Calibration-free retrieval of density information from computed tomography data

Abstract

In many research areas, particularly in materials science, retrieving a 3D distribution of the material density in a sample is of key interest. Although standard lab-based X-ray μCT imaging is capable of retrieving the 3D distribution of local attenuation coefficients, the local density cannot be determined unambiguously without the use of calibration standards due to the polychromaticity of the used X-ray tubes. In this paper, two methods are presented to retrieve this information based on accurate knowledge of the polychromatic properties of source and detector in the used μCT system. The methods use a very different approach, one being a polychromatic pre-processing method and one relying on a novel polychromatic iterative reconstruction method, yet produce equivalent results. They are limited to objects with a homogeneous composition, but are extremely powerful in a broad range of applications in materials characterization where the density distribution is of interest. Moreover, the method relying on iterative reconstruction has the potential to be applied for multi-material objects as well. A number of application examples illustrate the potential of the methods to not only retrieve quantitative estimates of density and sub-voxel porosity, but also to greatly reduce or eliminate the spectral artifacts that often hinder conventional calibration schemes.