MRI Contrasts in High Rank Rotating Frames

PurposeMRI relaxation measurements are performed in the presence of a fictitious magnetic field in the recently described technique known as RAFF (Relaxation Along a Fictitious Field). This method operates in the 2(nd) rotating frame (rank n=2) by using a nonadiabatic sweep of the radiofrequency effective field to generate the fictitious magnetic field. In the present study, the RAFF method is extended for generating MRI contrasts in rotating frames of ranks 1n5. The developed method is entitled RAFF in rotating frame of rank n (RAFFn). Theory and MethodsRAFFn pulses were designed to generate fictitious fields that allow locking of magnetization in rotating frames of rank n. Contrast generated with RAFFn was studied using Bloch-McConnell formalism together with experiments on human and rat brains. ResultsTolerance to B-0 and B-1 inhomogeneities and reduced specific absorption rate with increasing n in RAFFn were demonstrated. Simulations of exchange-induced relaxations revealed enhanced sensitivity of RAFFn to slow exchange. Consistent with such feature, an increased grey / white matter contrast was observed in human and rat brain as n increased. ConclusionRAFFn is a robust and safe rotating frame relaxation method to access slow molecular motions in vivo. Magn Reson Med 73:254-262, 2015. (c) 2014 Wiley Periodicals, Inc.