Correlation-weighted 23Na magnetic resonance fingerprinting in the brain

TL;DR

This paper introduces a novel sodium MRI fingerprinting technique that simultaneously maps multiple sodium relaxation parameters and density in the brain, with built-in corrections for RF inhomogeneities and frequency offsets, validated in phantoms and healthy volunteers.

Contribution

The study presents a new 3D sodium MRI fingerprinting method using a comprehensive dictionary and spherical tensor formalism, enabling accurate, simultaneous mapping of sodium parameters in the brain.

Findings

Comparable sodium relaxation times to reference methods in phantoms

Accurate mapping of sodium parameters in healthy brain tissue

Good agreement with literature values for sodium relaxation times

Abstract

We developed a new sodium magnetic resonance fingerprinting ($^\text{23}\text{Na}$ MRF) method for the simultaneous mapping of $\text{T}_\text{1}$, $\text{T}_\text{2,long}^{*}$, $\text{T}_\text{2,short}^{*}$ and sodium density with built-in $\Delta\text{B}_{1}^{+}$ (radiofrequency transmission inhomogeneities) and $\Delta\text{f}_\text{0}$ corrections (frequency offsets). We based our $^\text{23}\text{Na}$ MRF implementation on a 3D FLORET sequence with 23 radiofrequency pulses. To capture the complex spin ${\frac{\text{3}}{\text{2}}}$ dynamics of the $^\text{23}\text{Na}$ nucleus, the fingerprint dictionary was simulated using the irreducible spherical tensor operators formalism. The dictionary contained 831,512 entries covering a wide range of $\text{T}_\text{1}$, $\text{T}_\text{2,long}^{*}$, $\text{T}_\text{2,short}^{*}$, $\Delta\text{B}_\text{1}^{+}$ factor and $\Delta\text{f}_\text{0}$ 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 $^\text{23}\text{Na}$ 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, $^\text{23}\text{Na}$ 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.