Ultrasound therapy i

1. ULTRASOUND THERAPY I
Aditya Johan .R, M.Fis

2. INTRODUCTION
 Sound is defined as the periodic mechanical disturbance of an elastic medium such as
air
 Sound requires a medium for its transmission and cannot cross a vacuum
 Ultrasound refers to mechanical vibrations, which are essentially the same as sound
waves but of a higher frequency
 Such waves are beyond the range of human hearing and can therefore be called
ultrasonic

3. SOUND FREQUENCIES
 Vibration merges with sound at frequencies around 20 Hz; vibration below this
frequency is often called infrasound or infrasonic
 Audible sound – 20 to 20000 Hz
 Ultrasound – Greater than 20000 Hz
 Infrasound – Less than 20 Hz
 Therapeutic ultrasound – 0.5 to 5 MHz – 1 to 3 MHz

4.  Sonic waves are series of mechanical compressions and rarefactions in the direction of travel of the
wave, hence they are called longitudinal waves
 They can occur in solids, liquids, and gases and are due to regular compression and separation of
molecules

5.  The molecules of all matter are in constant random motion; the amount of molecular
agitation is what is measured as heat – the greater the molecular movement, the greater the
heat
 Sound waves will pass more rapidly through material in which the molecules are close
together, thus their velocity is higher in solids and liquids than in gases

7. PRODUCTION OF THERAPEUTIC US
 Piezo-electric effect: The production of a small electro motive force across certain
substances on being subjected to external pressure
 Reverse piezo-electric effect: Production of mechanical waves or vibrations due to the
application of electro motive force

8. PIEZOELECTRIC EFFECT
 Many types of crystal can be used but the most favored are quartz, which occurs
naturally, and some synthetic ceramic materials such as barium titanate and lead
zirconate titanate (PZT)
 These crystals deform when subjected to a varying potential difference – a piezo-
electric effect
 In order to apply the electric charges, metal electrodes must be fixed to the crystal

9. PIEZOELECTRIC EFFECT
 Piezoelectric effect generated by piezoelectric crystal
 Occurs when an electric current is passed through the crystal
 Crystal expands and contracts at frequencies that produce ultrasound

10. ULTRASOUND PROPERTIES
 Transducer (Sound head) : A crystal that converts electrical energy into
sound
 Power : The amount of acoustic energy per unit time (Watt)
 Intensity : The power per unit era of the sound head (Watt/cm2)

11. BEAM NON UNIFORMITY
 Spatial average intensity: Average intensity of
the US output over the area of the transducer
 Spatial peak intensity: Peak intensity of the
ultrasound output over the area of the
transducer. The intensity is usually great in the
centre of the beam and lowest at the edges
of the beam

12. BEAM NON UNIFORMITY
 Beam non-uniformity ratio (BNR):
Ratio between peak intensity and
average intensity in the beam.
 The lower the BNR the more
uniform the beam
 With BNR 5:1, when the spatial
average intensity is 1W/cm2, the
spatial peak intensity would be
5W/cm2

13.  Continuous ultrasound: continuous delivery of
US through out the treatment period
 Pulsed ultrasound: delivering US only during a
portion of the treatment period.
 Pulsing reduces the thermal effects

14. DUTY CYCLE
 Duty cycle: proportion of the total treatment time
that the US is on.
 This can be expressed in percentage or a ration
 20% or 1:5 duty cycle, is on for 20% of the time
and off for the 80% of time

15. FREQUENCIES
 Frequency: number of compression rarefraction
cycles per unit of time, usually expressed in cycles
per second (Hertz)
 Increasing the frequency of US causes a decrease in
its depth of penetration and concentration of the US
energy in the superficial tissues

16. EFFECTIVE RADIATING AREA (ERA)
 Effective radiating area (ERA): The area of the transducer from which the US energy
radiates. Since the crystal doesn’t vibrate uniformly , the ERA is always smaller than the
area of the treatment head.

17. CONVERGENCE AND DIVERGENCE AREA
 Some waves cancel out, others reinforce so that
the net result is a very irregular pattern of the
sonic waves in the region close to the
transducer face, called the near field or Fresnel
zone.
 Beyond this, the far field or Fraunhofer zone,
the sonic field spreads out somewhat and
becomes much more regular because of the
differing path lengths from points on the
transducer.

18. THE END