In any real NMR experiment, however, the transverse magnetization decays much faster than would be predicted by natural atomic and molecular mechanisms; this rate is denoted T2* ("T2-star"). T2* can be considered an "observed" or "effective" T2, whereas the first T2 can be considered the "natural" or "true" T2 of the tissue being imaged. T2* is always less than or equal to T2.
T2* results principally from inhomogeneities in the main magnetic field. These inhomogeneities may be the result of intrinsic defects in the magnet itself or from susceptibility-induced field distortions produced by the tissue or other materials placed within the field.
Certain MR sequences using gradient echoes and relatively long TE values are called T2*-weighted. They are used to accentuate local magnetic homogeneity effects to aid in the detection of hemorrhage or calcifications. T2*-sensitive sequences also form the basis for functional MRI (fMRI) using the BOLD (Blood Oxygen Level Dependent) technique.
If one makes a certain assumption about the line shapes contributed by these processes we may write
where 1/T2i = γ ΔBi is the relaxation rate contribution attributable to field inhomogeneities (ΔBi) across a voxel. Note that the equation is a sum of relaxation rates (1/T2's) rather than relaxation times (T2's).
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Additional notes about T2*
The assumption about line shapes required for the 1/T2* equation to hold true is that they are Lorentzian.
I have followed the usual definition of T2* here that only includes inhomogeneity effects (T2i). The effects of microscopic diffusion have been neglected. In some texts you will occasionally find a diffusion term included in the definition so that 1/T2* = 1/T2 + 1/T2i + 1/T2d
Chavhan GB, Babyn PS, Thomas B et al. Principles, techniques, and applications of T2*-based MR imaging and its special applications. Radiographics 2009;29:1433-1449.
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