Ultrasound

Ultrasound uses

1.) sound waves at high frequencies → acoustical energy

2.) Heating modality

3.) also has non-thermal effects → mechanical effects

Heating modality

1.) penetrates skin and subcutaneous fat

2.) can be used to heat muscles, tendons, ligaments, joint capsules, scar tissue

Electrical current is passed through a crystal causing it to vibrate

1.) sound waves generated by the vibration of the crystal

2.) most ultrasound machines for musculoskeletal treatments have a frequency range of 800,000 hz to 3.3 hz

3.) 1 Mz to 3.3 Mz, audible range is 15-20,000 Hz

Duty cycle

1.) calculated by dividing the time sound is delivered by the total treatment time. If applied for 150ms out of every second → 150/1000 = 15% (sound energy is applied to the patient 15% of the time)

2.) duty cycle of 100% = continuous US; 50% and 20% are common pulsed US duty cycles

3.) continuous ultrasound results in tissue healing. pulsed ultrasound has mechanical/non-thermal effects

Piezoelectric effect

mechanical deformation of a crystal causes an electrical current to form

Reverse Piezoelectric effect (indirect)

1.) alternating current is passed through crystal, results in very fast contraction and expansion of the crystal

2.) vibration produces high frequency sound waves

3.) requires high voltage therefore must have well insulated cables

Ultrasound beam: near field

treatment area and fresnel zone

Ultrasound beam: far field

fraunhofer zone

Spatial peak intensity (Isp)

peak intensity or maximum intensity

watts per cm2

Spatial average intensity (Isa)

Average intensity → Isp X duty cycle

1.4 W/cm2 X 50% = .7 W/cm2

ERA

effective radiating area

BNR

Beam nonuniformity ratio

Effective radiating area

1.) area of the sound head that produces sound energy

2.) always smaller than size of the ultrasound head (ideally only slightly smaller)

Bean nonuniformity ratio

1.) US beams are not uniform across the US head

2.) BNR = amount of variability of the beam

3.) ratio between the peak intensity of the US beam divided by the average intensity of the US beam

4.) use to compare quality of different machines

Ideal BNR is

1 (or 1:1) but withing range of 2:1-8:1 is acceptable (2:1 means at 1 W/cm2 the peak intensity is 2 w/cm2)

Lower the BNR the more

uniform the intensity of the sound waves → eliminates hot spots, allows for higher dosage without discomfort and allows for greatest comfort and safety

Conducting media

1.) air is a poor conductor of US, use coupling medium to prevent reflection of sound waves by air (US gel and gel pads are preferred)

2.) keep the sound head flat against skin (90 degrees of sound waves)

3.) some manufacturers recommend mixing analgesic creams with US gel → DO NOT recommend

Underwater ultrasound 1

1.) indirect ultrasound, use plastic basin or bucket (not metal, reflection)

2.) keep sound head parallel to part, distance .5-3.0 cm (increase intensity or increase time)

3.) wipe bubbles from skin and sound head occasionally during treatment

Amplitude of soundwaves

1.) amount of sound energy being emitted from sound head (watts)

2.) dosage is based on amount of energy in a square centimeter (w/cm2)

3.) more watts = more heating not greater penetration

Frequency

1.) typically 1-3MHz

2.) frequency dictates depth of heating

Duty Cycle

can be interrupted (pulsed) or continuous

Frequency affects

1.) depth at which the greatest amount of ultrasound energy is absorbed

2.) time required cause an increase in tissue temperature

3.) high frequency (3MHz) → absorbed more rapidly therefore affects superficial tissues

4.) low frequency (1MHz) → absorbed at a slower rate therefore affects deeper tissues

Absorption in tissue: continuous US

1.) more sound energy delivered, more sound energy absorbed, greater tissue heating

2.) higher protein concentration = higher absorption, higher density = higher absorption rate

3.) bone, tendon, cartilage, ligaments, and joint capsules absorb most. Skin and fat absorb the least

Scattering

when US wave encounters a boundary between tissues, energy will scatter by reflection or refraction

Reflection

1.) reversal of the direction or propagation of the ultrasound wave. Acoustic impedance of muscle, fat and water is low (~1% of the energy reflected)

2.) US energy is reflected at soft tissue bone interference leading to increased heating → tissue close to bones (increased heating)

Refraction

1.) change of US wave from a straight path when passing obliquely from one medium to another

2.) bending of US energy within tissue can lead to concentration of US at the point of refraction

Tissue Heating

1.) many of the benefits of US attributed to tissue heating

2.) causes increase in metabolic activity which increases oxygen demand which leads to increased blood flow

1 degree C increase in temperature is associated with

increase in metabolic activity

2-3 degrees C increase in temperature is associated with

reduction in muscle spasm, increases blood flow and reduction of chronic inflammation

4 degrees C increase in temperature

alters viscoelastic properties of collagen

Therapeutic effects

1.) increase extensibility of collagen fibers of tendons and joint capsules

2.) increase blood flow, increase cell metabolism

3.) increase collagen synthesis, decreased joint stiffness

4.) decreased muscle spasm and enhanced tendon, ligament, and muscle healing

Tissue Cooling

1.) thermal effect is only temporary → superficial layers cool more rapidly than deeper tissues

2.) any tissue manipulation or stretching should be performed immediately after ultrasound treatment

Non-thermal effects: pulsed US

1.) gives very little heating of tissues, has been shown to affect tissue healing and alter activity → stimulation of fibroblasts, increased blood flow, increased proteins associated with injury repair

2.) through nano motion or acoustical streaming and stable cavitation

Acoustical streaming

1.) movement of fluids along cell membranes due to mechanical pressure exerted by the sound waves

2.) movement occurs in direction of the sound waves (microstreaming)

3.) facilitates fluid movement and increases cell membrane permeability

Cavitation

1.) formation of gas bubbles from pressure changes in tissue fluids

2.) bubbles expand and contract with US waves

Stable cavitation

1.) rhythmic expansion and contraction of bubbles during repeated pressure changes over many acoustic cycles

2.) facilitates fluid movement and membrane transport

Unstable cavitation

1.) collapse of gas bubbles which may cause tissue damage

2.) associated with low frequency high intensity US (not therapeutic)

Contraindications

1.) malignancy

2.) hemorrhage

3.) ischemia

4.) infection

5.) thrombus

6.) gonads, eye,

7.) pelvic, abdominal and lumbar areas of pregnant women, spinal cord after laminectomy

8.) plastic and cemented implants (metal is okay), near or over electronic implants

9.) unknown etiology

Risks

1.) bony prominences → avoid, use indirect technique, smaller sound head

2.) epiphyseal plate → may alter bone growth

Intensity

1.) Use a range of .5-2.5 W/cm2 → higher intensities are not necesarily more effective

2.) do not exceed peak intensity of 8.0 W/cm2 → need to know BNR, lower BNR can use higher intensity

3.) in lab: 1.2 W/cm2 = low enough for safety

Application of US

1.) treat area approx. 2-3 times the size of ERA → larger areas heating won’t be achieved

2.) slow controlled movement in US head

Treatment time

1.) generally 5-8 minutes, sometimes as little as 1-2 minutes

2.) depends on size of treatment area, never treat for more than a total of 15 minutes (several treatment areas)

Phonophoresis

1.) sonophoresis → similar to iontophoresis

2.) uses sound energy to drive medication into the tissue, medication does not need to have a charge → lack of literature to support efficacy, problems determining amount of medication delivered

US and Electrical stimulation (COMBO)

1.) research is lacking but claims for use include → trigger points, epicondylitis, superficial pain areas, decrease adhesions

2.) US head becomes the treating electrode when used with a dispersal pad

Low intensity pulsed US

1.) LIPUS → stimulation of fx healing, good evidence for effectiveness

2.) reusable wearable batter powered unit

3.) self-adhesive disposable coupling bandages secure the applicators over or around treatment area