Non-destructive tree-ring measurements using a clinical 3T-MRI for archaeology

Abstract

For dendrochronology, both high spatial resolution and a wide imaging area are required. However, there has been no study to satisfy these two requirement for a tree-ring measurement method using magnetic resonance imaging (MRI). In this study, we developed a ring-width measurement method (ultra-high-resolution T2 weighted imaging: uHR-T2WI) using clinical MRI that can scan images at high resolution and over a wide sample area for waterlogged wood, by repeating small fields of view (FOV).Incidentally, until now, the method using X-ray computed tomography (X-CT) has been developed for non-destructive ring-width measurement for analysis of wooden cultural heritage and excavated wooden artifacts, and is mainly successful for dried samples with a low moisture content. However, the contrast of X-CT images tends to reduce as the water content increases, and it has been reported that the attempts to visualize waterlogged and PEG-impregnated archaeological oak wood were unsuccessful owing to a low degree of contrast between the water/PEG and the preserved wood (Bill et al., 2012).Actually, the archaeologically excavated wood generally has very high moisture content (400%–800%), thereby reducing the suitability of X-CT for tree-ring analysis.On the other hand, MRI equipment images the density and state of hydrogen nuclei, and so the expected quality of MRI improves as the moisture content of the wood increases.With these factors, to use both modality complementary will be very useful for dendroarchaeology. However, direct comparison of the suitable range of each modality has not been reported. Therefore, we examined the difference between X-CT and MRI related to moisture content of wood, and examined the range of adaptation for each modality as a function of moisture content.By using the proposed method, we then obtained a tree-ring width series and evaluated the usefulness of the proposed method. The uHR-T2WI method was able to obtain tree-ring width series comparable to those of an optical scanner from images acquired at a spatial resolution of 0.05 mm in high moisture content wood. The results show that wood with a high moisture content (e.g., waterlogged wood measured immediately after excavation, or high-water-content lacquered wooden cultural property) can now be successfully analyzed as non-destructive methods with MRI.In conclusion, the complementarity of both MRI and X-CT will be very useful as non-destructive inspection methods for dendroarchaeology. Although further research should consider more wood species, our study suggests that the proposed method will significantly advance the field of dendrochronology, particularly when related to dendroarchaeology.