Look-up Table Correction for Beam Hardening-Induced Signal of Clinical Dark-Field Chest Radiographs.

BACKGROUND:The microstructure of material at a μ m $\,\umu\mathrm{m}$ length scale leads to ultra-small-angle scattering of X-rays, which typically occurs, e.g., for lung tissue or some plastic foams. When using an interferometer, this effect alters the visibility of the fringe pattern, which can be detected and resolved by the detector. Thus, the ultra-small-angle scattering can be represented as a dark-field image. For a polychromatic source, the hardening of the source spectrum changes visibility as well, generating an additional fake dark-field signal by the attenuation of the material on top of the real ultra-small-angle scatter-related dark-field signal. Consequently, even homogeneous materials without microstructure typically exhibit a change in visibility. PURPOSE:The objective of this study is to develop a fast, simple, and robust method to correct dark-field signals and bony structures present due to beam hardening on dark-field chest radiographs of study participants. METHODS:The method is based on calibration measurements and image processing. BH by bones and soft tissue is modeled by aluminum and water, respectively, which have no microstructure and thus only generate an artificial dark-field signal. Look-up tables were then created for both. By using a weighted mean of these, forming a single LUT, and using the attenuation images, the artificial dark-field signal and thus the bone structures present are reduced for study participants. RESULTS:It was found that applying a correction using a weighted LUT leads to a significant reduction of bone structures in the dark-field image. The weighting of the aluminum component has a substantial impact on the degree to which bone structures remain visible in the dark-field image. Furthermore, a large negative bias in the dark-field image-dependent on the aluminum weighting-was successfully corrected. CONCLUSIONS:BH-induced signal in the dark-field images was successfully reduced using the method described. The choice of aluminum weighting to suppress rib structures, as well as the selection of bias correction, should be evaluated based on the specific clinical question.