C T2 and TE

Transverse (T2) relaxation is the process in which the finite components of the magnetization present in the transverse plane decay to zero (the equilibrium state). Often, T2 relaxation is discussed in terms of two separate processes. One of these is driven by molecular movements and is known as "pure" T2 relaxation. The other is the result of the presence of a magnetic field gradient within the voxel of interest and is known as T2* relaxation. To measure "pure" T2 relaxation independently of T2* relaxation, spin echo acquisitions (Fig. 4) are commonly used. The spin echo amplitude decays as the TE is lengthened, as illustrated in the graph in Fig. 4. This is generally found to be described by a decaying exponential function that has a characteristic time constant. To determine the value of the characteristic time constant for transverse relaxation (T2), one finds the point at which 63% of the signal has decayed and then projects that point onto the TE axis.

As is the case for T1, the T2 value depends on motional constraints imposed by tissue ultrastructure. However, the functional dependence of T2 relaxation differs from that for T1 relaxation. Tissues in which the water molecule motion is unhindered (e.g., CSF) tend to produce a very long T2. Increasing constraints on water movement lead to progressively shorter values for T2. WM shows a shorter T2 compared to that of GM, and both of these show a shorter T2 compared to that of CSF. The T2 relaxation characteristics of the three tissues are illustrated in the graph shown in Fig. 13. Spin echo signals produced by each of the tissues decay as TE is lengthened, with the differences becoming more pronounced as progressively longer TE values are used.

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