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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

Authors

  • Ragnar K. Sande,

    Corresponding author
    1. Fetal Medicine Unit, Department of Obstetrics and Gynecology, Haukeland University Hospital, Bergen, Norway
    • Clinical Fetal Physiology Research Group, Department of Clinical Medicine, University of Bergen, Bergen, Norway
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  • Knut Matre,

    1. Institute of Medicine, University of Bergen, Bergen, Norway
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  • Geir E. Eide,

    1. Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway
    2. Department of Public Health and Primary Health Care, University of Bergen, Bergen, Norway
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  • Torvid Kiserud

    1. Clinical Fetal Physiology Research Group, Department of Clinical Medicine, University of Bergen, Bergen, Norway
    2. Fetal Medicine Unit, Department of Obstetrics and Gynecology, Haukeland University Hospital, Bergen, Norway
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  • The probe used in this study was provided by General Electric Vingmed Ultrasound, Horten, Norway, and was unused prior to the study and lent to us free of charge for the duration of the study. That company tested the output power level of the probe before and after the study, also at no cost to us. There were no other potential conflicts of interest.

Correspondence

Ragnar Kvie Sande, Lysefjordgata 91, Stavanger 4015, Norway. E-mail: ragnar.sande@gmail.com

Abstract

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.

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