Work

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

Year: 2024

Type: article

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 (). These pulses are generally only designed for flip angle, however, this can lead to uncontrolled , which then leads to variable magnetization transfer (MT) effects. We demonstrate the hybrid design approach for quantitative imaging where both flip angle and are important. Theory and Methods A dual cost function optimization is performed containing the normalized mean squared errors of the flip angle and distributions weighted by a parameter . 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 (= 1/T1) mapping. Results Simulations showed that both flip angle and can be homogenized simultaneously without detriment to either when compared to an individual optimization. By homogenizing flip angle and , 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 mapping by controlling incidental MT effects, yielding more uniform maps.

Source: Magnetic Resonance in Medicine

DOI: 10.1002/mrm.30333