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 B1+ amplitude and length of the saturation pulse by acquiring NOE images with different B1+ values with multiple saturation lengths. Repeated NOE MRI measurements were made on five healthy volunteers by using optimized saturation pulse parameters including correction of B(0) and B1+ 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.