1. Improved Cardiac Shim using Field Map Estimate from Multi-echo Dixon Method
Motivation:for improved cardiac shim
•B0-field inhomogeneity affects images …
•GRE-EPI artifacts
•SSFP dark banding artifacts
•Chemical shift fat suppression
•T2* losses
•Rapid field variation across the heart
•caused by tissue-air interface & veins
Approach:Joint fat/water/field map estimates
•multi-echo GRE acquisition
•VARPRO solution for fat and water components &fieldmap estimate
•parallel imaging reconstruction
•real-time, free-breathing, non-ECG gated 2D
• multi-slice acquisition, acquiring the volume in ≈ 7s
water & fat separation:multi-echo Non-linear Least Squares methods
multi-echo
dataset
VARPRO
Signal model:
ML cost function:
VARPRO formulation:
Sequence:real-time, multi-echo GRE
Typical parameters:
matrix size:64x48
parallel imaging:rate 2 (GRAPPA)
number of echoes:6 (monopolar readout)
bandwidth:900 Hz/pixel
TE (TR):1.2, 2.7, 4.3, 5.8, 7.3, 8.9 ms (10.25 ms)
duration:246 ms/slice
spatial resolution:6.25 x 6.25 mm2 (8 mm thick with no gap)
# slices:20 slices typical (covering 160 mm)
1 2 3 M=48
N-echoes per PE line
Acquire auto-calibration data for parallel imaging
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Slice
approx 7 sec
Volume coverage2d multi-slice
Improved cardiac shim
SSFP
cine
fieldmap
before
after
before
after
0
20
40
60
80
100
120
140
-300
-200
-100
0
100
profile position (mm)
frequency (Hz)
Real-time fieldmap estimatesfree breathing, non-triggered
fieldmap
(±200 Hz)
Fieldmap profiles across heartfor 50 repetitions using shimsequence.
magnitude
std = 4 Hz
Summary
•Approach to field map estimation
• multi-echo fat/water separation imaging
• low spatial resolution fieldmap
• non-triggered, free-breathing acquisition
• real-time with parallel imaging
•2d-multislice volume coverage
•Performance
• reliable and fast (<7.5 s / volume)
• relatively insensitive to cardio-respiratory phase
