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/ 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

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 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

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 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

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)