Ultrasound Safety: Thermal and Mechanical Indices and the ALARA Principle
Mechanical and Thermal Indices
In practice, the user sees the active output of the device through two indices — mechanical (MI) and thermal (TI), which should be displayed on the monitor. Depending on individual settings, these indices change in real-time and should not exceed the maximum values established as 'Good Practice' for specific medical applications (EFSUMB Position Paper, Wüstner et al., 2022).
MI is associated with the potential for acoustic cavitation — the rapid and destructive collapse of a bubble capable of causing local tissue damage (WFUMB, Shiina et al., 2015). At MI > 0.7, there is a theoretical risk of inertial cavitation, which is more pronounced when using ultrasound contrast agents (Postema et al., 2018). Despite discussions about the validity of MI, there is currently no alternative indicator in clinical settings for assessing safety concerning cavitation damage.
TI is a rough indicator of tissue temperature increase under ultrasound exposure and is defined as the ratio of transmitted power to the estimated power required to heat tissue by 1 °C. It should be noted that TI does not reflect the actual temperature increase (Postema et al., 2018).
The ALARA Principle
ALARA (As Low As Reasonably Achievable) is a principle that should always be adhered to whenever possible (Kollmann et al., 2013). According to ALARA, diagnostic ultrasound can be considered safe at the lowest possible thermal and mechanical index values while maintaining the highest possible scanning quality (EFSUMB, Fodor et al., 2022). In unclear situations, ALARA is always the best choice within the benefit/risk analysis (Wüstner et al., 2022).
According to AIUM recommendations, each examination should consider potential benefits and risks, and the ALARA principle should be observed concerning factors affecting acoustic output, as well as dwell time and total scanning time (AIUM, 2023–2024).
Safety of Elastography and ARFI
When using acoustic radiation force (ARFI), significant temperature increases are possible, especially if bone is in the beam; with ARFI, the temperature maximum is at the focus, whereas in B-mode, it is near the transducer (ECMUS/EFSUMB, Fodor et al., 2022). Simulations have shown a possible temperature increase of about 5 °C in the presence of bone or sensitive tissues (eye, fetus), with the maximum at the focus (Săftoiu et al., 2019).
Radiation force-based methods typically use pulses with amplitudes 1.0 < MI < 1.9 and longer durations (several hundred cycles) compared to conventional diagnostic imaging (10–20 cycles for Doppler methods) (WFUMB, Shiina et al., 2015). Tracking beams, repeated at high frequency, use pressures close to the upper FDA limit (MI < 1.9). In ARFI imaging, displayed indices (MI and TI) may be underestimated. To ensure safety in ultrasound elastography, the ALARA principle should be applied (LoE 2b, GoR B) (EFSUMB, Săftoiu et al., 2019).
| Indicator | What it Reflects | Key Threshold/Value |
|---|---|---|
| MI (Mechanical Index) | Potential for Acoustic Cavitation | MI > 0.7 — theoretical risk of inertial cavitation; FDA upper limit MI < 1.9 |
| TI (Thermal Index) | Expected Tissue Heating (not actual temperature) | Ratio of transmitted power to power for heating by 1 °C |
Frequently asked questions
What does MI > 0.7 mean?
At MI > 0.7, there is a theoretical risk of inertial cavitation, which is more pronounced when using ultrasound contrast agents (Postema et al., 2018).
Does TI reflect the actual temperature in tissues?
No. TI is only a rough indicator of temperature increase; it does not show the actual rise and is defined as the ratio of transmitted power to the power needed to heat tissue by 1 °C (Postema et al., 2018).
What MI values are used in ARFI/elastography?
Radiation force-based methods typically use pulses with amplitudes 1.0 < MI < 1.9 and longer durations. In ARFI, displayed MI and TI may be underestimated (WFUMB, 2015; Săftoiu et al., 2019).
When is the risk of heating in ARFI maximal?
In the presence of bone in the beam or sensitive tissues (eye, fetus); simulations showed a possible increase up to ~5 °C with the temperature maximum at the focus (Săftoiu et al., 2019; EFSUMB, 2022).
What factors does a physician control according to ALARA?
Factors affecting acoustic output, as well as dwell time and total scanning time; MI and TI should be kept as low as possible without compromising quality (AIUM 2023–2024; EFSUMB 2022).