T2* decay refers to an exponential decrease in Mxy (i.e. signal strength) following the initial excitation pulse as a function of time constant T2*. A picture of the signal or free induction decay (FID) is shown on the right, occurring immediately after a 90 RF excitation pulse in a liquid phantom.
Following the excitation pulse, there is an immediate exponential loss of signal strength. This depends upon two factors:
- static field non-uniformity within each voxel: due to imperfections in the construction of the scanner magnet itself, as well as from magnetic susceptibility effects in the patient inside the field
- spin-spin interactions (T2 relaxation)
T2* decay is what actually a coil receiver detects immediately after termination of the induction pulse and is of much greater magnitude than T2 in tissues due to the inherent inhomogeneity of the magnetic field. If one had a "perfectly" uniform magnet and an object without susceptibility effects, the T2 and T2* would be equal.
The relationship between T2 and T2* can be illustrated by the multiecho spin echo sequence shown in Figure 2. 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 echoes reflects T2 decay. T2* decay can be refocused by 180 pulses as in the simplified spin echo sequence in the figure but not be gradient refocusing seen in gradient echo sequences.
The effects of T2* can therefore be seen and utilized in gradient echo imaging and in the FID signal following a 90 RF pulse.
T2* decay underlies all gradient echo imaging and as such makes detection of inherently inhomogeneous tissues with magnetic susceptibility effects such as calcium and blood easier - a feature exploited in detection of small hemorrhages with hemosiderin, for example, in Binswanger disease.