Analysis and correction of background velocity offsets in phase-contrast flow measurements using magnetic field monitoring.

The value of phase-contrast magnetic resonance imaging for quantifying tissue motion and blood flow has been long recognized. However, the sensitivity of the method to system imperfections can lead to inaccuracies limiting its clinical acceptance. A key source of error relates to eddy current-induced phase fluctuations, which can offset the measured object velocity significantly. A higher-order dynamic field camera was used to study the spatiotemporal evolution of background phases in cine phase-contrast measurements. It is demonstrated that eddy current-induced offsets in phase-difference data are present up to the second spatial order. Oscillatory temporal behaviors of offsets in the kHz range suggest mechanical resonances of the MR system to be non-negligible in phase-contrast imaging. By careful selection of the echo time, their impact can be significantly reduced. When applying field monitoring data for correcting eddy current and mechanically induced velocity offsets, errors decrease to less than 0.5% of the maximum velocity for various sequence settings proving the robustness of the correction approach. In vivo feasibility is demonstrated for aortic and pulmonary flow measurements in five healthy subjects. Using field monitoring data, mean error in stroke volume was reduced from 10% to below 3%.