Flow-gated phase-contrast MRI using radial acquisitions.

Phase-contrast (PC) MRI is a noninvasive method for imaging blood and tissue velocity. PC experiments are typically segmented into many cardiac cycles in order to obtain sufficient spatial and temporal resolution, and thus require a gating signal to properly combine the data from multiple cardiac cycles. Although the surface electrode ECG is the gating waveform of choice for cardiovascular MRI, alternative gating methods are required for fetal cardiac imaging (due to the unavailability of fetal ECG signals) and for cases of ECG contamination via the strong magnetic fields within the MR scanner. This study demonstrates the feasibility of using imaging data from flow-encoded MRI experiments to derive times for cardiac gating. An undersampled radial k-space acquisition method is used to measure an image-derived flow-gating waveform in real-time, from which the gating times are derived. These gating times are used to reconstruct a conventional gated-segmented image series by combining the real-time data from multiple heartbeats. Flow-gated PC experiments were performed on five normal volunteers with slice prescriptions in four anatomic regions. The standard deviation (SD) of the difference between the flow-gating and ECG gating times ranged from 5 ms to 12 ms.