Magnetic susceptibility in MRI - Epileptologie ...epileptologie-bonn.de/.../MOtero_Magnetic_susceptibility-in_MRI.pdf · Magnetic susceptibility artifact markedly distorts the orbit

Magnetic susceptibility in MRI

María José Otero Díaz

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 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.


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.


Macroscopic effects of χ

Artefacts between boundaries of substances with

different magnetic behaviours (e.g. air-tissue boundary).




Macroscopic susceptibility effects caused by air–tissue

interfaces.



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).






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).


Mapping susceptibility


Magnetic resonance

image

Phase image

Remove large scale

effects

Suscepti-bility

reconstruct.

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 k-

space at the magic angle (i.e., 54.7° from the B0 axis).



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.


Susceptibility imaging

Susceptibility weighted imaging.

Susceptibility tensor imaging.


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.




Unprocessed

original SWI

magnitude image.

HP-filtered phase

image.

Processed SWI

magnitude image.



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.


Susceptibility Tensor Imaging (STI)


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.


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


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).

Comparison of color-coded STI and DTI


Color codes:

Red: anterior–

posterior;

Green: left-

right;

Blue: dorsal–

ventral.


Thanks for your attention!

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.

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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


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