T1 relaxation
Spin–lattice relaxation is the mechanism by which the component of the magnetization vector along the direction of the static magnetic field reaches 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.
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 contained within a molecular 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).
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