Analytical Corrections for Beam-hardening and Object Scatter in Volumetric Computed Tomography Systems

The advent of the use of digital area detectors in Volumetric Computed Tomography (VCT) systems has brought with it the challenges of achieving image quality comparable to that provided by earlier planar industrial CT systems equipped with highly collimated and efficient linear detector arrays. Particularly for imaging with higher voltage x-ray sources up to 450kVp, among the most distinct imaging artifacts encountered are those associated with the phenomena of beam hardening and object scattering. Where large (20cm+) flat panel digital detectors with typical scintillator depths and no anti-scatter collimation are used for the highest throughput VCT, these effects can dominate image quality for certain applications. In the current work, analytical approaches have been developed to correct for both of these effects in VCT imaging. The corrections are based on calculations of the attenuation and scattering processes characteristic of, in the simplest case, an object composed of a single material, as is the situation with castings. The beam-hardening corrections can be applied a priori directly to projection data prior to reconstruction. The scattering corrections are derived in a twostep process, whereby object geometry is extracted from an initial three dimensional data set reconstructed with uncorrected projection data. Subsequently an innovative analytical method is utilized to extract specific scattering profiles for subtraction from the projection data on a viewby-view basis. These corrections have been applied successfully to empirical VCT data of several test samples and industrial components, composed of materials such as aluminum and steel. The use of the analytical corrections obviates the need for the conventional empirical correction approaches, which entail appreciable difficulty of implementation, limitations, and errors. Introduction: The advent of the use of area x-ray detectors, particularly digital flat panel detectors, in Volumetric Computed Tomography (VCT) systems highlights the need for improved object scatter reduction and correction techniques. Scatter can be significant for a wide range of x-ray energies, e.g. from 100kVp to 450kVp and, along with the phenomenon of beam-hardening, leads to underestimations of material penetrated and corresponding low-frequency artifacts that can severely compromise VCT image quality. Empirical approaches for correcting for scatter are very difficult to implement: simulation-based analytic corrections offer the promise for a flexible solution to this problem, as well as a more accurate and flexible alternative to empirical measurements for the purpose of correcting for the effects of beam-hardening. The phenomenon of beam-hardening (BH) arises due to the preferential attenuation of lower energy x-rays in a polychromatic spectrum as it penetrates through a material and the resulting change in spectral profile. In general the change is towards a profile with a higher average energy which penetrates more efficiently than does the initial spectrum, hence the term hardening applies. The average attenuation value as a function of depth of penetration departs from linearity as shown in Figure 1a. path length -ln (I /I o ) ideal actual -ln (I /I o )