Guidelines for the Safe Use of Ultrasound: Part I - Medical and Paramedical Applications (excerpt from Safety Code 23, 1989)

4. Safe Use Guidelines

4.1 Ultrasound Physical Therapy

4.1.1 Ultrasound Therapy Devices Regulations

A common conclusion from surveys conducted since 1974(Footnote 70,Footnote 74) on the clinical use of ultrasound therapy devices was that these devices were generally unable to deliver prescribed doses to patients with a reasonable degree of accuracy. This occurred because the indicated output of the device often had little relation to the actual acoustic output. Excessive exposures either lead to unnecessary risk or fail to achieve clinical benefit. Inadequate exposures fail to achieve clinical benefit resulting in an unnecessary exposure to ultrasound. Since the intensity levels used in ultrasound therapy are in the range where adverse biological effects have been observed in animal studies, it is essential that the treatment doses are indicated and delivered accurately.

To ensure that new devices can deliver prescribed exposures, the Canadian Ultrasound Therapy Devices Regulation(Footnote 75) was promulgated under the Radiation Emitting Devices Act(Footnote 76) in 1981 and amended in 1984.

The requirements include a number of specifications for the safe use of ultrasound therapy devices. They may be divided into the following four areas: (1) indicators; (2) labelling; (3) output control; and (4) timer specifications.

An important feature of the regulation is that the maximum temporal average effective ultrasonic intensity must not exceed 3 W/cm2 . This value was chosen for several reasons: (i) higher intensities do not seem to be required for efficacy. Survey data indicate that 3 W/cm2 is commonly found as the maximum nominal intensity available for most devices. This intensity value has been accepted by European manufacturers for many years as the maximum necessary for therapy. (ii) Higher intensities may be painful or damaging. For example, the work of Payton et al.(Footnote 43) suggests that intensities above 3 W/cm2 applied near a bone would either be unbearable or cause bone damage. Other studies noted by Lehmann(Footnote 26) suggested that damage tended to occur for intensities greater than 3 W/cm2 . Spatially non-uniform beams with temporal average effective ultrasonic intensities greater than 3 W/cm2 could have I(SPTA) in situ intensities substantially higher than the focal lesion curve of Figure 2.1.(Footnote 1)

4.1.2 Care of Equipment

Calibration - It is recommended that ultrasound therapy devices be calibrated by suitably trained personnel at least once each month(Footnote 74) to ensure that the ultrasonic power is indicated with an accuracy of ±20%. The timer accuracy should also be checked to ensure that it has the accuracy stated in Radiation Emitting Devices Regulations. These procedures should also be performed upon delivery of the device. A list of power meters and hydrophones is available from the Consumer and Clinical Radiation Protection Bureau on request.

Maintenance - Mechanical shocks and overheating of either the applicator or housing must be avoided. Abrasives or chemicals must not be used on the applicator face. If such action occurs, or is suspected, the ultrasonic power calibration should be checked.

Testing of Applicator - To avoid applicator equipment damage, it should not be tested by placing droplets of water on its face and by seeing if they vaporize in order to ascertain the emission of ultrasound. A better practice is to immerse the applicator head in a basin of water, point the beam towards the side of the container, and observe whether the sound waves cause ripples at the edge of the basin.

4.1.3 Operator Safety Precautions

Operators of ultrasound therapy devices should minimize exposure
to themselves as follows:

  1. The applicator face must not be touched by the operator when the device is emitting ultrasound.
  2. The operator must not immerse any part of his/her body in the water bath in the path of the direct transmitted beam, while an ultrasound therapy device is operating in that water bath.
  3. Ultrasound therapy devices should only be turned on when the face of the applicator is in acceptable acoustic contact with the patient, and the operator is holding the applicator by its handle. Adherence to this guideline also reduces the possibility of thermal damage to the applicator.
  4. To help avoid any reflected or scattered ultrasound from entering the operator's hand during underwater treatments, a dry soft knit glove may be worn inside a rubber glove to provide air-gap protection.

4.1.4 Patient Safety Precautions

To minimize potential adverse health effects, the operator should use the minimum patient exposure required to achieve the desired benefit.

The operator should be present all the time during an ultrasound exposure, so the intensity can be reduced or the treatment can be terminated if the patient shows the least sign of distress. Records should be kept of each patient, noting the exposure levels, times, and couplant used. Maintaining well documented, reproducible exposure conditions, should help minimize unnecessary exposure.

Since ultrasound is almost totally reflected at an air-tissue interface, coupling media must always be used between the applicator surface and the patient. Poor coupling could lead to all or most of the ultrasound energy being dissipated in the applicator. Subsequent heating of the applicator could either damage it and/or burn the patient. Furthermore, where a significant fraction of the ultrasound beam is expected to propagate to the site of exit from the body, it may be advisable to avoid undesirable reflections by ensuring that the region being treated is coupled to, and rests on, an absorber.

The transducer should be kept moving slowly, angled at 90° to the treatment area, during the course of treatment to minimize the risk of causing hot spots (undue temperature rise in a single volume of tissue receiving excess exposure). Another reason to avoid the use of a stationary transducer concerns the danger that the peak intensity will remain over the same tissue for the full treatment and prolonged standing waves could develop. Prolonged standing waves might result in blood flow arrest and cause possible damage to the endothelial cells in the blood vessel walls. This could also lead to the formation of blood clots(Footnote 47).

If any pain or uncomfortable "prickly" sensation is felt by the patient, this may be an indication that the bones or nerve endings in the vicinity of the ultrasonic beam are becoming, or are already, overheated. In this case the ultrasound power or intensity must be immediately reduced.

4.1.5 Contraindications

Contraindications to ultrasound therapy have been listed in several publications(Footnote 1,Footnote 26,Footnote 64). Some of these are based on a general understanding of the principles and practice of ultrasound therapy. Others may arise from extrapolation of specific scientific experiments or are based on the personal clinical experience of the physiotherapists listing the contraindications. The following list is similar to that of Reid(Footnote 77) and Oakley(Footnote 78). Where possible, other references or rationales for the contraindications are also given.

Ultrasound physical therapy should not be applied to any patient with obtunded reflexes or to any area with significantly diminished pain sensitivity or heat sensitivity(Footnote 1,Footnote 26,Footnote 77,Footnote 78).

No pregnant or potentially pregnant patient should ever receive ultrasound therapy in any area of the body which is likely to result in exposure to the fetus. Overheating of the fetus could result. The fetus is at particularly high risk during the first trimester, during the period of organogenesis(Footnote 1,Footnote 57,Footnote 77,Footnote 78).

Ultrasound should not be applied to the eye in physiotherapy procedures since the lens has limited means (due to being avascular), for removing heat and has a relatively high absorption coefficient. Similarly, any region of significantly diminished blood circulation should not undergo irradiation, except at low intensities where wound healing may be expected(Footnote 1,Footnote 26,Footnote 77,Footnote 78).

Ultrasound should not be applied to the brain, spinal cord or large subcutaneous peripheral nerves(Footnote 77,Footnote 78).

To avoid the possibility of spinal cord damage, it is advisable to avoid using ultrasound over the vertebral column following laminectomies or when any anaesthetic area is involved(Footnote 26,Footnote 77).

Ultrasound physical therapy should not be applied to the reproductive organs(,Footnote 26,Footnote 77,Footnote 78).

Care should be taken not to irradiate neoplastic tissues as there is some evidence that inappropriate heating patterns, giving rise to temperatures less than 42°C, may stimulate tumor growth or promote metastases(Footnote 1,Footnote 77,Footnote 79).

Care should be taken not to irradiate epiphyseal lines in children(Footnote 26).

Treatment of acute infection of bone or tissue should not be carried out as the treatment could force areas of pus into surrounding tissue, thereby spreading infection(Footnote 26,Footnote 77,Footnote 78).

Ultrasound should not be used in the thoracic area if the patient has a cardiac pacemaker of any kind(Footnote 1,Footnote 78).

Blood vessels in poor condition should not be treated as the vessel walls may rupture as a result of the exposure(Footnote 78).

Patients suffering from cardiac disease should not receive treatment over the cervical ganglia, the stellate ganglion, the thorax in the region of the heart, or the vagus nerve, as a reflex coronary vasospasm might result. Only low intensities and short treatment times should be used if these patients are treated in other areas since the stimulation of practically any afferent autonomic nerve (especially the vagus nerve) in the body may cause a change in cardiac rate(Footnote 77,Footnote 78).

Patients with thrombophlebitis or other potentially thromboembolic diseases should not be treated since a partially disintegrated clot could result in an obstruction of the arterial supply to the brain, heart or lungs(Footnote 77,Footnote 78).

4.1.6 Operator Training

Skilled personnel using reliable and accurately calibrated equipment are necessary to deliver prescribed doses of ultrasound to patients both safely and effectively. Improper training increases the risk of incorrect usage which can, at best, reduce the benefits of ultrasound and, at worst, result in tissue damage.

Therefore operators should have successfully completed a recognized course in ultrasound therapy which should include instruction in: basic physics, instrumentation, biological effects of ultrasound, indications and contraindications, dosage prescription, and application techniques. A sound knowledge of anatomy, particularly surface anatomy, is an essential prerequisite.

Courses are available in eleven universities across Canada in their programs of Physical Therapy or Rehabilitation. A list of these courses is available from the Canadian Physiotherapy Association in Toronto.

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