Ultrasound

Characteristics and use

Ultrasound consists of mechanical (acoustic) waves, historically characterised as frequencies above the upper human audible limit. This limit varies in the human population, and for people with normal hearing there is a steep rise in hearing threshold over frequencies of 10-20 kHz. Ultrasound is broadly categorised into ultrasound that travels in the air, termed airborne ultrasound, and ultrasound that travels in condensed media such as solids, liquids and biological tissue.

Ultrasound is widely used in medicine for both diagnostic and therapeutic purposes. Diagnostic ultrasound has advanced remarkably in the past 50 years. It can provide excellent visualization of internal anatomy and is used extensively in clinical practice. Therapeutic applications include physical therapy, treatment of kidney stones, cancer therapy and dental applications. Ultrasound is also used for cosmetic treatments including skin tightening, wrinkle removal and body sculpting.

Airborne ultrasound is produced in industry by various applications, such as cleaning, drilling, welding and emulsifying, and is used in various commercial products, such as pest repellents, burglar alarms, remote controls and guidance devices for the blind. Airborne ultrasound is also increasingly being used in virtual reality technology to create the experience of touch.

Ultrasound effects on the body and health implications

Exposure to high-intensity ultrasound can heat biological tissue and potentially lead to heat-related health effects. Diagnostic ultrasound typically operates at low levels where no significant heating occurs. High intensity ultrasound may be used for therapeutic and cosmetic purposes where heating is required for the efficacy of the treatment.

High-intensity ultrasound exposure to the body can also cause cavitation, which is the formation, growth and collapse of bubbles in tissue. Ultrasound cavitation can break down tissue and is used for therapeutic and cosmetic purposes.

The evidence does not point to any serious adverse effects in patients from exposure to diagnostic ultrasound as used in normal clinical practice, in the absence of ultrasound contrast agents. Relatively little information is available on the interaction between ultrasound and contrast agents and further research is required.

Airborne ultrasound, although characterised as being acoustic waves in air that generally can’t be heard, may contain high-frequency audible components. Prolonged exposure to high-intensity audible sound can cause adverse auditory effects, such as hearing impairment and tinnitus.

Airborne ultrasound has also been associated with the occurrence of non-specific symptoms such as headaches, fatigue, dizziness and feelings of discomfort or annoyance. It is currently unclear whether non-specific symptoms are the direct result of audible or inaudible ultrasound and further research is required.

Protection

Safety standards and procedures on the safe use of ultrasound for medical applications are usually available from relevant national and international medical authorities. ICNIRP has published a statement on diagnostic devices using non-ionizing radiation (NIR) that includes ultrasound, and has considered the use of ultrasound in medicine in a previous international workshop. ICNIRP has also published a statement on NIR exposure for cosmetic purposes, including the use of ultrasound.

The International Radiation Protection Association (IRPA) published interim guidelines on limiting human exposure to airborne ultrasound in 1984, based on the limited scientific evidence that was available at that time. ICNIRP has published a statement on the validity of the IRPA guidelines which makes a number of recommendations for future research on airborne ultrasound.