Discuss the principles and production of Ultrasound. Explain about the method of Application and a note on effects on tissues.

Discuss the principles and production of Ultrasound.  Explain about the method of Application and a note on their effects on tissues.


Principle of Ultrasound
Ultrasound is a type of sound waves that transmit energy by alternately compressing and rarefying material.
Properties of Waves	
Sonic waves are a series of mechanical compression and rarefactions in the direction of travel of the wave, hence they are called longitudinal waves.
Attenuation of Ultrasound is the gradual reduction in intensity of the ultrasonic beam once it has left the treatment head. 
Production of Ultrasound
Ultrasound is generated by applying a high-frequency alternating electrical current to the crystal in the transducer of an ultrasound unit. The crystal is made of a material with piezoelectric properties, causing it to respond to the alternating current by expanding and contracting at the same frequency. When the crystal expands, it compresses the material in front of it, and when it contracts, it rarefies the material in front of it. This alternating compression-rarefaction produces the ultrasound wave.
Pulsed ultrasound is produced when the high-frequency alternating electrical current is delivered to the transducer for only a limited proportion of the treatment time, as determined by the selected duty cycle. E.g. 20% or 50%.
The method of application of Ultrasound
Frequency
The frequency is selected according to the depth of tissue to be treated. 1 MHz is used for tissue up to 5 cm deep, , and 3 MHz is used for tissue 1 to 2 cm deep.
Pulse Ultrasound
When the goal is to increase tissue temperature, a continuous duty cycle should be used. Ultrasound is applied where only the nonthermal effects without tissue heating are desired, pulsed ultrasound with a 20% or lower duty cycle should be used.
Intensity
When the goal is to increase tissue temperature, the patient should feel some warmth within 2 to 3 minutes of initiating ultrasound application.
When 1 MHz frequency ultrasound is used, an intensity of 1.5 to 2.0 W/cm2 is used. When 3 MHz frequency is used, an intensity of about 0.5 W/cm2 is generally sufficient.
Duration
Treatment duration is selected according to the treatment goal, the size of the area to be treated, and the effective radiating area of the sound head. For most thermal or nonthermal applications, ultrasound should be applied for 5 to 10 minutes for each treatment area that is twice the effective radiating area of the transducer.
Number and Frequency of Treatments
The recommended number of treatments depends on the goals of treatment and the patient’s response. If the patient is making progress, treatment should be continued. In most cases, an effect should be detectable within 1 to 3 treatments.
Thermal level ultrasound is usually applied only during the subacute or chronic phase of healing, when treatment 3 times a week is recommended; ultrasound at nonthermal levels may be applied at earlier stages, when treatment may be as frequent as daily. 
Sequence of Treatment
In most cases, ultrasound may be applied before or after other interventions; however, when ultrasound is used to heat tissue, it should not be applied after any intervention that may impair sensation, such as ice. Also, when thermal level ultrasound is used to increase collagen extensibility to maximize the increase in length produced by stretching, the ultrasound must be applied directly before and, if possible, during application of the stretching force. 
Moving the Head
The sound head is moved at approximately 4 cm/second— quickly enough to maintain motion and slowly enough to maintain contact with the skin. 
The pattern of movement can be a series of overlapping parallel strokes, circles or figures of eight.
Techniques of Application
1)Direct contact Method
If the surface to be treated is fairly regular then a coupling medium is applied to the skin.
2)Water Bath Method
When direct contact is not possible because of irregular shape of part or because of tenderness, a water bath may be used. As the part to be treated is immersed in water this can only reasonably be applied to the hand, ankle and foot. Degassed water is used if possible. 
Ordinary tap water presents the problem that gas bubbles dissociate out from the water, accumulate on the patient skin and the treatment head, and reflect the US beam. If tap water has to be used then the gas bubbles must be wiped from these surfaces frequently.
The treatment head is held 1 cm from the skin and moved in small concentric circles, keeping the front parallel to the skin.
3)Water bag method
Ultrasound therapy to irregular surface which cannot conventionally be placed in a water bath is treated with a plastic or rubber bag filled with water forming a water cushion between the treatment head and the skin.
A coupling medium has to be placed both between the rubber bag and skin and between the rubber bag and the treatment head to eliminate any air.
Effects of Ultrasound on tissues
Ultrasound has a variety of biophysical effects. It can increase the temperature of deep and superficial tissues and has a range of nonthermal effects.
THERMAL EFFECTS 
The thermal effects of ultrasound, including acceleration of metabolic rate, reduction or control of pain and muscle spasm, alteration of nerve conduction velocity, increased circulation, and increased soft tissue extensibility. 
Ultrasound heats tissues with high ultrasound absorption coefficients are generally those with high collagen content, Thus ultrasound is particularly well suited to heating tendons, ligaments, joint capsules, and fasciae while not overheating the overlying fat.
NONTHERMAL EFFECTS
These effects are the result of the mechanical events produced by ultrasound, including cavitation, microstreaming, and acoustic streaming.
Ultrasound has shown to increase intracellular calcium levels and to increase skin and cell membrane permeability.
Ultrasound increases mast cell degranulation and the release of chemotactic factor and histamine. Ultrasound also promotes macrophage responsiveness and increases the rate of protein synthesis by fibroblasts and tendon cells.