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Purpose: To minimize the sensitivity of inhomogeneous magnetization transfer gradient‐echo (ihMT‐GRE) imaging to radiofrequency (RF) transmit field (B1+) inhomogeneities at 3 T. Methods: The ihMT‐GRE sequence was optimized by varying the concentration of the RF saturation energy over time, obtained by increasing the saturation pulse power while extending the sequence repetition time (TR). Different protocols were tested using numerical simulations and human in vivo experiments in the brain white matter (WM) of healthy subjects at 3 T. The sensitivity of the ihMT ratio (ihMTR) to B1+ variations was investigated by comparing measurements obtained at nominal transmitter adjustments and following a 20% global B1+ drop. The resulting relative variations (δihMTR) were evaluated voxelwise as a function of the local B1+ distribution. The reproducibility of the protocol providing minimal B1+ bias was assessed in a test‐retest experiment. Results: In line with simulations, ihMT‐GRE experiments conducted at high concentration of the RF energy over time demonstrated strong reduction of the B1+ inhomogeneity effects in the human WM. Under the optimal conditions of 350‐ms TR and 3‐µT root mean square (RMS) saturation power, 73% of all WM voxels presented δihMTR below 10%. Reproducibility analysis yielded a close‐to‐zero systematic bias (ΔihMTR = −0.081%) and a high correlation (ρ² = 0.977) between test and retest experiments. Conclusion: Concentrating RF saturation energy in ihMT‐GRE sequences mitigates the sensitivity of the ihMTR to B1+ variations and allows for clinical‐ready ihMT imaging at 3 T. This feature is of particular interest for high and ultra‐high field applications. [ABSTRACT FROM AUTHOR]

Purpose: To implement, characterize, and optimize an interleaved inhomogeneous magnetization transfer (ihMT) gradient echo sequence allowing for whole‐brain imaging within a clinically compatible scan time. Theory and Methods: A general framework for ihMT modelling was developed based on the Provotorov theory of radiofrequency saturation, which accounts for the dipolar order underpinning the ihMT effect. Experimental studies and numerical simulations were performed to characterize and optimize the ihMT‐gradient echo dependency with sequence timings, saturation power, and offset frequency. The protocol was optimized in terms of maximum signal intensity and the reproducibility assessed for a nominal resolution of 1.5 mm isotropic. All experiments were performed on healthy volunteers at 1.5T. Results: An important mechanism driving signal optimization and leading to strong ihMT signal enhancement that relies on the dynamics of radiofrequency energy deposition has been identified. By taking advantage of the delay allowed for readout between ihMT pulse bursts, it was possible to boost the ihMT signal by almost 2‐fold compared to previous implementation. Reproducibility of the optimal protocol was very good, with an intra‐individual error < 2%. Conclusion: The proposed sensitivity‐boosted and time‐efficient steady‐state ihMT‐gradient echo sequence, implemented and optimized at 1.5T, allowed robust high‐resolution 3D ihMT imaging of the whole brain within a clinically compatible scan time. Magn Reson Med 79:2607–2619, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]