Dependence of rotating frame relaxation rates on frequency offset: spin-lock versus adiabatic rotation

Introduction Rotating frame relaxation rates (R1ρ and R2ρ) measured during continuous-wave spin-lock pulse depend on the frequency offset (ΔΩ) between the pulse and the isochromat of interest [1]. This characteristic impairs a time-efficient utilization of this method for MRS applications in vivo, since data need to be acquired separately for each metabolite of interest. The present work investigates the frequency offset dependency of R1ρ and R2ρ during Hyperbolic Secant (HS) adiabatic full passage (AFP) pulses [2,3], comparing it to the case of continuous-wave irradiation. We specifically considered the relaxation channels of dipolar interactions in the weak field limit, and anisochronous exchange (i.e., exchange between spins with different chemical shifts, δω ≠ 0) in the fast exchange regime (FXR). Phantom measurements were performed to validate the theoretical predictions for adiabatic pulses.