Spin-lattice relaxation time
- Spin-lattice relaxation time
Spin-lattice relaxation time, known as T1, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. 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. It is thus the time it takes for the signal to recover 63% [1-(1/e)] of its initial value before being flipped into the magnetic transverse plane. Hence the relation:
Mz(t)= M0(1-e-t/T1)
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 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 time it takes for the spins to give the energy they obtained from the RF pulse back to the surrounding lattice, thereby restoring their equilibrium state.
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 interact with excited nuclei. However, at extremely high mobilities, the probability of a component of the lattice field being able to interact with excited nuclei decreases.Different tissues have different T1 values. For example, fluids have long T1s (1500-2000 mSec), and water based tissues are in the 400-1200 mSec range, while fat based tissues are in the shorter 100-150 mSec range.
=T1 weighted
In MRI, T1 weighted images can be obtained by setting short TR ( < 750mSec ) and TE ( < 40mSec ) 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 mSec.
ee also
*Relaxation (NMR)
References
*McRobbie D., et al. "MRI, From picture to proton." 2003
*Hashemi Ray, et al. "MRI, The Basics" 2ED. 2004.
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