Magnetization transfer
Encyclopedia
Magnetization transfer (MT), as commonly used in biomedical MRI, refers to the transfer of longitudinal magnetization from the hydrogen nuclei of water that have restricted motion to the hydrogen nuclei of water that moves with many degrees of freedom. The water with restricted motion is generally conceived as being bound to macromolecules,such as proteins and lipids through a series of hydrogen bond
s. The free water pool is the bulk water of the cytosol
. In this context, the hydrogen nuclei are typically referred to simply as protons.
In magnetic resonance imaging
of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and bound (hydration), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequencies lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). The high rotational frequency also results in fewer interactions with the environment so that the transverse magnetization dephasing is slower and the T2
is long. Conversely, hydrated water molecules are slowed down by extensive interactions with the protons in the local macromolecules and hence magnetic field inhomogeneities are created that lead to wider resonance frequency spectrum. This results in faster dephasing of the magnetization that produces the NMR
signal and much shorter T2 values(<200 μs). Because the T2 values are so short the NMR
signal from the protons of bound water is not typically observed in MRI.
However, using an off-resonance pulse excitation to saturate protons in the restricted pool can have a detectable effect on NMR signal from the mobile (free) proton pool. The transverse magnetization created is rapidly dephased and the longitudinal magnetization requires some time (approximately 5 times T1
) to return to equilibrium. Since the bound water may exchange magnetization with the free water, the loss of longitudinal magnetization will also be introduced into the pool of free water. This causes an increase in the T1 of the free water and reduced signal from the free water in tissues in which the magnetization transfer mechanism is prevalent. Since the extent of signal decay depends on the exchange rate between free and hydration water, MT can be used to provide an alternative contrast method in addition to T1,T2, and proton density differences.
MT is believed to be a nonspecific indicator of the structural integrity of the tissue being imaged.
An extension of MT, the magnetization transfer ratio (MTR) has been used in neuroradiology
to highlight abnormalities in brain structures. (The MTR is (Mo-Mt)/Mo.)
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s. The free water pool is the bulk water of the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
. In this context, the hydrogen nuclei are typically referred to simply as protons.
In magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
of molecular solutions, such as protein solutions, two types of water molecules, free (bulk) and bound (hydration), are found. Free water protons have faster average rotational frequency and hence less fixed water molecules that may cause local field inhomogeneity. Because of this uniformity, most free water protons have resonance frequencies lying narrowly around the normal proton resonance frequency of 63 MHz (at 1.5 teslas). The high rotational frequency also results in fewer interactions with the environment so that the transverse magnetization dephasing is slower and the T2
Spin-spin relaxation time
thumb|right|T2 relaxation curveSpin–spin relaxation is the mechanism by which Mxy, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value of zero, in nuclear magnetic resonance and magnetic resonance imaging...
is long. Conversely, hydrated water molecules are slowed down by extensive interactions with the protons in the local macromolecules and hence magnetic field inhomogeneities are created that lead to wider resonance frequency spectrum. This results in faster dephasing of the magnetization that produces the NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
signal and much shorter T2 values(<200 μs). Because the T2 values are so short the NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
signal from the protons of bound water is not typically observed in MRI.
However, using an off-resonance pulse excitation to saturate protons in the restricted pool can have a detectable effect on NMR signal from the mobile (free) proton pool. The transverse magnetization created is rapidly dephased and the longitudinal magnetization requires some time (approximately 5 times T1
Spin-lattice relaxation time
Spin–lattice relaxation is the mechanism by which the z component of the magnetization vector comes into thermodynamic equilibrium with its surroundings in nuclear magnetic resonance and magnetic resonance imaging. It is characterized by the spin–lattice relaxation time, a time constant known as T1...
) to return to equilibrium. Since the bound water may exchange magnetization with the free water, the loss of longitudinal magnetization will also be introduced into the pool of free water. This causes an increase in the T1 of the free water and reduced signal from the free water in tissues in which the magnetization transfer mechanism is prevalent. Since the extent of signal decay depends on the exchange rate between free and hydration water, MT can be used to provide an alternative contrast method in addition to T1,T2, and proton density differences.
MT is believed to be a nonspecific indicator of the structural integrity of the tissue being imaged.
An extension of MT, the magnetization transfer ratio (MTR) has been used in neuroradiology
Neuroradiology
Neuroradiology is a subspecialty of radiology focusing on the diagnosis and characterization of abnormalities of the central and peripheral nervous system, spine, and head and neck. Primary imaging modalities include computed tomography and magnetic resonance imaging...
to highlight abnormalities in brain structures. (The MTR is (Mo-Mt)/Mo.)