The accuracy of medical CAT-scan images for non-destructive density measurements in small volume elements within solid wood

Abstract

A comparison between identically sampled CAT-scan images of five wooden test pieces (Pinus sylvestris) showed that the CT-number in each pixel varied with a standard deviation of ±3.9–11.1 CT numbers. This inaccuracy in CT-numbers is called “noise”. As long as the direction of rotation can be controlled the noise in CAT-scan images of wood can be assumed to be approx. ±4 CT-numbers in each pixel. A calculation showed the average CT-number must differ ±1 unit to distinguish average CT-numbers in 2 × 2 × 1.5 mm volumes within solid wood. It has previously been shown that a change of ±1 CT-number corresponds approx. to a change in density of ±1 kg/m3. On the other hand, there is a difference in X-ray absorption coefficients between wood and water. Thereby dry wood densities in corresponding volumes must differ approx. ±2 kg/m3 to significantly be distinguished. The corresponding figure is approx. ±6 kg/m3 for wet wood densities with moisture content levels ranging from 6–100%.