Therapeutic Ultrasound

ByBeverly 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)