Parallel transmit hybrid pulse design for controlled on-resonance magnetization transfer in R1 mapping at 7T

Abstract

Purpose: This work proposes a "hybrid" RF pulse design method for parallel transmit (pTx) systems to simultaneously control flip angle and root-mean-squared $B_1^{+}$ ( $B_1^{\mathrm{rms}}$ ). These pulses are generally only designed for flip angle, however, this can lead to uncontrolled $B_1^{\mathrm{rms}}$ , which then leads to variable magnetization transfer (MT) effects. We demonstrate the hybrid design approach for quantitative imaging where both flip angle and $B_1^{\mathrm{rms}}$ are important.

Theory and methods: A dual cost function optimization is performed containing the normalized mean squared errors of the flip angle and $B_1^{\mathrm{rms}}$ distributions weighted by a parameter $\lambda$ . Simulations were conducted to study the behavior of both properties when simultaneously optimizing them. In vivo experiments on a 7T MRI system with an 8-channel pTx head coil were carried out to study the effect of the hybrid design approach on variable flip angle $R_1$ (= 1/T₁) mapping.

Results: Simulations showed that both flip angle and $B_1^{\mathrm{rms}}$ can be homogenized simultaneously without detriment to either when compared to an individual optimization. By homogenizing flip angle and $B_1^{\mathrm{rms}}$ , $R_1$ maps were more uniform (coefficient of variation 6.6% vs. 13.0%) compared to those acquired with pulses that only homogenized flip angle.

Conclusion: The proposed hybrid design homogenizes on-resonance MT effects while homogenizing the flip angle distribution, with only a small detriment in the latter compared to a pulse that just homogenizes flip angle. This improved $R_1$ mapping by controlling incidental MT effects, yielding more uniform $R_1$ maps.

Keywords: B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity; RF pulse design; magnetization transfer; parallel transmit; ultra high field.