What values of TI and MI are recommended for fetal Ultrasound studies

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

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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

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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.

Recommended Thermal Index (TI) and Mechanical Index (MI) for Fetal Ultrasound

To minimize any potential bioeffects during obstetric ultrasound, professional societies recommend strict monitoring of the on-screen TI and MI values and adherence to the ALARA (As Low As Reasonably Achievable) principle.

Thermal Index (TI)

Three TI variants may be displayed:

• TIS: soft-tissue TI (used up to 10 weeks’ gestation)
• TIB: bone TI (used after 10 weeks when bone is near focus)
• TIC: cranial bone TI (for transcranial applications)

Exposure Time Recommendations^[1]

TI Range	Maximum Dwell Time (minutes)
0.0 – 0.7	No specific time limit
0.7 – 1.0	60 min
1.0 – 1.5	30 min
1.5 – 2.0	15 min
2.0 – 2.5	4 min
2.5 – 3.0	1 min
> 3.0	Scanning not recommended

• First trimester (up to 13 + 6 weeks): TI (soft tissue) should be kept ≤ 0.7, preferably beginning scans at 0.7 or lower and avoiding any increase above 1.0.^[2][3]
• Doppler modes: TI should be kept as low as possible; routine use of spectral Doppler in the first trimester is discouraged. When necessary, exposures should be limited to under 5–10 minutes and TI ≤ 1.0.^[3][2]

Mechanical Index (MI)

• General Upper Limit: MI ≤ 1.9 per FDA regulations.^[4]
• Clinical Practice: MI should be kept as low as is consistent with obtaining diagnostic images; target MI ≤ 1.0 for fetal scanning and ≤ 0.3 when using contrast agents.^[5][3]
• Exposure Monitoring: Continually monitor MI on-screen; avoid prolonged dwell at high MI values to minimize cavitation risk.

Key Points

• Always begin fetal scans at the lowest TI possible (ideally ≤ 0.7) and avoid TI > 1.0, especially in early pregnancy.
• Adhere strictly to recommended maximum dwell times for each TI range.
• Keep MI values as low as achievable, well below 1.9, and observe MI ≤ 1.0 for routine fetal imaging.
• Follow ALARA by minimizing both acoustic output indices and scan duration, particularly when using Doppler modes in the first trimester.
  
  Sources

1. https://www.aium.org/resources/official-statements/view/recommended-maximum-scanning-times-for-displayed-thermal-index-(ti)-values
2. https://www.aium.org/resources/official-statements/view/prudent-use-and-safety-of-diagnostic-ultrasound-in-pregnancy 
3. https://radiopaedia.org/articles/thermal-index  
4. https://www.aium.org/resources/official-statements/view/statement-on-biological-effects-of-ultrasound-in-vivo
5. https://www.bmus.org/static/uploads/resources/BMUS-Safety-Guidelines-2009-revision-FINAL-Nov-2009.pdf
6. https://obgyn.onlinelibrary.wiley.com/doi/full/10.1002/uog.17969
7. https://onlinelibrary.wiley.com/doi/10.1002/jum.15188
8. https://www.sciencedirect.com/topics/medicine-and-dentistry/thermal-index
9. https://www.aium.org/resources/official-statements/view/safety-in-diagnostic-ultrasound-educational-activities-using-nonpregnant-participants
10. https://medultrason.ro/medultrason/index.php/medultrason/article/download/2372/1583
11. https://www.aium.org/resources/official-statements/view/as-low-as-reasonably-achievable-(alara)-principle
12. https://aiumthescan.blog/tag/mi/
13. https://fetalmedicine.org/var/uploads/web/Doppler/Doppler Ultrasound – Safety.pdf
14. https://pmc.ncbi.nlm.nih.gov/articles/PMC11545858/
15. https://pubmed.ncbi.nlm.nih.gov/39518728/
16. https://www.ncbi.nlm.nih.gov/books/NBK570574/