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Recommendations for Improved Safety of Lung Ultrasound

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Lung ultrasound (LUS) has been emerging as a vital clinical tool. LUS aids in diagnosing a range of conditions, from pneumonia to respiratory distress syndrome or pulmonary edema. LUS was also very significant at the height of the COVID-19 pandemic, when point-of-care lung monitoring modalities were crucial.  

Diagnostic ultrasound standards and safety guidelines were established in the late 20th century to ensure the safety of ultrasound imaging and avoid ultrasound bioeffects in tissues. The Thermal Index (TI) and Mechanical index (MI) are two ultrasound exposure indices that respectively indicate the risks of tissue heating and cavitation and which must be displayed in real time during scanning. However, the lung is a tissue like no other, and the bioeffects observed in animal studies (in mice, rabbits, pigs, and monkeys) are very different from the bioeffects observed in other tissues. Capillary pulmonary hemorrhage is a unique bioeffect that is correlated to the MI. In order to avoid such specific ultrasound bioeffects, a new safety paradigm must be created for LUS.

Despite guidelines recommending MI ≤ 0.4, recent research suggests that a further reduction to MI ≤ 0.3 for enhanced safety might be needed. In addition, it is critical to account for the actual MI in situ, which is influenced by the thickness of the chest wall. This is particularly concerning in neonatal LUS safety, due to thin chest walls and intensive use.

Existing safety education varies among practitioners, and surveys indicate a lack of knowledge regarding lung ultrasound safety. In the absence of an appropriate preset, pre-installed on all machines, for neonatal LUS guaranteeing an MI ≤ 0.3, the risk of error and exposure to higher MI is significant. In pediatric and adult patients with a thicker chest wall, a higher MI would be acceptable, as long as adherence to the “as low as reasonably achievable” (ALARA) safety principle is maintained.

Overall, the recommendations for Improved Safety  of Lung Ultrasound are:

  1. To install a preset on all ultrasound machines limiting MI to ≤ 0.3 for neonatal cases.
  2. To provide a user-friendly means for practitioners to select the safety preset without manual adjustments.
  3. To allow higher MI values for pediatric and adult patients when needed for optimal imaging, considering higher ultrasound attenuation in thicker chest walls.
  4. To guide practitioners in adhering to the As Low As Reasonably Achievable (ALARA) principle and by considering the chest wall attenuation for MI > 0.3.
  5. To develop a specific Mechanical Index for Lung (MIL). The creation of a unique MIL for LUS, displayed on-screen to estimate pleural exposure accurately would increase safety and safety awareness among practitioners.

Enhancing safety in LUS requires a multifaceted approach, encompassing preset implementation, practitioner education, and technological advancements. The proposed recommendations aim to address current safety challenges, ensuring the continued effectiveness and safety of lung ultrasound in diverse clinical settings and for diverse populations (from neonates to high BMI patients). By combining technological innovations with user-friendly controls, the proposed safety paradigm seeks to strike a balance between optimal imaging outcomes and patient safety in the evolving landscape of LUS.

For more information, see the “Statement and Recommendations for Safety Assurance in Lung Ultrasound” from the American Institute of Ultrasound in Medicine (AIUM)

Marie Muller, PhD, is an Associate Professor of Mechanical and Aerospace Engineering at NC State University.

Ultrasound: How to respond to questions about its safety

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“Is ultrasound safe for my baby?” and “I know someone whose baby was born too small because of all the ultrasound she received during her pregnancy”. These are two sentences that you might hear during your busy day in the ultrasound unit. The AIUM Official Statement “Conclusions Regarding Epidemiology for Obstetric Ultrasound” states: “Based on the available epidemiologic data, there is insufficient justification to warrant conclusion of a causal relationship between diagnostic ultrasound and recognized adverse effects in humans. The epidemiologic evidence is based primarily on exposure conditions existing prior to 1992, the year in which maximum recommended levels of acoustic output for ultrasound machines were substantially increased for fetal/obstetric applications. Some older studies have reported effects of exposure to diagnostic ultrasound during pregnancy, such as low birth weight, delayed speech, dyslexia, and non-right-handedness. Other more recent studies have not demonstrated such effects. The absence of definitive epidemiologic evidence does not preclude the possibility of adverse effects of ultrasound in humans.”

Why is this statement important to all practitioners of ObGyn ultrasound?

Because knowing the information will enable you to answer patient questions and comments mentioned at the beginning of this post. What the AIUM statement explains is that studies performed on specific large human populations, with defined methods did not show that diagnostic ultrasound is responsible for harm in humans. (Studies such as this are what epidemiology does: examine how often diseases occur in different groups of people and why.)

While in the past, there were some publications that suggested some effects, such as low birth weight, more controlled studies have not been able to demonstrate such effects in humans. An important point is that many studies are relatively old and were performed before maximum recommended output of ultrasound machines meant for OB use was increased from 94mW/cm2 to 720mW/cm2.  This increase was intended to obtain more detailed images. The US Food and Drug Administration (USFDA) agreed with ultrasound instrument manufacturers’ requests to allow this increase, on the condition that two numbers were displayed in real-time on the monitor of the ultrasound system:

  • The thermal index (TI) to show the possibility of increased temperature, secondary to energy absorption by the tissues, and
  • The mechanical index (MI) to convey the risk of direct effects of the sound waves.

If these are kept low, no noxious effects are demonstrable, as expressed in the Epidemiology statement. This includes physical as well as mental effects. What are low indices? If the TI is <1 (the scientific number is 0.7, but 1 is easier to remember), there appears to be no risk of thermal effects for exposure under 1 hour. Regarding non-thermal or mechanical effects, based on the absence of gas bubbles in the fetal lungs and bowels (the two organs where effects were shown in animals after birth), no effects are expected in human fetuses. Demonstrating long-term effects or lack thereof, particularly if subtle, is much more complicated.

The statements issued by the AIUM’s Bioeffects Committee are intended as baseline considerations in practice. As ultrasound continues to be adopted into clinical use, the Bioeffects Committee will continue to monitor outcomes in order to inform and educate the community.

Jacques S. Abramowicz, MD, is a professor in the Department of Obstetrics and Gynecology at the University of Chicago.

Interested in learning more about the bioeffects of ultrasound? Check out the following AIUM Official Statements:

Also:

Abramowicz JS, Fowlkes JB, Stratmeyer ME, Ziskin MC. Bioeffects and Safety of Fetal Ultrasound Exposure: Why do we Need Epidemiology? In: Sheiner E, (ed.): Textbook of Epidemiology in Perinatology. New York: Nova Science Publishers, Inc.; 2010.