Synopsis
Target Audience
Physicists,
engineers or scientists with training in MRI who are interested in contrast
mechanisms.
Learning Objectives
- Understand the main components of
the Bloch equations
- Understand how variations in
relaxation times affect image contrast
- Appreciate how tissue microstructure
affects relaxation
- Learn about examples how image
contrast is exploited in neuroimaging
Brief Overview
- In most biological applications the MRI
signal originates from protons within water
- Bloch equations phenomenologically
describe the evolution of magnetization over time including relaxation behavior
described by T1, T2 and T2* time constants
- Bloembergen-Purcell-Pound (BPP)
theory describes dipole-dipole and spin-lattice relaxation and thus T1, T2
- Relaxation in tissue can be
significantly enhanced due to exchange between bulk water and bound water at
macromolecular sites
- Governing T1, T2, T2* relaxation
time constants are tissue dependent and thus generate image contrast
- Contrast between tissue types and
pathological tissue states is the basis for clinical diagnostics
- Contrast is increasingly used to
make inferences on the underlying tissue microstructure, such as myelin and
iron mapping in the brain
Discussion
Magnetic
resonance imaging (MRI) relies on the phenomenon of nuclear magnetic resonance
due to the spin of the nucleus. In medicine and biology typically the net
magnetization of an ensemble of protons, i.e. nucleus of the hydrogen atom, is
imaged. The phenomenological Bloch equations [1] describe the evolution of the net magnetization,
and its components, and captures such phenomena as the excitation due to
radio-frequency (RF) fields, precession in a static magnetic field (B0),
and relaxation of magnetization.
Relaxation
processes, and in particular the tissue-dependency of relaxation, are a
cornerstone of MRI in medical and biological applications, since they underlie
the exquisite contrast between tissue types and pathological states. In the
state of equilibrium, the magnetization is aligned with the axis of the main
static magnetic field, i.e., only longitudinal but no transverse magnetization
is present. An RF pulse can excite the spin system, reducing longitudinal and
generating transverse magnetization. Relaxation is the return of the
magnetization to the state of equilibrium. Two types of relaxation behavior are
included in the Bloch equations and characterized by relaxation time constants.
1) The longitudinal relaxation (longitudinal relaxation time T1) describes the recovery
of the longitudinal magnetization component aligned with the main field. 2) The
transverse relaxation (transverse relaxation time T2, or in the presence of
inhomogeneities in the static magnetic field the apparent/effective transverse
relaxation time T2*) describes the decay of the transverse magnetization
component perpendicular to the main field.
Longitudinal
relaxation is driven by the exchange of energy between the nuclear spin system
and the surrounding environment, i.e. lattice – thus called spin-lattice
relaxation. The transverse relaxation is primarily driven by interaction
between the magnetic dipoles of spins – thus called spin-spin relaxation. The
Bloembergen-Purcell-Pound (BPP) theory provides an intuition for the mechanisms
behind spin-lattice and spin-spin
relaxation in homogeneous liquids and
other model systems [2] and predicts their relaxation times
T1 and T2.
Unlike the
model systems that [2] focused on, the structure of
biological tissue is highly complex with an abundance of membranes and
macromolecules, leading to further refinements of relaxation theory in tissue [3]–[5]. Although the relaxation mechanisms
in tissue are not fully understood, it is known that relaxivity is increased
due to water-binding sites at the protein-water interface where significant
relaxation and solute-solvent energy transfer occur [3]. Such sites are found in macromolecules
in the brain, particularly the cholesterol in myelin [6].
The
dependence of T1 on local myelination [7] is widely used in T1-weighted
anatomical imaging to delineate differently myelinated gray from white matter.
Recently, quantitative T1 maps have been used as a standardized means of elucidating
the subtle myeloarchitecture of the human cortex at high spatial resolution [8]. In a similar vein, T2 and T2* have
been used as markers of myelination [9] or of iron content [10].
The simultaneous acquisition of multiple relaxation parameters was used
to disentangle contrast mechanisms and provide a better characterization of
brain tissue [11]–[13]. In addition, endogenous contrast
may be complemented by exogenous contrast agents, such as injected Gadolinium
compounds used for characterizing vasculature or the perfused status of tumours.
Conclusion
MRI
provides exquisite soft tissue contrast due to the tissue-dependent nature of
magnetization relaxation. Characteristic relaxation times T1, T2 and T2* can
offer insights into tissue microstructure at scales significantly smaller than
the image voxel size. The consequent image contrast is widely exploited, e.g.,
in neuroimaging of gray and white matter structures.
Acknowledgements
No acknowledgement found.References
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