T2* relaxation is the loss of signal seen with dephasing of individual magnetizations. It is characterized macroscopically by loss of transverse magnetization at a rate greater than T2. It is caused by magnetic field inhomogeneity an occurs in all magnets. The relationship between T2 and T2* can be illustrated by the multiecho spin echo sequence shown in the diagram below. The 180 degree RF pulses used to generate the echo are rephasing the spins that have undergone T2* decay. The gradual decline in signal from subsequent echos reflects T2 decay (See Figure). Unlike spin echo sequences, gradient echo sequences do not refocus T2* decay. Therefore, gradient echo sequences are more susceptible to ferromagnetic foreign bodies that distort the main magnetic field homogeneity.
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