Ultrasound

• Ultrasound (US)


  A form of acoustic vibration propagated in the form of longitudinal compression waves at frequencies too high to be heard by human ear
• Ultrasound


  • Produces longitudinal mechanical waves
  • Frequencies below 20 Hz are infrasonic, 20 Hz - 20,000 Hz are audible, above 20,000 Hz - 10 GHz are ultrasonic
• Longitudinal waves


  Cause the particles of the medium to oscillate to and fro in the direction of the propagation of the wave, giving rise to alternate conditions of compression and rarefaction (relaxation)

  
• Ultrasound in physical therapy

  One of the deep heating modalities used by the therapist, type of heat transmission is conversion (converts sound waves into thermal energy)
• Physical therapy utilizes the following ultrasound frequencies: 0.75 MHz, 0.87 MHz, 1 MHz, 1.5 MHz, 3 MHz
• Piezoelectric effect


  When a quartz crystal is stressed, a potential difference is produced across its surfaces
• Reverse piezoelectric effect


  A high frequency alternating current is introduced for the vibration of quartz crystal and ultrasound is produced, this is the principle wherein all PT ultrasound generators are constructed
• Components of ultrasound generators

  • Power supply
  • Oscillating circuit
  • Transducer head/treatment head/probe/applicator head
• Transducer head


  • Contains crystal inserted between two electrodes or metal electrode and metal end plate, many machines use barium titanate crystals
• Production of ultrasound


  Alternating current applied to crystal makes it vibrate and produce soundwaves from the transducer
• Reflection


  Occurs when there is acoustic mismatch between two media as ultrasound passes through, acoustic mismatch occurs when each medium has different acoustic impedance

  
• Acoustic impedance


  Z = PV, where Z is acoustic impedance, P is density of tissue, and V is velocity of propagation
• Refraction


  Ultrasound energy would be refracted as it passes to another medium, only at angles of incidence less than 15 degrees will any energy pass into the tissues
• Factors affecting absorption of ultrasound

  • Acoustic impedance of tissues
  • Propagation velocity of sound
  • Density of tissues
  • Frequency of ultrasound
  • Protein content
  • Fat and water content
  • Angle of incidence
  • Viscosity of fluid
  • Reflection
  • Refraction
• Attenuation


  Progressive loss of acoustic power as ultrasonic energy travels through a medium as result of absorption, it is directly proportional to the frequency and inversely proportional to the wavelength

  
• Micromassage (mechanical effect)


  Alternate compression and relaxation of tissues by pressure of sound waves
• Oscillation of particles


  Due to oscillations of particles, this can cause friction which can produce thermal effects
• Cavitation


  Dissolved gases present in biological media produce cavities during rarefaction which can collapse during compression, creating high concentration of energy, occurs more readily in fluids of low viscosity and low cellular content, and more likely at lower frequencies
• Stable cavitation


  Cavities don't collapse
• Unstable/transient cavitation


  Cavities collapse, producing high concentration of energy, more dangerous
• Near field (Fresnel zone)


  Propagated unidirectionally as non-uniform beam of acoustic energy, distribution of power has maxima and minima due to interference between adjacent wave fronts
• Constructive interference


  Two or more waves arrive at a point in phase, combining their energy
• Destructive interference


  Two or more waves arrive at a point out of phase, cancelling each other out
• Far field (Fraunhoffer zone)


  Homogeneous or uniform field of the transducer head, located at the periphery
• Beam Non-uniformity Ratio (BNR)


  Ratio of maximum point intensity to average intensity across transducer surface, normal value is 2-6, lower BNR means more even energy distribution
• Sound beam is cylindrical in shape, propagation does not occur in vacuum, beam spread is inversely proportional to transducer head diameter and frequency
• Continuous ultrasound


  Energy is continuously radiated towards tissues, more thermal effect, maximum duration 10 minutes
• Pulsed ultrasound


  Energy is intermittently radiated towards tissues, thermal effects reduced but pressure and amplitude remain unaltered, maximum duration 15 minutes
• Mark space ratio


  Pulsed ultrasound expressed as a ratio (e.g. 1:5)

  
• Duty cycle ratio


  Pulsed ultrasound expressed as a percentage = on phase / (on + off phase) x 100
• Intensity/Power


  Expressed in watts or watts/cm2, calculated as energy output of transducer head / effective radiating area (ERA) of transducer head
• Greatest dose is 3 watts/cm2, commonly 1.5-2 watts/cm2
• Frequency


  Everyday or every other day, should not exceed 2 courses of treatment sessions if patient is not improving
• No need to progress dosage, if progression is necessary it should be stabilized within 2-3 treatments
• Duration


  10 minutes maximum for continuous US, 15 minutes maximum for pulsed US, longer duration for larger treatment areas
• Stationary technique


  Transducer head is fixed and steady during treatment, can be used for small localized areas with small dosages, more danger for cavitation
• Stroking technique


  Transducer head is moved slowly and steadily over the treatment area, reduces risk of cavitation
• Transducer head


  ERA (effective radiating area of transducer head)
• 15 watts (power output); ERA = 5 cm2
• Greatest dose


  • 3 watts/cm2; commonly is 1.5 – 2 watts/cm2