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Free Induction Decay

After the RF transmitter is turned off, the protons immediately begin to re-radiate the absorbed energy. If nothing is affecting the homogeneity of the magnetic field all of the protons will be spinning at the same resonance frequency. The initial amplitude of the signal is determined by the portion of the magnetization vector (Mø) that has been tipped onto the XY plane. This, in turn, is determined by the sine of the flip angle, a. The maximum signal is obtained when the flip angle is 90°. (Remember, sin(0°) = 0, sin(90°) = 1.0) The signal unaffected by any gradient is known as a Free Induction Decay (FID). The time constant that determines the rate of decay is called T2. An FID has no positional information.

FID: Free Induction Decay. An NMR Signal in the absence of any magnetic gradients.



An FID decays exponentially.
At t = T2, 63.2% of the signal has been lost.

The decay curve is the signal envelope. The actual signal is oscillating at the resonance frequency in the MHz range.

An Imperfect World

T2* Decay

In the real world, the NMR signal decays faster than T2 would predict. Pure T2 decay is a function of completely random interactions between spins. The assumption is that the main external Bø field is absolutely homogeneous. In reality, there are many factors creating imperfections in the homogeneity of a magnetic field. The main magnet itself will have flaws in its manufacture. Every tissue has a different magnetic susceptibility which distorts the field at tissue borders, particularly at air/tissue interfaces. Patients may have some type of metal on or in them (dental work, clips, staples, etc.). The sum total of all of these random and fixed effects is called T2* (pronounced T - Two star).



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