Quantitative Flow Imaging in Human Umbilical Vessels In Utero Using Nongated 2D Phase Contrast MRI

Abstract

Background: Volumetric assessment of afferent blood flow rate provides a measure of global organ perfusion. Phase-contrast magnetic resonance imaging (PCMRI) is a reliable tool for volumetric flow quantification, but given the challenges with motion and lack of physiologic gating signal, such studies, in vivo on the human placenta, are scant.

Purpose: To evaluate and apply a nongated (ng) PCMRI technique for quantifying blood flow rates in utero in umbilical vessels.

Study type: Prospective study design.

Study population: Twenty-four pregnant women with median gestational age (GA) 30 4/7 weeks and interquartile range (IQR) 8 1/7 weeks.

Field strength/sequence: All scans were performed on a 3.0T Siemens Verio system using the ng-PCMRI technique.

Assessment: The GA-dependent increase in umbilical vein (UV) and arterial (UA) flow was compared to previously published values. Systematic error to be expected from ng-PCMRI, in the context of pulsatile UA flow and partial voluming, was studied through Monte-Carlo simulations, as a function of resolution and number of averages.

Statistical tests: Correlation between the UA and UV was evaluated using a generalized linear model.

Results: Simulations showed that ng-PCMRI measurement variance reduced by increasing the number of averages. For vessels on the order of 2 voxels in radius, partial voluming led to 10% underestimation in the flow. In fetuses, the average flow rates in UAs and UV were measured to be 203 ± 80 ml/min and 232 ± 92 ml/min and the normalized average flow rates were 140 ± 59 ml/min/kg and 155 ± 57 ml/min/kg, respectively. Excellent correlation was found between the total arterial flow vs. corresponding venous flow, with a slope of 1.08 (P = 0.036).

Data conclusion: Ng-PCMRI can provide accurate volumetric flow measurements in utero in the human umbilical vessels. Care needs to be taken to ensure sufficiently high-resolution data are acquired to minimize partial voluming-related errors.

Level of evidence: 2 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.

Keywords: Monte Carlo simulations; fetal umbilical vessels; nongated phase contrast MRI; partial volume effects; pulsatility; volumetric umbilical flow.