Ultrasound acoustic output during third trimester biophysical profile

Diagnostic ultrasound (DUS) has the potential for bioeffects, mainly temperature elevation and cavitation of gas bubbles, expressed, respectively, by the Thermal Index (TI) and Mechanical Index (MI). Since fetal lungs and bowels do not contain gas, the mechanical risk is probably nonexistent. The biophysical profile (BPP) involves static, relatively prolonged scanning. Bones have high levels of energy absorption. Thus, there may be concern of temperature elevation in their vicinity. However, no human clinical data exist regarding fetal exposure. The present study was aimed at examining the acoustic output of clinical ultrasound instruments, as expressed by TI and MI, during routine third trimester BPP testing. A prospective observational study was conducted in patients undergoing third trimester BPP scans (iU22, Philips Medical Systems). Color and spectral Doppler were not included. Data were collected by an obstetrician. Sonographers were unaware of the data being sought. Data included gestational age, duration of the exam, and specific time duration spent at each MI and TI. Each MI and TI variation was recorded during the exam. Statistical analysis was performed using the SPSS-12 package (SPSS, Chicago, IL). Ninety-one TI and MI variations during 30 BPP examinations were evaluated. Mean gestational age was 34.0 ± 3.7 weeks. Mean duration of DUS was 12.8± 4.0 minutes. During the examinations the mean TI was 0.3±0.1 (range 0.2-0.7) and mean MI was 1.1±0.1 (range 0.7-1.2). The highest TI (0.7) recorded lasted for two minutes. The risk of fetal exposure to an increase in temperature during routine third trimester BPP exams, as expressed by TI, is extremely low, and probably lacks clinical significance.