Therapeutic Ultrasound

Therapeutic ultrasound is the transmission of high frequency soundwaves (0.7-3.3 MHz) through the body to produce thermal and non-thermal effects. These high frequency soundwaves are generated by the Reverse Piezoelectric effect. The Piezoelectric effect is the ability of a material to generate electricity when mechanically deformed. In reversing this, there is an electrical input to a material that causes a mechanical deformation and produces soundwaves. The material in ultrasound is a crystal found in the head of the transducer.

As ultrasound is transmitted it attenuates through the medium of the body, becoming weaker. The main factors affecting attenuation are:

Reflection: redirection of an incident beam away from a reflecting surface
Refraction: redirection of a wave at an interface
Absorption: conversion of mechanical energy to heat

Ultrasound can have a thermal effect through vibration of molecules to increase heat. This would be accomplished through a 100% or continuous duty cycle. A non-thermal effect is achieved by acoustic streaming, cavitation, and microstreaming to alter cell membrane permeability. To obtain this non thermal effect a pulsed, generally 20% duty cycle would be used.

When treating, the sound head should only move in an area that is 2x the Effective Radiating Area (ERA) in order to get a therapeutic response. Moving the head is important because of the Beam Non-uniformity Ratio (BNR). Imperfections of the crystal in the sound head create spikes of intensity. The sound head needs to move around to more evenly disperse the treatment and to prevent burns. The BNR should not exceed a 6:1 ratio. A normal treatment time for 2x ERA will be 5-10 minutes.

Thermal Ultrasound

As tissue absorbs ultrasound and kinetic energy increases, vibrating molecules. This friction between molecules results in heat production. Similar to other heating modalities, this creates the physiological effects of accelerated metabolic rade, reduced pain/spasm, altered NCV, increased circulation, and increased soft tissue extensibility. Thermal ultrasound is well suited for healing tendons, ligaments, and capsules as these collagen filled strucutres have a high absorption coefficient. Absorption of the energy causes an increase in tissue temperature, however this is only really suited for smaller areas.

The amount of temperature increase is important for different therapeutic effects. Lehmen and Lehman outlined with temperature/effect relationship:

1˚C increases metabolic rate
2-3˚C reduces muscle spasm and pain and increases blood flow
>4˚C increases collagen extensibility and inhibits sympathetic activity

For a comprehensive chart of parameter settings to temperature change, refer to Draper et al.

Several Factors affect the amount of heat delivered to a tissue including:

Absorption coefficient
- Increases with increased collagen and in proportion to the frequency
Sound head speed
- 4 cm/sec, however is doesn't seem to matter
rate of heating
- Proportional to tissue absorption coefficient and higher frequency
Frequency
- 3 MHz will increase temp 3-4 times more than 1 MHz, but less depth of penetration
  - 5 cm depth with 1 MHz
  - 1-2 cm depth with 2 MHz
Duration/Intensity
- Increasing intensity = increased temperature
- Increasing duration = increased temperature
- Tissue temperature increases by ~.2˚C/minute at 1 W/cm2
Local cooling effects
Patient comfort level
- another variable to take into account when setting treatment time

Indications for thermal ultrasound would include soft tissue shortening, pain control, and tendon and ligament injuries

Ultrasound can be particularly effective for stretching soft tissue. The tissue should be heated by 4˚C for tissue elongation. Draper et al found there was an optinum window for stretching following 3 MHz ultrasound treatment.

Non-thermal Ultrasound

The non-thermal effects of ultrasound are generated at 20% duety cycle and low intensity. The mechanical effects of caviation, microstreaming, and acoustic streaming are created. Cavitation is the formation, growth, and pulsation of bubbles cause by ultrasound. Microstreaming is a small current caused by these cavitations. Acoustic streaming is the flow of fluids on a larger scale than microstreaming in a circular flow and believed to alter cellular activity. The altered cellular activity seems to cellular level processes that are key to healing such as increased intracellular calcium, increased skinand cell membrane permeability, increased mast cell degranulation and histamine release, and the promotion of normal cell function by increasing mast cell degranulation, promoting macrophage responsiveness, and increasing the rate of protein synthesis by fibroblasts.

Non-thermal ultrasound can be used for:

Dermal ulcers
Surgical incisions
Fracture healing
Phonophoresis
Calcium deposits
Plantar warts

Physical

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