Chemical exchange rotation transfer (CERT) on human brain at 3 Tesla

To test the ability of a novel pulse sequence applied in vivo at 3 Tesla to separate the contributions to the water signal from amide proton transfer (APT) and relayed nuclear Overhauser enhancement (rNOE) from background direct water saturation and semisolid magnetization transfer (MT). The lack of such signal source isolation has confounded conventional chemical exchange saturation transfer (CEST) imaging.

We quantified APT and rNOE signals using a chemical exchange rotation transfer (CERT) metric, MTR_double . A range of duty cycles and average irradiation powers were applied, and results were compared with conventional CEST analyses using asymmetry (MTR_asym ) and extrapolated magnetization transfer (EMR).

Our results indicate that MTR_double is more specific than MTR_asym and, because it requires as few as 3 data points, is more rapid than methods requiring a complete Z-spectrum, such as EMR. In white matter, APT (1.5 ± 0.5%) and rNOE (2.1 ± 0.7%) were quantified by using MTR_double with a 30% duty cycle and a 0.5-µT average power. In addition, our results suggest that MTR_double is insensitive to B₀ inhomogeneity, further magnifying its speed advantage over CEST metrics that require a separate B₀ measurement. However, MTR_double still has nontrivial sensitivity to B₁ inhomogeneities.

We demonstrated that MTR_double is an alternative metric to evaluate APT and rNOE, which is fast, robust to B₀ inhomogeneity, and easy to process.