The effects of reducing the thermal index for bone from 1.0 to 0.5 and 0.1 on common obstetric pulsed wave Doppler measurements in the second half of pregnancy

Objective To test the hypothesis that clinically relevant vessels can be visualized and interrogated with Doppler recording during the second half of pregnancy at an output energy below the currently advocated limits without loss of information. Design Observational cross‐sectional study. Setting Tertiary fetal medicine center. Sample Based on a power calculation for equivalence studies, we recruited 65 pregnant women. Methods Ultrasound examination was performed at 18, 24 or 36 weeks of gestation. The umbilical artery, middle cerebral artery, ductus venosus, and both uterine arteries were identified using color Doppler, and the blood velocities were measured using pulsed wave Doppler at a thermal index for bone (TIB) of 1.0. This procedure was repeated at TIB values of 0.5 and 0.1. The depth of Doppler recording was noted. Main outcome measures Visualization of the vessels by color Doppler at all power levels and any systematic changes or increased variance of the recorded parameters with decreasing power level. Results All vessels could be visualized by color Doppler and their flow velocities measured using pulsed wave Doppler in all participants and at all power levels. There were no systematic changes or increased parameter variance when reducing the power level, despite the insonation depth being significantly greater than in early pregnancy. Conclusions Reducing the ultrasound power from TIB 1.0 to 0.1 does not alter color Doppler visualization or pulsed wave Doppler measurements in the second half of pregnancy. The lower power level can be recommended as a starting point for clinical examinations throughout pregnancy.