Referenceless 4D flow MRI using radial balanced SSFP at 0. 6 T

Abstract Purpose To implement four‐dimensional‐flow MRI using phase‐contrast balanced steady‐state free precession (bSSFP) at 0.6 T using a free‐running three‐dimensional (3D) radial trajectory and referenceless background phase correction. Methods A free‐running, wobbling Archimedean spiral approach including bipolar velocity‐encoding gradients (3D PC‐bSSFP) was implemented on a 0.6T prototype scanner. Bipolar rewinder gradients were added to ensure first‐moment nulling per repetition time. Velocity encoding was performed using a three‐point encoding scheme (i.e., omitting a reference measurement). Advanced computer simulations were carried out to validate the approach. Image reconstruction was performed using a locally low‐rank approach. Results for anatomical visualization and flow quantification were reconstructed separately with different regularization factors. Background phase correction was achieved using phase estimation on time‐averaged reconstructions. In vivo data were acquired in 6 healthy subjects during free breathing. Additional two‐dimensional (2D) phase‐contrast spoiled gradient‐echo (2D PC‐GRE) breath‐hold data were obtained for reference to compare flow values in the ascending aorta, descending aorta, and pulmonary trunk. Results Velocity data acquired with 3D PC‐bSSFP compared well with 2D PC‐GRE (root mean square error = 3.96 cm/s), with minor underestimation of velocities (−0.52 cm/s). Cardiac phase‐dependent signal‐to‐noise ratios normalized for differences in scan time and resolution between 3D PC‐bSSFP and 2D PC‐GRE demonstrate relatively steady values for 3D PC‐bSSFP when compared to 2D PC‐bSSFP with some reduction during phases of high flow. Conclusion Free‐running, referenceless, four‐dimensional‐flow MRI using radial 3D PC‐bSSFP is feasible on a lower‐field 0.6T system, producing adequate flow quantification while yielding simultaneously reasonable cine images for concurrent flow and functional assessment of the heart and great vessels.