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Safely Using Diagnostic Ultrasound

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The clinical applications for diagnostic ultrasound have expanded tremendously since its introduction in the late 1950s thanks to technological advancements in both hardware and software, enabling rapid diagnoses at the patient bedside. With this expansion, the medical specialties employing ultrasound as a diagnostic tool have also increased substantially, resulting in a consistently growing group of new users across all levels of medical training and practice.

Ultrasound has long been understood as a low-cost, portable, and ionizing radiation-free imaging method, which has, in part, fueled this rapid expansion. However, ultrasound is ultimately a type of mechanical energy that is able to penetrate tissue, yielding the potential for bioeffects. Practically, the potential for bioeffects is measured through the thermal index (TI) and mechanical index (MI), which provide indicators of the temperature elevation and likelihood of cavitation, respectively, at a particular scan setting. While there have been no independently confirmed adverse effects in humans caused by current diagnostic instruments without contrast agents, biological effects have been reported in pre-clinical mammalian systems, emphasizing the importance of proper clinical use. As diagnostic ultrasound expands to new users and clinical applications, it is imperative that we continue to understand and assess these potential bioeffects and educate new ultrasound users to continue to use ultrasound safely.

The AIUM bioeffects committee has long undertaken this task, examining emerging technologies and making recommendations based on findings. Recently, the bioeffects committee updated its statement on the “Prudent Clinical Use and Safety of Diagnostic Ultrasound”. This statement reaffirms the promise of ultrasound as a safe and effective tool for diagnostic imaging when used properly by qualified health professionals.

Specifically, we emphasize three main ways to ensure diagnostic ultrasound is used safely:

  1. Monitor acoustic outputs—The likelihood of bioeffects can increase by increasing acoustic outputs, indicated by the thermal and mechanical indices. Exposure time should also be monitored, as increased exposure time can also increase the likelihood of bioeffects.
  2. Follow the ALARA principle—The as low as reasonably achievable (ALARA) principle maintains that users employ the lowest acoustic output and shortest scanning time to reasonably achieve diagnostic-quality images.
  3. Only allow qualified professionals to use ultrasound—Ultrasound should be used only by qualified health professionals to provide medical benefit to the patient.

As new diagnostic ultrasound technologies are developed and evaluated, it will continue to be critical to ensure new users understand the proper use of diagnostic ultrasound and the potential for bioeffects, particularly as the use of ultrasound expands beyond traditional use cases and into the future—perhaps even one day into the home!

Alycen Wiacek, PhD, is an engineer, ultrasound researcher, and educator, working to develop new ultrasound-based imaging technologies and improve the quality and diagnostic accuracy of ultrasound. She is a member of the AIUM Bioeffects Committee and is passionate about developing technology to increase access to high quality ultrasound.

Understanding the Basics of Medical Ultrasound Safety in Musculoskeletal Ultrasound

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Musculoskeletal ultrasound (MSK US) is an invaluable diagnostic tool that provides real-time, dynamic imaging of muscles, tendons, ligaments, joints, and soft tissues. Its advantages include being non-invasive, relatively low-cost, and free of ionizing radiation. However, to maximize its benefits and ensure patient safety, it is crucial for practitioners to understand and apply certain fundamental principles, including ALARA (As Low As Reasonably Achievable) and the Mechanical Index (MI). Here, we provide an overview of these concepts and other essential information for new users of MSK US.

ALARA Principle

The ALARA principle stands for “As Low As Reasonably Achievable” and is a cornerstone of safe ultrasound practice. It emphasizes minimizing the patient’s exposure to ultrasound energy while still obtaining the necessary diagnostic information.

Key Strategies to Apply ALARA:

1. Optimize Scanning Parameters: Use the lowest possible settings for power, gain, and exposure time that still yield diagnostic quality images. Avoid unnecessary Doppler applications, which use higher energy levels.

2. Adjust the Probe Position and Angle: Efficient probe manipulation can improve image quality without increasing power output. Use proper ergonomics to maintain consistent and effective contact with the patient’s skin.

3. Limit Scan Duration: Conduct scans efficiently to minimize exposure time. Pre-plan the examination to focus on areas of interest and avoid prolonged scanning.

By adhering to the ALARA principle, practitioners ensure that ultrasound procedures are both effective and safe.

Mechanical Index (MI)

The Mechanical Index (MI) is a parameter used to evaluate the potential for mechanical bioeffects, such as cavitation, which can occur during ultrasound procedures. It is calculated based on the peak negative pressure of the ultrasound wave and the frequency of the ultrasound.

Understanding MI Values:

  • Low MI (<0.3): Safe for sensitive tissues; minimal risk of cavitation.
  • Moderate MI (0.3–0.7): Generally considered safe for routine diagnostic imaging.
  • High MI (>0.7): Increased risk of mechanical bioeffects; should be used with caution and justified by clinical need.

To maintain patient safety, it is essential to monitor and adjust the MI, especially during prolonged or intensive scans.

Thermal Index (TI)

Another crucial parameter in MSK US is the Thermal Index (TI), which estimates the potential for tissue heating. The TI is influenced by the duration of the ultrasound exposure and the intensity of the ultrasound beam.

Categories of TI:

  • TIS (Soft Tissue): Applies to imaging of soft tissues and abdominal organs.
  • TIB (Bone): Relevant for imaging near bone structures.
  • TIC (Cranial): Pertains to imaging the fetal skull or neonatal head.

For MSK US, TIB is the most relevant as it applies to imaging around bones and joints. Maintaining an appropriate TI helps prevent thermal damage to tissues.

Essential MSK US Techniques

1. Probe Selection: Use the appropriate probe for the area being examined. High-frequency linear probes (7–15 MHz) are commonly used for superficial structures like tendons and muscles, while lower-frequency probes are better for deeper structures.

2. Patient Positioning: Proper patient positioning is crucial for optimal imaging. Ensure the area of interest is accessible and the patient is comfortable to avoid movement that can degrade image quality.

3. Image Optimization: Adjust the depth, focus, gain, and time-gain compensation (TGC) to enhance image quality. Clear visualization of the anatomy is essential for accurate diagnosis.

4. Dynamic Examination: Utilize the dynamic nature of ultrasound to assess the movement and function of musculoskeletal structures. Real-time imaging can help identify abnormalities that static imaging may miss.

5. Documentation: Capture and store high-quality images and clips of the relevant findings. Proper documentation supports clinical decisions and facilitates communication with other healthcare providers.

Conclusion

Performing musculoskeletal ultrasound requires a solid understanding of key safety principles, such as ALARA and MI, as well as technical skills in image optimization and patient positioning. By adhering to these guidelines, practitioners can ensure safe and effective use of MSK US, providing valuable insights into musculoskeletal conditions and enhancing patient care.

Cynthia Owens, BA, is the Publications Coordinator for the American Institute of Ultrasound in Medicine (AIUM).

Interested in learning more about the basics of ultrasound? Check out these resources from the American Institute of Ultrasound in Medicine:

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.