Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T
David Leitão, Raphael Tomi-Tricot, Pip Bridgen, Tom Wilkinson, Patrick Liebig, Rene Gumbrecht, Dieter Ritter, Sharon L. Giles, Ana Baburamani, Jan Sedlacik, Joseph V. Hajnal, Shaihan J. Malik
Abstract
Purpose
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 (𝐵+1)B1+ inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied 𝐵+1B1+ field saturating but not rotating its magnetization; thus, standard pTx pulse design methods do not apply.
Theory and Methods
Pulse design for saturation homogeneity (PUSH) optimizes pTx RF pulses by considering uniformity of root-mean squared 𝐵+1B1+, 𝐵rms1B1rms, 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 𝐵rms1B1rms and of the MT contrast by acquiring MT ratio maps.
Results
Simulations and in vivo experiments showed up to six and two times more uniform 𝐵rms1B1rms 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.
Conclusion
The proposed PUSH method obtains more uniform and higher MT contrast than CP mode within the same specific absorption rate (SAR) budget.