Therapeutic Ultrasound Flashcards
What is therapeutic ultrasound? (TUS)
-Sound wave > acoustic energy (mechanical pressure wave) due to alternately compressing and rarefying material -energy proprogation -frequency greater than 20,000 Hz (cycles/sec) -inaudible to human ear
How is TUS generated?
The generator - high frequency electrical generator - oscillator circuit: produces electrical current of specific frequency -Crystal: frequency causes this to vibrate which then sets off sound wave -Control panel
The Crystal
- -Synthetic ceramic (2-3mm thick) - produces uniform beam
- -Transducer
- * converts electrical energy to mechanical acoustic energy
- * reverse piezoelectric effect
- * creates vibration at frequency of electrical oscillation of generator
- * vibrating crystal causes soundhead to vibrate
Reverse Piezoelectric effect
The mechanical deformation of certain crystals or ceramics when exposed to an electric field.
Piezoelectric effect
the generation of an electric charge by subjecting certain crystals or ceramics to mechanical strain (ex hitting material with hammer will produce electric charge)
The sound head
- -Transfers acoustic energy to tissues
- -ceramic, aluminum, or stain-less steel plate attached to crystal
- generally matches frequency of crystal thus not interchangable
Applicator
- -housing for crystal and soundhead
- -held by PT to apply ultrasonic wave to pt - made of hard insulated plastic
- -should be ergonomically designed for therapist comfort
Electromagnetic wave
Photon moves along the wave
Mechanical wave
Molecules bump into their neighbor resulting in propagation of energy
Wave types
- -Longitudinal waves
- -Transverse waves
Longitudinal waves
- movement of molecules is along the direction the wave travels
- * seen in liquids and gases
- movement of molecules is along the direction the wave travels
- molecular flow occurs parallel to direction of wave prorogation and is known as MICROSTREAMING
- near field (fresnel zone)
- area of US beam closest to transducer
- * sound wave transmitted in straight line -Far field (Fraunhofer zone) area of US beam where wave begins to diverge
- * sound wave begins to diverge or scatter
- area of US beam closest to transducer
Power
- -The amount of acoustic energy per unit of time
- -measured in watts (W) - 1 W= 1 Joule/sec
1 Joule is equal to…
- a unit of work equivalent to the energy expended in one second by an electric current of one ampere in a resistance of one ohm.
- one joule is = to 0.738 foot pounds
Effective radiating area (ERA)
- the size of the area of the transducer that radiates the US beam
- -Dependent upon surface area of the crystal -generally less than the size of the tranducer cover -measured in cm^2
Spatial intensity
-Relates to size of transducer ERA
Temporal intensity
Relates to time transducer is “on”
Spatial average intensity
- -Acoustic power (W) divided by the effective radiating area (ERA) of the transducer
- -measured in W/cm^2
Spatial peak intensity
-Peak intensity of the US beam over the area of the transducer -generally greatest at the center
Spatial average temporal peak (SATP) intensity
- the spatial average intensity of the US during the on time of the pulse
- -generally displayed on US generator as W/cm^2
Spatial average temporal average (SATA) intensity
- -SATP x duty cycle=SATA
- measures amount of energy delivered to tissue
duty cycle
- -The percentage the US beam is on -Continuous= 100%
- pulsed= generally expressed as 10%, 20%, 50%
Beam nonuniformity ratio (BNR)
- -Ratio of spatial peak intensity to spatial average intensity
- -ratio of the intensity of the highest peak to that of the average intensity of all the peaks
- the lower the BNR the lower the intensity of the highest peak - the lower the BNR the lower the probability of the pt feeling undesirable hot spots
Range for beam nonuniformity ratio (BNR)
- Recommended to be between 2:1 and 8:1
- ex: with transducer with BNR of 5:1 * if SATA is 1 W/cm^2 then SATP could be as high as 5 W/cm^2
Frequency
- The number of compression-rarefraction cycles per unit of time -expressed in cycles per sec (Hz)
- TUS frequency range: 0.75 to 3.3 MHz (3.3 million cycles/sec)
- -effects of depth penetration
- * 1 MHz- penetrates 5 cm * 3 MHz- penetrates 1-2 cm
Why does absorption increase as frequency increases?
- Less available for transmission to deeper tissues (more absorbed superficially)
- absorption occurs in 1/3 the time for 3 Hz vs 1 MHz US
Absorption
- -Conversion of mechanical energy of US wave into kinetic or heat energy
- -Expressed by absorption coefficient
- * tissue and frequency dependent
- * highest in tissue with high collagen content (tendon, ligament, bone)
- * increase in proportion to intensity
Absorption coefficients
bone > cartilage> tendon> skin> muscle (perpendicular)> muscle (parallel)> nerve > fat > blood
Reflection
- Redirection of the US beam away from the surface -US reflected at tissue interfaces
- * 35% @ soft tissue bone interfaces
- * 100% at skin air interface
- * .1% at couplant skin interface
- ***cosine law
- * 35% @ soft tissue bone interfaces
US rate of healing per minute
Ultrasound Rate of Heating Per Minute
Intensity (W/cm2) 1 MHz 3MHz
.5 .040C .30C
- 0 .20C .60C
- 5 .30C .90C
- 0 .40C 1.40C
Refraction
- wave enters tissue at one angle and continues through at another angle
attenuation
- measures the decrease in US intensity as the wave travels through tissues
- -Due to either absorption of US wave due to reflection and refraction
- -penetrates easily through tissues high in water content and is absorbed best in dense tissues of high protein content -expressed as attenuation constant
- * higher for tissues with higher collagen content (protein)
- penetration & absorption are inversely related
- -absorption increases as frequency increases thus less energy is available for transmission to deeper tissues
Attenuation constant
bone>cartilage>tendon> skin> blood vessels> muscle > nerve (no value) > fat > blood
standing waves
- -Reflected wave interacts with wave moving in opposite direction
- if both are in phase energy will stimulate and be more intense
- must keep soundhead moving to minimize standing waves
cavitation
- -Formation, growth, pulsation of bubbles exposed to US beam
- compression phase- bubbles compress
- -rarefraction phase- bubbles expand
- may be stable or unstable
- * stable=bubbles intact
- * unstable= too much energy put in and bubbles burst
- may be stable or unstable
- stable cavitation is thought to produce non thermal US effects
- true unstable cavitation does not occur at therapeutic levels
Microstreaming
- -Minute unidirectional flow of fluid in the vicinity of pulsating bubbles
- -results from pressure of sound wave that moves ions or small molcules
- -produces stresses that can effect change cell membrane structure and function
Characteristics of US waves
- -Velocity of sound wave is dependent upon the physical properties of the medium through which it travels -changes as densities of tissue changes
- * 3360 m/sec-bone
- * 1500 m/sec- water
- * 330 m/sec- air
Couplant
- -Any agent that allows for smooth transmission of US from sound head to tissue (skin)
- mediums that transmit US well
- * water- 100%
- * mineral oil- 97%
- * US gel (ultraphonic)- 96%
- * US lotion ( polysonic)- 90%
- mediums that transmit US well
Therapeutic US physiological effects
- -Thermal
- increases tissue temp
- -Non-thermal
- alter cell membrane permeability
Physiological effects: thermal effects
- occurs when using any mode US
- causes molecules to vibrate
- higher intensities and more continuous beam produces more vigorous vibration
- increased extensibility of collagen fibers
- reduced viscosity of fluid elements in tissues
- decreased jt stiffness
- reduced muscle spasm
- diminished pain preception
- increased metabolism
- increased blood flow
Absorption coefficent
- the amount of absorption of US energy that occurs in a tissue type at specific frequency
- higher for tissues with higher collagen content
- increase proportionally with US frequency
