Relayed nuclear Overhauser enhancement imaging with magnetization transfer contrast suppression at 3 T

Purpose: To develop a pulsed CEST magnetization-transfer method for rapidly acquiring relayed nuclear Overhauser enhancement (rNOE)–weighted images with magnetic transfer contrast (MTC) suppression at clinical field strength (3 T). 
Methods: Using a pulsed CEST magnetization-transfer method with low saturation powers (B₁) and long mixing time (t_mix) to suppress contributions due to strong MTC from solid-like macromolecules, a low B₁ also minimized direct water saturation. These MTC contributions were further reduced by subtracting the Z-spectral signals at two or three offsets by assuming that the residual MTC is a linear function between −3.5 ppm and −12.5 ppm. 
Results: Phantom studies of a lactic acid (Lac) solution mixed with cross-linked bovine serum albumin show that strong MTC interference has a significant impact on the optimum B₁ for detecting rNOEs, due to lactate binding. The MTC could be effectively suppressed using a pulse train with a B₁ of 0.8 μT, a pulse duration (t_p) of 40 ms, a t_mix of 60 ms, and a pulse number (N) of 30, while rNOE signal was well maintained. As a proof of concept, we applied the method in mouse brain with injected hydrogel and a cell-hydrogel phantom. Results showed that rNOE-weighted images could provide good contrast between brain/cell and hydrogel. 
Conclusion: The developed pulsed CEST magnetization-transfer method can achieve MTC suppression while preserving most of the rNOE signal at 3 T, which indicates the potential for translation of this technique to clinical applications related to mobile proteins/lipids change.