Parallel transmit pulse design for saturation homogeneity (PUSH) for magnetization transfer imaging at 7T

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 $\left(B_1^{+}\right)$ inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied $B_1^{+}$ 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 $B_1^{+}$ , $B_1^{\mathrm{rms}}$ , 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 $B_1^{\mathrm{rms}}$ 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 $B_1^{\mathrm{rms}}$ 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.

Keywords: $B_1^{+}$ inhomogeneity; RF pulse design; magnetization transfer; parallel transmit; ultrahigh-field.