New methods for robust continuous wave T relaxation preparation.

Measurement of the longitudinal relaxation time in the rotating frame of reference (T ) is sensitive to the fidelity of the main imaging magnetic field (B ) and that of the RF pulse (B ). The purpose of this study was to introduce methods for producing continuous wave (CW) T contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T contrast generation methods with the B and B field inhomogeneities. Four hard-pulse and four adiabatic CW-T magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin-lock amplitudes under ideal and non-ideal conditions, as well as theoretical analysis of the hard-pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω  << ΔB ) and high (ω >> ΔB ) spin-locking field strengths. In simulations, previously reported single-refocus and new triple-refocus hard-pulse and double-refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non-ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non-refocused. The findings promote the use of the previously reported hard-pulse single-refocus ΔB and B insensitive T as a robust method with minimal RF energy deposition. The double-refocus adiabatic B insensitive rotation-4 CW-T preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications.