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

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PurposeTo investigate the effect of chemical exchange and multicomponent relaxation on the rapid T2 mapping method, DESPOT2 (driven equilibrium single pulse observation of T2) and the steady-state free precession (SSFP) sequence upon which it is based. Although capable of rapid T2 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 T2 decay curve is more accurately described by the summation of more than one relaxation species.Materials and MethodsThe 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.ResultsSimulations show the measured multicomponent SSFP signal may be expressed as a linear summation of signal from each species under usual imaging conditions where the repetition time is much less than T2. 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.ConclusionSSFP and DESPOT2 may provide a new method for investigating multicomponent systems, such as human brain, and disease processes, such as multiple sclerosis. J. Magn. Reson. Imaging 2008;27:1421–1429. © 2008 Wiley-Liss, Inc.

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