Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC TR 62799: Models for Evaluation of Thermal Hazard in Medical Diagnostic Ultrasonic Fields
Exploring advanced thermal indices and dosimetry models for assessing temperature rise risks during diagnostic ultrasound examinations
Introduction to IEC TR 62799
IEC/TR 62799:2013 is a Technical Report that provides background information and detailed analysis of potential replacements for the conventional Thermal Index (TI) used in diagnostic ultrasound safety. Prepared by IEC Technical Committee 87 (Ultrasonics), this report addresses known limitations of the existing TI approach as defined in IEC 60601-2-37 and IEC 62359, and explores several alternative models for more accurately assessing thermal hazard during medical ultrasound examinations.
The clinical importance of this work cannot be overstated. Diagnostic ultrasound is one of the most widely used medical imaging modalities, with billions of examinations performed globally. While generally safe, ultrasound energy is absorbed by tissue and converted to heat, and the potential for thermally induced bio-effects — particularly in sensitive tissues such as the developing fetus, neonatal brain, or eye — requires careful monitoring. The TI was introduced as a real-time display parameter to alert operators to potential thermal risk, but research has increasingly shown that the current TI formulation has significant limitations.
The existing TI has several known limitations: it uses a linear display scale that compresses clinically relevant variations, assumes equilibrium temperature conditions that may not be reached during short exposures, uses simplified tissue parameters, and does not adequately account for transducer self-heating.
Alternative Models Evaluated
IEC TR 62799 evaluates five distinct alternative approaches to thermal hazard assessment, each with specific advantages and trade-offs:
| Model | Acronym | Basic Principle | Key Advantage |
|---|---|---|---|
| Exponential TI Display | TInew | Logarithmic/exponential scale for better resolution at low TI values | Improved visual differentiation near safety thresholds |
| Thermally Equivalent Time Index | TETI | Converts temperature history into equivalent time at 43°C | Accounts for cumulative thermal dose |
| Time to Threshold | TT | Predicts time required to reach a critical temperature rise | Directly relevant to clinical decision-making |
| Safe Use Time | SUT | Maximum exposure duration not exceeding a reference hazard level | Simple, intuitive safety limit format |
| Thermally Equivalent Time Displayed | TETD | Real-time display of cumulative t₄₃ thermal dose | Provides running risk estimate during scanning |
Each model is thoroughly analyzed in the report with theoretical foundations, illustrative results, a comparison of advantages and shortcomings, and representative exposure scenarios. The thermal dose concept (t₄₃) — the equivalent exposure time at 43°C that produces the same bio-effect as the actual time-varying temperature — serves as a common reference across multiple models.
The thermal dose concept (t₄₃) is borrowed from hyperthermia oncology research. It uses a thermal normalization constant R = 4.0 for temperatures at or below 43°C and R = 2.0 for temperatures above 43°C, reflecting the different rates of thermal damage accumulation in tissue.
Scientific Foundations and Tissue Parameters
A major contribution of IEC TR 62799 is its detailed analysis of the tissue parameters that influence ultrasonic heating. The report examines acoustic absorption coefficients, perfusion effects, and the thermal properties of different tissue types (soft tissue, bone, and fetal tissue). It specifically addresses the limitations of using homogeneous tissue models and explores the impact of blood perfusion as a cooling mechanism that varies significantly between tissue types and physiological states.
The report also presents temperature-time curves based on both theoretical models (Pennes bioheat equation) and empirical measurements. These curves form the basis for establishing conservative boundaries for safe exposure durations, with separate recommendations for fetal and non-fetal exposures. The distinction between scanning and non-scanning modes is emphasized, as scanning distributes energy over a larger volume, reducing peak temperature rise.
The time-to-threshold (TT) model provides clinically actionable information: it estimates how long an ultrasound examination can continue at current output levels before the temperature rise reaches a predefined threshold (e.g., 4°C). This gives operators a clear safety margin to work within.
Recommendations and Future Directions
After systematic evaluation, IEC TR 62799 recommends the Thermally Equivalent Time Index (TETI) as the most promising replacement for the current TI. TETI combines the advantages of thermal dose concepts with the simplicity of a numerical index, providing better correlation with actual biological risk than the existing TI. The report also notes that different models may be appropriate for different clinical scenarios — for example, TT may be more suitable for interventional procedures where specific temperature thresholds are critical, while TETD is better for long-duration monitoring.
The report serves as essential reading for ultrasound system manufacturers developing next-generation safety displays, regulatory bodies reviewing safety standards, and clinical researchers investigating ultrasound bio-effects.
FAQs
Q: What is the thermal index (TI) and why does it need replacement?
A: TI is a real-time display parameter showing the ratio of emitted acoustic power to the power needed to raise tissue temperature by 1°C. Current TI has limitations: linear display compresses important variations, assumes equilibrium not reached in short scans, uses simplified models, and doesn’t account for transducer self-heating.
A: TI is a real-time display parameter showing the ratio of emitted acoustic power to the power needed to raise tissue temperature by 1°C. Current TI has limitations: linear display compresses important variations, assumes equilibrium not reached in short scans, uses simplified models, and doesn’t account for transducer self-heating.
Q: What is the significance of the threshold temperature of 43°C?
A: 43°C is established from decades of hyperthermia research as the temperature above which thermal damage accumulates more rapidly. The Arrhenius relationship shows that the rate of thermal damage doubles for every 1°C increase above 43°C (R=2), compared to a quadrupling every 1°C below 43°C (R=4).
A: 43°C is established from decades of hyperthermia research as the temperature above which thermal damage accumulates more rapidly. The Arrhenius relationship shows that the rate of thermal damage doubles for every 1°C increase above 43°C (R=2), compared to a quadrupling every 1°C below 43°C (R=4).
Q: Does the report distinguish between different types of ultrasound examinations?
A: Yes. A key distinction is between fetal exposures (where thermal sensitivity is highest) and non-fetal exposures. The report also differentiates between scanning and non-scanning modes, as scanning distributes energy spatially and reduces peak heating.
A: Yes. A key distinction is between fetal exposures (where thermal sensitivity is highest) and non-fetal exposures. The report also differentiates between scanning and non-scanning modes, as scanning distributes energy spatially and reduces peak heating.
Q: Have any of these alternative models been adopted in current ultrasound safety standards?
A: The report was published to inform ongoing standards development. As of the standard’s publication, the alternatives were under consideration for future revisions of IEC 62359 and IEC 60601-2-37. Users should check the latest editions for current requirements.
A: The report was published to inform ongoing standards development. As of the standard’s publication, the alternatives were under consideration for future revisions of IEC 62359 and IEC 60601-2-37. Users should check the latest editions for current requirements.
