In vivo reproducibility of 3D relayed NOE in the healthy human brain at 7 T

Abstract

Purpose: Nuclear Overhauser effect (NOE) is based on dipolar cross-relaxation mechanism that enables the indirect detection of aliphatic protons via the water proton signal. This work focuses on determining the reproducibility of NOE magnetization transfer ratio (NOE_MTR ) and isolated or relayed NOE (rNOE) contributions to the NOE MRI of the healthy human brain at 7 Tesla (T).

Methods: We optimized the $B_1^{+}$ amplitude and length of the saturation pulse by acquiring NOE images with different $B_1^{+}$ values with multiple saturation lengths. Repeated NOE MRI measurements were made on five healthy volunteers by using optimized saturation pulse parameters including correction of B₀ and $B_1^{+}$ inhomogeneities. To isolate the individual contributions from z-spectra, we have fit the NOE z-spectra using multiple Lorentzians and calculated the total contribution from each pool contributing to the overall NOE_MTR contrast.

Results: We found that a saturation amplitude of 0.72 μT and a length of 3 s provided the highest contrast. We found that the mean NOE_MTR value in gray matter (GM) was 26%, and in white matter (WM) was 33.3% across the 3D slab of the brain. The mean rNOE contributions from GM and WM values were 8.9% and 9.6%, which were ∼10% of the corresponding total NOE_MTR signal. The intersubject coefficient of variations (CoVs) of NOE_MTR from GM and WM were 4.5% and 6.5%, respectively, whereas the CoVs of rNOE were 4.8% and 5.6%, respectively. The intrasubject CoVs of the NOE_MTR range was 2.1%-4.2%, and rNOE range was 2.9%-10.5%.

Conclusion: This work has demonstrated an excellent reproducibility of both inter- and intrasubject NOE_MTR and rNOE metrics in healthy human brains at 7 T.

Keywords: CEST; MTR; NOE; brain; cross-relaxation; rNOE.