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.

Figure 1.

(a) B₀ corrected Z-spectra of the white matter region of subject 3 using parameters: B₁⁺ = 0.72 μT and saturation lengths = 0.5, 0.8, 1, 2, 3, 4, and 5 sec. Using a saturation length of 3 seconds yielded the highest NOE_MTR contrast. (b) B₀ and B₁⁺ corrected NOE_MTR maps from five subjects over three scans. Scan 1 for Subject 3 was done 3 years and 18 days before scans 2 and 3. Scan 1 for other two subjects were done a week before scans 2 and 3. Scans 2 and 3 were done one week apart and the last scan for subjects 4 and 5 were done eight months later.

Figure 2.

(a) Segmentation map of an exemplary slice from a 3D slab of one of the subjects based on white matter (yellow) and grey matter (cyan). (b) NOE_MTR maps of the used slices (2^nd-11^th) in the 3D slab from Subject 1. Slices 1 and 12 were not used due to phase overlapping.

Figure 3.

(a) An example five-pool Lorentzian fit of a ROI region in WM illustrating the contribution of the different pools. (b) Reproducibility of rNOE amplitude maps of all subjects.

Figure 4.

Histograms of the amplitudes of each respective pool with respect to the unsaturated signal (100 ppm) after performing a multi-pool separation. The left column shows the histogram shows the amplitudes from the NOE, MT, and DS pools which provide the bulk of the MTR signal on the same scale based on GM and the right column is WM. The bottom histogram shows the amplitudes from Amide and Amine pools which contribute less than 10% of overall NOE_MTR signal.