Improved magnetization alignment schemes for spin-lock relaxation experiments

A pair of pulse schemes that spin-lock magnetization efficiently are presented. The design of the sequences benefited from a particularly simple relation that is derived describing to first order the evolution of any magnetization component due to the application of an off-resonance 90 degrees pulse. The sequences are shown theoretically and experimentally to significantly outperform the 90 degrees-delay-90 degrees element that is often used in current applications. It is shown that alignment of magnetization to within 1 degrees of the effective field can be obtained over a bandwidth extending between [-omega(SL), omega(SL)], where omega(SL) is the strength of the spin-lock field using a simple scheme that is an order of magnitude shorter than an adiabatic pulse that might also be used for a similar purpose.