Publication
Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T
Publisher:
Wiley
Date:
10-03-2022
DOI:
10.1002/MRM.29199
Abstract: This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for magnetization transfer (MT) contrast in the presence of transmit field inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied field saturating but not rotating its magnetization thus, standard pTx pulse design methods do not apply. Pulse design for saturation homogeneity (PUSH) optimizes pTx RF pulses by considering uniformity of root‐mean squared , , which relates to the rate of semisolid saturation. Here we considered designs consisting of a small number of spatially non‐selective sub‐pulses optimized over either a single 2D plane or 3D. Simulations and in vivo experiments on a 7T Terra system with an 8‐TX Nova head coil in five subjects were carried out to study the homogenization of and of the MT contrast by acquiring MT ratio maps. Simulations and in vivo experiments showed up to six and two times more uniform compared to circular polarized (CP) mode for 2D and 3D optimizations, respectively. This translated into 4 and 1.25 times more uniform MT contrast, consistently for all subjects, where two sub‐pulses were enough for the implementation and coil used. The proposed PUSH method obtains more uniform and higher MT contrast than CP mode within the same specific absorption rate (SAR) budget.