Accelerated sodium MRI using undersampled 3D SPARKLING at 7T

Submitted to Magnetic Resonance in Medicine; Purpose: To evaluate the benefits of SPARKLING, a stochastic k-space sampling approach over the deterministic sampling scheme TPI (Twisted Projection Imaging) in the context of accelerated cerebral $^{23}$Na MRI and assessing its impact on tissue sodium concentration (TSC) estimation.Methods: Guided by simulation results, in vitro and in vitro UTE $^{23}$Na MRI datasets were acquired at a 4 mm isotropic resolution from healthy volunteers using both TPI or SPARKLING trajectories with different acceleration factors (AF) on a 7T MR scanner equipped with a 32 channels head coil. Following reconstruction using NUFFT or Proximal Optimized Gradient Method (POGM) algorithms with or without regularization, respectively, the resulting sodium images were compared in terms of effective resolution (FWHM of the PSF), SNR and overall quality. with an in vitro assement of the accuracy of the TSC was performed via external referencing using a 4-point calibration approach. Results: In vivo cerebral sodium images acquired using SPARKLING with an acceleration factor of 32 (TA= 5 min 38 s) are similar to those obtained using TPI at AF=8 (TA= 22 min 34 s) with minimal impact on the accuracy of our TSC quantification. Conclusion: In conditions compatible with clinical examination, undersampled SPARKLING $^{23}$Na MRI can outperform the conventional TPI k-space sampling scheme allowing for shorter acquisition times.