Dimensional standard for micro X-ray computed tomography

The decrease in the cost of high end computing and the availability of high quality X-ray sources in the laboratory environment has led to an increased use of three-dimensional (3D) X-ray micro computed tomography (μCT). In the medical community, the primary concern for CT is calibrating for X-ray absorption and ascertaining the difference between healthy tissue and cancerous tissue or examining fractures. Absorption calibration is also important in the materials community, however confirming dimensional accuracy of voids, defects, machined parts, cracks, or the distribution of dispersed particles is typically more important. One key aspect of μCT that is often overlooked in the literature is the number of radiographs required for dimensional accuracy of the 3D reconstruction and minimization of image noise. In μCT, a number of radiographs are collected in theta increments as the sample is rotated at least 180°. They are typically collected in 1° increments (or 181 radiographs), 0.25° increments (721 radiographs), or some other multiple. The question that arises, especially in a laboratory based instrument, where the required exposure times are longer to get high-quality signal-to-noise compared to synchrotron sources, is what is the optimal number of images required to reach the volumetric statistics of the sample, and minimize the noise while not overly scanning the sample at a cost in time? A dimensional standard based upon NIST certified glass microspheres dispersed in a low density poly(styrene) matrix to answer this question is proposed. Experiments are shown that describe the microsphere size statistics as a function of number of radiographs calculated using a commercial software package, AvizoFire. These results are important in understanding the distribution of voids in a foam and confirming the accuracy of the 3D measurements obtained.