (20,000 cycles per second).
  • Ultrasound is mechanical vibration, beyond this upper limit
  • Therapeutic frequencies of ultrasound are in the region of 1MHz or 3MHz.

Production of Sound Waves

Production of sound waves

Sound waves are vibrations that travel through the air or any other medium, like water or solids. They move in a back-and-forth motion, and we call this motion “longitudinal.” To make sound waves, we need something that vibrates, like a tuning fork, which shakes the air molecules around it. These vibrations create a series of pressure waves that move through the air and cause the molecules to alternate between being pushed together (compression) and pulled apart (rarefaction).

Factors Related to Waves


The number of times a particle undergoes a complete cycle (compression and rarefaction) in one second.



It is the distance between the two closest points on the waveform in the particular medium.


The velocity of a wave is the speed at which the wave moves through the medium. It varies depending on the physical nature of the medium.

Production of Ultrasound

The Ultrasound instrument consists of a high-frequency generator. This is connected to a Piezoelectric crystal (treatment head or transducer circuit) by a co-axial cable. for the production of ultrasound waves.1 MHz or 3MHz frequency is needed When these varying potential differences or frequencies are applied to quartz crystal or barium titanate crystal, via a linking electrode, the crystal is fused to the metal front plate of the treatment head. any changes in the shape of the crystal(Compressed and Relaxed), produce an ultrasonic wave. it generates ultrasound waves by a Piezoelectric effect.

The piezoelectric effect refers to the application of an electric field to a crystal, which causes a realignment of the internal dipole structure. this realignment results in crystal lengthening or contraction, converting electrical energy into kinetic or mechanical energy. This is how the ultrasound transducer produces sound waves.

Continuous and Pulsed ultrasound

  • Most ultrasound equipment can generate both continuous and pulsed ultrasound energy.
  • In continuous mode, the treatment head continuously produces ultrasonic energy.
  • In pulse mode, the period of ultrasound is separated by a period of silence.
  • A pulsed ultrasound setting will be used, When a thermal effect is not desired, such as during acute inflammation or around a bony surface,

Duty Cycle (Pulse Mark: Space Ratio)

When ultrasound is applied in its pulsed mode, the ratio of the time on to time off should be expressed. This is the mark: space ratio(M:S) or duty cycle. Mark being the time ultrasound is on, space being the time silence. M: S Ratio has a variable range of 1:1, 1:2, 1:3, 1:4, 1:7.

Ultrasound Beam, Far Field & Near Field

  • The US beam is not uniform and changes in its nature with distance from the transducer.
  • If it is nearest the treatment head is called the Near field, the Interference field, or the Frenzel zone.
  • If it’s beyond the near field, away from the transducer head is called the far field or the Fraunhofer zone.
  • Near Field=r²/λ.  
  • Here r= the radius of the ultrasound head and λ = the US wavelength
  • Wavelength(λ) and Frequency(f) are inversely related.
  • Near Field=r²×f
  • The depth of the near field varies with the frequency of ultrasound.
  • For 1 MHz frequency, near field less than 3 MHz and vice versa.

ultrasound field

Reflection of Ultrasound

Ultrasound follows the laws of optics. If an ultrasonic beam traveling through one medium encounters another medium that will not transmit it, reflection takes place.

Air will not transmit ultrasonic waves, so in ultrasonic treatment great focus to avoid leaving air between the treatment head and patient, to minimize reflection.

However, there will always be some reflection at each interface that the ultrasound beam encounters. this gives rise to the term ACOUSTIC IMPEDANCE(Z)

Acoustic Impedance (Z) – It is the ratio between the reflected and transmitted ultrasound at an interface. If the acoustic impedance is low, the transmission is high, and vice versa.

Attenuation of Ultrasound

The intensity of ultrasound waves decreases as they travel through tissue, it’s known as attenuation. it is the progressive loss of acoustic power. Two major factors contribute to attenuation absorption and scatter

Absorption –

Ultrasound is absorbed by the tissue and is converted to heat at the point. absorption of ultrasound takes place at a molecular level. the tissues with the higher protein content will absorb the US to a greater extent.

us absorption in tissue
Fig: Absorption of ultrasound in tissues


Scattering of ultrasound in the body occurs due to two phenomena, divergence in the far field and reflection. Especially because of reflection, the ultrasound beam may spread in the body, so that effect can develop not only in the direction of the sound beam but also outside it.
The ultrasonic beam is reduced in intensity the deeper it passes. this gives rise to the expression ‘HALF VALUE DISTANCE’ . which is the depth of soft tissue that reduces the ultrasonic beam to half its surface intensity.

Coupling media

Ultrasonic waves are not transmitted by air. some couplant used to improve contact and reduce friction between the transducer and skin during ultrasound treatments. example- Aquasonic gel.
Air is wholly unsuitable as a contact medium, because of the almost complete reflection of the ultrasound.

Effects of Ultrasonic Waves on Tissue

The effects of Ultrasonic Waves effect on Tissue can be categorized in the following three parts-

  1. Thermal effects
  2. Non-thermal effects
  3. Biological effects

1) Thermal effects of ultrasound-

 When ultrasonic waves are absorbed by the tissues, they generate heat in the tissue. The amount of heat developed depends upon:

  • Components of the tissue- protein absorbs ultrasound efficiently and therefore produces much heat.
  • The number of times the treatment head passes over the part.
  • The efficiency of the circulation of the part.
  • When using continuous ultrasound, the amount of heat developed,
  • The amount of heat developed is directly proportional to the intensity and duration of insonation.
  • Thermal effect = pulsed < continuous.

2) Non-thermal effects of ultrasonic waves-

Non-thermal effects of ultrasound are Cavitation, Acoustic streaming, Standing waves, and Micro massage.

a) Cavitation:

In the simplest sense, it relates to the formation of gas-filled voids within the tissue and body fluids as a result of ultrasound vibration. There are two types of cavitation stable and unstable.

Stable cavitation- it is not dangerous and could be beneficial as it causes micro-streaming or Acoustic streaming, where the permeability of cell membranes and the direction of movement of molecules into a cell in influenced.

Unstable cavitation – It is potentially dangerous to the tissue. as the collapse of the bubbles caused a great local rise in temperature. it is avoided by moving the treatment head, using a low intensity, and using a high frequency (1 or 3 MHz).

b) Acoustic streaming:

This is the unidirectional movement of a fluid, in an ultrasound field. This is a steady circular flow of cellular fluid. It stimulates cell activity if it occurs at the boundary of the cell membrane. which results in therapeutically advantageous changes needed for repair, such as; increase protein synthesis, increased secretion from the mast cells, fibroblast motility changes, and increased uptake of the second messenger calcium.

c) Standing waves:

Standing waves are also called stationary waves. A combination of two waves moving in the opposite direction, each wave having the same amplitude and frequency. In ultrasound also standing wave occurs when the reflected wave superimposes on the incident waves.

Standing wave fields can cause transient cavitation and consequently, the formation of free radicals, so it’s important to minimize the hazards of it the therapist moves the ultrasound head continuously.

d) Micro massage-

When an ultrasonic beam applies it produces compression-rarefaction of cells and affects the movement of tissue fluid in interstitial spaces. This can help reduce edema.

3) Biological Effects-

Ultrasound accelerates the normal resolution time of the inflammatory process. ultrasound can have a useful effect in all three stages of repair.

a) Inflammatory Stage:

Ultrasound increases the fragility of lysosomes membrane and thus enhances the release of their contained enzymes, these enzymes will help to clear the area or debris and the body begins to put new collagen in the area of injury.

b) Proliferative Stage:

Ultrasound increases the mobility of fibroblasts and myofibroblasts and encourages their movements toward the area of repair. To form, the scar the fibroblasts are stimulated to produce collagen fibers, and myofibroblast contract to pull the edge together. The U.S. accelerates the rate of angiogenesis.

c) Remodeling Stage:

Ultrasound increases the tensile strength of the scar by affecting the direction, strength, and elasticity of the fibers, which make up the scar.

Uses of Ultrasound(Indications)

  • Relief of pain
  • Muscle spasm
  • Joint contracture
  • Adhesive capsulitis
  • Calcific bursitis
  • Myositis
  • Tendinopathy
  • Ligament sprains
  • Chronic indurated edema
  • Iontophoresis
  • Soft tissue injuries from sports or other causes


  • Burn
  • Cavitation
  • Overdose (excessive treatment may cause an exacerbation of symptoms)
  • Damage to equipment (when the treatment head is held in the air, while switched on)


  • Tumour
  • Pregnancy
  • Vascular condition (thrombophlebitis)
  • Acute sepsis
  • Radiotherapy ( at least six months after radiation
  • Cardiac pacemaker (not treated with u.s. in the area of the chest)
  • Direct application to the EYE, BRAIN, SPINAL CORD, HEART, and REPRODUCTIVE ORGAN.

Techniques of Application

Direct contact:

Apply ultrasound by direct contact method

This method may be used when the treated surface is regular. Then apply the coupling medium between the skin and the transducer head to avoid air. Move the ultrasound head in a circular movement for transmitting the ultrasonic beam to the tissue.

Water bath application:

This method may be used When direct contact is not possible due to irregularity of the part or tenderness at the site. this method is used on distal parts of the body such as the hand, foot, ankle, etc.

Water bag therapy:

  • Uneven surface
  • Skin lesion
  • Traumatic condition e.g. sprain and strain of the ligament, tendon, or muscle. usually start 24 -48 hours after the occurrence, to allow time for healing injured capillaries.