Use of gadolinium chloride as a contrast agent for imaging spruce knots by magnetic resonance
Treatments of knot-containing spruce wood blocks with a paramagnetic salt, gadolinium (III) chloride, in combination with solvent pretreatments, were evaluated as strategies to enhance the visualization of wood features by magnetic resonance imaging (MRI). Initial experiments with clear wood and excised knot samples showed differences in moisture uptake after pretreatments with selected solvents. For knot-containing spruce wood blocks, increased detail in the images with an ethanol pretreatment was attributed to the removal of extractives thereby resulting in higher moisture contents for the knot wood. Incorporation of the gadolinium-based contrast agent resulted in an abrupt loss in signal for a zone around each knot. Accordingly, the retention of gadolinium ions appears to be selective, thereby allowing the demarcation of what is likely to be compression wood known to surround softwood knots. Applications include studies on wood anatomy by MRI and the modeling of wood defects. The treatment of wood with contrast agents as such also shows promise as a technique to improve our understanding of the localization of different cell-wall chemistries, especially as they relate to ion exchange capacity.