Proton Spin–Lattice Relaxation in Wood—T1Related to Local Specific Gravity Using a Fast-Exchange Model

The spin–lattice relaxation and cross relaxation of western red-cedar sapwood (moisture contents 216 to 15%) and western hemlock sapwood (moisture contents 144 to 15%) were investigated using proton magnetic resonance. Below the fiber saturation point (FSP), solid wood and cell-wall water were found to have the sameT1indicating a fast exchange of proton magnetization between these two environments. Above the FSP, multiexponentialT1decay was observed. A one-dimensional profile across the growth rings showed that the multiple components of spin–lattice relaxation in wood were due to variations in local specific gravity. Within a small volume, a singleT1was observed for protons in lumen water, cell-wall water, and solid wood. The mixing of proton reservoirs was attributed to water diffusion between the lumen and cell wall, and cross relaxation between the protons in the cell-wall water and the solid wood. This cross-relaxation time was found to be a few milliseconds, which is fast on theT1time scale of tens of milliseconds. A fast-magnetization-exchange model which relatedT1values to local specific gravity was verified for protons in all environments in wood. Using this model, the distribution of local specific gravity was derived for the western red-cedar and hemlock samples.