The use of sodium polytungstate as an X-ray contrast agent to reduce the beam hardening artifact in hydrological laboratory experiments

Abstract

Iodine is conventionally used as a contrast agent in hydrological laboratory experiments using polychromatic X-ray computed tomography (CT) to monitor two-phase Darcy flow in porous geological media. Undesirable beam hardening artifacts, however, render the quantitative analysis of the obtained CT images difficult. CT imaging of porous sand/bead packs saturated with iodine and tungsten-bearing aqueous solutions, respectively, was performed using a medical CT scanner. We found that sodium polytungstate (Na 6 H 2 W 12 O 40 ) significantly reduced the beam hardening compared with potassium iodide (KI). This result is attributable to the location of the K absorption edge of tungsten, which is nearer to the peak of the polychromatic X-ray source spectrum than that of iodine. As sodium polytungstate is chemically stable and less toxic than other heavy element bearing compounds, we recommend it as a promising contrast agent for hydrological CT experiments.

Optimizing contrast agents with respect to reducing beam hardening in nonmedical X-ray computed tomography experiments

“…We prepared a range of virtual contrast agents by mixing heavy elements in different weight fractions and suspending them in bulk water, and injected them into virtual porous geological samples. This process mimics the sample preparation for the laboratory CT experiments of Darcy flow in porous geological samples [4,15,27]. Two-dimensional (2-D) CT images of the porous geological samples saturated with the various contrast agents were reconstructed assuming the use of a polychromatic medical CT scanner.…”

“…Toyoura sand from Yamaguchi, Japan, which is a common standard sand used in geotechnical research [27,28], was chosen as the virtual geo-material for this study (grain density, 2.65 g/cm 3 ; chemical composition, SiO 2 , 92.6; Al 2 O 3 , 3.7; Fe 2 O 3 , 0.7; CaO, 0.5; MgO, 0.2; loss on ignition, 0.5 wt.% (total, 98.2 wt.%)). The sand was packed in a cylindrical tube of negligible thickness to produce a sand pack with a porosity of 39 vol.%.…”

“…Nakashima [27] found that an aqueous solution of 74 W significantly reduces beam hardening in geotechnical CT experiments compared with 53 I, suggesting that elements heavier than iodine may be promising for nonmedical research. To examine the feasibility of this method, we performed a series of CT simulations.…”

“…To examine the feasibility of this method, we performed a series of CT simulations. The simulations mimic the cupping artifact which often occurs in the CT images of homogeneous porous geological samples saturated with heavy element-bearing fluids [4,27]. Assuming the use of a polychromatic medical CT scanner, synthetic CT images were reconstructed for porous sand-pack samples saturated with aqueous suspensions of various concentrations of different heavy elements.…”

Advancing the visualization of pure water transport in porous materials by fast, talbot interferometry-based multi-contrast x-ray micro-tomography

“…First of all, the use of the solution changes the interfacial and fluid dynamic properties of the involved liquid, compared with water. Second, it leads to the increase of beam hardening artefacts 16 . Third and more importantly, it is unfeasible to use waterbased salt solutions instead of pure water when the transport process is chemically reactive, e.g., water transport through cement-based materials, due to the cement hydration reaction, or water transport in polymer electrolyte membrane fuel cells under operating conditions.…”