Investigating exchange and multicomponent relaxation in fully-balanced steady-state free precession imagaing

Purpose: To investigate the effect of chemical exchange and multicomponent relaxation on the rapid T-2, mapping method, DESPOT2 (driven equilibrium single pulse observation of T-2) and the steady-state free precession (SSFP) sequence upon which it is based. Although capable of rapid T-2 determination, an assumption implicit of the method is single-component relaxation. In many biological tissues (such as white and gray matter), it is well established that the T-2 decay curve is more accurately described by the summation of more than one relaxation species. Materials and Methods: The effects of exchange were first incorporated into the general SSFP magnetization expressions and its effect on the measured SSFP signal investigated using Bloch-McConnell simulations. Corresponding imaging experiments were performed to support the presented theory. Results: Simulations show the measured multicomponent SSFP signal maybe expressed as a linear summation of signal from each species under usual imaging conditions where the repetition time is much less than T-2. Imaging experiments performed using dairy cream demonstrate strong agreement with the presented theory. Finally, using a dairy cream model, we demonstrate quantification of multicomponent relaxation from multiangle SSFP data for the first time, showing good agreement with reference spin-echo values. Conclusion: SSFP and DESPOT2 may provide a new method for investigating multicomponent systems, such as human brain, and disease processes, such as multiple sclerosis.