Simultaneous proton magnetic resonance fingerprinting and sodium MRI

Purpose: The goal of this work is to demonstrate a method for the simultaneous acquisition of proton multiparametric maps (T₁, T₂, and proton density) and sodium density images in 1 single scan. We hope that the development of such capabilities will help to ease the implementation of sodium MRI in clinical trials and provide more opportunities for researchers to investigate metabolism through sodium MRI. Methods: We developed a sequence based on magnetic resonance fingerprinting (MRF), which contains interleaved proton (¹H) and sodium (²³Na) excitations followed by a simultaneous center-out radial readout for both nuclei. The receive chain of a 7T scanner was modified to enable simultaneous acquisition of ¹H and ²³Na signal. The obtained signal-to-noise ratio (SNR) was evaluated, and the accuracy of both proton T₁, T₂, and B⁺ ₁ and sodium density maps were verified in phantoms. Finally, the method was demonstrated in 2 healthy subjects. Results: The SNR obtained using the simultaneous measurement was almost identical to single-nucleus measurements (<1% change). Similarly, the proton T₁ and T₂ maps remained stable (normalized root mean square error in T₁ ≈ 2.2%, in T₂ ≈ 1.4%, and B⁺ ₁ ≈ 5.4%), which indicates that the proposed sequence and hardware have no significant effects on the signal from either nucleus. In vivo measurements corroborated these results and demonstrated the feasibility of our method for in vivo application. Conclusions: With the proposed approach, we were able to simultaneously acquire sodium density images in addition to proton T₁, T₂, and B⁺ ₁ maps as well as proton density images.