Acoustic output as measured by thermal and mechanical indices during fetal nuchal translucency ultrasound examinations

Objective: Nuchal translucency (NT), performed between 11+0 and 13+6 weeks' gestation as part of a screening test for fetal chromosomal abnormalities, involves relatively steady ultrasound scanning of the fetus. The present study was aimed at measuring the acoustic output, as expressed by thermal index (TI) and mechanical index (MI), during routine NT examinations. Methods: A prospective, observational study was conducted in patients undergoing routine NT screening. Examinations were performed between 11+0 and 13+6 weeks' gestation. Fetuses with anomalies were excluded from the analysis. Data were collected by an obstetrician, including duration of the examination, and each MI and TI during ultrasound examinations. Sonographers were unaware of the data being sought. Results: A total of 50 ultrasound NT examinations were evaluated. Mean gestational age was 12.3 ± 0.6 weeks. Mean duration of the ultrasound examination was 11.6 ± 4.2 min. There were 109 variations of the TI during the examinations. Mean TI was 0.2 ± 0.1; the median was 0.2, with a range of 0.1-0.7. There were 105 variations of the MI during the examinations. Mean MI was 1.1 ± 0.1 (median 1.1, range 0.7-1.3). Mean NT was 1.4 ± 0.4 mm. Conclusions: Acoustic exposure levels during NT ultrasound examination, as expressed by TI and MI, are generally low.