Spontaneous Decay of Transverse Magnetization
Following a
radio
frequency excitation pulse, the coherent rotation of the ensemble of protons forms a detectable signal. The signal decays spontaneously, at a characteristic rate (T2). At the time the MR signal is sampled (ET), the signal intensity from tissues with a long T2 will be greater than that from tissues with a short T2. Differences in effective T2 form the basis of the contrast for most fMRI methods.