Spin-lattice relaxation time
Encyclopedia
Spin–lattice relaxation is the mechanism by which the z component of the magnetization vector comes into thermodynamic equilibrium with its surroundings (the "lattice") in nuclear magnetic resonance
and magnetic resonance imaging
. It is characterized by the spin–lattice relaxation time, a time constant known as T1. It is named in contrast to T2, the spin-spin relaxation time
.
Or, for the specific case that
It is thus the time it takes for the longitudinal magnetization to recover approximately 63% [1-(1/e)] of its initial value after being flipped into the magnetic transverse plane by a 90° radiofrequency pulse.
Nuclei are held within a lattice structure, and are in constant vibrational and rotational motion, creating a complex magnetic field. The magnetic field caused by thermal motion of nuclei within the lattice is called the lattice field. The lattice field of a nucleus in a lower energy state can interact with nuclei in a higher energy state, causing the energy of the higher energy state to distribute itself between the two nuclei. Therefore, the energy gained by nuclei from the RF pulse is dissipated as increased vibration and rotation within the lattice, which can slightly increase the temperature of the sample. The name spin-lattice relaxation refers to the process in which the spins give the energy they obtained from the RF pulse back to the surrounding lattice, thereby restoring their equilibrium state. The same process occurs after the spin energy has been altered by a change of the surrounding static magnetic field (e.g. prepolarization by or insertion into high magnetic field) or if the nonequilibrium state has been achieved by other means (e.g. hyperpolarization
by optical pumping).
The relaxation time, T1 (the average lifetime of nuclei in the higher energy state) is dependent on the gyromagnetic ratio of the nucleus and the mobility of the lattice. As mobility increases, the vibrational and rotational frequencies increase, making it more likely for a component of the lattice field to be able to stimulate
the transition from high to low energy states. However, at extremely high mobilities, the probability decreases as the vibrational and rotational frequencies no longer correspond to the energy gap between states.
Different tissues have different T1 values. For example, fluids have long T1s (1500-2000 ms), and water based tissues are in the 400-1200 ms range, while fat based tissues are in the shorter 100-150 ms range. The presence of strongly magnetic ions or particles (e.g. ferromagnetic, paramagnetic) also strongly alter T1 values and are widely used as MRI contrast agent
s.
uses the resonance of the protons to generate images. Protons are excited by a radiofrequency pulse of an appropriate frequency and then emit energy in the form of radiofrequency (RF) signal as they return to their original state. The RF signal decays with an exponential curve characterized by a parameter T1 (see Relaxation (NMR)
).
T1 weighted images can be obtained by setting short TR ( < 750 ms ) and TE ( < 40 ms ) values in conventional spin echo
sequences, while in Gradient Echo Sequences they can be obtained by using flip angles of larger than 50o while setting TE values to less than 15 ms.
T1 is significantly different between grey matter
and white matter
and is used when undertaking brain scans.
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
and 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...
. It is characterized by the spin–lattice relaxation time, a time constant known as T1. It is named in contrast to T2, the spin-spin relaxation time
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...
.
Nuclear physics
T1 characterizes the rate at which the longitudinal Mz component of the magnetization vector recovers exponentially towards its thermodynamic equilibrium, according to equation:Or, for the specific case that
It is thus the time it takes for the longitudinal magnetization to recover approximately 63% [1-(1/e)] of its initial value after being flipped into the magnetic transverse plane by a 90° radiofrequency pulse.
Nuclei are held within a lattice structure, and are in constant vibrational and rotational motion, creating a complex magnetic field. The magnetic field caused by thermal motion of nuclei within the lattice is called the lattice field. The lattice field of a nucleus in a lower energy state can interact with nuclei in a higher energy state, causing the energy of the higher energy state to distribute itself between the two nuclei. Therefore, the energy gained by nuclei from the RF pulse is dissipated as increased vibration and rotation within the lattice, which can slightly increase the temperature of the sample. The name spin-lattice relaxation refers to the process in which the spins give the energy they obtained from the RF pulse back to the surrounding lattice, thereby restoring their equilibrium state. The same process occurs after the spin energy has been altered by a change of the surrounding static magnetic field (e.g. prepolarization by or insertion into high magnetic field) or if the nonequilibrium state has been achieved by other means (e.g. hyperpolarization
Hyperpolarization (physics)
Hyperpolarization is the nuclear spin polarization of a material far beyond thermal equilibrium conditions. It is commonly applied to gases such as 129Xe and 3He which are then used, for instance, in hyperpolarized magnetic resonance imaging of the lungs....
by optical pumping).
The relaxation time, T1 (the average lifetime of nuclei in the higher energy state) is dependent on the gyromagnetic ratio of the nucleus and the mobility of the lattice. As mobility increases, the vibrational and rotational frequencies increase, making it more likely for a component of the lattice field to be able to stimulate
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
the transition from high to low energy states. However, at extremely high mobilities, the probability decreases as the vibrational and rotational frequencies no longer correspond to the energy gap between states.
Different tissues have different T1 values. For example, fluids have long T1s (1500-2000 ms), and water based tissues are in the 400-1200 ms range, while fat based tissues are in the shorter 100-150 ms range. The presence of strongly magnetic ions or particles (e.g. ferromagnetic, paramagnetic) also strongly alter T1 values and are widely used as MRI contrast agent
MRI contrast agent
MRI contrast agents are a group of contrast media used to improve the visibility of internal body structures in magnetic resonance imaging . The most commonly used compounds for contrast enhancement are gadolinium-based. MRI contrast agents alter the relaxation times of tissues and body cavities...
s.
T1 weighted images
Magnetic resonance imagingMagnetic 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...
uses the resonance of the protons to generate images. Protons are excited by a radiofrequency pulse of an appropriate frequency and then emit energy in the form of radiofrequency (RF) signal as they return to their original state. The RF signal decays with an exponential curve characterized by a parameter T1 (see Relaxation (NMR)
Relaxation (NMR)
In nuclear magnetic resonance spectroscopy and magnetic resonance imaging the term relaxation describes several processes by which nuclear magnetization prepared in a non-equilibrium state return to the equilibrium distribution. In other words, relaxation describes how fast spins "forget" the...
).
T1 weighted images can be obtained by setting short TR ( < 750 ms ) and TE ( < 40 ms ) values in conventional spin echo
Spin echo
In magnetic resonance, a spin echo is the refocusing of precessing spin magnetisation by a pulse of resonant radiation. Modern nuclear magnetic resonance and magnetic resonance imaging rely heavily on this effect....
sequences, while in Gradient Echo Sequences they can be obtained by using flip angles of larger than 50o while setting TE values to less than 15 ms.
T1 is significantly different between grey matter
Grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil , glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts...
and white matter
White matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
and is used when undertaking brain scans.