Simultaneous 3D acquisition of ¹H MRF and ²³Na MRI

Purpose: To develop a 3D MR technique to simultaneously acquire proton multiparametric maps (T₁, T₂, and proton density) and sodium density weighted images over the whole brain. Methods: We implemented a 3D stack-of-stars MR pulse sequence which consists of interleaved proton (¹H) and sodium (²³Na) excitations, tailored slice encoding gradients that can encode the same slice for both nuclei, and simultaneous readout with different radial trajectories (¹H, full-radial; ²³Na, center-out radial). The receive chain of our 7T scanner was modified to enable simultaneous acquisition of ¹H and ²³Na signal. A heuristically optimized flip angle train was implemented for proton MR fingerprinting (MRF). The SNR and the accuracy of proton T₁ and T₂ were evaluated in phantoms. Finally, in vivo application of the method was demonstrated in five healthy subjects. Results: The SNR for the simultaneous measurement was almost identical to that for the single-nucleus measurements (<2% change). The proton T₁ and T₂ maps remained similar to the results from a reference 2D MRF technique (normalized RMS error in T₁ ≈ 4.2% and T₂ ≈ 11.3%). Measurements in healthy subjects corroborated these results and demonstrated the feasibility of our method for in vivo application. The in vivo T₁ values measured using our method were lower than the results measured by other conventional techniques. Conclusions: With the 3D simultaneous implementation, we were able to acquire sodium and proton density weighted images in addition to proton T₁, T₂, and (Formula presented.) from ¹H MRF that covers the whole brain volume within 21 min.