Correlation-weighted Na magnetic resonance fingerprinting in the brain.

We developed a new sodium magnetic resonance fingerprinting (Na MRF) method for the simultaneous mapping of and sodium density with built-in (radiofrequency transmission inhomogeneities) and corrections (frequency offsets). We based our Na MRF implementation on a 3D FLORET sequence with 23 radiofrequency pulses. To capture the complex spin dynamics of the Na nucleus, the fingerprint dictionary was simulated using the irreducible spherical tensor operators formalism. The dictionary contained 831,512 entries covering a wide range of factor and parameters. Fingerprint matching was performed using the Pearson correlation and the resulting relaxation maps were weighted with a subset of the highest correlation coefficients corresponding to signal matches for each voxel. Our Na MRF method was compared against reference methods in a 7-compartment phantom, and applied in brain in five healthy volunteers at 7 T. In phantoms, Na MRF produced values comparable to those obtained with reference methods. Average sodium relaxation time values in cerebrospinal fluid, gray matter and white matter across five healthy volunteers were in good agreement with values previously reported in the literature.