First-trimester sonography: Is the fetus exposed to high levels of acoustic energy?

Purpose. As a form of energy, diagnostic ultrasound has bioeffects on living tissues. The thermal index (TI), TIS (TI for soft tissue), TIB (TI for bone), TIC (TI for cranial bone) expresses the potential for rise in temperature at the ultrasound beam's focal point. The mechanical index (MI) indicates the potential for the ultrasound beam to induce inertial cavitation in tissues. The goal of this study was to characterize the acoustic output of clinical ultrasound instruments, as expressed by TI and MI, during routine first-trimester sonographic examinations. Methods. A prospective observational study was conducted. First-trimester patients were randomly selected from those scheduled for viability scans. An obstetrician collected data. Sonographers were blinded to the data being sought, which included gestational age, duration of the examination, and every variation in the MI and TI during each sonographic examination. Results. A total of 52 first-trimester examinations were evaluated. The mean gestational age was 8.9 ± 1.9 weeks. The mean duration of the sonographic examinations was 8.1 ± 1.4 minutes. During the examinations, there were 178 MI variations (mean ± SD, 0.9 ± 0.3) and 167 TI variations (mean ± SD, 0.2 ± 0.1). Conclusion. First-trimester sonographic examinations are associated with a negligible rise in TI.