T2*, relaxation time constant

Relaxation is an inherent property of all nuclear spins. There are two predominant types of relaxation processes in NMR of liquids. These relaxation processes are denoted by the longitudinal (Ti) and transverse (T2) relaxation time constants. When a sample is excited from its thermal equihbrium with an RF pulse, its tendency is to relax back to its Boltzmann distribution. The amount of time to re-equilibrate is typically on the order of seconds to minutes. T, and T2 relaxation processes operate simultaneously. The recovery of magnetization to the equilibrium state along the z-axis is longitudinal or the 7 relaxation time. The loss of coherence of the ensemble of excited spins (uniform distribution) in the x-, y-plane following the completion of a pulse is transverse or T2... 

Figure 17 Bulk signal amplitude for samples containing various liquid to solid ratios. The signal decays exponentially with a T2 relaxation time constant.

Ti) and spin-spin (T2) relaxation time constants. Fourier transformation of the time domain data produces a frequency spectrum where the amplitude at each frequency is a measure of the number of nuclei in the corresponding region in space. In magnetic resonance imaging a multivariate slice for one pulse sequence is used if spatial resolution is most important. A full NMR spectrum can be produced for a single point inside the material if the spectral aspect is most important. 

Note that T recovery occurs continually and causes some protons to realign with the Bo field. However, when subsequent n or 180° pulses are applied, as described below in Step 4, these protons are randomized relative to then-orientation with respect to the Bo field. This implies that T2 relaxation time constant must be less than or equal to Ti relaxation time constant... 

However, T2 is sensitive to the molecular interactions of spins and dependent on the molecular environment [60]. Thus, T2 may overlap for different components in certain materials and this technique alone may not be sufficient to identify the components. The relaxation time distributions are often broad, e.g., in meat [21], thus making it more difficult to associate the relaxation time constants with the components. 

The real power of MRI is the ability to exploit the inherent NMR properties of different tissues and tissue pathologies. The main MR parameters are known as longitudinal-relaxation time-constant (Tl), describing the time for the magnetization to recover to its equilibrium along the main-field axis after RF perturbation transverse-relaxation time-constant (T2), describing the... 

Physical parameters Molecular 0.1-1 nm Dipole-dipole interaction Second moment, fourth moment of lineshape Incoherent magnetization transfer characteristic times for cross-polarization and exchange Mesoscopic lnm-0.1 p,m Longitudinal relaxation time Ti Transverse relaxation time T2 Relaxation time Tip in the rotating frame Solid-echo decay time T2e Spin-diffusion constant Microscopic 0.1-lOp.m Molecular self-diffusion constant D Macroscopic 10 p,m and larger Spin density... 

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