Purpose of Presentation
• To provide a basic foundation of the physics involved in therapeutic ultrasound(US)
• Analyse the transmission of sound energy in biologic tissue
• Explain the thermal and non thermal effects of US
• Choose the most appropriate and clinically effective uses for US
What is Frequency ?
• The number of oscillations a molecule undergoes in one second defines the frequency of sound waves.
• Frequency is expressed in Hertz (Hz)• 1MHz – deep penetration/deep heating/ 1-
5cm• 3MHz– Superficial penetration/superficial
heating/1-3cm
Generation of ultrasound
• Ultrasound can be generated with special crystals that possess the property of piezoelectricity and are able to vibrate very rapidly
• There are 2 forms of piezoelectric effect, direct and indirect or reverse
Piezoelectric property
• The direct piezoelectric effect is the generation of an electric voltage across a crystal when the crystal is compressed
• The reverse piezoelectric effect is the contraction or expansion of a crystal to a voltage applied across its face. Thus ,the reverse piezoelectric effect is used to generate ultrasound at a desired frequency
Thermal effects
• Increase temperature by 1 C , increases ̊�metabolism & healing
• 2- 3 C decreases pain and muscle spasm ̊�• 4 C -- greater increase in extensibility of ̊�
collagen and decreases joint stiffness• Greater than 4 C temperature elevation, ̊�
damage the tissues
Non thermal effects
• Results from cavitation and microstreaming• Modifies the inflammatory response• Enhance the healing of injured tissues
Effective Radiating Area(ERA)
• The total area of the surface of the Transducer that actually produces the sound wave
Pulsed versus Continuous wave
• When continuous wave ultrasound is used, the sound intensity remains constant throughout the treatment
• When pulsed ultrasound wave is used , the intensity is periodically interrupted with no ultrasound energy being produced during the off period.
Duty cycle
• Is selected according to treatment goal• To increase tissue temperature a 100%
(continuous) duty cycle is used• When non thermal effects are the goal, pulsed
ultrasound with a 20% or 50% duty cycle should be used
Intensity
• The strength of the sound beam is determined by its intensity
• Intensity is expressed in units of watts per square centimetre(w/cm²)
• Intensities employed in PT range from 0.25w/cm² - 1.5w/cm²
• Higher intensities results in greater tissue temperature increase
Clinical implications of Ultrasound
• Soft tissue healing and repairs• Scar tissue and joint contracture• Stretching of connective tissue• Absorption of calcium deposits• Bone healing• Pain reduction• Assessing stress fracture• Chronic inflammation
Phonophoresis
• A technique in which ultrasound is used to enhance delivery of selected medication into the tissues
• Most common medications used are anti-inflammatory drugs ,e.g. hydrocortisone 1% or 10% , dexamethasone
Treatment parameters
• Frequency of treatment• Duty cycle• Intensity • Area to be treated• Duration of Treatment• Coupling methods• Number & frequency of treatments• Moving the sound head
Frequency
• Is selected according to the depth of tissue to be treated
• Tissue at a depth of 5cm- 1MHz is used• Superficial tissue to a depth of 1-2cm- 3MHz is
used• Acute conditions may use low intensity pulsed
US ,daily
Intensity
• Selected according to the goal of treatment• To increase tissue temperature, the patient
should feel mild warmth within 2-3 minutes of starting application
Treatment area
• Recommended that area to be treated be roughly twice the size of the sound head for 5- 10 minutes
• Larger areas will need longer time• Higher intensity applied in w/cm – shorter
time and vice versa
Duration of treatment
• Selected based on size of area to be treated• The ERA of the sound head• Whether thermal or non-thermal application• 5 – 10 minutes duration for an area twice the
size of the sound head• Little research defining the duration
Coupling medium
• Purpose of the medium is to exclude air from the region between the patient and the transducer so that the sound waves get to the targeted area
• Transfer of sound energy by conversion requires an intervening material to be a good transmitter of sound energy
• Reflection is reduced at the air-tissue interface
Coupling medium
• Should be viscous enough to act as a lubricant as the transducer moves over the surface of the skin
• Should be applied to the skin surface and the transducer should be in contact with the coupling medium before the power is turned on
• If the transducer is not in contact with the skin & coupling medium , the piezoelectric crystal may be damaged and the transducer can overheat
Coupling medium
• Water is an effective coupling medium• Water – soluble gels have the most desirable
properties for a good coupling medium• Salts in the gel may damage the metal face of
the transducer with improper cleaning
Moving the transducer/sound head
• Due to the nonuniformity of the ultrasound beam ,energy distribution is uneven in the tissue
• Potential for creating tissue- damaging “hot spots”
• Moving the transducer during treatment leads to a more even distribution of the energy
Moving the transducer/sound head
• Overlapping circular motions or longitudinal stroking pattern can be used
• Slow movement of the transducer is recommended
Number & frequency of treatments
• Depends on goals of treatment and patient’s response
• An effect should be detectable within 1 to 3 treatments
• If no improvement after 3 or 4 applications, US should be discontinued or the parameters adjusted
Suggested Readings
• Physical Agents in Rehabilitation by Michelle H. Cameron
• Thermal Agents in Rehabilitation by Susan L. Michlovitz
• Therapeutic Modalities in Rehabilitation by William E.Prentice
Number & frequency of treatments
• It is recommended that US be limited to 14 treatments.
• Not documented ,scientifically