From the Guidelines
A thermal index (TI) value of 3 in ultrasound indicates a significant potential for tissue heating, where the temperature of the exposed tissue could rise by approximately 3°C above normal body temperature during the examination, as it represents the ratio of acoustic power being used to the power required to raise tissue temperature by 1°C 1. This level of thermal index is considered high and requires careful consideration during diagnostic procedures. When performing ultrasound with a TI of 3, clinicians should minimize exposure time, particularly for sensitive tissues like embryonic or fetal structures, the eye, or the brain.
Key Considerations
- The ALARA principle (As Low As Reasonably Achievable) should be followed, meaning that scanning time should be limited to what is necessary for diagnostic purposes 1.
- Understanding the thermal index value helps sonographers balance the need for quality diagnostic images against the potential risk of tissue heating effects during ultrasound examinations.
- The thermal index is a measure of relative risk to the patient, providing a useful guide but not representing the absolute thermal risk to exposed tissue, which requires an accurate measurement of temperature rise in targeted tissues 1.
Clinical Implications
- A maximum TI of 1 has been suggested for use in ocular ultrasound to ensure that the applied ultrasound intensities do not likely increase the temperatures of ocular structures by more than 1 °C 1.
- Sonicated tissue will absorb a proportion of the delivered acoustic energy as heat, which may cause damage if allowed to accumulate, particularly in tissues with minimal blood flow like the lens, cornea, aqueous humour, and vitreous humour 1.
From the Research
Thermal Index (TI) Value of 3
- A TI value of 3 signifies a higher potential risk of thermal bioeffects during ultrasound imaging 2.
- The TI is a relative indicator of thermal risk, providing feedback to clinicians or sonographers to assess potential risks to patients 2.
- However, the provided studies do not specifically mention a TI value of 3, but rather discuss the general concept of TI and its limitations 3, 4, 2, 5, 6.
- The American Institute of Ultrasound in Medicine recommends that the TI formulations be updated to address inconsistencies and incorporate time dependence, but this has not been implemented yet 2.
- Other studies discuss the importance of observing MI/TI indexes and adhering to the ALARA principle to minimize potential risks during obstetric ultrasound examinations 5.
- A new index, the thermal dose index (TDI), has been proposed to assess the risk of adverse thermal effects, taking into account the examination duration and TI value 6.