Journal Club: mcDESPOT with B0 & B1 Inhomogeneity

1. Journal Club: mcDESPOT with B0 & B1 Inhomogeneity

2. Papers • Deoni et al. Gleaning multicomponent T1 and T2 information from steady-state imaging data. Magn. Reson. Med. (2008) vol. 60 (6) pp. 1372-1387 • Deoni. Correction of main and transmit magnetic field (B0 and B1) inhomogeneity effects in multicomponent-driven equilibrium single-pulse observation of T1 and T2. Magn. Reson. Med. (2010)

3. mcDESPOT • A whole-brain quantitative mapping technique • Idea: collect SPGR and SSFP scans at several flip angles • These have a known theoretical steady-state signal equation • Fit the equation as it varies with flip angle to the collected data at each voxel • Gives us: T1, T2, and more • Uses a two-compartment model for the signal equation: a fast and slow relaxing species in exchange

4. scDESPOT Theory • DESPOT1: SPGR equation • Find M0 and T1, minimize (SSPGR-ŜSPGR)2 • DESPOT2: SSFP equation • Given T1, find M0 and T2, minimize (SSSFP-ŜSSFP)2

5. mcDESPOT Theory • 2 component SPGR equation • Find fF,fS, T1,F, T1,S, kFS=1/τF, kSF=1/τS

6. mcDESPOT Theory • 2 component SSFP equation • Find M0 and T1, minimize (SSPGR-ŜSPGR)2

7. mcDESPOT Theory

8. Simplifying Assumptions • 2 component model • Only need to find fF (the fast volume fraction) • Chemical equilibrium • Allows us to eliminate finding kSF=1/τS • Both components are on the same resonance

9. Fitting Method • Genetic Algorithm • Previous method, proved to be too slow • Stochastic Region of Contraction • Current method, processing time is still substantial (about 24 hrs. for a 2mm isotropic brain) • Supposedly good for avoiding local minima • Not much literature on it (Berger and Silverman. Microphone Array Optimization byStochastic Region Contraction.)

10. Stochastic Region of Contraction • Has been offered as an alternative to simulated annealing, which can be slow but is very general • SRC is good for objective functions with these characteristics: • Few large valleys, many small local minima is fine • The neighborhood around the global minimum is still lower than any other local minima • Depends on <100 variables

11. Stochastic Region of Contraction • Given an initial N-dimensional, rectangular, search volume containing the global optimum • Explore the objective function with random points in the space • Systematically contract the volume until it reaches a satisfactorily small region that traps the global optimum

12. Stochastic Region of Contraction • Algorithm • Define initial search space for: T1,F&S, T2,F&S, fF, τF, Δωs • Treat this rectangular box as a uniform distribution and sample N times • Compute the objective function for each sample • Keep M of the best samples and define the new box based on the ranges of the variables in these samples • Rinse and repeat until convergence

13. Stochastic Region of Contraction

14. When Things Go Wrong • Simulation of artifacts • B0 effects • B1 and slab profile effects • The key assumption of mcDESPOT is that the prescribed flip angle is achieved everywhere in the volume • Need to account for this if not the case • In vivo at 1.5T (and some proof of concept 3T) • 4 normal volunteers

15. B0 Solutions • B0 mapping • DESPOT2-FM with phase-cycled SSFP • Requires collection of another set of 9 SSFP images • Modified signal equation and objective function as presented earlier

16. When Things Go Wrong: B0 Artifacts

17. When Things Go Wrong: B0 Artifacts

18. B1 Solutions • B1 mapping • AFI, niDALL, Bloch-Siegert • DESPOT1-HIFI with IR-SPGR (1 component) • Requires addition of 1 IR-prepped scan • Details of signal equation to derive B1 map not covered here • Gives κ(r):

19. When Things Go Wrong: B1 Artifacts

20. When Things Go Wrong: B1 Artifacts

21. When Things Go Wrong: B1 Artifacts

22. When Things Go Wrong: B1 Artifacts

23. In Vivo

24. In Vivo

25. In Vivo • B0 and B1 effects are not enough to account for the difference in MWF between mcDESPOT and T2-MCRI (usu. in range of 8-9%)

26. Conclusions • DESPOT1-HIFI does well even thought slab profile changes with angle and assumes single component • Alternatives should be considered though since anatomical structures are visible on the maps: not the best B1 map • Bloch-Siegert seems compelling but need a way to incorporate slab profile as well • DESPOT2-FM and phase-cycled SSFP has been a part of the protocol at 1.5T and should also stay when we move to 3T+ • Alternative B0 mapping methods should be considered if they offer a significant benefit in acquisition time

27. Other Avenues to Explore • 3 component model, is another pool skewing the MWF? • Are the 2 components actually on the same resonance?