Slice‐selective extended phase graphs in gradient‐crushed, transient‐state free precession sequences: An application to MR fingerprinting

Purpose Slice‐selective, gradient‐crushed, transient‐state sequences such as those used in MR fingerprinting (MRF) relaxometry are sensitive to slice profile effects. Whereas balanced steady‐state free precession MRF profile effects have been studied, less attention has been given to gradient‐crushed MRF forms. Extensions of the extended phase graph (EPG) formalism, called slice‐selective EPG (ssEPG), are proposed that model slice profile effects. Theory and methods The hard‐pulse approximation to slice‐selective excitation in the spatial domain is reformulated in k‐space. Excitation is modeled by standard EPG shift and transition operators. This ssEPG modeling is validated against Bloch simulations and phantom slice profile measurements. ssEPG relaxometry accuracy and variability are compared with other EPG methods in phantoms and human leg in vivo. The role of ∆ B 0 interactions with slice profile and gradient crushers is investigated. Results Simulations and slice profile measurements show that ssEPG can model highly dynamic slice profile effects of gradient‐crushed sequences. The MRF ssEPG T 2 estimates over 0 < T 2 < 100 ms improve accuracy by > 10 ms at some values relative to other modeling approaches. Small deviations in B 0 can produce substantial bias in T 2 estimations from a range of MRF sequence types, and these effects can be modeled and understood by ssEPG. Conclusion Transient‐state, gradient‐crushed sequences such as those used in MRF are sensitive to slice profile effects, and these effects depend on RF pulse choice, gradient crusher strength, and ∆ B 0 . It was found ssEPG was the most accurate EPG‐based means to model these effects.