Rotating frame relaxation during adiabatic pulses vs. conventional spin lock: simulations and experimental results at 4 T

Spin relaxation taking place during radiofrequency (RF) irradiation can be assessed by measuring the longitudinal and transverse rotating frame relaxation rate constants (R-1 rho and R-2 rho). These relaxation parameters can be altered by utilizing different settings of the RF irradiation, thus providing a useful tool to generate contrast in MRI. In this work, we investigate the dependencies of R-1 rho and R-2 rho due to dipolar interactions and anisochronous exchange (i.e., exchange between spins with different chemical shift delta omega not equal 0) on the properties of conventional spin-lock and adiabatic pulses, with particular emphasis on the latter ones which were not fully described previously. The results of simulations based on relaxation theory provide a foundation for formulating practical considerations for in vivo applications of rotating frame relaxation methods. Rotating frame relaxation measurements obtained from phantoms and from the human brain at 4 T are presented to confirm the theoretical predictions. (C) 2009 Elsevier Inc. All rights reserved.