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
• 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)
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
Normalized SPGR Signal vs. Prescribed
Normalized SPGR Signal vs. “Actual”
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
Plotter – spgr_fa18_100us
Plotter – spgr_fa18_200us
Plotter – spgr_fa18_300us
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?
100us Fermi Pulse
Presc. Flip Angle
ia_rf1
xmtaddScan
xmtadd
18
32767
0
13.5209
12
32767
21.6974
6
32767
81.9034
1
2270
200
• R1/R2/TG = 12/29/148
• 18/10^(-xmtaddScan/200) = 18 deg.
200us Fermi Pulse
Presc. Flip Angle
ia_rf1
xmtaddScan
xmtadd
18
32767
103.47
0
12
32767
138.688
6
32767
198.894
1
5531
200
• R1/R2/TG = 11/29/132
• 18/10^(-xmtaddScan/200) = 59.241 deg.
300us Fermi Pulse
Presc. Flip Angle
ia_rf1
xmtaddScan
xmtadd
18
32767
175.057
0
12
29111
200
6
14555
200
1
2425
200
• R1/R2/TG = 11/29/132
• 18/10^(-xmtaddScan/200) = 135.0696 deg.
Thoughts?
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