A comparison between acoustic output indices in 2D and 3D/4D ultrasound in obstetrics

Three-dimensional (3D) ultrasound is gaining popularity in prenatal diagnosis. While there are no studies regarding the safety of 3D ultrasound, it is now widely performed in non-medical facilities, for non-diagnostic purposes. The present study was aimed at comparing the acoustic output, as expressed by thermal index (TI) and mechanical index (MI), of conventional two-dimensional (2D) and 3D/4D ultrasound during pregnancy.

A prospective, observational study was conducted, using three different commercially available machines (iU22, Philips Medical Systems; Prosound Alfa-10, Aloka; and Voluson 730 Expert, General Electric). Patients undergoing additional 3D/4D ultrasound examinations were recruited from those scheduled for fetal anatomy and follow-up exams. Fetuses with anomalies were excluded from the analysis. Data were collected regarding duration of the exam, and each MI and TI during 2D and 3D/4D ultrasound exams.

A total of 40 ultrasound examinations were evaluated. Mean gestational age was 31.1 +/- 5.8 weeks, and mean duration of the exam was 20.1 +/- 9.9 min. Mean TIs during the 3D (0.27 +/- 0.1) and 4D examinations (0.24 +/- 0.1) were comparable with the TI during B-mode scanning (0.28 +/- 0.1, P = 0.343). The MIs during the 3D volume acquisitions were significantly lower than those in the 2D B-mode ultrasound studies (0.89 +/- 0.2 vs. 1.12 +/- 0.1, P = 0.018). The 3D volume acquisitions added 2.0 +/- 1.8 min of actual ultrasound scanning time (i.e. not including data processing and manipulation, or 3D displays, which are all post-processing steps). The 4D added 2.2 +/- 1.2 min.

Acoustic exposure levels during 3D/4D ultrasound examination, as expressed by TI, are comparable with those of 2D B-mode ultrasound. However, it is very difficult to evaluate the additional scanning time needed to choose an adequate scanning plane and to acquire a diagnostic 3D volume.