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Therapeutic Ultrasound By Beverly Nelson 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 ultrasound ? • Sound waves with a Frequency above 20,000 Hz 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/ 15cm • 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 Biologic effects of Ultrasound • 2 classifications; • (1) Thermal effects • (2) non thermal effects- 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 Attenuation • Energy contained within a sound beam decreases as it travels through the tissue 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 antiinflammatory 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 Duty cycle • Selected according to the goal of treatment 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 510 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 Contraindications & precautions • Physical Agents in Rehabilitation – Chapter 7/page 189 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 Method of application • Direct contact • Immersion • Bladder technique Recording Ultrasound treatment • • • • • • • Area of the body treated Ultrasound frequency Ultrasound intensity Duty cycle Treatment duration Technique of application (Read documentation on page 194)