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Correcting for Center Frequency Variations in MRSIData Using the Partially Suppressed Water Signal
Lawrence P Panych, Ph.D., Joseph R Roebuck, Ph.D., M.D., Nan-Kuei Chen, Ph.D.
Department of Radiology, Brigham and Women’s Hospital,Harvard Medical School, Boston MA
This work was supported by NIH U41-RR019703, R21-CA11092, 5R25-CA089017.
ISMRM Workshop on Data Processing for MR Spectroscopy and ImagingWarrenton, Virginia 11-13 November, 2006
• Several authors have reported methods using interleaved navigators to dynamically estimate the center frequency during a spectroscopic acquisition using a short acquisition without water suppression to correct spatial and spectral artifacts that would otherwise occur.
• Thiel et. al. Magn Reson Med. (2002)47:1077
Interleaved navigator during single voxel 1H-MRS acquisition
• Elbel and Maudsley. Magn Reson Med. (2005)53:465.
Interleaved navigator during 1H 3D-EPSI acquisition
• In preliminary work reported here, we explored using the raw phase-encoded data from a MRSI acquisition with partial water suppression to estimate the center frequency variations.
INTRODUCTION
INTRODUCTION: Spatial Misregistration of MRSI Signal1
241 24
x
y
Reconstructed MRSI of a pre-selected rectangular voxel placed at the center of the field of view (shown in red) within a homogeneous phantom. The signal is largely localized to the pre-selected voxel but spatial misregistration is seen in both directions, x > y.
INTRODUCTION: The Magnetic Field Drift
Center frequency variation with time estimated by interleaving MRSIs with and without PE.
Top: Plots from same magnet, separate acquisitions, without and with water suppression.Bottom: Plots from two 1.5T magnets acquired synchronously with water suppression.
Fre
quen
cy (
Hz)
Without suppression With suppression
Stable ωo Fluctuating ωo
METHODS
Magnet: General Electric Signa 1.5T 12.0 hardware/software.
Localization: PRESS TR 1000/TE30 Rectangular voxel pre-selected in the axial plane of magnet isocenter
MRSI: 24 x 24 or 16 x 16 sequential phase encoding in x, y1 x 1 x 1 cm in-plane resolution
Navigators: Interleaved with MRSI with Gx = Gy = 0
Phantom: General Electric MRS brain phantom
NameName SymbolSymbol Conc.Conc.
Myo-inositolMyo-inositol mImI 7.5 mM7.5 mM
CholineCholine ChoCho 3.0 mM3.0 mM
CreatineCreatine CrCr 10.0 mM10.0 mM
GlutamateGlutamate GluGlu 12.5 mM12.5 mM
N-acetyl-L-aspartateN-acetyl-L-aspartate NAANAA 12.5 mM12.5 mM
LactateLactate LacLac 5.0 mM5.0 mM
Post-processing: MatLab, Power Mac
METHODS: Interleaved MRSI and Navigators (Gx=Gy=0)
1 M
1
N
kx
ky
1 M
1
N
n=1
kx=N
t = TR
t = TR
t = TR
MR
SI
NA
VIG
AT
OR
SINTERLEAVED
m
2
3
2
n=N
3
t = 0
t = 2N×M×TR
n
kx=1
t = 2N×TR
n, kx
METHODS: Uncorrected Data
ky
kxINTERLEAVED
MRSI
NAVIGATORS
y
x2D-FT
2D-FT
y
x
(Gx=Gy=0)
METHODS: Uncorrected Data
ky
kxINTERLEAVED
MRSI
NAVIGATORS
2D-FT
2D-FT
y
x
y
x
(Gx=Gy=0)
METHODS: Center Frequency Drift Estimation and CorrectionMethod Using Solvent Suppressed Navigators
t = 32 s
n, kx
ΔtΔωn
tiωe(t)ff nn
corrn
INTERLEAVED
samples used to estimate n(t)
t = 0
METHODS: Center Frequency Drift Estimation and CorrectionUsing MRSI Data
t = 0
ky
kx
MRSI
INTERLEAVED
kx=2, ky=2
kx=9, ky=7
samples used to estimate n(t)
t = 32 s
1
161 16
METHODS: Self Correction Using Unsuppressed Water
Shift parameter estimated from MRSIwhen values close to ‘true’ value (Hz)
Maximum signal level in orderof phase encoding steps
Shift parameter estimatedfrom navigators (Hz)
Uncorrected MRSI Self-corrected MRSI
kx=8, ky=1, 2, …, 16
METHODS: Correction Strategies Using Interleaved Data
samples used to estimate n(t)
INTERLEAVED
NAVIGATORS MRSI
Self-Correction
NAVIGATORS MRSI
Navigated Correction
METHODS: Correction Strategies Using MRSI Data
Reordered Correction Hybrid Correction
kx = 8, 9, and 10 reordered in data acquisition order to provide more equally distributed high SNR k-space data for estimating center frequency drift.
5
9
13
ky
kx
MRSI
5
9
13
kx
ky
MRSI1
In addition to reordering kx = 8, 9, and 10, Gx and Gy were set to 0 for kx = 1 to provide high SNR navigators every 16 TRs for estimating center frequency drift.
Water
RESULTS: Comparison of Correction Strategies
No correction ReorderedWithout Correction
ReorderedWith Correction
HybridCorrection
NavigatedCorrection
RESULTS: Comparison of Correction Strategies
No correction ReorderedWithout Correction
ReorderedWith Correction
HybridCorrection
NavigatedCorrection
NAA
RESULTS: Comparison of Correction Strategies
No correction ReorderedWithout Correction
ReorderedWith Correction
HybridCorrection
NavigatedCorrection
NAA
Lac
NAACr
Cho
mIGlu
CrmI
Lac
NAACr
ChomI
Glu
CrmI
UncorrectedUncorrected Reordered correction
FUTURE WORK: Additional Correction Strategies Using MRSI Data
Reordered Correction UsingAdditional Lines of kx
5
9
13
ky
kx
MRSI
5
9
13
ky
kx
MRSI11
Reordered Correction UsingCentral Portion of k-space
Error in the frequency shift parameter defined as the difference between the parameter estimated from the MRSI and the interleaved navigators. Low error is represented as white and high error as black.
• We have demonstrated that it is possible to significantly reduce spatial artifact encountered in MRSI data resulting from fluctuations in the center frequency during MRSI data acquisition. These variations may be due to instrumentation, environmental or physiological factors
• In this work, the MRSI data itself was used to determine parameters used in the correction. This self-navigated correction approach is feasible when there is strong enough signal from water present in the raw data, either because residual water is intentionally left to provide a reference or because the data is acquired without solvent suppression.
• Future work will focus on improving the temporal resolution of the center frequency estimates by additional reordering of the high SNR points in k-space, validating the concept with multi-compartment phantoms, and evaluating the method using clinically relevant models.
SUMMARY
Spectrum from uncorrected MRSI data
Spectrum from corrected MRSI data
Lac
NAACr
Cho
mIGlu
CrmI
Lac
NAACr
ChomI
Glu
CrmI
