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The Problem

The Problem. B0 inhomogeneity is an issue for all SSFP based relaxometry methods How to solve for the optimal protocol in the presence of off resonance for DESPOT2 and PCVFA? Previously DESPOT2-FM collected a 0-phase-cycle bSSFP at the same flip angles as in DESPOT2

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The Problem

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  1. The Problem • B0 inhomogeneity is an issue for all SSFP based relaxometry methods • How to solve for the optimal protocol in the presence of off resonance for DESPOT2 and PCVFA? • Previously DESPOT2-FM collected a 0-phase-cycle bSSFPat the same flip angles as in DESPOT2 • There are two main approaches: • Estimate B0 from the data, as in FM • Assume that the B0 map is given, choose α and phase-cycling to be robust to inhomogeneity

  2. Setup • DESPOT scenario – assume T1 is given by DESPOT1 • Choose SSFPs to estimate T2 and nuisance parameters M0 (and B0) • Free scenario– choose any combination of SPGRs and SSFPs to jointly estimate T1 and T2

  3. Sample Tissue • 3T GM: T1 = 1650ms, T2=85ms • B0 = 8 sample points uniformly distributed from [0, 2π) • Then for each evaluation of the CRLB, we get 8x3 precision values for T1, T2, and M0 at each off resonance • How to combine these into a single value for a cost function?

  4. Cost Functions • λT1=1, λT2=1, λM0=0, λB0=0 • There are different ways to combine the vector or precisions into a single value • Minimize the mean, this is the same as doing a sum but preferable because the units are comparable to past results • Minimizing the maximum, i.e. the worst case, probably makes the most sense • This was done in Li et al. “Optimized IR Sequences for Quantitative T1 and MT Imaging” • Note that scaling of the off resonance precision is a special case because θ=0 when on resonance with undefined CoV • Instead I leave it in its base units • This has no effect on the optimization since λB0=0 • But just changes how I display the results

  5. Approach • DESPOT Scenario • Exhaustively search SSFPs up to 8 total images • Free Scenario • First, searched combinations of SPGR and SSFP up to 6 • Found that they only used SSFP • However, surmised that more than 6 are needed as there was no “redundancy” • After that, I switched to only finding SSFPs up to 10 to reduce the computation time, instead of trying different combinations of both sequences

  6. Summary

  7. For reference, the optimal DESPOT2 π-phase-cycled bSSFP angles are: [ 10.759, 57.522 ] deg.

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