Do you really know what we should about bioeffects?

Blog posting By Jessica Farrow
1st year DMS student

Montgomery College

Do you really know what we should about bioeffects?

As professionals, diagnostic medical sonographers keep abreast on new and current issues, but how often do they renew their understanding on topics that affect them and their patients on a daily basis?

The ultrasound machines used every day, have greatly improved over the past ten years. They have more diagnostic capabilities and applications. The main reasons for these improvements have been the permission from the FDA, in 1992, to allow higher acoustic intensity limit (720 mW/cm2), in addition to technological advancements.

We are all happy for these improvements but with improvements to our machines come more experiments on bioeffects. It is our responsibility to maintain the excellent safety record of diagnostic medical sonography. The only way we can do this is to reeducate ourselves by reading new studies and by learning how to use our equipment in the safest and most efficient way.

Unfortunately, a recent study conducted by Sheiner, MD, Shoham-Vardi, PhD, and Abramowicz, MD, showed only 15.9 % of physicians and 14% of other end users, Including sonographers, knew where to find the TI (Thermal Index) and MI (Mechanical index) during an ultrasound examination, When both of these values are located in the upper right hand side of the display screen.

These values should be monitored after each scanner adjustment for all patients especially during fetal scans. Even though a temperature elevation of 2 degrees Celsius above normal exposure for less then fifty hours has no confirmed bioeffects reported, we should still operate under the ALARA (As Low As Reasonably Achievable) principle to ensure our patients safety. To use the ALARA principle effectively, decrease the out put power and increase the amplification enough to still get an optimal image.

The Thermal index (TI) in prenatal examinations should not exceed 0.5 in the first trimester. If the TI is greater than 0.5 to 1.0 the time frame sound not exceed thirty minutes, and TI values grater than 2.5 should not be used for more then 1 minute. Use a mechanical index (MI) less than 0.4 when gas bodies are present such as fetal lungs.

In postnatal examinations, try to keep the TI under 2.0 but can go up to six for scans under thirty minutes. Limit scan time for TI over 6 to one minute. Use a mechanical index less then 0.4 if gas bodies are present and can go up to 1.9 (FDA Limit) if gas bodies are not present. These values can also be used for adult patients. The TI and MI can be increased in order to complete an effective study but it is always good to keep in mind the safest levels of TI and MI.

The AIUM reports that there are currently no harmful bioeffects according to ultrasound exposure reports. However, some bioeffects may be a concern in the future, but at this point, the benefit of ultrasound is far greater then the risk, as long as the machinery is maintained and operated by trained and competent individuals who use the power of ultrasound prudently.

Do you know where to look to determine if you are scanning to find the Thermal Index and Mechanical Index for the exam you are performing? Would you know what to do to decrease the output power? Think about ALARA the next time you are scanning.

Resources:

“Sonography Introduction To Normal Structure And Function” Second Edition: Curry, Tempkin :Elsevier, 2004: (Page 22)

“Understanding Ultrasound Physics” Third Edition: Edelman, Ph.D : ESP, Inc, 2007 : (page 369-382)

Ultrasound Biosafety Considerations for the Practicing Sonographer and SonologistNelson et al. J Ultrasound Med.2009; 28: 139-150

J Ultrasound Med 200/;26:319-325