Download Magnetic susceptibility in MRI

Magnetic susceptibility in MRI
María José Otero Díaz
Summary
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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
Macroscopic susceptibility effects caused by air–tissue
interfaces.
Magnetic susceptibility in MRI - María J. Otero
Macroscopic effects of χ
Magnetic susceptibility artifact markedly distorts the orbit and
frontal parenchyma in this patient with prior sinus reconstructive
surgery.
Magnetic susceptibility in MRI - María J. Otero
Microscopic effects of χ
Related to:
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microstructure of tissues (geometry, orientation)
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chemical composition (e.g. iron content).
Magnetic susceptibility in MRI - María J. Otero
Microscopic effects of χ
Magnetic susceptibility in MRI - María J. Otero
Microscopic effects of χ
Effect of iron extraction on MRI contrast in postmortem brain tissue. Iron
extraction strongly reduces intracortical magnetic susceptibility–based
contrast.
Magnetic susceptibility in MRI - María J. Otero
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
resonance
image
Remove
large
scale
effects
Phase
image
Magnetic susceptibility in MRI - María J. Otero
Susceptibility
reconstruct.
From phase to susceptibility
Magnetic susceptibility in MRI - María J. Otero
From phase to susceptibility
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 kspace at the magic angle (i.e., 54.7° from the B0 axis).
Magnetic susceptibility in MRI - María J. Otero
From phase to susceptibility
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 zerocone surfaces associated with any two-angle sampling.
Magnetic susceptibility in MRI - María J. Otero
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.
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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.
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Consequently, data that cannot be calculated due to the
cone becomes available at the new orientations.
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Magnetic susceptibility in MRI - María J. Otero
Susceptibility imaging
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Susceptibility weighted imaging.
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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)
Unprocessed
original SWI
magnitude image.
HP-filtered phase
image.
Processed SWI
magnitude image.
Magnetic susceptibility in MRI - María J. Otero
Susceptibility weighted imaging (SWI)
Conventional
gradient echo
T2*-weighted
image
Susceptibility
weighted image
SWI phase image
Magnetic susceptibility in MRI - María J. Otero
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
Susceptibility Tensor Imaging (STI)
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
Fiber tracking
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.
Magnetic susceptibility in MRI - María J. Otero
Fiber tracking
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).
Magnetic susceptibility in MRI - María J. Otero
Comparison of color-coded STI and DTI
Color codes:
Red: anterior–
posterior;
Green: leftright;
Blue: dorsal–
ventral.
Magnetic susceptibility in MRI - María J. Otero
Thanks for your attention!
Magnetic susceptibility in MRI - María J. Otero
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.
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Duyn JH, et al. High-field MRI of brain cortical substructure based on signal phase. PNAS July 10, 2007
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Reson Med 62:1510–1522.
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inhomogeneity due to spatial variation of magnetic susceptibility.
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Webb’s Physics of Medical Imaging. Second edition. CRC Press
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Magnetic susceptibility in MRI - María J. Otero