magnetic susceptibility in mri - epileptologie...
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Summary
What is magnetic susceptibility?
Artefacts due to susceptibility in MRI
How to measure susceptibility
Susceptibility imaging
Applications: tractography.
Magnetic susceptibility in MRI - María J. Otero
Magnetic susceptibility
Dimensionless proportionality constant that indicates
the degree of magnetization, M, of a material in
response to an applied magnetic field, H.
Magnetic susceptibility in MRI - María J. Otero
Magnetic behaviours
Four different types of behaviour may be
distinguished:
Diamagnetism: χ is negative and of the order of 10-6.
Paramagnetism: χ is positive and typically in the
range 10-5-10-3.
Superparamagnetism: appears in small ferromagnetic
nanoparticles. Their magnetic susceptibility is much larger
than the one of paramagnets.
Ferromagnetism: χ is positive and extremely large,
typically greater than 100.
Magnetic susceptibility in MRI - María J. Otero
Artefacts
Between two areas with different susceptibility, a
small magnetic field gradient will exist.
These gradients accelerate the dephasing between
the protons on either side of the boundary, which leads
either to signal attenuation via T2* or to severe image
distortion.
Magnetic susceptibility in MRI - María J. Otero
Macroscopic effects of χ
Artefacts between boundaries of substances with
different magnetic behaviours (e.g. air-tissue boundary).
Magnetic susceptibility in MRI - María J. Otero
Macroscopic effects of χ
Magnetic susceptibility in MRI - María J. Otero
Macroscopic susceptibility effects caused by air–tissue
interfaces.
Macroscopic effects of χ
Magnetic susceptibility in MRI - María J. Otero
Magnetic susceptibility artifact markedly distorts the orbit and
frontal parenchyma in this patient with prior sinus reconstructive
surgery.
Microscopic effects of χ
Related to:
microstructure of tissues (geometry, orientation)
chemical composition (e.g. iron content).
Magnetic susceptibility in MRI - María J. Otero
Microscopic effects of χ
Magnetic susceptibility in MRI - María J. Otero
Effect of iron extraction on MRI contrast in postmortem brain tissue. Iron
extraction strongly reduces intracortical magnetic susceptibility–based
contrast.
BOLD
Blood oxygenation level dependent (BOLD) contrast is
used to depict neuronal activation.
Oxygenated haemoglobin is diamagnetic, while
deoxygenated haemoglobin is paramagnetic and thus has
a shorter T2*, driving to differences in BOLD contrast.
The technique is also sensitive to other sources of T2*-
induced signal losses: e.g. boundaries between substances
with different susceptibility (air/tissue).
Magnetic susceptibility in MRI - María J. Otero
Mapping susceptibility
Magnetic susceptibility in MRI - María J. Otero
Magnetic resonance
image
Phase image
Remove large scale
effects
Suscepti-bility
reconstruct.
From phase to susceptibility
Magnetic susceptibility in MRI - María J. Otero
High values of F lead to streaking artifacts and noise
amplification in the images calculated using this equation.
Problematic high values of F occur where its denominator
is close to or equal to zero, namely, on or near a cone in k-
space at the magic angle (i.e., 54.7° from the B0 axis).
From phase to susceptibility
Magnetic susceptibility in MRI - María J. Otero
Sampling from two orientations is insufficient because
the solid angle of each cone is >90° (≈2·54.7°),
leading to inevitable interceptions among the four zero-
cone surfaces associated with any two-angle sampling.
Calculation Of Susceptibility through Multiple Orientation Sampling
COSMOS solves the inverse problem by oversampling
from multiple orientations making use of some facts:
The zero cone surface in the Fourier domain is fixed at the
magic angle with respect to the B₀ field.
If an object is rotated with respect to the B₀ field, then in
the object's frame, the B₀ field is rotated and thus the cone.
Consequently, data that cannot be calculated due to the
cone becomes available at the new orientations.
Magnetic susceptibility in MRI - María J. Otero
Susceptibility imaging
Susceptibility weighted imaging.
Susceptibility tensor imaging.
Magnetic susceptibility in MRI - María J. Otero
Susceptibility weighted imaging (SWI)
This method exploits the susceptibility differences
between tissues to generate a unique contrast.
A high-pass filtered phase image is used to detect
these differences. The magnitude and phase data are
combined to produce an enhanced contrast magnitude
image which is exquisitely sensitive to venous blood,
hemorrhage and iron storage.
Magnetic susceptibility in MRI - María J. Otero
Susceptibility weighted imaging (SWI)
Magnetic susceptibility in MRI - María J. Otero
Unprocessed
original SWI
magnitude image.
HP-filtered phase
image.
Processed SWI
magnitude image.
Susceptibility weighted imaging (SWI)
Magnetic susceptibility in MRI - María J. Otero
Conventional
gradient echo
T2*-weighted
image
Susceptibility
weighted image
SWI phase image
Susceptibility Tensor Imaging (STI)
Magnetic response M is dependent upon the orientation of
the sample and can occur in directions other than that of the
applied field H. In these cases, volume susceptibility is
defined as a spatial tensor
where i and j refer to the directions of the applied field
and magnetization, respectively.
Magnetic susceptibility in MRI - María J. Otero
Applications: Fiber tracking
There is a direct link between the orientation of the
nerve fibers in white matter (WM) and the contrast
observed in magnitude and phase images acquired
using gradient echo MRI.
Dominant source of this contrast: effect of the myelin
sheath on the evolution of the NMR signal.
Magnetic susceptibility in MRI - María J. Otero
A, nerve fibers are
modeled as infinite hollow
cylinders oriented at angle,
θ, to B0.
B, two-pool model.
C, the susceptibility of the
myelin sheath is anisotropic
and described by a
cylindrically symmetric
tensor in which the principal
axis is radially oriented.
Fiber tracking
Magnetic susceptibility in MRI - María J. Otero
Fiber tracking
Magnetic susceptibility in MRI - María J. Otero
Calculated field perturbations due to the hollow cylinder model populated with isotropic susceptibility (A, D), exchange-related field offsets (B, E), and radially oriented anisotropic susceptibility (C, F).
Comparison of color-coded STI and DTI
Magnetic susceptibility in MRI - María J. Otero
Color codes:
Red: anterior–
posterior;
Green: left-
right;
Blue: dorsal–
ventral.
References Liu T et al. Calculation of susceptibility through multiple orientation sampling (COSMOS): a method for conditioning the inverse problem from measured magnetic field map to susceptibility source image in MRI. Magn Reson Med 2009;61:196–204.
Duyn JH, et al. High-field MRI of brain cortical substructure based on signal phase. PNAS July 10, 2007 vol. 104 no. 28 11796-11801
Shmueli K, et al.(2009) Magnetic susceptibility mapping of brain tissue in vivo using MRI phase data. MagnReson Med 62:1510–1522.
Liu C (2010) Susceptibility tensor imaging. Magn Reson Med 63(6):1471–1477.
Marques JP, Bowtell R (2005) Application of a Fourier-based method for rapid calculation of field inhomogeneity due to spatial variation of magnetic susceptibility.
LiW, Wu B, Avram AV, Liu C (2012) Magnetic susceptibility anisotropy of human brain in vivo and its molecular underpinnings. Neuroimage 59(3):2088–2097.
Wharton, S. and R. Bowtell, Fiber orientation-dependent white matter contrast in gradient echo MRI. PNAS, 2012. 109(45): p. 18559-18564.
Gary H. Glover, 3D z-Shim Method for Reduction of Susceptibility Effects in BOLD fMRI. Magnetic Resonance in Medicine 42:290–299 (1999)
Jianqi Li et al. Reducing the Object Orientation Dependence of Susceptibility Effects in Gradient Echo MRI Through Quantitative Susceptibility Mapping. Magn Reson Med(2011)
Yu-Chung N. Cheng. Limitations of Calculating Field Distributions and Magnetic Susceptibilities in MRI using a Fourier Based Method. Phys Med Biol. 2009 March 7; 54(5): 1169–1189
E.M. Haacke, et al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1. AJNR January 2009 30: 19-30
Webb’s Physics of Medical Imaging. Second edition. CRC Press
Magnetic susceptibility in MRI - María J. Otero