MRI of Wood

Living wood is comprised of functionally specialized cells arranged in axial and radial directions. Vessels are water conducting axial elements in range of diameter from 8 to 85 μm and embedded in supporting tissue represented by fiber tracheids. In the study we investigated intact and traumatic structures of a beech branch as well as water distribution in these tissues by series of planar images extracted from a 3D image set and by volume rendered images calculated at different view angles. Three dimensional magnetic resonance microscopy in researches of morphology and water content in living tree tissue has numerous advantages in comparison to conventional histology and gravimetric measurements of the moisture content.
    A beech (Fagus sylvatica L.) branch with two small dead branches (knots) was cut from a tree and coated with air and water impermeable coating to prevent it form drying. The sample was imaged by the 3D spin-echo technique and processed by ImageJ (NIH Image) image processing software for volume rendering. Cross-sectional planar images of intact tissue revealed high moisture in pith, in radially oriented xylem rays, in water conducting early wood and in wood producing a cambial zone. In xylem growth rings we could distinguish late (autumn) wood from early (spring) wood. The cross-section image of a dead branch (left image) clearly revealed a position of the protection zone delimiting sound wood from the branch cavity. This zone is protecting living wood against dehydration and against ingression of atmospheric oxygen and pathogenic microorganisms.

twig MR image

Volume rendered images (right images) clearly displayed anatomical structures of the branch. On the surface can be seen cavities at the position of two shed branches and increased increment of wood trying to close these two wounds. The images also enabled insight into spatial distribution of the moisture content. Surprisingly, the images showed that a part of the pith of dead branches still contained high moisture content. Our research demonstrated that structural response is associated with passive water distribution in mechanically injured tree tissues [1,2].

Another study done in our laboratory was focused on testing effects of different wood impregnation methods on wood protection against surface water. Samples of different wood spices (fir, spruce, larch, chestnut) in a shape of 12 mm cube were vacuum impregnated with oil or surface treated with vax or left unprotected. Then each of the samples was immersed for half an hour in water and then inserted in our 400 MHz MRI scanner and started scanning. The samples were scanned dynamically in 1D in three different interchanging directions with respect to the wood growing rings (axial, radial and tangential direction). The 1D scanning was interrupted after 1, 2.3, 18.6, 35 and 68 hours for acquisition of high-resolution 3D images using 3D gradient-echo imaging methods. While sequences of 1D profiles in different directions have good temporal resolution and provide deficient spatial information, 3D images have excellent spatial resolution for the price of poor temporal resolution. However, combination of both methods, 1D profiles and 3D images, provides all needed information. For demonstration of the method, two images of fir wood are shown below. In the first one it is shown by a sequence of 1D profiles in radial, tangential and axial directions how dynamics of sample drying is different in each of the three directions. It can also be seen that the amount water that migrated into the sample after the soaking period is quite substantial for the nonimpregnated sample of which mass has after the soaking increased by 40%.
MC profiles of nonimpregnated fir
Quite different soaking and drying dynamics was observed with the fir sample which was vacuum impregnated with oil. The mass of the sample increased after the soaking only for 4% which is tenfold improvement in comparison to the nonimpregnated sample. However, the impregnation had also a negative effect on drying dynamic which became substantially slower than with the nonimpregnated sample.
MC profiles of impregnated fir

  1. MERELA, Maks, SEPE, Ana, OVEN, Primož, SERŠA, Igor. Three-dimensional in vivo magnetic resonance microscopy of beech (Fagus sylvatica L.) wood. MAGMA, 2005, vol. 18, p. 171-174. [PDF]
  2. MERELA, Maks, SERŠA, Igor, MIKAC, Urška, OVEN, Primož. Uporaba magnetno resonančnega slikanja za raziskave anatomije in vlaľnosti lesa (Application of magnetic resonance imaging for research of anatomy and moisture content of wood). Zb. gozd. lesar., 2006, vol. 79, p. 75-84. [PDF]
  3. OVEN, Primož, MERELA, Maks, SERŠA, Igor, MIKAC, Urška. Raziskave zgradbe lesa s tri-dimenzionalnim magnetno resonančnim slikanjem (Research of wood structure by three dimensional magnetic resonance imaging). Les, 2008, vol. 60, p. 7-11. [PDF]
  4. OVEN, Primož, MERELA, Maks, MIKAC, Urška, SERŠA, Igor. 3D magnetic resonance microscopy of a wounded beech branch. Holzforschung, 2008, vol. 62, p. 322-328. [PDF]
  5. MERELA, Maks, OVEN, Primož, SERŠA, Igor, MIKAC, Urška. A single point NMR method for an instantaneous determination of the moisture content of wood. Holzforschung, 2009, vol. 63, p. 348-351. [PDF]
  6. MERELA, Maks, PELICON, Primož, VAVPETIČ, Primož, REGVAR, Marjana, VOGEL-MIKUŠ, Katarina, SERŠA, Igor, POLIČNIK, Helena, POKORNY, Boštjan, LEVANIČ, Tom, OVEN, Primož. Application of micro-PIXE, MRI and light microscopy for research in wood science and dendroecology. Nuclear instruments & methods in physics research B, 2009, vol. 267, no. 12/13, p. 2157-2162. [PDF]
  7. OVEN, Primož, MERELA, Maks, MIKAC, Urška, SERŠA, Igor. Application of 3D magnetic resonance microscopy to the anatomy of woody tissues. IAWA journal, 2011, vol. 32, no. 4, p. 401-414. [PDF]
  8. ŽLAHTIČ ZUPANC, Mojca, MIKAC, Urška, SERŠA, Igor, MERELA, Maks, HUMAR, Miha. Distribution and penetration of tung oil in wood studied by magnetic resonance microscopy. Industrial crops and products, 2017, vol. 96, p. 149-157. [PDF]
  9. ŽLAHTIČ ZUPANC, Mojca, MIKAC, Urška, SERŠA, Igor, MERELA, Maks, HUMAR, Miha. Water distribution in wood after short term wetting. Cellulose, 2019, vol. 26, no. 2, p. 703-721. [PDF]
  10. MIKAC, Urška, MERELA, Maks, OVEN, Primož, SEPE, Ana, SERŠA, Igor, MR Study of Water Distribution in a Beech (Fagus sylvatica) Branch Using Relaxometry Methods. Molecules, 2021, vol. 26, 4305. [PDF]