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:
- To install a preset on all ultrasound machines limiting MI to ≤ 0.3 for neonatal cases.
- To provide a user-friendly means for practitioners to select the safety preset without manual adjustments.
- To allow higher MI values for pediatric and adult patients when needed for optimal imaging, considering higher ultrasound attenuation in thicker chest walls.
- 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.
- 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.
