Distinguishing Among Soil Solid Phases Using Micro-CT Scanning.
Richard J. Heck and Thomas Elliot. Univ of Guelph, Guelph, ON N1G2W1, Canada
The utilization of computerized axial X-ray tomography (CT), to understand soil space, has gained considerable attention over the past decade. Because of the relatively large difference between the X-ray attenuation of voids (air or water-filled) and soil solid phases in general, much of this work has focused on quantifying soil porosity, then relating it to the retention and transport characteristics of soil moisture. The major shortcomings have been the coarse spatial resolution and low X-ray energy levels (as clinical scanners were frequently employed), as well as limited image processing capacity; these are being overcome as technologies evolve. The discrimination of solid phases has, however, presented a greater challenge, due primarily to the nature of the X-ray radiation and its interaction with materials. CT imaging is based on the attenuation of X-rays (according to Beer's Law) as they pass through materials which exhibit specific intrinsic linear attenuation coefficients. The reconstruction of a 3D model, from a sequence of radial projections, involves determining the apparent linear attenuation coefficient for each volume element (voxel). Typically, these values are converted to Hounsfield Units, which are based on a scale whereby the value of air is set at –1000 and the value of water is set at 0; on this scale, solids usually exhibit positive values. Within the energy level ranges attainable by current micro-CT scanners, the total X-ray attenuation of a given material is considered to be a function of ‘coherent scatter', ‘incoherent scatter' as well as ‘photoelectron adsorption'. The magnitude of these effects is a function of the energy level of the X-ray radiation. Unfortunately, X-ray tubes do not generate ‘monochromatic' radiation, but rather a spectrum that includes both discrete components (characteristic of the target element) and a continuous component called ‘breaking radiation'.Thus, filtering maybe imperfect when employed to narrow the spectrum. Consequently, when combining the effects of multiple energy levels and the finite resolution (>5 microns) of real detection arrays, regions of different composition can generate very similar apparent attenuation coefficients. This can severely limit the ability to discriminate among solid phases in complex systems such as soils. The limitations related to the finite resolution of the imaging array, is a question of current electrical engineering technology. That being said, however, improvements to the spatial resolution capability have just transferred the phase discrimination problem to a finer scale. Theoretical consideration suggest that, while the total attenuation of different material may be practically indistinguishable at certain energy levels, the relative change in attenuation, in response to changing X-ray energy levels (i.e. the various components of attenuation) may be noticeably different. Consequently, it is theoretically possible to use, not only the absolute attenuation at a given energy level, but also the relative response of attenuation to changing energy levels, to distinguish solid phases. This research is focused on the practical application of this concept using a state-of-the-art micro-CT imaging system. A wide selection of solid materials commonly encountered in soils, including silicates, secondary metal oxides, carbonates, sulfates, coal and plant fiber, as well as selected anthropogenic material such as rubber, plastic and glass, were subject to systematic imaging by an EVS-MS8 micro-CT scanner (6 micron voxel size) at X-ray energy levels ranging from 80 to 130 kV. Imagery was co-registered, to permit ratioing and subsequent cluster analysis (such as that employed in multichannel image analysis software), rather than just standard level thresholding of monochromatic image histograms. The primary objectives are to identify solid phases that can be discriminated in this manner, and the optimal instrumental parameters for this purpose. Such capability will be fundamental to the characterization of the 3D configuration of solid phases in intact soil.