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1. Thermal Bioeffects & Index Selection
Ultrasound energy absorbs into fetal tissue as heat. The Thermal Index (TI) indicates the ratio of transmitted acoustic power to the power required to raise tissue temperature by 1°C.
TIS (Soft Tissue Thermal Index): Used before 10 weeks of gestation prior to bone ossification.
TIB (Bone Thermal Index): Used after 10 weeks when fetal bone absorbs ultrasound energy significantly faster.
TIC (Cranial Bone Thermal Index): Used when scanning the skull and neurocranium.
Clinical rule: Keep TI < 0.7 for routine obstetrics; if TI is between 1.0 and 2.0, limit continuous scanning to under 15 minutes.
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2. Mechanical Bioeffects & Mechanical Index (MI)
The Mechanical Index relates to acoustic cavitation, radiation force, and microstreaming in tissues containing gas or microbubbles.
In obstetric ultrasound, gas bodies are virtually absent in the fetus, making cavitation negligible in standard diagnostic modes.
Keep MI < 1.0 to prevent microvascular injury or petechial bleeding in fetal capillary beds.
Never utilize ultrasound contrast agents during pregnancy.
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3. Practical ALARA Scanning Protocols
Optimizing image quality through proper knobology allows lower acoustic output while producing crisp diagnostic scans.
Increase receiver gain and TGC before increasing acoustic output power.
Minimize use of spectral Doppler in the first trimester (<10 weeks); prefer B-mode and M-mode for fetal heart rate confirmation.
Do not linger in one spot; keep the probe in gentle motion to distribute thermal dissipation.