Sound is the mechanical disturbance of an elastic medium such as air. Vibrations which travel through the air or another medium are called as sound. The frequency of hearing sound is 20Hz – 20,000Hz.
Ultrasound means the above the frequency of sound or the sound waves which are produced above the 20,000 Hz.
Therapeutic Ultrasound

Production of Ultrasound:

For a one mega Hz frequency machine vibrating source with a frequency of one million pulses/s (million cycles/s) is needed. This is achieved by using either quartz crystal or barium crystal. These crystals deformed when subjected to various potential differences (Piezoelectric effect). The basic components of Ultrasound apparatus are the following.
o   Source if high frequency current
o   Cable
o   Link electrodes
o   Barium or Quartz crystal
o   Metal plate


Source of high frequency current which is conveyed via co – axial cable to a transducer circuit or ultrasound head. Inside the ultrasound head the high frequency current is applied to a quartz or barium crystal through link electrode, the meal plate being fused with the quartz crystal. Any change in the shape of quartz crystal causes movement of the metal plate which in turn produces ultrasound waves. Strict control of high frequency current (1 MHz or 3 MHz) produces a steady or regular ultrasound waves.

Parameters of ultrasound:

Intensity (watt):

The unit of intensity when using ultrasound is watt. But this is a gross measure of power transmitted by the treatment head.  So, an average intensity will be used. There are two types of intensities.

Space average intensity:

The average intensity over a specified area is given (watt/cm²). This is used for continuous ultrasound.

Time average/space average intensity:

It can be given when ultrasound being applied in a pulsed mode and gives average intensity over the whole treatment time for a specified area e.g. if 0.5 watt/cm² is applied in a pulsed mode at 1 : 4 then in 1 second, the average intensity will be 0.1 watt/cm², if ultrasound is continuous.

Reflection of ultrasound:

Sound waves obey the laws of reflection and if an ultrasound beam travelling through one medium passes to another medium (encounter) which will not transmit this ultrasound beam, reflection will take place. For example, air will not transmit the ultrasound waves. So in ultrasound treatment care should be taken that air will not left between the treatment head and the body surface to minimize reflection.
However, there will be some reflection at interface that ultrasound beam encounter; this will give rise to acoustic impedance. If acoustic impedance is low transmission will be high and vice versa.

Transmission of ultrasound:

If the ultrasound beam encounter between two media and is transmitted it may be refracted deflection from original path when travelling from one medium in which its velocity is low to a medium having high velocity, it might be refracted from its normal path.
Significance of refraction is that if x is a target refraction would cause the ultrasound beam to miss it. Refraction will not occur when the incident wave travelling along the normal direction, treatment should be given with majority of wave travelling along the normal direction. Refraction will not occur if the ultrasound head is used perpendicular to the body surface.

Attenuation of ultrasound:

Reduction in the intensity of ultrasound beam once it has left the ultrasound head is called attenuation of ultrasound. Reduction may occur by the following two processes.
·         Absorption
·         Scatter
Ultrasound beam are absorbed and converted into heat and reduced its intensity is called absorption. This occur when normal ultrasound beam is deflected from its path by refraction i.e. air bubbles in the tissues. The overall affect of these two as such that the ultrasound beam is reduced in intensity, the deeper it passes, this give rise to the expression of half values distance  which the depth of the soft tissues that reduces the ultrasound beam to half its surface intensity. The half value distance for soft tissue is different for different frequencies i.e. for 1MHz is 4cm and for 3MHz is 2.5cm. In practice (treatment practice) when treating deep structures consideration needs to be given to the frequency and intensity of ultrasound.

Coupling media:

Ultrasound cannot be transmitted through air therefore medium is used for it. 100 %waves cannot be transmitted but only %age can be transmitted through skin having medium. The following are the media which transmit the ultrasound waves.

Aqua sonic gel
72.6 %
67 %
Distal water
59 %
Liquid paraffin
19 %
0 %
0 %

Air will in fact reflect the ultrasound waves back into the treatment head and this could set up the standing waves which might damage the crystal.

Physiological effects of ultrasound:

Thermal effects
Non thermal effects

Thermal effects:

The heat absorbed by the tissue depends upon the following.
a.       Absorption of tissue:
Protein absorbs more heat. So, tissue having more protein will absorb heat.
b.       Insonated tissues:
Those tissues which are exposed to ultrasound produce much heat which have power of increase circulation.
c.       Number of treatment sessions
d.       Continuous produce ultrasound
e.      Periosteum will reflect the ultrasound waves leading to standing waves. It causes burning sensation in bony area. So avoid ultrasound on bony prominence.

Non thermal effects:

        i.            Mechanical effects (micro-massage):

This is where the longitudinal compression waves of the ultrasound produce compression/rarefaction of the cells and effects the movement of the tissue fluid in the interstitial space. This can help to reduce the oedema combined with the thermal effect, the extensibility of the scar and adhesions could be affected in such away to make stretching them easier.

      ii.            Biological effects:

It helps to reduce inflammation.
·         Inflammation: ultrasound increases the fragility of lysosome and thus enhances the release of their enzymes. These enzymes will help to clear of debris (waste product) and allow the next stage to occur.
·         Proliferative stage: Fibroblast and myofibroblast may have calcium ions driven into these injured area and the collagen fibers are formed. This is called proliferative stage.
·         Remodeling stage

Uses of ultrasound:

Recent injury and inflammation:

Mechanical effect causes the removal of exudates. Thermal effects accelerate protein synthesis which helps in the repair of the damage tissue.

Chronic scars:

The thermal effect of the ultrasound is also use for the removal of chronic scar by stretching the tissues.

Chronic oedema:

Mechanical effect and thermal effect of ultrasound help in the removal of oedema.

Dangers of ultrasound:


If the continuous ultrasound is used with stationary head will cause burn in the tissue. When ultrasound is use o bony prominence, the waves cannot be absorb by the Periosteum and reflect them causing burn in the underlying tissues.

Damage to the equipment:

When the head of ultrasound treatment remains in air will produce standing waves which damage the treatment head crystal of ultrasound.

Contraindications of ultrasound:

·         Thrombophlebitis
·         Acute sepsis (the presence of pus-forming bacteria or their toxins in the blood or tissues)
·         Tumors
·         Radiotherapy (up to 6 month) has devitalizing effect on the tissue so ultrasound is contraindicated
·         Pregnancy
  • Cardiac diseases