Investigating the effect of exchange and multicomponent T1 relaxation on the short repetition time spoiled steady-state signal and the DESPOT1 T1 guantification method

Purpose: To examine the spoiled steady-state (spoiled gradient-recalled echo sequence [SPGRI) signal arising from two-compartment systems and the role of experimental parameters, in particular TR for resolving signal from each compartment. Materials and Methods: Using Bloch-McConnell simulations, we examined the SPGR signal from two-component systems in which T-1 is much greater than the mean residence time (tau(m)) of proton spins in each component. Specifically, we examined the role of TR on the ability to resolve each components signal, as well as the influence of experimental parameters on derived DESPOT1 T-1 values. Results: Results revealed that when TR <= 0.01 tau(m), the measured SPGR signal may be modeled as a summation of signal from each species using a no-exchange approximation. Additionally, under this short TR condition, the driven equilibrium single pulse observation of T-1 (DESPOT1) mapping approach provides T-1 values preferentially biased toward the short or long T-1 species, depending on the choice of flip angles. Conclusion: The ability to model the SPGR signal using a no-exchange approximation may permit the quantification multicomponent T-1 relaxation in vivo. Additionally, the ability to preferentially weight the DESPOT1 T-1 value toward the short or long T-1 may provide a useful window into these components.