Effect of Diffusion and Exchange

As already noted, diffusion of molecules from one part of an inhomogeneous magnetic field to another inhibits complete refocusing of magnetizations and can significantly reduce the intensity of the echo according to Eq. 9.1  

Fortunately, the variable t can be adjusted, independent of the desired total length of the echo train, so that during the period r little diffusion occurs and the second exponential factor can be made arbitrarily small, so that valid measurements of T2 can be made. 

---