PHYS1008 Study Guide - Final Guide: Larmor Precession, Spin–Lattice Relaxation, Exponential Growth
2 Aug 2018
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T2 relaxation occurs whenever there is t1 relaxation. Additional processes such as static local field inhomogeneities and spin flip-flops cause t2 relaxation without affecting t1. T1 is only heat loss to the lattice. Tissues that are very rigid have small t2 times because they will lose their energy straight away. Fat has a relatively short t1 value compared to most other tissues. T1 + t2 as a function of molecular size and tumbling rate. The minimum value of t1 and dip in the t2 curve occurs when motion is at the larmor frequency. The most important determinant of t1 + t2 values is size and motion of molecule which the hydrogen nucleus resides. Small, rapidly rotating molecules (free water) have long t1 and t2 times. As molecular motion slows (proteins and dense solids) t2 shortens and t1 increases. Molecules that are bound cannot tumble as much.
