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Hard Pulses at 3T

Hard Pulses at 3T. Jason Su Oct. 10, 2011. Issues. Been having problems getting correct flip angles from modified width Fermi hard pulses Used rfstat to generate/calibrate 200us, 300us, and 400us pulses This is a critical problem for DESPOT-related experiments

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Hard Pulses at 3T

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  1. Hard Pulses at 3T Jason Su Oct. 10, 2011

  2. Issues • Been having problems getting correct flip angles from modified width Fermi hard pulses • Used rfstat to generate/calibrate 200us, 300us, and 400us pulses • This is a critical problem for DESPOT-related experiments • Initial thoughts on solutions: • nom_pw was incorrectly set to 100us (“fix”) • nom_bw did not scale with the pulses when ns3d_flag was on (“fix2”, includes nom_pw correction)

  3. Experiment • 3T2, agar phantom • Measure signal curve vs. flip angle with mean of central ROI • Slab profile shouldn’t be big issue • Using 801 soft pulse as our target curve (800us min phase, low BW pulse) • This is what was being used at 1.5T • Shown in blue

  4. Normalized SPGR Signal vs. Prescribed

  5. Normalized SPGR Signal vs. “Actual”

  6. Comments • 200us and 300us hard pulses are underflipping by a factor of 2x and 3x compared to 100us • The “actual” flip angle axis is scaled by lining up the max of the curves • Predicts that hp200 is underflipping by 0.6 compared to 801 • Neither of the proposed fixes had much effect

  7. Plotter – spgr_fa18_100us

  8. Plotter – spgr_fa18_200us

  9. Plotter – spgr_fa18_300us

  10. Comments • Pulsewidth seems to be as we expect • The prewind gradient appears to overlap with the RF pulse or is this an artifact of plotter?

  11. 100us Fermi Pulse • R1/R2/TG = 12/29/148 • 18/10^(-xmtaddScan/200) = 18 deg.

  12. 200us Fermi Pulse • R1/R2/TG = 11/29/132 • 18/10^(-xmtaddScan/200) = 59.241 deg.

  13. 300us Fermi Pulse • R1/R2/TG = 11/29/132 • 18/10^(-xmtaddScan/200) = 135.0696 deg.

  14. Thoughts?

  15. ISMRM Abstracts • kT points with DESPOT1 mapping @ 7T • Observed modest improvements with 1ch kT pts. • Correction with a B1 map is better than kT points alone • Need get back and quantify improvement • Try to apply kT+B1 correction with our AFI data • Accelerated DESPOT1 • View sharing with proper scaling accelerates collection of SPGR DESPOT angles • LCAMP may go even faster but still some work to be done • MSmcDESPOT – baseline and 1yr MS study • Not much new since last time even with full 1yr set for normals • Progressive patients have greater increase in DV than CIS or RR • TBSS? • DEV/CISmcDESPOT – longitudinal MS studies with 1-3 month sampling interval • Christine and Nora are now editing lesion segmentation • Potential questions: • How does MWF/DV in a lesion change over time? • Greater shifts in EDSS than MSmcDESPOT, potential for more interesting longitudinal correlations

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