Ultrasound Safety – Revisited

by Kevin T. Rooker, RT(R), RDMS, RVT, Sonography Consultantsultrasound1

Is ultrasound safe? Will it hurt my baby? These are questions we sometimes hear from our clients. We need to be able to answer those questions with confidence for several reasons. First, because our patients deserve an honest answer, and second because we never know who is listening. We know that there are some that think you should not be performing limited OB ultrasound and will always be looking for reasons to justify their position. Let's not give them that opportunity on the issue of ultrasound safety. Unfortunately, we, the medical community, have not done as well as we can at educating ourselves on the safety of ultrasound1.

Ultrasound is a wave of mechanical energy that penetrates human tissue as an oscillating (alternating) wave of high and low pressure. As it does so, there are two potential types of biological effects; Mechanical and Thermal. In 1993, the FDA allowed ultrasound manufacturers to significantly increase the amount of ultrasound energy created in diagnostic ultrasound systems, as long as they displayed the MI (Mechanical Index) and TI (Thermal Index) on the screen for the operator (and our clients) to see. The premise being that if we know what the MI and TI are, what their limits are, and most importantly how to lower them, then we are being as safe as we possibly can.

The Mechanical Index is a safety metric which lets the operator know how much energy is being transmitted into the patient during a sonography examination. Remember that sound is created by pressure waves, so mechanical energy is transmitted into any object which receives sound. Sound waves can be quite powerful. For example, think of the thump on your chest when sitting next to a teenager's car with the high dollar stereo system. It is defined as the peak negative pressure (PNP) of the ultrasound wave (point of maximal rarefaction). In easier terms; think pressure change divided by time. Lots of pressure change over short periods of time can be damaging. The FDA has established a maximum MI of 1.9 for diagnostic imaging. Any machine capable of generating MI greater than 1.0 must display the MI onscreen. The FDA MI limit for obstetric sonography is 1.0.

The Thermal Index is another safety metric which lets the operator know the potential of creating heat (hyperthermia) with the ultrasound beam. Many assumptions are made in this calculation, and it is often thought that the heating potential is underestimated. So keep in mind that the TI formulation was not intended to, and cannot provide an accurate measure of temperature rise within a specific patient. Instead it was designed to provide the operator with a relative measure of risk for a particular imaging mode. A Thermal Index of one (TI 1) indicates conditions under which the rise in temperature would be likely to be 1°C. The thermal index is different for different types of tissue, and can be displayed on your system accordingly: soft tissue (TIs), bone (TIb) and cranium (TIc). In the first trimester, when using Doppler to hear and demonstrate the fetal heart, the TIs setting us used. The limit for TI varies with time, please reference the chart below from the British Medical Ultrasound Society, the entire document may be found here.

Ultrasound Safety

Enough about what the MI and TI are, how do we as operators keep them at safe levels? There are two basic concerns to remember.

First is the AIUM ALARA policy2; which is an acronym for As Low As Reasonably Achievable. Simply translated, it means to keep the output power settings as low as possible, that still allows for adequate images. Most ultrasound systems can operate with output power settings at about 50% and still produce quite satisfactory OB images. Have your system presets adjusted so that when you are performing OB sonography both (abdominally and transvaginally), the output power settings are set low. You can always increase them if clinically necessary. And keep in mind that you can increase the gain to make your image brighter, as gain is just how well the system is "listening", it has no effect on the TI.

The second concern is time. Keep the overall examination as short as is reasonable. If using Doppler to allow Mom to hear her baby's heartbeat, depending upon your ceter's policy, keep the Doppler exposure to about 5-10 seconds. Know where to find the MI and TI displays on your system. If you don't know, consult your operator's manual or contact the manufacturer of your ultrasound system for more information.

Diagnostic ultrasound in obstetrics has been around for the better part of 40 years. To date, no one has been able to prove (and many have tried) that diagnostic ultrasound, when used prudently (MI,<1.0 and TI<0.7), has had any adverse effects developing fetuses; and this includes the limited use of Doppler in the first trimester "Thus far, there have been no significant thermal effects documented in humans and at this time the possibility of having all the factors present to is highly unlikely 3." However, it has been shown that aborting a living fetus is fatal, every time.


1. Ultrasound Is Safe . . . Right? Resident and Maternal-Fetal Medicine Fellow Knowledge
Regarding Obstetric Ultrasound Safety. J Ultrasound Med 2011; 30:21–27
Sheiner E, Abramowicz JS. Clinical end users worldwide show poor knowledge regarding safety issues of ultrasound during pregnancy. J Ultrasound Med. 2008;27(4):499-501

2. As Low As Reasonably Achievable (ALARA) Principle Approved 4/2/2014 aium.org/officialStatements/39

3. FDA Recommendations for the Safe Use of Ultrasound in Obstetrics CNE article authored by Sherri A. Longo, M.D. Assistant Professor in the Department of Obstetrics and Gynecology, Division of Maternal-Fetal Medicine, Tulane University School of Medicine in New Orleans, Louisiana; e-edcredits.com/nursingcredits/article.asp?testID=29