Ultrasound

Description of waves

Amplitude	Maximum value of oscillating quantity
Intensity	Amount of energy transported per unit time + unit area  → energy transmitted :proportional to ( amplitude )2
Wavelength	repeat distance of wave pattern
Period	Repeat time of wave pattern  Wave travels 1 wavelength in 1 period
Frequency	1/ period ( repeat time of wave pattern ) → number of complete waves over a period of time

What happens in the path of ultrasound

Attenuation: 

→ loss of intensity ( energy transported ) as wave travels through substance 

→ increase in thickness of substance ( path length ) → greater attenuation 

→ smaller attenuation coefficient → less energy absorbed 

→ higher frequency of wave → more energy absorbed 

→ half-value depth: distance at which the energy the wave carries ( intensity )  is halved 

What happens in the boundary of ultrasound

Reflection: 

→ cause: materials have different acoustic impedance → meet → boundary 

→ amount of energy reflected + amount of energy transmitted = incident amount of energy 

→ larger difference in acoustic impedance → larger amount of energy reflected 

→ acoustic impedance of air + bone vs water + flesh is large 

What happens to ultrasound waves in body

Superposition of waves 

→ effects of pre-existing waves summated

Production of ultrasound waves 

1. Crystals in ultrasound transducer → piezoelectric effect 

→ crystal dimensions change slightly to put pressure on surroundings → alternating current → pressure wave

Optimisation of ultrasound therapy

	Components
Conduction	Ultrasound gel/ water bath when application of ultrasound  → air is poor conductor
Transmission	Reflection + refraction occurs at boundaries of tissue  → intensity + direction of beams affected  → * ensure the sound head is perpendicular to skin → reduce attenuation
Attenuation	Intensity of ultrasound reduces due to absorption/reflection/ refraction

Treatment parameters ( mode )

Considerations	Treatment effect
Continuous: sound energy constant  Pulsed: periodically interrupted energy  Duty cycle: percentage of time ultrasound is generated ( pulse duration ) : pulse period  Mark: space ratio: ratio of pulse length to interval	Continuous: heating at higher intensity  Pulsed: X heat effects  Immediate post-injury: 1:3  Subacute: 1:1→ e.g. 5s on + 5s off → duty cycle: 50%

Treatment parameters  ( frequency )

Considerations	Treatment effect
→ depth of tissue penetration determined  → same amount of energy transmitted  → measured by half value depth  Half value depth: depth of penetration at which intensity reduced by half  → High protein low water content tissue absorb more ultrasound waves	1Hz: deep tissue  → half value depth: 6.5 cm  3 Hz: superficial tissue  → half value depth: 3 cm

Treatment parameters ( intensity )

Considerations	Treatment effect
Rate at which energy is delivered per unit area  → Unit: W/cm2  → Spacial average intensity: total power output/ total effective radiating area  → SAI: 3 W/cm2 → safe for therapy  → SATP: spaced average temporal peak  → all Uq machines  → if average intensity to be 1 w/cm2 w/ duty cycle 1:1 → multiply by 2  → SATA: spaced average time averaged	Acute: 0.5 W/cm2  Chronic: 0.5-1 W/cm2  → ***** in water → increase intensity by 20%  → lowest intensity + highest freq

Treatment parameters ( duration )

Considerations	Treatment effect
30s - 2 mins per effective radiating area	Shorter time for acute

Treatment parameters for ultrasound ( effective radiating area + treatment area size )

Considerations
Surface area that transmits sound wave from crystal to tissues	ERA
Surface area to deliver ultrasound energy to  → number of ERAs to cover treatment area	Treatment area size

Prescription of treatment ( ultrasound )

1. Recent injury: ASAP → 1-2 x daily 

2. Chronic: alternate days 

3. Improvement: same dose 

4. Worsen: reduce 

5. X change: increase one parameter 

Thermal effect of ultrasound + advantage

	Advantage
Amount of heating dependent on heat capacity of tissues  Heat capacity of tissues  Frequency  Intensity  Duration  Local blood flow  Depth of fat/ skin/ muscle thickness	Selective heating w/o significant temp increase in skin  Heating in chronic phase to reduce muscle spasm

Nonthermal effects of ultrasound

1. Cavitation 

• Stable: oscillation of bubbles in pressure waves w/ acoustic streaming 

• Unstable: volume of bubbles increase quickly  collapse → rapid pulsing US + head movement

2. Acoustic streaming: 

• Steady unidirectional movement of fluid in US field 

→ microstreaming vs bulk streaming ( visible ) 

→ affects cell membrane permeability → helpful for chronic by stimulating cell repair process when bubbles burst 

→ alter rate of diffusion of ions across cell membrane

Dsiadvantage of ultrasound usage

1. Periosteal pain + burns: 

• Sharp pain due to heating when using US at bone surface 

• Caused by reflection at bone/ soft tissue interface —> standing waves ( reflected wave superimposed on incident wave )

2. Infection control 

• Cross contamination + infection 

• Universal infection control precautions: 

→ clean transducer head w/ alcohol wipe 

→ wound care → through plastic dressing/ water bag 

Therapeutic effects of ultrasound 

Pain relief	increased blood flow → removal of chemical irritants  Affects electrical activity of nerves ( pain gating )  Speeding up inflammatory process
Tissue healing	Acute inflammatory:  Increased calcium ion diffusion across cells  Release of histamine  Increased normal state of healing  Granulation stage:  Increased collagen by fibroblasts  Better organisation of collagen bundles  Remodelling:  Effect of heat on deeper tissue
Scar tissue effects
Fracture healing

Clinical indications

1. Acute soft tissue injury 

2. Localised MSK conditions → tendons + ligament injuries 

3. Scar tissue → plantar fasciitis 

4. Wound healing → pressure sores 

5. Pain relief 

6. Mastitis + postnatal perineal

Contraindications for ultrasound

1. Malignant tumours 

2. Pelvic region in pregnancy 

3. Over inbuilt stimulator ( pacemaker ) 

4. Lack of thermal sensation 

5. Circulatory insufficiency 

6. Exacerbation of existing condition 

7. Unable to communicate 

8. Eyes/ testes

Precautions for ultrasound therapy

1. Metal implants 

2. Joint replacement 

3. Superficial bone 

4. Bony epiphysis in children 

5. Longitudinal movement along blood vessels 

6. Breast implants

Principles for application of ultrasound 

Coupling agent	Prevention of reflection into transducer → damage crystals
Perpendicular to skin	Maximum penetration  Reduction of reflection
Moving sound head constantly	Evening out pattern of energy absorption of ultrasound waves  Prevention of formation of standing waves  Prevent unstable cavitation

Principles for application of ultrasound 2 ( techniques )

1. Direct contact 

2. Water bath → if surface contour makes it hard to follow w/ ultrasound head 

• immerse limb into warm water bath 

• Hold head from skin surface by 1 cm 

• Keep head moving 

3. Water bag 

• Prevent water bubbles 

• Put gel on patient + bag 

1. Solid sterile gel