Download
1 / 7
70 likes | 183 Vues
B0 inhomogeneity poses significant challenges for SSFP-based relaxometry techniques such as DESPOT2 and PCVFA. This study delineates two primary methods for overcoming these challenges: estimating B0 from data or assuming a predefined B0 map. The analysis involves choosing optimal flip angles and phase cycling to ensure robustness against inhomogeneity. By evaluating precision values for T1, T2, and M0 across varying off-resonance conditions, the research presents strategies for combining these into a unified cost function, ultimately identifying optimal DESPOT2 phase-cycled bSSFP angles.
E N D
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
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
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?
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
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
For reference, the optimal DESPOT2 π-phase-cycled bSSFP angles are: [ 10.759, 57.522 ] deg.
