Ultrasound Thermal Index (TI) and Mechanical Index (MI) Limits for Adult and Pregnant Patients
For safe ultrasound imaging in both adult and pregnant patients, the Thermal Index (TI) should not exceed 1.0 and the Mechanical Index (MI) should not exceed 0.3 for ophthalmic applications, while general adult ultrasound examinations permit MI values up to 1.9. 1, 2
Thermal Index (TI) Limits
Definition and Significance
- TI is defined as the ratio of a device's output acoustic power to the power required to raise tissue temperature by 1°C 1
- TI indicates the potential for heating in various tissues during sonication 1
- Temperature effects become more dangerous with both magnitude and duration of exposure 1
Specific TI Limits
- For ophthalmic ultrasound: Maximum TI of 1.0 has been approved by the US-FDA and recommended by the British Medical Ultrasound Society (BMUS) 1
- For general adult ultrasound: Accepted TI range is 0-2.0 2
- For obstetric/pregnancy ultrasound: Recommended TI range is 0-1.0 2, 3
Clinical Implications of Thermal Effects
- Toxic thermal effects become apparent between 39-40°C 1
- Protein coagulation occurs between 44-46°C 1
- Lethal dose at 40°C is measured in hours, while at 46°C it's measured in minutes 1
- Tissues with minimal blood flow (like ocular structures) are particularly sensitive to overheating due to limited heat dissipation 1
Mechanical Index (MI) Limits
Definition and Significance
- MI describes the likelihood of cavitation occurring in tissue exposed to ultrasound 1
- Lower MI values cause stable cavitation; higher values cause inertial cavitation with more damaging effects 1
- Inertial cavitation can result in high temperatures, microjet formation, and shock waves 1
Specific MI Limits
- For ophthalmic ultrasound: Maximum MI values set by US-FDA and BMUS are ≤0.23 and ≤0.3, respectively 1
- For general adult ultrasound: Accepted MI range is 0-1.9 2
- For obstetric/pregnancy ultrasound: Recommended MI range is 0-0.4 2
Clinical Implications of Mechanical Effects
- MIs above 0.3 in ophthalmic applications have been associated with:
- MIs below 0.3 generally show either no adverse effects or only reversible changes 1
Special Considerations for Pregnant Patients
- Doppler ultrasound studies during pregnancy can reach higher TI levels (mean 1.5) compared to B-mode imaging (mean 0.3) 3
- Doppler procedures should be performed with caution and be as brief as possible during obstetric sonography 3
- Research suggests that reducing ultrasound power from TI 1.0 to 0.1 does not alter Doppler visualization or measurements in the second half of pregnancy 4
- Studies have not identified consistent patterns of alterations in birth size when TI or MI intermittently exceeded 1.0 during prenatal ultrasounds 5
Practical Application
- Always monitor and minimize both TI and MI values displayed on ultrasound equipment 6
- Keep scan duration as short as possible while obtaining necessary diagnostic information 2
- Be particularly cautious with Doppler studies, which typically generate higher TI values than B-mode imaging 3
- For ophthalmic applications, maintain strict adherence to lower MI limits (≤0.3) due to the eye's sensitivity to mechanical effects 1
- Consider starting at lower power settings (TI 0.1) for obstetric examinations and increasing only if necessary 4
Common Pitfalls and Caveats
- The displayed TI is a relative risk measure and does not represent absolute thermal risk to exposed tissue 1
- Tissue heating may persist beyond the cessation of sonication, particularly in structures with poor blood flow 1
- The fast-healing nature of some tissues (like the eye) may mask initial damage in delayed assessments 1
- Microbubble contrast agents significantly increase the risk of mechanical bioeffects and should be used with additional caution 1
- Different tissue models (Soft Tissue Thermal Index, etc.) are used for different applications, which affects the interpretation of TI values 1