Spin–lattice relaxation
During nuclear magnetic resonance observations, spin–lattice relaxation is the mechanism by which the longitudinal component of the total nuclear magnetic moment vector (parallel to the constant magnetic field) exponentially relaxes from a higher energy, non-equilibrium state to thermodynamic equilibrium with its surroundings (the "lattice").
There is a different parameter, T2, the spin–spin relaxation time, which concerns the exponential relaxation of the transverse component of the nuclear magnetization vector (perpendicular to the external magnetic field).
Measuring the variation of T1 and T2 in different materials is the basis for some magnetic resonance imaging techniques.
[1] T1 characterizes the rate at which the longitudinal Mz component of the magnetization vector recovers exponentially towards its thermodynamic equilibrium, according to equation
Nuclei are contained within a molecular structure, and are in constant vibrational and rotational motion, creating a complex magnetic field.
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. pre-polarization by or insertion into high magnetic field) or if the nonequilibrium state has been achieved by other means (e.g., hyperpolarization by optical pumping).
[citation needed] 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.
The magnetization of the proton ensemble goes back to its equilibrium value with an exponential curve characterized by a time constant T1 (see Relaxation (NMR)).
[citation needed] T1 weighted images can be obtained by setting short repetition time (TR) such as < 750 ms and echo time (TE) such as < 40 ms 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.
A strong T1 contrast is present between fluid and more solid anatomical structures, making T1 contrast suitable for morphological assessment of the normal or pathological anatomy, e.g., for musculoskeletal applications.
Spin–lattice relaxation in the rotating frame is the mechanism by which Mxy, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value of zero, under the influence of a radio frequency (RF) field in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
[2] T1ρ MRI is an alternative to conventional T1 and T2 MRI by its use of a long-duration, low-power radio frequency referred to as spin-lock (SL) pulse applied to the magnetization in the transverse plane.
Quantitative T1ρ MRI relaxation maps reflect the biochemical composition of tissues.
[3] T1ρ MRI has been used to image tissues such as cartilage,[4][5] intervertebral discs,[6] brain,[7][8] and heart,[9] as well as certain types of cancers.
