How fast a proton wobbles or precesses depends on the magnetic field that it experiences. An isolated proton, far from any other proton (or electron) only feels (is affected by) the main Bø field. As protons (or spins) move together (due to random motion for example), their magnetic fields begin to interact. If the field from one proton increases the field that the second proton feels then the second proton will precess at a slightly faster rate. If the first field opposes the main field then the second proton will precess more slowly. As soon as the spins move farther apart their fields no longer interact and they both return to the original frequency but at different phases! This type of interaction is called spin-spin interaction. These temporary, random interactions cause a cumulative loss of phase across the excited spins resulting in an overall loss of signal.
Spin-Spin Relaxation: The temporary and random interaction between two excited spins that causes a cumulative loss in phase resulting in an overall loss of signal. Also known as transverse or T2 relaxation.
T2 Decay Curve
Mxy = Mø * e-t/T2
Similar to T1 relaxation, the signal decay resulting from transverse or spin-spin relaxation is described mathematically by an exponential curve, identical in concept to radioactive decay with a half-life measured in tens of msecs. The value T2 is the time after excitation when the signal amplitude has been reduced to 36.8% of its original value (or has lost 63.2%. - This is the opposite of T1 where 63.2% of Mz is recovered in one T1 period.) The value of T2 is unique for every kind of tissue and is determined primarily by its chemical environment with little relation to field strength. In chapter 5, we will discuss in more detail how these unique T2 values are used to produce different types of image contrast.
T2 Decay: The exponential loss of signal resulting from purely random spin-spin interactions in the transverse or XY plane.
In general, T2 values are unrelated to field strengths (unlike T1 values).