An increased thermal index can be achieved when performing Doppler studies in obstetric sonography

The present study was aimed at evaluating acoustic outputs during clinical ultrasound examinations, as expressed by the thermal index (TI) and the mechanical index (MI), during the second half of pregnancy and comparing acoustic outputs between B-mode and Doppler examinations.

Patients with suspected fetal growth problems undergoing Doppler studies of the fetal circulation in addition to B-mode sonography were selected. Examinations took place between 21 and 40 weeks' gestation. An obstetrician collected data prospectively. Sonographers were unaware of the data being sought. The analysis of variance test was applied for differences in continuous variables.

A total of 63 examinations were evaluated. The mean gestational age +/- SD was 31.6 +/- 5.1 weeks. The mean duration of the total examinations was 17.6 +/- 8.6 minutes, whereas the Doppler studies lasted 0.9 +/- 0.8 minutes. The TI was significantly higher in the pulsed wave Doppler studies (mean, 1.5 +/- 0.5; range, 0.9-2.8) and color flow imaging studies (mean, 0.8 +/- 0.1; range, 0.6-1.2) compared with B-mode sonography (mean, 0.3 +/- 0.1; range, 0.1-0.7; P < .01). During the examination, 190 B-mode MI variations were recorded (mean, 1.1 +/- 0.1), which were comparable with those of the 31 color flow Doppler studies (mean, 1.0 +/- 0.1; P = .09) but higher than the 190 pulsed wave Doppler MI variations (mean 0.9 +/- 0.2; P < .001).

Increased acoustic output levels, as expressed by TI levels, are reached during obstetric Doppler studies. In particular, TI levels may reach 1.5 and higher. Doppler procedures should be performed with caution and be as brief as possible during obstetric sonography.