High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications
Introduction
Since the discovery of a new type of radiation by Wilhelm Röntgen, X-rays have been used extensively in various research fields. A pertinent feature of this radiation type is its capability to penetrate material in varying degrees. This is mathematically formulated by Beer's law, which expresses the transmitted intensity I of a monochromatic X-ray passing an object:where I0 is the incident beam intensity and μ(s) is the local linear attenuation coefficient along the raypath s. The energy-dependent linear attenuation coefficient μ is determined by four effects, i.e. photoelectric effect, incoherent (Compton) scattering, coherent (Rayleigh) scattering and pair production. The latter can only occur at energies above 1.022 MeV and is thus not relevant in most X-ray CT setups. More information on this topic can be found in (Attix, 1986, Knoll, 2000).
This property was soon used for medical (Frost, 1896, Miller, 1896) and non-medical (Brühl, 1896) applications. In geosciences, the internal structure of a great diversity of geological samples has been examined by radiographic imaging mainly in the last 50 years (Calvert and Veevers, 1962, Hamblin, 1962, Bouma, 1964, Howard, 1968, Baker and Friedman, 1969, Herm, 1973, Sturmer, 1973, Bjerreskov, 1978, Monna et al., 1997, Louis et al., 2007, Schmidt et al., 2007). Constant improvement of the equipment still makes it a very extensively used technique in a wide range of applications, of which the most known are medical radiography and security systems.
A major drawback of this technique is the loss of information in one dimension. Radiographs, which are sometimes called projection or shadow images, project a 3D object on a 2D detector plane, losing depth information. This can lead to misinterpretation of the images.
A new technique to overcome this disadvantage was developed in the 1970s called Computerized transverse axial tomography (Hounsfield, 1973, Ambrose, 1976, Ommaya et al., 1976) (abbreviated CAT or CT). By acquiring projection images from different directions, a 3D volume is reconstructed using dedicated computer algorithms. This 3D reconstruction technique was almost immediately used for medical applications, allowing visualisation of the human body and brain (Gawler et al., 1974, Ledley et al., 1974, Paxton and Ambrose, 1974). Applications in other research domains such as wood technology (Onoe et al., 1983, Taylor et al., 1984), palaeontology (Conroy and Vannier, 1987, Zollikofer et al., 1998), soil science (Petrovic et al., 1982, Crestana et al., 1985, Crestana et al., 1986, Anderson et al., 1988, Braz et al., 2000), marine sciences (Boespflug et al., 1995) and geosciences in general (Vinegar and Wellington, 1987, Wellington and Vinegar, 1987, Coles et al., 1991), as well as industrial applications (Hopkins et al., 1981) followed shortly.
From Eq. (1), it can be understood that the integrated linear attenuation coefficient can be easily derived at each point of a radiograph:
By application of a rotational movement of the sample relative to the X-ray source and detector system, a number of different angular projection images are made. By using appropriate reconstruction algorithms (Herman, 1980, Herman and Natterer, 1981, Kak and Slaney, 1988), the local value of μ can be calculated for each point inside the scanned volume. The value of μ depends on the material density ρ and the mass attenuation coefficient μ/ρ, which is a tabulated and energy-dependent value and is approximately proportional to Z3 in the X-ray energy range typically used for CT, with Z as the atomic number (Attix, 1986, Knoll, 2000). Knowledge of this value thus does not allow a unique identification of the material or its density, unless one of them is known in advance.
It must be noted that this is only valid for monochromatic X-rays which follow a straight path. It will be demonstrated in following sections that these assumptions are not met, resulting in reconstruction artefacts.
X-ray CT has become more commonplace in the earth sciences for imaging geological samples at ambient conditions (Ketcham and Carlson, 2001, Rivers et al., 2004, Lesher et al., 2009). Medical CT and industrial CT systems, with typical spatial resolutions of 250 μm voxel size, are often used for their large core scanning capabilities (Baraka-Lokmane et al., 2009) and dual energy scanning possibilities for the chemical analysis of core samples (Purcell et al., 2009). When one is performing the study of core samples, the surface as well as the internal features, including bedding features, sedimentary structures, natural and coring-induced fractures, cement distribution, small-scale grain size variation and density variation can now be analysed (Coles et al., 1991, Orsi et al., 1994, Coles et al., 1998). Extensive research has included applications on the complex porosity and pore geometry of carbonate reservoirs (Purcell et al., 2009), rock-fluid analysis (PyrakNolte et al., 1997, Purcell et al., 2009, Wennberg et al., 2009), the performance of diverting agents in unconsolidated sandstones (Vinegar and Wellington, 1987, Wellington and Vinegar, 1987, Ribeiro et al., 2007a, Ribeiro et al., 2007b), the physical properties of permafrost layers (Calmels and Allard, 2008), gas hydrate dissociation (Denison et al., 1997, Okui et al., 2003), and many other topics in geosciences.
Although X-rays and gamma-rays are the most commonly used type of radiation in CT, the same principle can be applied to protons (Ito and Koyamaito, 1984, Takada et al., 1988), neutrons (Koeppe et al., 1981, Overley, 1983, Baechler et al., 2002, Lehmann and Wagner, 2010) and heavy particles (Crowe et al., 1975, Ohno et al., 2004, Shinoda et al., 2006) as radiation source. These techniques are beyond the scope of this paper, and are therefore not discussed further.
Over the years, medical CT scanners have been drastically improved in terms of image quality, imaging speed and deposited radiation dose. Following technological advances, different generations of CT scanners have been conceived (Goldman, 2007), with recent developments towards dual-energy (Flohr et al., 2006, Primak et al., 2007, Graser et al., 2009) and energy selective CT (Barber et al., 2011). Temporal resolution has improved to less than 100 ms (Flohr et al., 2009). In contrast, spatial resolution remains limited to several hundreds of micrometres due to the dimension of the investigated object, i.e. a human patient.
A new research field emerged in high-resolution X-ray tomography, commonly called micro-CT. This method was first discussed in the 1980s, using X-ray tubes (Sato et al., 1981, Elliott and Dover, 1982, Elliott and Dover, 1985), gamma-ray sources (Gilboy et al., 1982, Gilboy, 1984) and synchrotron radiation (Grodzins, 1983, Bonse et al., 1986, Flannery et al., 1987) as X-ray source. Due to their low brilliance, gamma-ray sources are rarely used in micro-CT, and throughout the years both synchrotron-based and lab-based (using X-ray tubes) micro-CT have developed rapidly. The high brilliance of synchrotron radiation results in a clear superiority in terms of achievable spatial resolution and signal-to-noise ratio (Baruchel et al., 2006), but the number of synchrotron facilities is limited and the operational cost is very high. Lab-based micro-CT systems (Jakubek et al., 2006, Masschaele et al., 2007) on the other hand have lower X-ray flux but are more cost-efficient. Nowadays, a large number of desktop CT systems exist (Sasov and Van Dyck, 1998) which are commercially available, making micro-CT accessible for a large number of researchers. Recent developments in terms of X-ray optics made lab-based micro-CT comparable to synchrotron-based micro-CT in terms of spatial resolution but at the drawback of a heavily reduced X-ray flux and thus increased measuring time.
Although there is no clear distinction between CT and micro-CT, only applications of the latter will be discussed in this article. The boundary between both techniques will be defined here at a spatial resolution of 200 μm, which is typically not achievable by medical CT devices. Sample sizes for micro-CT range from as large as 40 cm to as small as several micrometres, with typical sample sizes for micro-CT in geosciences ranging from 1 mm to 5 cm. Despite this difference, several of the limitations and advantages of X-ray micro-CT which will be discussed in following sections are also applicable to standard (medical) CT, although this will not always be mentioned explicitly.
A large difference between conventional medical CT and micro-CT can be found in the rotational movement. In medical CT, the patient remains stationary, whilst the complete X-ray source and detector system rotates around the patient. In most micro-CT systems, it is the object under investigation that rotates, and X-ray source and detector remain stationary. This setup achieves a better mechanical stability which is required at high resolutions. An exception on the rotating sample is in-vivo small animal micro-CT scanners (Ritman, 2004, Schuster et al., 2004). These are usually designed for high-speed scanning at reduced radiation dose, and are as such not optimized for geological applications. Nevertheless, their high scanning speed and stationary sample positioning can be useful for imaging dynamical processes in geological specimens.
The most common lab-based setup is the standard cone-beam micro-CT (Feldkamp et al., 1984, Turbell, 2001). In this setup, the conical X-ray beam makes geometrical magnification possible by positioning the object under investigation at any position between X-ray source and detector (Fig. 1a). This way, the highest achievable resolution is mainly limited by the focal spot size of the X-ray source. A trade-off has to be made between low-flux transmission-type X-ray tubes which allow for a focal spot size below 1 μm, and high-flux reflection-type X-ray tubes where the focal spot is larger (Vlassenbroeck, 2009). More generally, a smaller focal spot size requires a reduced X-ray flux. This trade-off makes very high resolution micro-CT (< 1 μm) difficult and time-consuming at lab-based setups. On the detector side, a range of technologies can be used due to the geometrical magnification. In most cases, a large a-Si flat-panel detector combined with a relatively thick scintillator screen is used to obtain a high dynamic range. Alternatively, X-ray optics can be used in combination with a high-flux source to achieve very high spatial resolution (Feser et al., 2008, Gelb et al., 2009).
At synchrotron sources, the X-ray beam is almost parallel, making geometrical magnification impossible without X-ray optics but has a high X-ray flux. This high flux can be detected by a thin scintillator screen, converting the X-rays to visible light (Fig. 1b). This allows the use of optical magnification lenses to achieve high resolution (Koch et al., 1998). A trade-off exists between a very thin scintillator screen, increasing the obtained resolution but reducing the detection efficiency, or a thicker scintillator which increases the detection efficiency but reducing the highest achievable resolution (Stampanoni et al., 2002). Geometrical magnification can be achieved using Fresnel zone plates (Chao et al., 2005, Chu et al., 2008), resulting in very high resolution. Recently, ptychography has gained importance in the field of X-ray microscopy to achieve an even smaller voxel size down to 10 nm (Dierolf et al., 2010, Godard et al., 2011, Schropp et al., 2012).
Section snippets
Advantages and limitations of X-ray micro-CT
In this section, some of the advantages and limitations of high-resolution X-ray CT with respect to other techniques are discussed. Although some of the limiting effects are intrinsic to the technique, several others are the subject of current research and are likely to have a reduced impact in the near future.
X-ray micro-CT used in geosciences
In the following, a brief and limited overview of applications of micro-CT in geosciences is given, categorized in specific disciplines. It must be emphasized that many other references exist, and that this overview is merely an indication of the wide range of possibilities of micro-CT.
Conclusion
Although the possibilities of X-ray micro-CT are increasingly well-known and used in almost all research fields in earth sciences, several challenges remain to be tackled. Despite technological and computational advances, discretization effects and the consequences thereof, such as the partial volume effect and the relation between sample size and voxel size, are issues that are to be kept in mind when performing X-ray CT analysis. Like in many other microscopical techniques, working at high
Acknowledgements
The authors acknowledge the entire UGCT team for the fruitful discussions on all aspects of micro-CT. Prof. Neil Davies, Prof. Luc Van Hoorebeke and Dr. Manuel Dierick are gratefully acknowledged for their review of the manuscript. J. Dewanckele, R. Ketcham, A. Alajmi, R. Sok, J. Kyle, E. Charalampidou, K. Singh, and H. Henderickx are acknowledged for providing the original figures for reprint.
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