ULTRASOUND Chapter 7. What is Ultrasound? US is a type of sound wave that transmits energy by alternately compressing and decompressing (rarefying) material.

ULTRASOUND

Chapter 7

What is Ultrasound?

• US is a type of sound wave that transmits energy by alternately compressing and decompressing (rarefying) material.

How does US transfer thermal energy?

• Heat is transferred by conversion of soundwaves. Mechanical energy is converted into heat.

• When using US energy is absorbed 2 to 5 cm

• As US travels thru tissue it decreases in intensity

Generation of US

• US is generated by applying high freq AC (plug in the wall) to the crystal in the transducer.

• The crystal is made of material with piezoelectric properties .

• The AC current causes the crystal to expand (compress molecules) and contract (molecules rarefy or spread apart)

Compression >>rarefaction = complete cycle of a soundwave (fig 7-3, p 178)

Ultrasound

• US can produce a variety of physical effects. They can be classified as thermal and nonthermal.

Thermal Effects: caused by vibration of molecules that bounce into each other cause a release in energy and an increase in tissue temperature.

US

• Nonthermal Effects: do not generate an increase in tissue temperature, due to less movement of molecules.

• Acoustic streaming, microstreaming, and cavitation which may alter cell membrane permeability are nonthermal effects of US.

Cavitation (page 199)

• Formation, growth, and pulsation of gas-filled bubbles caused by US.

• Cavitation can be stable or unstable.

Microstreaming (p 200)

• Micoscale eddying that takes place near any small, vibrating objects.

• Microstreaming occurs around the gas bubbles set into oscillation by cavitation.

Acoustic Streaming (p 199)

• Steady current in a fluid driven by the absorption of ultrasound waves.

• The flow is larger the microstreaming and is thought to alter cellular activity by transporting material from one part of the ultrasound field to another.

US waves will be transmitted, absorbed , reflected or refracted.

• Review terms on pp 199-200

Effects of Ultrasound

Thermal Effects

Page 179

Tissue Affected• Ultrasound can reach deeper tissue

than superficial thermal agents

• The thermal effects of US can :– Accelerate metabolic rate– Modify pain– Modify mm spasm– Alter nerve conduction velocity– Increase circulation– Increase soft tissue extensibility

• Ultrasound will heat tissue with high ultrasound coefficients (see page 199). This would be tissue with high collagen content.– i.e. tendons, ligaments, joint capsules and

fascia

Factors Affecting the Amount of Temperature Increase

• Tissue of application• Frequency• Average intensity• Duration of application

Frequency

• Unit of measure = megahertz (MHz)

• Frequency is the parameter that affects depth of penetration of US waves.

• US can penetrate as deep as 5 cm.

Frequency

• The depth of penetration if a sound wave is inversely related to frequency, therefore the lower the freq. the deeper the penetration.

Frequency• As the freq. increases

there is more scattering of US energy and less energy is available for penetration into deeper tissue.

• 1 MHz will penetrate deeper than 3.3 MHz

Mode of ApplicationContinuous vs. Pulsed Modes• Continuous mode

– acoustic energy that flows w/o interruption

• Allow for a build up of thermal energy

Intensity (spatial average intensity)

• Intensity of US is the rate at which sound energy is applied or the strength of the acoustic energy.

• Unit of measure is watts per centimeter squared (w/cm2).

Pulsed Mode

• Interrupted flow of acoustic energy.

• Duty Cycle – ratio of pulse duration; on and off time of US flow

Pulsed Mode• The amount of “on” time will determine how

much thermal energy is created.

• If the duty cycle is high (i.e. 80% or 50%) some thermal energy will be generated.

If the If the duty cycle is low (i.e. 20%) no thermal energy will be generated and will result in non themal effect

Thermal Effects of US

• Can be used to increase tissue temp

• Can reach deeper and heat smaller areas compared to superficial heat modalities.

• Can heat tissue with high collagen content (ie. Tendon ligament, jt. Capsule, fascia)

Thermal Effects of US

• The amount of tissue temp increase will depend on the type of tissue being heated, frequency, intensity and duration of US.

What are the physiological effects from thermal US?

• Metabolic rate• Pain• Mm spasm/tone• Nerve conduction• Circulation• Soft tissue extensibility

Nonthermal Effects of US

• Micromassage – due to microstreaming (pg 192) and acoustic streaming(pg 192).

• Increased cell permeability – US causes a “stirring” effect in fluid near biomembrane which can increase the rate of ion diffusion across the membrane.

• Cavitation (pg 192)

Methods of Application

• Coupling agent (gel, mineral oil, glycerin, degassed water) must be used to enhance transmission of US waves.

• Direct Contact on area of treatment.

• Immersion – body part of Rx can be immersed in water. Useful in treating superficial areas, small irregular surfaces, sensitive areas and wounds (no direct contact).

Clinical Applications of USPage 181

• Soft Tissue Shortening• Pain Control • Dermal Ulcers• Surgical Skin Incisions• Tendon Repairs• Resorption of Ca+ Deposits

Clinical Applications of US

• Bone Fractures• Carpal Tunnel Syndrome (CTS)• Phonophoresis• Plantar Warts • Herpes Zoster Infection

How Do I Decide What Parameters to Use?

• Paramters for US depend on desired effect (thermal vs non-thermal), state on condition (acute vs chronic), Size of treatment area, type of target tissue, initial Rx vs follow up Rx.

Considerations for Duration of US Rx

• The 1st Rx is generally shorter in duration compared to follow up Rx. Once response is assessed then time can be increased.

• Acute conditions require less time.• Smaller areas require less time

• Effective Radiating Area (ERA) of the sound head can be used to determine US duration.

ERA is found in the manual of the US machine.

Rx time = 2 x ERA

If area is 20 cm2 and the ERA is 10cm2 the Rx time would be 5 – 10 minutes

Considerations for Intensity of US Rx

• Intensity is documented in W/cm2

• Set according to Rx goal• Superficial lesions should be

treated with lower intensity (I.e. hand v. back)

• Acute condition should be Rx’s with lower intensities.

Considerations for Duty Cycle of US Rx

• Duty Cycle selected according to Rx goal

• Duty cycle can be 100%, 50% (pulsed), or 20%

(this refers to the “on” time of the US)To increase tissue temp (generate heat)

use 100%.If nonthermal effect is desired used 50%

or 20%

PHONOPHORESIS

• US using meds ( hydrocortisone, dextamethasone, lidocaine).

• Meds are RX by MD.• Cream is applied to skin, then US

gel

Theories of Phonophoresis

• 1. Heating superficially increases vasodilation and absorption through circulation

• 2. Increase in cell permeability enhances diffusion of meds across the cell membrane

• 3. Pressure of US drives the drug into skin.

Contraindication (pg 189)

• Malignant Tumor• Pregnancy• CNS tissue• Joint Cement• Plastic components• Pacemaker• Thrombophlebitis• Eyes • Reproductive Organs

Precautions for US (page 191)

• Acute inflammation• Epiphyseal Growth Plate• Fractures• Beast Implants

Adverse Effects of US

• Burns – especially if superficial• Cavitation can cause tissue

damage• Pain – 3 reasons 1. Sharp pain = sign of periosteal

over heating caused by sound head moving over bony prominence or moving too slow over bone

Pain with US

• Dull ache = intensity is too high

• Prickling/stinging = not enough coupling medium

Documentation for US

• In the objective document: - Specific location of RX

- Intensity (w/cm2)- Duty Cycle/ Mode (pulsed 20% vs. continuous)

- Freq ( 1 MHz or 3 MHz)- Duration ( always put a time measure I.e. minutes)

Guideline For Selecting Ultrasound Intervention Parameters

Intensity Greater than 1.0 w/cm2 for thermal effects

Less than 1.0 w/cm2

for nonthermal effects

Frequency 1 MHz for targeting deep tissue

3 MHz for targeting superficial tissue

Duty Cycle 100% (continuous) for thermal effects (elevate tissue temperature)

20% for nonthermal effects (prevent build up of heat)

Treatment Time Longer treatment time to get maximal heating..4 X ERA

Shorter times for more acute and smaller areas.67 x ERA