What is Ultrasound?
ULTRASOUNDChapter 7What 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 USUS 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)
UltrasoundUS 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.USNonthermal 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 199-200Effects of UltrasoundThermal EffectsPage 179Tissue AffectedUltrasound can reach deeper tissue than superficial thermal agents
The thermal effects of US can :Accelerate metabolic rateModify painModify mm spasmAlter nerve conduction velocityIncrease circulationIncrease soft tissue extensibilityUltrasound 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 IncreaseTissue of applicationFrequencyAverage intensityDuration of applicationFrequencyUnit of measure = megahertz (MHz)
Frequency is the parameter that affects depth of penetration of US waves.
US can penetrate as deep as 5 cm.FrequencyThe depth of penetration if a sound wave is inversely related to frequency, therefore the lower the freq. the deeper the penetration.
FrequencyAs 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 ModesContinuous 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 ModeInterrupted flow of acoustic energy.
Duty Cycle ratio of pulse duration; on and off time of US flow
Pulsed ModeThe 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 USCan 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 USThe 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 ratePainMm spasm/toneNerve conductionCirculationSoft tissue extensibilityNonthermal Effects of USMicromassage 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 ApplicationCoupling 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 181Soft Tissue ShorteningPain Control Dermal UlcersSurgical Skin IncisionsTendon RepairsResorption of Ca+ DepositsClinical Applications of USBone FracturesCarpal Tunnel Syndrome (CTS)PhonophoresisPlantar Warts Herpes Zoster InfectionHow 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 RxThe 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 RxIntensity is documented in W/cm2Set according to Rx goalSuperficial lesions should be treated with lower intensity (I.e. hand v. back)Acute condition should be Rxs with lower intensities. Considerations for Duty Cycle of US RxDuty Cycle selected according to Rx goalDuty 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%PHONOPHORESISUS using meds ( hydrocortisone, dextamethasone, lidocaine).Meds are RX by MD.Cream is applied to skin, then US gel
Theories of Phonophoresis1. Heating superficially increases vasodilation and absorption through circulation2. Increase in cell permeability enhances diffusion of meds across the cell membrane3. Pressure of US drives the drug into skin.Contraindication (pg 189)Malignant TumorPregnancyCNS tissueJoint CementPlastic componentsPacemakerThrombophlebitisEyes Reproductive Organs
Precautions for US (page 191)Acute inflammationEpiphyseal Growth PlateFracturesBeast Implants
Adverse Effects of USBurns especially if superficialCavitation can cause tissue damagePain 3 reasons 1. Sharp pain = sign of periosteal over heating caused by sound head moving over bony prominence or moving too slow over bonePain with USDull ache = intensity is too high
Prickling/stinging = not enough coupling mediumDocumentation for USIn 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 ParametersIntensityGreater than 1.0 w/cm2 for thermal effects Less than 1.0 w/cm2 for nonthermal effectsFrequency1 MHz for targeting deep tissue
3 MHz for targeting superficial tissueDuty Cycle100% (continuous) for thermal effects (elevate tissue temperature)
20% for nonthermal effects (prevent build up of heat)Treatment TimeLonger treatment time to get maximal heating..4 X ERAShorter times for more acute and smaller areas.67 x ERA