Essential Concepts in MRI. Yang XiaЧитать онлайн книгу.
or much longer) while T2 becomes short (tens of milliseconds or much shorter). This random field model is valid in the “weak collision” case where τc < T2; the theory will fail at the situation in relatively rigid materials. Some of the longest T1 are commonly found in solids, on the order of tens of minutes; some of the shortest T2 are on the order of 10 µs, which occurs in solids as well as well-organized connective tissues such as tendon and ligaments. Typical ranges of T1 and T2 for biological tissues are marked in Figure 3.4.
Figure 3.4 Schematic log/log trends of relaxation times on the correlation time τc, which also depend upon the resonance frequency and temperature (not shown). T1 relaxation time has a minimum, occurring when ω ~ 1/τc. This minimum separates two regimes of relaxation. On the left of the minimum is the extreme narrowing regime (ω ≪ 1/τc) where T1 ~ T2, where one can find the reduction of the homogeneous line width (1/πT2); one commonly finds simple and less viscous liquids of small molecules in this regime. On the right of the minimum there is a regime (ω ≫ 1/τc) where T1 and T2 diverge significantly; one can find in this regime viscous liquids and polymers of relatively large and complex molecules, as well as a variety of biological tissues.
References
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Notes
1 1 The Zeeman effect is named after Dutch physicist Pieter Zeeman for his 1896 observation of splitting of the optical spectral lines in a magnetic field.
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