Medical CT-scanners for non-destructive wood density and moisture content measurements

Abstract: Most methods to measure wood density and moisture content are destructive. One non-destructive technique is X-ray computed tomography (CT). The actual physical variable measured is the X-ray linear attenuation coefficient which is highly density dependent. The primary purpose of this thesis is to establish the accuracy of medical CT-scanners for wood density measurements in small volume elements. As wood moisture content has an effect on wet wood density, the secondary purpose of the thesis is to suggest and evaluate an image processing method to perform moisture content measurements using CT-images. The thesis contains a method for calculating X-ray attenuation coefficients and CT-numbers for wood and describes how the CT-numbers correlate to wood densities. Conventional methods are not suitable for measuring densities of small volumes inside solid wood. Therefore, a method for calculating the accuracy in CT-images depending on image noise is suggested for medical CT-scanners. The results are experimentally compared to density measurements made with a microtomography CT-scanner and show that wood density can be measured with an accuracy of +/-1kg/m3 in a 2 x 2 x 1.5 mm volume element using a medical CT-scanner at a significance level of 0.05. However, the area within which measurements are made must not contain large density variations deriving from annual growth rings. The method suggested for moisture content measurements indicates an accuracy of +/-1% in moisture content in 3 x 3 x 5 mm volumes. In practice an accuracy of +/-1.4% or better below fiber saturation point and +/-4% or better above fiber saturation point can be expected in corresponding volumes.