September 9, 2008

ULTRAHIGH FREQUENCY SOUND^©

Lyn Paul Taylor, A.A., B.A., M.A., R.P.T.

(Editing Assistant and Computer Consultant: Joanna Soon, B.S.)

Ultrahigh frequency sound (ultrasound) is a form of energy derived from mechanical vibration.  It is produced when a crystal set in an ultrasonic transducer is electrically stimulated causing the electrical energy to be changed into a mechanical vibration that produces a focused beam of ultrasonic waves.  The waveform can be continuous or pulsed (intermittent).  The ultrasonic waves pass through a metal cap that covers the crystal (together called the sound head) and, under the right conditions, are passed into biological tissues.

The depth of penetration of ultrasound into the tissues depends on the frequency of the sound (the number of vibrations per second) and the density of the tissues.  The therapeutic effects of ultrasound are said to depend on frequency, intensity (amplitude), and duration of application.

There are two basic types of ultrasound heads, moveable and stationary.  The moveable ultrasound head was developed before the arrival of the pulsed waveform.  Because of the heating effect produced by the continuous waveform, comfort dictated that the ultrasound head be continuously moved over a fairly large area (several times the ultrasound head size) to reduce thermal effects on the tissues sounded (the sound head heated up).  The pulsed (intermittently interrupted) ultrasound waveform does not have the same tissue heating effect and can sometimes be applied with a stationary (fixed) ultrasound head, or with relatively little sound head movement.  This allows the ultrasound to be concentrated in a more concise area.  Sound heads vary in size, ranging from four inches to less than one half inch in diameter.

A coupling agent is necessary to conduct the ultrasound from the sound head to the skin because of the minuscule air gap that always exists between the uncoupled sound head and the skin, even when the sound head is firmly pressed against the skin.  The coupling agent's job is essentially to seal the sound head and the skin together.  The ideal coupling agent should be fluid enough to allow for free transmission of the sound into the skin but viscous enough to prevent run off if it warms up during application.  Substances as fluid as mineral oil, or as viscous as soluble colloid gels, have been shown to be adequate couplants.

It is axiomatic that the longer ultrasound passes through a tissue, the greater the thermal and nonthermal physiological effects.  Application duration of ultrasound in clinical practice usually ranges from four to eight minutes, varying as the size of the treatment site varies.  As a rule of thumb, an area varying from 16 to 72 cm² is sounded for six minutes and more than 72 cm² for eight minutes.  Smaller areas may be sounded for two to four minutes.

An ultrahigh frequency sound instrument
featuring 1 Mhz and 3 Mhz heads

The lower the frequency of ultrasound, the deeper the penetration of significant levels of sound into biological tissues.  Absorption (attenuation) of the ultrasonic beam decreases as the frequency decreases.  At three megahertz (Mhz), only 50% of the ultrasonic beam reaches a soft tissue depth of one centimeter (cm), and only 3% reaches a depth of five cm.  At one Mhz (the frequency most commonly used by ultrasound units produced in the United States), 82% of the ultrasonic beam penetrates to a soft tissue depth of one cm and 32% reaches five cm.  Lower frequencies, of course, penetrate deeper, but the depth of most soft tissues (until bone is reached) would seem to make greater penetration generally unnecessary and probably undesirable since greater penetration increases the risk of stimulating uninvolved structures (especially if treating abdominal structures).

Intensity (current amplitude) is the amount of power (watts or W) applied to a definitive surface area (per cm²).  Intensities applied therapeutically range from 0.6 to 2 W/cm².  As the intensity of the ultrasound increases, the temperature of the tissues being sounded rises in proportion to the amount of sound absorbed by the respective tissue; the greater the tissue absorption of the sound, the greater the heating. Early researchers found that lower treatment intensities often produced better recipient responses than higher levels.  This follows the Amd-Schulz Law that states that low dosages of any form of energy are likely to produce beneficial physiological reactions within stimulated tissues, while medium to high dosages (more than 2.0 W/cm²) may produce pathological results (heightened sensitivity or pain).  Levels below 0.6 W/cm² have little or no therapeutic value.

The density of the tissues will affect attenuation of the ultrasound beam, with greater densities causing more attenuation then less dense tissues.  The degree of attenuation varies from one type of tissues to another.  Blood attenuates the ultrasound beam at a rate of 3% per cm, fat 13%, muscle 24%, blood vessel 32%, skin 39%, tendon 59%, cartilage 68% and bone 96%.

The number of treatments within a given time span plays an important role in the effectiveness of ultrasound treatment.  Some authorities have recommended that ultrasound treatment of a given treatment site be conducted as frequently as twice a day (as close as one half hour apart).  As a rule, the more frequently a site can be treated, the greater the effect of the ultrasound.  It should be noted, however, that different conditions will require a different frequency of treatment, and the total number of treatments required for successful resolution will also vary.

Ultrasound can be applied in a continuous or pulsed waveform.  The continuous waveform has the greater thermal effect and has been reported to be effective at increasing collagen tissue elasticity, altering blood flow, changing nerve conduction velocity, increasing pain threshold and enzyme activity, and changing contractile activity in skeletal muscles.  The pulsed waveform has reportedly been used when the nonthermal mechanisms of ultrasound can be of benefit and when heating needs are minimized (as in the treatment of stasis ulcers).  The nonthermal mechanisms include acoustical streaming, increasing cellular permeability, exertion of a propelling force to drive chemicals across the cell wall barriers, and the dispersion of fluids.  The thermal effect of the pulsed waveform is minimized because when in the off-phase of the pulse, the intensity is zero.  Any expression of intensity, when speaking of the pulsed waveform, must be an average of the temporal peak value of the intensity and the zero value of the off-phase.

Application:

• The timer, waveform and dosage levels of the ultrasound machine should be preset.

• The coupling agent should be liberally applied over the treatment site.

• The sound head should be placed on the coupling agent covering the treatment site with the flat surface of the sound head against the skin (in bony prominent areas this may only be possible with a small sound head).

• The ultrasound unit should be turned on.

• If a moveable sound head and a continuous waveform are being used, the sound head should be moved over the treatment site (being careful to keep the sound head flat against the skin) with a continuous circuitous motion, or with continuous stroking back and forth motion. The sound head should be moved at a slow, steady rate.  If a stationary sound head and a pulsed waveform are used, no motion of the sound head is necessary, but the practitioner should remain in constant attendance and the amplitude adjusted to maintain recipient comfort.

• Following treatment, the sound head should be thoroughly cleansed in preparation for the next application.

Applying a coupling agent in preparation for Ultrasound treatment

Ultrasounding a treatment site on the low back

Precautions:

Because blood supply to the lens is poor, therapeutic levels of ultrasound should not be applied over the eye.  The poor circulation doesn't allow the heat generated by ultrasound to dissipate adequately and may lead to cataract formation.  The retina may also sustain damage from therapeutic levels of ultrasound because any ultrasound entering the eye will be only slightly attenuated by the aqueous and vitreous humors, allowing the ultrasound to have full impact on delicate retinal tissues.

Therapeutic levels of ultrasound should not be applied over a pregnant uterus. Animal studies have suggested that the temperature elevation produced by an ultrasound beam may cause low birth weight, brain size reduction, and various orthopedic deformities.

Ultrasound applied over the testes may produce a prolonged elevation of testicular temperature that may result in temporary sterility.

Malignant tissue should not be ultrasounded.  In vitro studies have suggested the possibility that ultrasound may promote malignant cellular detachment and metastasis.

When treating an immature recipient, caution should be exerted to avoid exposing long bone ends to ultrasound because of the remote possibility that the epiphysis may be damaged by exposure to ultrasound.  It should be noted, however, that the literature reviewed suggested that epiphyseal lines are, in fact, safe from normal therapeutic levels of ultrasound.  The evidence suggests that only ultrasound intensities above 3.0 W/cm², applied with a stationary sound head for periods of three minutes or more may damage epiphyseal plates and retard bone growth (also true for demineralization of the bone).

Thrombus formations (blood clots) should not be ultrasounded because of the danger of increasing thrombus formation and promoting emboli production.

Caution should be exercised when applying ultrasound to areas that suffer from inadequate circulation.  Elevated tissue temperatures may threaten such tissues since the heat transfer normally provided by the circulatory system is missing.  In such cases, the continuous waveform of ultrasound should be avoided and only the pulsed waveform used and that with caution.

Ultrasound should not be applied over areas containing prosthetic implants or other hardware.  Any stimulated metal implant may expand from the heat imparted to it by ultrasound.  This may cause it to push out against the supporting tissues (bone), only to shrink again when the heat has dissipated, leaving the implant loose within supportive tissues.  As a consequence, the implant may lose its stability and require removal and replacement (if possible).

References:

W.T. Coakley, "Biophysical Effects of Ultrasound at Therapeutic Intensities", Physiotherapy, 64:6, June 1978. Pp. 166-168

M. Dyson, J.B. Pond, J. Joseph, and R. Warwick, "Stimulation of Tissue Regeneration by Pulsed Plane-Wave Ultrasound", IEEE Transactions on Sonics and Ultrasonics, July 1970. Pp. 133-139

M. Dyson and J. Suckling, "Stimulation of Tissue repair by Ultrasound: a Survey of the Mechanisms Involved", Physiotherapy, 64:4, 1978. Pp. 105-108

C.S. Enwemeka, "The Effects of Therapeutic Ultrasound on Tendon Healing", Am J Phys Med Rehabil, vol. 68, 1989. Pp. 283-287

B.H. Ferguson, A Practitioners Guide to the Ultrasonic Therapy Equipment Standard, U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration, Rockville, Maryland, July 1985.

G.T. Haar, "Basic Physics of Therapeutic Ultrasound", Physiotherapy, 64:4, April 1978. Pp. 100-104

A. Hartley, Therapeutic Ultrasound, Anne Hartley Agency, Etobiocoke, Ontario, 1987.

J.F. Lehmann, B.J. DeLateur and D.R. Silverman, "Selective Heating effects of Ultrasound in human beings", Archives of Physical Medicine and Rehabilitation, June 1966.

F.E. Miller and J.B. Weaver, "Ultrasound Therapy", The Physical Therapy Review, 34:11, 1954. p. 562

E.M. Oakley, "Application of Continuous Beam Ultrasound at Therapeutic Levels", Physiotherapy, 64:6, June 1978. Pp. 169-172

D.C. Reid and G.E. Cummings, "Efficiency of Ultrasound Coupling Agents", Physiotherapy, 63:8, August 1977. Pp. 255-257

H.P. Schwan and E.L. Carstensen, "Advantages and Limitations of Ultrasonics in Medicine", JAMA, vol. 149, May 1952. Pp. 121-125

L.P. Taylor, T. Hui, The Taylor Technique of Soft Tissue Management, Inflammation: Evaluation & Treatment, 2002.  Pp. 44-63

Therapeutic Ultrasound, Reprinted from Physiotherapy, March/April, 1987, Journal of the Chartered Society of Physiotherapy, London, England.

F.S. Zach, B. Boynton, K. Phillips, and E. Smith, "Localized Application of Ultrasonic Energy", British Journal of Physical Medicine, August 1957.

M.C. Ziskin and S.L. Michlovitz, "Therapeutic Ultrasound", Thermal Agents in Rehabilitation, F.A. Davis Co., Philadelphia, Pa., 1986. Pp. 141

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