Gamma-Ray Computed Tomography in Soil Science: Some Applications

Abstract

The first computed tomography (CT) apparatus was developed by Godfrey Hounsfield, at the beginning of the seventies of the last century (Hounsfield, 1973). For its development Hounsfield and Allan M. Cormack (Cormack, 1963), who developed the mathematical basis of image reconstruction, were rewarded by the Nobel Prize in medicine in 1979. Cormack’s developments were basically focused in the reconstruction of bodies with geometries with no medical interest, like for instance, the human head. After these first developments, some CT scanners were developed and gradually introduced into other areas of knowledge like engineering, agronomy, biology, physics, chemistry, etc. Most of the modern scanners make use of gamma and X-ray sources. However some instruments make use of neutron and positron ray sources. Recent developments describe the use of synchrotron X-ray beams to investigate fluid transport at the pore scale (Coles et al., 1998). Some other applications of CT involve: investigation on morphological changes in small animals (Stenstrom et al., 1998); determination of soil macroporosity by chemical mapping (Brandsma et al., 1999); and dental analysis of the anatomy and some restorative materials through X-ray microtomography (Braz et al., 2001). More recently, Vontobel et al. (2006) and Winkler et al. (2006) presented the use of neutron tomography to investigate the morphology or structure of rocks and metal melts. CT allowed demonstrating the behaviour of metal melts with different densities. Voronov et al. (2010) used the micro CT (with micrometric resolution) to analyze the distribution of flow-induced stresses in highly porous media of interest for bioengineering. Flow-induced stresses have been found to stimulate the growth of cells. Reports describing the use of the CT in soil science were published few years later of the invention of the first tomography scanner system by Hounsfield. Petrovic et al. (1982) followed by Hainsworth and Aylmore (1983) and Crestana et al. (1985) successfully reported the use of CT scanners for soil bulk density studies, soil water content and water movement measurements. CT is a non destructive and non invasive investigation technique used to assess some attributes in the interior of an object of interest. It is essentially based on the principle of the attenuation of an electromagnetic radiation beam by the object. CT technique has been well