Spin-Echo Decay of Spins Diffusing in a Bounded Region

An approximate expression for the spin-echo-decay envelope is obtained from Torrey's equation for spins diffusing in a magnetic field gradient $G$ and in a region that is small compared with ${\left(\frac{D}{\gamma G}\right)}^{\frac{1}{3}}$ in the direction of the large uniform magnetic field, and large in the directions perpendicular to the field. (Here $D$ is the diffusion constant and $\gamma$ the nuclear gyromagnetic ratio.) In a preceding paper, Wayne and Cotts compare this expression with experiment for regions that are large compared with ${\left(\frac{D}{\gamma G}\right)}^{\frac{1}{3}}$ but small enough that the simple spin-echo-decay envelope for an infinitely large region does not apply.