B1+ inhomogeneity mitigation in CEST using parallel transmission

PURPOSE: In order to benefit from the increased spectral bandwidth at ultrahigh field (UHF), the use of parallel transmission (pTx) to mitigate flip-angle inhomogeneity in chemical exchange saturation transfer (CEST) imaging is investigated.

THEORY AND METHODS: A pTx basis pulse is homogenised by magnitude least-squares (MLS) optimization and expanded to form a frequency-selective saturation pulse for CEST. The pTx saturation pulse was simulated with a three-pool Bloch-McConnell equation to evaluate the impact of pTx on CEST contrast. In vivo CEST imaging performance (7 T) of the pTx saturation pulse and the standard Gaussian saturation in circularly polarized mode were compared. Two-spokes pTx homogeneous excitation was used in all in vivo experiments to ensure fair comparison of the two saturation pulses. Magnetization transfer ratio and inverse Z-spectrum analyses were used as metrics in evaluating the data from 3 healthy volunteers.

RESULTS: Bloch-McConnell simulations showed that side bands of the pTx saturation pulse at ±20 ppm did not affect any CEST contrast. Improved homogeneity in contrasts and relaxation-compensated CEST metrics were observed in our in vivo data when the pTx saturation pulse was used.

CONCLUSION: A pTx-based pulsed CEST presaturation scheme is proposed and validated by simulations and 7T in vivo imaging. Magn Reson Med, 2017. © 2017 International Society for Magnetic Resonance in Medicine.