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Fat and water separation techniques in
NMR imagingNMR imaging
Noura Azzabou
30 April 2013
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A vector representation of the complex signal S for a a given
pixel with two spectral components, water (W) and fat (F).
Fat and water separation techniques : chemical shift principle
The phase angle of fat relative to
that of water due to their chemical
shift difference
The error phase due to the magnetic
field in homogeneity
another error phase due to other system imperfections
such as the spatial dependence of RF penetration and
different signal delay in the receiver chains.
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A vector representation of the complex signal S for a a given
pixel with two spectral components, water (W) and fat (F).
Fat and water separation techniques : chemical shift principle
The phase angle of fat relative to
that of water due to their chemical
shift difference
The error phase due to the magnetic
field in homogeneity
another error phase due to other system imperfections
such as the spatial dependence of RF penetration and
different signal delay in the receiver chains.
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Assuming that water-fat chemical shift (σσσσ) is 3.5 ppm, and B0 = 3T
3.5 ppm @ 3T = σ B0 γ = 3.5 x 3 x 42.58 MHz/T= 447 Hz
relative phase = = TE x 447 x 2π
w
f
w
f
Fat and water separation techniques : 2 point Dixon
Acquisition : Echos time selection
TEi= 1/447 s = 2.24 ms
TEi= 2/447 s = 4.48 ms
(“in phase” condition)
TEo= 1.5/447 s = 3.36 ms
TEo= 3/447 s = 6.71 ms
(“opposed phase” condition)
f
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Assuming a prior knowledge about fat water content and that for example there is
Fat and water separation techniques : 2 point Dixon
Water and Fat signal reconsutruction
Assuming a prior knowledge about fat water content and that for example there is
more fat than water in the pixel we can write
In the opposite case we have
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Fat and water separation techniques : 2 point Dixon
Reconstruction results
Fat ratio map, for the thigh of healthy volunteer.
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Fat and water separation techniques : 2 point Dixon
Reconstruction results
Fat ratio map, for the thigh of healthy volunteer. For the one leg water and
fat signal are swapped
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Fat and water separation techniques : 3 point Dixon
Reconstruction technique
After computing B0 inhomogenieties, fat and water signal are computed through
linear equation resolving.
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Fat and water separation techniques : 3 point Dixon
Phase wrapping problem
Phase of the first in-phase image S1
Phase of the second in-phase image S3
Difference between phases.
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Fat and water separation techniques : 3 point Dixon
Phase wrapping problem
10
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Fat and water separation techniques : 3 point Dixon
Phase wrapping problem
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Fat and water separation techniques : 3 point Dixon
Example of reconstructed images
in-phase opposed-phase F W
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Fat and water separation techniques : 3 point Dixon
Quantitative analysis
Reference : Hines, C. D. G., Yu, H., Shimakawa, A., McKenzie, C. a, Brittain, J. H., & Reeder, S. B. (2009). T1 independent, T2* corrected
MRI with accurate spectral modeling for quantification of fat: validation in a fat-water-SPIO phantom. Journal of magnetic resonance
imaging: JMRI, 30(5), 1215-22.
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Fat and water separation techniques : 3 point Dixon
Quantitative analysis
Reference : Hines, C. D. G., Yu, H., Shimakawa, A., McKenzie, C. a, Brittain, J. H., & Reeder, S. B. (2009). T1 independent, T2* corrected
MRI with accurate spectral modeling for quantification of fat: validation in a fat-water-SPIO phantom. Journal of magnetic resonance
imaging: JMRI, 30(5), 1215-22.
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Fat and water separation techniques : 3 point Dixon
Quantitative analysis
Reference : Hines, C. D. G., Yu, H., Shimakawa, A., McKenzie, C. a, Brittain, J. H., & Reeder, S. B. (2009). T1 independent, T2* corrected
MRI with accurate spectral modeling for quantification of fat: validation in a fat-water-SPIO phantom. Journal of magnetic resonance
imaging: JMRI, 30(5), 1215-22.
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voxelDixon technique is based on the chemical shift difference between water protons and lipid –CH2 protons.
Fat and water separation techniques : 3 point Dixon
Quantitative analysis
~3.5 ppm ~ 450 Hz @ 3T
Fat (-CH)
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Fat and water separation techniques : Fat spectrum
Representative localized in vivo proton spectrum (PRESS: TR = 1.8 s, TE = 20 ms, voxel size = 2 mm × 2 mm × 2 mm, NEX = 500) of adipose tissue of a mouse.
Strobel K et al. J. Lipid Res. 2008;49:473-480
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Fat and water separation techniques : Multi-peak Model
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Fat and water separation techniques : Multi-peak Model
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IDEAL - ALGORITHM
We can formulate this as a linear system with respect to fat and water signal.
In order to resolve this system we have to acquire at least M+2 images (with M total
number of fat peaks).
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IDEAL - ALGORITHM
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Fat and water separation techniques : IDEAL technique
Iterative reconstruction process
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Fat and water separation techniques : IDEAL technique
Iterative reconstruction process : field map error estimation
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Fat and water separation techniques : IDEAL technique
Iterative reconstruction process : field map error estimation
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Fat and water separation techniques : IDEAL technique
Signal Decay
The decay is due to the spin-spin relaxation (T2) and an additional component due to B0
inhomogeniey. The over all decay is called T2* (R2*=1/T2*)
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Fat and water separation techniques : IDEAL technique
Signal Decay
The decay is due to the spin-spin relaxation (T2) and an additional component due to B0
inhomogeniey. The over all decay is called T2* (R2*=1/T2*)
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Fat and water separation techniques : IDEAL technique
Signal Model with R2* decrease
The same iterative algorithm is applied with the difference that the unknow field
map is a complex number.
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Fat and water separation techniques : IDEAL technique
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Fat and water separation techniques : IDEAL technique
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Fat and water separation techniques : IDEAL technique
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� IDEAL technique assumes that the initial guess of R2* and B0 field inohomogeniety
are close to real values.
� The reconstruction technique can lead to local minimums which can result in false
estimation of water and fat signal.
� The algorithm, is a voxel by voxel resolution, no regularization term.
Fat and water separation techniques : IDEAL technique
Iterative reconstruction process : limitations
Results of IDEAL technique on mayonnaise samples with different fat ratio : from left to
right : field map, water map and fat map.
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The main objective is to
address the problem of
regularization as well as
local minimum problems local minimum problems
related to the IDEAL
resolution.
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Fat and water separation techniques : VARPRO technique
10
12
14x 10
4 Error of the fit as a function of the field map value
VARPRO: reconstruction amounts to 1D function minimization
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10
2
4
6
8
dB0
err
or
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Fat and water separation techniques : VARPRO technique
VARPRO: Algorithm
The algorithm is composed of two main step
1- Minimization
2- Regularization
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VARPRO: comparaison with IDEAL
Fat and water separation techniques : VARPRO technique
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An extension of the VARPRO approach, it performs minimization and regularization at
the same step
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Fat and water separation techniques : VARPRO + Graph cut
Discrete optimization technique like graph
cut or other approaches
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Fat and water separation techniques : VARPRO + Graph cut
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Fat and water separation techniques : Summary
Two element are crucial in the estimation of fat and water signal in the tissue :
1- Accurate signal model : R2* decay and multi-peak fat spectrum
2- Efficient global optimization approach, on complex data.
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Fat and water separation techniques : IDEAL technique
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Fat and water separation techniques : Impact of fat spectrum
Taking into account M peaks, requires the acquisition of M+2 images
� Exam duration is increased
Pre-calibration of fat spectrum :
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Fat and water separation techniques : Impact of fat spectrum
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Fat and water separation techniques : Impact of fat spectrum
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Fat and water separation techniques : Echo time selection
The variance of the estimator verifies
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T1-weighted spin-echo MR images (TR/TE, 400/10)
obtained in 10-year-old patient with functional score
of 3.
1 = vastus lateralis, 2 = biceps femoris, 3 =
semitendinosus, 4 = vastus medialis, 5 = rectus
femoris, 6 = vastus intermedius.
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Functional grade was strongly inversely correlated with fat percentage
measured with the Dixon technique (p < 0.05)
Fig. 3—Graphs show Dixon MRI measurements of intramuscular fat percentage increased significantly with
decreasing functional level in all muscles examined.