  Purpose. It is a long standing pursuit in sodium (23Na) MRI to separate
signals between mono and bi exponential T2 decays in the human brain, due to
lack of clinically translational solutions under the restriction of
intrinsically low signal to noise ratio (SNR). Here we propose a new technique
called multi TE single quantum (MSQ) sodium MRI to address the challenge.
Methods. We exploit an intrinsic difference in T2 decay between mono and bi
exponential sodium signals by acquiring SQ images at multiple TEs and
performing voxel based matrix inversions on these SQ images. The MSQ method was
then investigated on numerical models, agar phantoms, and human brains for the
feasibility on clinical scanners at 3T. Results. The whole brain T2* spectrum
of FID signals from the study subjects showed sparse peaks (2 to 4 peaks),
suggesting a global set of T2* values (T2*fr, T2*bs, T2*bl) applicable to the
separation. The simulations indicated a small impact (3.9 to 5.6 percent) of
T2* variation on accuracy of the separation, and the phantom experiments showed
a high accuracy of the separation, 95.8 percent for mono T2 sodium and 72.5 to
80.4 percent for biT2 sodium. The human studies demonstrated feasibility of the
separation and potentials of highlighting abnormal brain regions in the biT2
sodium images. Conclusion. The MSQ technique has been shown, via the numerical
simulations, phantom experiments, and human brain studies, to be able to
separate mono and bi T2 sodium signals using a two TE sampling scheme and a
global set of T2* values. However, MSQ has limitations and requires cautions in
practice. Keywords. sodium MRI, single quantum MRI, triple quantum MRI,
neuroimaging, neurodegeneration
