What values of TI and MI are recommended for fetal Ultrasound studies?
TI and MI recommendations for fetal studies include:
- • TIs values are used for first trimester studies.
- • TIb values are used for second and third trimester studies.
- • TI <0.5 should be used particularly for first trimester studies.
- • TI <0.5 may likely be used for extended scan times.
- • TI >0.5 and <1.0 should be limited to scan times of less than 30 minutes.
- • TI >2.5 should be limited to scan times of less than one minute.
- • MI ≤1.9 (the current FDA limit) can be used as needed in the absence of gas bodies.
In general, as for ionizing radiation, the principle of “as low as reasonably achievable (ALARA)” should be employed for ultrasound, particularly for obstetric studies.
For fetal ultrasound studies, the Thermal Index (TI) and Mechanical Index (MI) are important safety parameters that help minimize potential risks from ultrasound energy exposure.
Recommended Values (Safety Guidelines):
1. Thermal Index (TI)
- Recommended TI: ≤ 0.7
- TI reflects the potential for tissue heating.
- Limit use of TI > 1.0, especially in:
- First trimester (organogenesis phase)
- Doppler studies
2. Mechanical Index (MI)
- Recommended MI: ≤ 1.0
- MI reflects the potential for non-thermal effects, especially cavitation (formation of gas bubbles).
- MI should be kept as low as possible, particularly during:
- Early pregnancy
- Use of pulsed Doppler or 3D/4D ultrasound
General Principle: ALARA
“As Low As Reasonably Achievable”
- Always minimize exposure time
- Use lowest power settings that provide adequate image quality
- Avoid prolonged Doppler use in the first trimester
Guidelines from Organizations:
- AIUM (American Institute of Ultrasound in Medicine)
- BMUS (British Medical Ultrasound Society)
- WHO and ISUOG (International Society of Ultrasound in Obstetrics and Gynecology)
These organizations strongly recommend keeping both TI and MI as low as possible to ensure fetal safety.
