CEST imaging at 9.4 T using adjusted adiabatic spin-lock pulses for on- and off-resonant T1ρ-dominated Z-spectrum acquisition

Purpose: The CEST experiment, with its correlation to rare proton species that are in exchange with the water pool, is very similar to the off-resonant water spin-lock (SL) experiment. In particular, low-power SL Z-spectrum acquisition allows insight into T-1 rho and exchange effects with decreased direct water saturation. Because the available SL methods either require high B-1 power or are instable in the presence of strong B-1 and B-0 inhomogeneity present at ultra-high fields, the goal of this study was to find a robust adiabatic SL pulse for on- and off-resonant application in the human brain at 9.4 T. Methods: A series of Bloch simulations were used to find optimal pulse shape parameters of an adjusted hyperbolic secant pulse applicable in the low power regime typically used for exchange-weighted SL experiments. The optimized pulse was implemented and tested in phantom and in vivo experiments on a 9.4 T human scanner for on- and off-resonant T-1 rho- and Z-spectrum measurements. Results: The simulation yielded a feasible pulse shape, which yielded robust images, less sensitivity to B-1 and B-0 inhomogeneity compared with previous SL approaches and less direct water saturation, as well as a higher chemical exchange weighting compared with conventional CEST approaches. Conclusion: By adapting a pulse shape for low-power SL experiments, we were able to acquire robust on- and off-resonant adiabatic SL prepared images in vivo at 9.4 T. This development leads directly to SL Z-spectrum acquisition, beneficial for chemical-exchange-weighted MRI.