Real‐time multi‐directional flow MRI using model‐based reconstructions of undersampled radial FLASH – A feasibility study

The purpose of this work was to develop an acquisition and reconstruction technique for two‐ and three‐directional (2d and 3d) phase‐contrast flow MRI in real time. A previous real‐time MRI technique for one‐directional (1d) through‐plane flow was extended to 2d and 3d flow MRI by introducing in‐plane flow sensitivity. The method employs highly undersampled radial FLASH sequences with sequential acquisitions of two or three flow‐encoding datasets and one flow‐compensated dataset. Echo times are minimized by merging the waveforms of flow‐encoding and radial imaging gradients. For each velocity direction individually, model‐based reconstructions by regularized nonlinear inversion jointly estimate an anatomical image, a set of coil sensitivities and a phase‐contrast velocity map directly. The reconstructions take advantage of a dynamic phase reference obtained by interpolating consecutive flow‐compensated acquisitions. Validations include pulsatile flow phantoms as well as in vivo studies of the human aorta at 3 T. The proposed method offers cross‐sectional 2d and 3d flow MRI of the human aortic arch at 53 and 67 ms resolution, respectively, without ECG synchronization and during free breathing. The in‐plane resolution was 1.5 × 1.5 mm2 and the slice thickness 6 mm. In conclusion, real‐time multi‐directional flow MRI offers new opportunities to study complex human blood flow without the risk of combining differential phase (i.e., velocity) information from multiple heartbeats as for ECG‐gated data. The method would benefit from a further reduction of acquisition time and accelerated computing to allow for extended clinical trials. This work demonstrates the feasibility of multidirectional phase‐contrast flow MRI in real time, i.e. without any prospective or retrospective gating. Combining highly undersampled radial FLASH with model‐based reconstructions based on nonlinear inversion, cross‐sectional MRI movies of 2d and 3d flow in the human aorta are obtained at 53 and 67 ms resolution, respectively.