ultrasound

ultrasound definition

sound waves ≥20,000 Hz (.02 MHz) - above human hearing range

Sound waves require

a medium

therapeutic ultrasound is typically between

.7-3.3 MHz

Typical depth of absorption in ultrasound

2-5cm

Attenuation

as US travels through material it decreases in intensity

Ultrasound causes… as the waves are transmitted

circular motion of the material

thermal TTR

up to 5cm deep

non-thermal US aims to

alter cellular activity (acoustic streaming, micro streaming, cavitation)

causes of attenuation

absorption by non-target tissue, refraction, reflection

attenuation is greatest in

tissue with high collagen content

tissues with high attenuation coefficients

tendon, ligament, cartilage, scar tissue, joint capsule, bone

low attenuation coefficient materials

materials with water (e.g. muscle)

since some sound waves are reflected towards the sound head, there is the risk of

periosteal overheating

periosteal overheating is avoided by

keeping the US head moving

tissues from low to high attenuation coefficient

blood, fat, nerve, muscle, blood vessels, skin, tendon, cartilage, bone

the crystal in the US head has

piezoelectric properties (it vibrates)

to generate US

high frequency AC current is applied to crystal inside the transducer

the crystal is able to respond to electrical current by

expanding and contracting as the electrical current alternates

When the crystal expands it

compresses the material in front of it

when the crystal contracts it

rarefies the material in front of it (resulting in an US wave)

continuous US

electrical current is delivered continuously to transducer

has thermal benefits

continuous US

Pulsed US

electrical current is delivered for limited portion of time to transducer

good for non-thermal benefits (metabolism, healing, etc.)

pulsed US

duty cycle

the amount of time that the current is being delivered

good choice for duty cycle

20%

in pulsed US the pt should feel

nothing (just the motion of the head on their skin)

Acoustic streaming

steady, circular flow of cell material caused by US waves

Attenuation

decrease in US intensity as it travels through tissue

cavitation

gas bubbles in tissue are made smaller during compression phase of US and expand during rarefaction phase

microstreaming

very small flow of material (gas bubbles from cavitation), if these implode it can cause tissue damage (lithotripsy)

rarefaction

decrease in density of material as US waves pass through it

piezoelectric

able to change shape in response to electrical current

reflection

redirection of wave opposite to angle of incident, mostly occurs between tissue interfaces (air-skin, soft tissue-bone)

refraction

redirection of wave as it continues to enter tissue

Effective radiating area (ERA)

area of transducer from which US energy radiates, 1/2 the size of the transducer head

frequency

number of compression-rarefaction cycles/second

frequency is measure in

Hz

therapeutic US frequency

1-3MHz (1-3 million/sec)

power

amount of energy per unit of time

power is measured in

watts (W)

Intensity

power per unit of area

intensity is measured in

W/cm2

duty cycle

proportion of total treatment time that US is on

1 MHz depth

~5cm

3 MHz depth

~2cm

greater wavelength results in

deeper penetration

continuous duty cycle (100%) is

thermal

pulsed duty cycle is

non-thermal

spatial peak intensity

peak intensity of US output over the transducer area, not always the same)

spatial average intensity

average intensity of US output over the transducer

spatial average temporal average (SATP)

spatial average intensity averaged over the amount of on time of the US (measuring how much energy is delivered)

Beam non-uniformity ratio (BNR)

ratio of spatial peak intensity:spatial average intensity

acceptable BNR

5:1 or 6:1

SATP

spatial average temporal peak

intensity

power/area

intensity is adjusted

by user depending on goal of intervention

intensity for deeper tissue and thermal effect

1.2-2 W/cm²

intensity for superficial tissue and thermal effect

0.3-1 W/cm²

intensity for non-thermal effect, superficial to deep

0.5 W/cm²

ERA

effective radiating area

effective radiating area (ERA)

area of the sound head that is functioning to produce the sound wave

treatment time should be

3-5 minutes per ERA

ERA is based on

the size of the sound head

most common frequencies for ultrasound heads

1 and 3 MHz

when it comes to BNR

lower is better

BNR

how uniform the intensity is throughout the treatment time

near field

convergence of beam

far field

divergence of beam

interference in the beam causes

variation in US intensity

near field is dependent on

ERA and frequency

thermal effects of US

reduction of pain, tissue extensibility, increase circulation, reduce muscle spasms, alter nerve conduction

thermal ultrasound has the same

benefits as superficial heating agents, but can get to deeper structures and has smaller treatment area effected

Factors that influence amount of temperature change

• absorption coefficient of tissue

• frequency

• average intensity

  • duration of treatment

higher absorption coefficient results in

higher temperature changes

higher absorption coefficient has to do with

collagen and water

lower absorption coefficient results in

less temperature changes

tissues with lower absorption coefficients

water, muscle, fat

higher frequency results in

higher temperatures of tissue

lower frequencies result in

not as high temperatures of tissues

needed intensity with higher frequency

less intensity is needed

treatment area should not exceed

2x ERA

treatment time for one region should be

6-10 minutes

non thermal effects of US

increase skin & cell permeability, macrophage responsiveness, release of chemotactic factors & histamine, protein synthesis by fibroblast, nitric oxide synthesis in endothelial cells, proteoglycan synthesis in cartilage cells

non-thermal is effective for

inflammation phase of healing, wound healing, blood flow (local area), increase skin permeability to topical medications

clinical uses of us

soft tissue shortening, pain control, dermal ulcers, surgical skin incisions, tendon/ligament injuries, resorption of calcium deposits, bone fractures, carpal tunnel syndrome, phonopohresis

high intensity over bone fracture

may cause pain

best evidence of US use is for

soft tissue shortening

US medium examples

gel, balloon (bony area), underwater(bony/distal area)

when applying us underwater you ned to

leave a little bit of room between the transducer and the skin

When applying US for tissue extensibility

position so you can get a stretch easily (prolonged stretch when applied is best)

effectiveness of US for tissue extensibility depends on

absorption coefficient (collagen)

heat increases the

viscoelasticity of collagen matrix

increasing tissue temperature

temporarily improves extensibility (5 min)

general parameters for soft tissue shortening

• superficial - 3MHz, .5-1 W/cm², 100% for 5-10 minutes

• deep - 1MHz, 1.5-2.5 W/cm², 100% for 5-10 minutes