Medical Computed Tomography Scanner Research Articles

Dendrochronology with a medical X-ray photon counting computed tomography scanner

Dendrochronology traditionally involves invasive techniques that might harm cultural heritage artifacts. Non-invasive approaches using X-ray technology and computed tomography (CT) have emerged, but dedicated CT systems are limited in availability. This study explored the viability of using a recently developed photon counting medical CT scanner for growth ring analysis. Six wood samples from pine, oak and beech with varying growth rates were studied. Comparisons were made between traditional methods and CT scanning with measurements performed on the untreated surface, a prepared surface and on CT images. For samples with annual rings wider than 0.3–0.4 mm, CT scanning performed well, while samples with thinner rings or indistinct ring boundaries presented challenges. The combination of traditional tree-ring measurements and analyses of CT scanned images might be a superior study approach generating extra data and information.

In situ dynamic X-ray imaging of fluid-rock interactions inside tight sandstone during hydraulic fracturing: fluid flow process and fracture network growth

Hydraulic fracturing plays a fundamental role in unconventional hydrocarbon production and the development of enhanced geothermal systems. Thus, studying the fluid flow process and fracture network growth during hydraulic fracturing is necessary to provide guidance for creating and sustaining fracture networks in target reservoirs. The actual formation stress state and hydraulic fracturing process can be simulated in the laboratory for small-scale rock samples in hydraulic fracturing experiments. High-energy industrial X-ray computed tomography (CT) has been used to directly track the fracture propagation in millimeter-scale samples under triaxial stress loading. In this study, a large-scale fracture network with economic benefits for unconventional energy exploitation was the target of the experiment; therefore, centimeter-scale samples were used. A hydraulic fracturing simulation experiment was conducted on tight sandstone by applying triaxial compression and injection pressures. With the assistance of the PEEK core holder and third-generation medical X-ray CT scanner, in situ dynamic X-ray imaging of the fluid-rock interactions inside the sample was conducted for the first time. The fluid-rock interactions and fracture network growth can be dynamically observed, revealing the fluid-rock interactions during the hydraulic fracturing process and how macroscopic failure develops from the microfracture.

Dual-energy CT scan protocol optimization to monitor transient fluid saturation distributions during three-phase flow in sand columns

Surfactant foam injection is a promising in situ remediation technique for aquifers impacted by Non-Aqueous Phase Liquids (NAPL). Though the viscous properties of foam can promote immiscible displacement, pore plugging by foam is a concern in an environmental remediation (ER) context, as maintaining flow conditions may require the application of pressures that exceed soil matrix failure thresholds. Optimization of the technology for ER may be furthered by gaining a better understanding of the relationship between foam morphology and NAPL desaturation. This relationship is poorly understood, partly because the interactions between organic phases and foam occurring at specific positions within the flow field at any given time are difficult to assess quantitatively. A Dual-energy X-ray Computed Tomography (CT) technique was developed to enable the measurement of sand column characteristics, as well as transient fluid saturation profiles within the porous medium during foam injection. The available settings of a medical CT scanner were systematically explored in order to identify the acquisition and reconstruction parameters best suited to image the fluids of interest: 0.4 mm slice thickness, a medium kernel (U40u), peak tube voltages of 140 kVp and 80 kVp, and the addition of a doping agent to the NAPL at a concentration of 20 wt%. Using datasets produced by imaging sand columns at various stages of foam injection experiments, an algorithm was developed in order to extract saturation profiles from CT images. The algorithm included multiple scan averaging to reduce image noise, image registration, and specific matrix operations required for calculating fluid saturation values in two- and three-phase systems. The accuracies of the CT-extracted porosity and fluid saturation data were compared to, and found to be in good agreement with, experimentally determined average porosity and saturation values. The scan parameter optimization method developed here can be applied to future studies involving any combination of site-specific natural materials.

Soil macroporosity and water flow in the root zone of oases in hyper‐arid regions

AbstractThe characteristics of soil macropores and water infiltration are closely connected to the growth of plant roots and their root zone environment. However, it is unclear how the root zone environment of oasis farmlands regulates the development of soil macropores and water flow in hyper‐arid regions. The objective of this study was to investigate soil macropores and their effect on water flow under irrigated oasis farmlands using a combination of X‐ray computed tomography (CT) and dye tracer. It was hypothesized that the integration of CT and dye tracer could clearly reveal preferential flow through biopores and large pores of oasis soils. A helical medical CT scanner was used to quantify more information about soil macropores in the root zone, along with an in situ single‐ring dye infiltration experiment to reveal water flow in three different oasis farmlands (piedmont oasis farmland, marginal oasis farmland, and old oasis farmland). Soil macroporosity was 0.44% under crop rows, while soil macroporosity in the interrows was only 0.30% across the oasis farmlands. Biopores contributed 73% of the volume of the total macropores under crop rows. The stable infiltration rate in the interrows was 0.3 mm min−1, which was significantly (p < 0.05) less than that under crop rows (0.7 mm min−1). Water flow under crop rows were mainly transported in biopores and large pores. The contribution of macropores to preferential flow under crop rows was 4.8 times larger than interrows. The integration of CT and dye tracer was a more holistic technique, which adequately revealed that oases had preferential flow affected by biopores and large pores, resulting in higher solute and contaminant transport.Key Points Soil macropores and preferential flow affected by crop roots were investigated in hyper‐arid regions. The integration of CT and dye tracer adequately identified preferential flowpaths of oasis soils. Biopores were larger under crop rows than interrows. Preferential flow transported in biopores and large soil pores under crop rows.

A New Statistical Reconstruction Method for the Computed Tomography Using an X-Ray Tube with Flying Focal Spot

Abstract This paper presents a new image reconstruction method for spiral cone- beam tomography scanners in which an X-ray tube with a flying focal spot is used. The method is based on principles related to the statistical model-based iterative reconstruction (MBIR) methodology. The proposed approach is a continuous-to-continuous data model approach, and the forward model is formulated as a shift-invariant system. This allows for avoiding a nutating reconstruction-based approach, e.g. the advanced single slice rebinning methodology (ASSR) that is usually applied in computed tomography (CT) scanners with X-ray tubes with a flying focal spot. In turn, the proposed approach allows for significantly accelerating the reconstruction processing and, generally, for greatly simplifying the entire reconstruction procedure. Additionally, it improves the quality of the reconstructed images in comparison to the traditional algorithms, as confirmed by extensive simulations. It is worth noting that the main purpose of introducing statistical reconstruction methods to medical CT scanners is the reduction of the impact of measurement noise on the quality of tomography images and, consequently, the dose reduction of X-ray radiation absorbed by a patient. A series of computer simulations followed by doctor’s assessments have been performed, which indicate how great a reduction of the absorbed dose can be achieved using the reconstruction approach presented here.

Improved PET/MRI accuracy by use of static transmission source in empirically derived hardware attenuation correction

BackgroundAccurate quantification of radioactivity, measured by an integrated positron emission tomography (PET) and magnetic resonance imaging (MRI) system, is still a challenge. One aspect of such a challenge is to correct for the hardware attenuation, such as the patient table and radio frequency (RF) resonators. For PET/MRI systems, computed tomography (CT) is commonly used to produce hardware attenuation correction (AC) maps, by converting Hounsfield units (HU) to a linear attenuation coefficients (LAC) map at the PET energy level 511 keV, using a bilinear model. The model does not address beam hardening, nor higher density materials, which can lead to inaccurate corrections.PurposeIn this study, we introduce a transmission-based (TX-based) AC technique with a static Germanium-68 (Ge-68) transmission source to generate hardware AC maps using the PET/MRI system itself, without the need for PET or medical CT scanners. The AC TX-based maps were generated for a homogeneous cylinder, made of acrylic as a validator. The technique thereafter was applied to the patient table and posterior part of an RF-phased array used in cardiovascular PET/MRI imaging. The proposed TX-based, and the CT-based, hardware maps were used in reconstructing PET images of one cardiac patient, and the results were analysed and compared.ResultsThe LAC derived by the TX-based method for the acrylic cylinder is estimated to be 0.10851 ± 0.00380 cm−1 compared to the 0.10698 ± 0.00321 cm−1 theoretical value reported in the literature. The PET photon counts were reduced by 8.7 ± 1.1% with the patient table, at the region used in cardiac scans, while the CT-based map, used for correction, over-estimated counts by 4.3 ± 1.3%. Reconstructed in vivo images using TX-based AC hardware maps have shown 4.1 ± 0.9% mean difference compared to those reconstructed images using CT-based AC.ConclusionsThe LAC of the acrylic cylinder measurements using the TX-based technique was in agreement with those in the literature confirming the validity of the technique. The over-estimation of photon counts caused by the CT-based model used for the patient table was improved by the TX-based technique. Therefore, TX-based AC of hardware using the PET/MRI system itself is possible and can produce more accurate images when compared to the CT-based hardware AC in cardiac PET images.

The conditioning regime in industrial drying of Scots pine sawn timber studied by X-ray computed tomography: a case-study

Industrial drying of sawn timber is a process driven by a difference in moisture content (MC) between the core and the surface as moisture moves from the wet inner region towards the drier surface. After drying, the timber surface is always drier than its core, and stresses have developed within the wood volume. If the timber is to be further processed, these stresses and the moisture gradient need to be reduced to avoid unwanted distortion, i.e. the timber needs to be conditioned. Conditioning is usually accomplished by exposing the timber to a hot and humid climate after the drying regime. The conditioning regime is essential for timber quality, and it is energy and time consuming; therefore of interest for optimisation. This research was a case study where for the first time the MC during conditioning was studied in an X-ray computed tomography (CT) scanner. The aim was to test a previously developed algorithm and investigate the influence of MC and heartwood-sapwood proportion on the effectivity of the moisture equalisation in 30 mm thick Scots pine boards. The MC was estimated from CT data acquired during the drying and conditioning of the boards in a lab-scale kiln adapted to a medical CT scanner. Results show that the algorithm can provide relevant data of internal MC distribution of sawn timber at the pixel level. Furthermore, for the drying schedules studied, the conditioning at low MC (8%) does not need to be longer than 3 h, while higher MC (18%) requires a longer conditioning.

Determination of binary diffusion coefficients between hot liquid solvents and bitumen with X-ray CT

Abstract The Steam Assisted Gravity Drainage (SAGD) process has been used for in-situ thermal bitumen recovery in Canada. Nevertheless, there are several drawbacks that could impair the SAGD performance. One of the main drawbacks of SAGD is nontrivial consumption of energy to generate steam. To curtail the consumption of steam, many SAGD variants have been proposed. For example, Vapor Extraction (VAPEX) was proposed as a solvent-based cold production method and Solvent-Assisted SAGD (SA-SAGD) was proposed as a solvent-assisted hybrid method. In those solvent-based/assisted methods, the dissolution of solvents in bitumen enhances the in-situ oil mobility. The speed of the mixing between solvents and bitumen in solvent-based/assisted recovery processes is dictated by dispersive mass flux which is proportional to the transverse dispersion coefficient. Diffusion coefficient, tortuosity and transverse dispersivity should be quantified to characterize the transverse dispersion coefficient. In this research, the diffusion coefficients of solvents in Athabasca bitumen were determined with X-ray computed tomography (CT). A medical X-ray CT scanner was used to capture the diffusion phenomenon occurring between fresh liquid solvents and bitumen. The experiments were designed to reproduce the in-situ high pressure, high temperature condition (3.5 MPa, 135C) where solvent and bitumen actually interact at the vapor chamber edge in the SA-SAGD process. Using our new °experimental system, we successfully measured molecular diffusion coefficients at the condition of the vapor chamber edge for the first time. In conclusion, the binary diffusion coefficients between solvents and bitumen were successfully measured at the condition where the mixing of solvents and bitumen actually occurs in the SA-SAGD process. The characterized diffusion coefficients will be input parameters for the reservoir simulation and semi-analytical models.

A brief introduction to the use of X-ray computed tomography (CT) for analysis of natural deformation structures in reservoir rocks

AbstractX-ray computed tomography (CT) is frequently used for non-destructive imaging and analysis of internal features in rock samples. In this paper we review the method for analysis of subseismic deformation structures in reservoir rocks, and provide some examples of different types of structures. Both medical CT and high-resolution µCT have great potential for identification of small-scale deformation structures in reservoir rocks and samples from outcrop analogues. The CT imaging techniques provide 3D data that are used in combination with 2D information from core or outcrop, thin-section and scanning electron microscopy (SEM). CT and µCT are used for quantitative and qualitative analysis of individual fractures and fracture networks, and for imaging and analysis of internal heterogeneities of fault rocks and deformation bands. The benefit of CT is that 3D properties (e.g. structure size, connectivity and variation in aperture) are actually characterized in 3D, contrary to traditional 2D methods using core surface, thin-section and outcrop. Limitations and uncertainties arise from artefacts during acquisition and processing, scale of observation and resolution, and manual steps involved in the segmentation of the CT volume. Increased availability of medical CT and µCT scanners and improved resolution should in the future lead to improved description and modelling of small-scale reservoir structures.

Comparison of medical and industrial X-ray computed tomography for non-destructive testing

Industrial X-ray computed tomography (CT) is an emerging laboratory-based non-destructive testing technique used in a variety of applications for samples ranging from 1 mm to usually 300 mm in diameter. Usually, microCT scanners are used for industrial non-destructive testing due to the superior resolution possible compared to medical CT scanners, but it is not generally known that medical CT scanners can produce reasonable results when high resolution is not needed. As demonstrated in this case study of very dense objects, far shorter scan time is required, compared to conventional laboratory industrial CT systems, consequently being a better solution for applications such as quick scout-scans, high throughput applications and larger objects. This case study makes use of four typical industrial test objects, specifically chosen as candidates which would be expected to be too dense for relatively low-voltage medical scanners. The respective test objects were scanned with both medical and microCT scanners and the results compared for the purpose of industrial non-destructive analysis. The test objects are a steel turbine blade, a titanium casting, a concrete cylinder with aggregate stones and porosity, and a concrete block with metal fiber reinforcement.

Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

Abstract. Various geoscientific applications require a fast prediction of fracture permeability for an optimal workflow. Hence, the objective of the current study is to introduce and validate a practical method to characterize and approximate single flow in fractures under different stress conditions by using a core-flooding apparatus, in situ X-ray computed tomography (CT) scans and a finite-volume method solving the Navier–Stokes–Brinkman equations. The permeability of the fractured sandstone sample was measured stepwise during a loading–unloading cycle (0.7 to 22.1 MPa and back) to validate the numerical results. Simultaneously, the pressurized core sample was imaged with a medical X-ray CT scanner with a voxel dimension of 0.5 × 0.5 × 1.0 mm3. Fracture geometries were obtained by CT images based on a modification of the simplified missing attenuation (MSMA) approach. Simulation results revealed both qualitative plausibility and a quantitative approximation of the experimentally derived permeabilities. The qualitative results indicate flow channeling along several preferential flow paths with less pronounced tortuosity. Significant changes in permeability can be assigned to temporal and permanent changes within the fracture due to applied stresses. The deviations of the quantitative results appear to be mainly caused by both local underestimation of hydraulic properties due to compositional matrix heterogeneities and the low CT resolution affecting the accurate capturing of sub-grid-scale features. Both affect the proper reproduction of the actual connectivity and therefore also the depiction of the expected permeability hysteresis. Furthermore, the threshold value CTmat (1862.6 HU) depicting the matrix material represents the most sensitive input parameter of the simulations. Small variations of CTmat can cause enormous changes in simulated permeability by up to a factor of 2.6 ± 0.1 and, thus, have to be defined with caution. Nevertheless, comparison with further CT-based flow simulations indicates that the proposed method represents a valuable method to approximate actual permeabilities, particularly for smooth fractures (< 35 µm). However, further systematic investigations concerning the applicability of the method are essential for future studies. Thus, some recommendations are compiled by also including suggestions of comparable studies.

Letters to the Editor.

Letters to the Editor.

X‐ray CT‐Derived Soil Characteristics Explain Varying Air, Water, and Solute Transport Properties across a Loamy Field

Core Ideas We used CT‐derived parameters to explain solute, water, and air transport at field scale. CTmatrix was found to be the best parameter to explain solute transport. Limiting macroporosity gave the highest correlations with water and air transport. Combining macroporosity and CTmatrix improved the relationships of water and air flow. The characterization of soil pore space geometry is important for explaining fluxes of air, water, and solutes through soil and understanding soil hydrogeochemical functions. X‐ray computed tomography (CT) can be applied for this characterization, and in this study CT‐derived parameters were used to explain water, air, and solute transport through soil. Forty‐five soil columns (20 by 20 cm) were collected from an agricultural field in Estrup, Denmark, and subsequently scanned using a medical CT scanner. Nonreactive tracer leaching experiments were performed in the laboratory along with measurements of air permeability (Ka) and saturated hydraulic conductivity (Ksat). The CT number of the matrix (CTmatrix), which represents the moist bulk density of the soil matrix, was obtained from the CT scans as the average CT number of the voxels in the grayscale image excluding macropores and stones. The CTmatrix showed the best relationships with the solute transport characteristics, especially the time by which 5% of the applied mass of tritium was leached, known as the 5% arrival time (t0.05). The CT‐derived macroporosity (pores >1.2 mm) was correlated with Ka and log10(Ksat). The correlation improved when the limiting macroporosity (the minimum macroporosity for every 0.6‐mm layer along the soil column) was used, suggesting that soil layers with the narrowest macropore section restricted the flow through the whole soil column. Water, air, and solute transport were related with the CT‐derived parameters by using a best subsets regression analysis. The regression coefficients improved using CTmatrix, limiting macroporosity, and genus density, while the best model for t0.05 used CTmatrix only. The scanning resolution and the time for soil structure development after mechanical activities could be factors that increased the uncertainty of the relationships. Nevertheless, the results confirmed the potential of X‐ray CT visualization techniques for estimating fluxes through soil at the field scale.

Investigating Aperture-Based Permeability and Capillary Pressure in Rock Fractures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170819, “Investigating Aperture-Based Stress-Dependent Permeability and Capillary Pressure in Rock Fractures,” by Da Huo, SPE, Boxiao Li, SPE, and Sally M. Benson, SPE, Stanford University, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed. Laboratory measurements of fracture-aperture distributions have been made by use of computed-tomography (CT) scanning under various conditions of effective stress. The stress-dependent aperture-distribution data demonstrate that increasing stress results in two effects: The mean aperture will decrease and the variance of aperture distribution will increase. By analyzing the stress-dependent mean and variance of aperture distributions, this paper provides a more straightforward method for estimating stress-dependent permeability and capillary pressure in rock fractures. Experimental Setup A Berea sandstone is cut to create a fracture in the middle of the core. The sample is moderately homogeneous, with little porosity variation. The fracture surface is very rough and has large aperture variations. The length of the sample is 8 cm, and the radius is 5 cm. The porosity of the sample is 0.22. The rock sample is wrapped in a sleeve and then placed in an aluminum core holder. Water is injected around the sleeve to create the confining stress. Two high-accuracy pressure transducers are used to measure pressure at the inlet and outlet of the core, and electric heaters maintain the temperature of the core. Pore pressure is kept constant at atmospheric pressure, and temperature stays constant at 50°C. For permeability measurements, a dual-pump system is used to inject water into the core. A pressure-regulating pump is connected to the outlet of the core to maintain 2.07 MPa at the downstream end of the core throughout the experiment. The experimental setup is shown in Fig. 1. Permeability is measured with the steady-state method. Fluid is injected at three different rates while the pressure drop across the core is measured. On the basis of core size, fluid viscosity, slope of pressure drop, and fluid-injection rate, Darcy’s law is used to calculate permeability. The intact-core permeability is measured at different stress levels and is subtracted from the fractured-core permeability. Permeability is calculated using a mean aperture of 0.1 mm. The core holder is placed in a medical X-ray CT scanner. Five repeated scans are taken and averaged to reduce the uncertainty of the CT scanning.

X-ray tomography of large wooden artworks: the case study of "Doppio corpo" by Pietro Piffetti

X-ray computed tomography (CT) is now used in the cultural heritage field because it is non-invasive and it can give a large amount of information on the inner structure of the object under study. Until recently mainly medical CT scanners or micro-CT setups have been used, limiting the analysis to relatively small artworks or requiring multiple acquisition and difficult image-joining for objects larger than detector dimensions. To overcome the limitations of ordinary CT devices, a facility for the X-ray tomography of large size artefacts has recently been designed and installed in a protected area of the Fondazione Centro Conservazione e Restauro "La Venaria Reale", a Centre for Preservation and Restoration. This facility, based on a X-ray source, a linear X-ray detector and a high precision mechanical system, has been and will be used to gather information on materials, manufacturing techniques and conservative conditions of artworks undergoing the restoration process. In this paper the results of the tomography of the first analyzed large artistic object are presented, giving an idea of the wealth of information obtained from the CT scan. The presented artwork is the writing cabinet called "doppio corpo", a masterpiece of furniture more than 3 m high, inlaid for Savoy Residences by Pietro Piffetti, the most famous cabinet-maker in Piedmont in the XVIII century. The artwork is now housed in the Quirinale Palace, the official residence of the Italian President in Rome. The CT analysis permitted us to obtain valuable information about the conservative conditions, the presence of previous interventions, the distribution of various materials and the dimensions and arrangement of several wooden pieces, thus allowing for interesting hypotheses about the building technique of this masterpiece.

Relationship between Hounsfield Unit in CT Scan and Gray Scale in CBCT.

Background and aims. Cone-beam computed tomography (CBCT) is an imaging system which has many advantages over computed tomography (CT). In CT scan, Hounsfield Unit (HU) is proportional to the degree of x-ray attenuation by the tissue. In CBCT, the degree of x-ray attenuation is shown by gray scale (voxel value). The aim of the present study was to investigate the relationship between gray scale in CBCT) and Hounsfield Unit (HU) in CT scan.Materials and methods. In this descriptive study, the head of a sheep was scanned with 3 CBCT and one medical CT scanner. Gray scales and HUs were detected on images. Reconstructed data were analyzed to investigate relationship between CBCT gray scales and HUs.Results. A strong correlation between gray scales of CBCT and HUs of CT scan was determined.Conclusion. Considering the fact that gray scale in CBCT is the criteria in measurement of bone density before implant treatments, it is recommended because of the lower dose and cost compared to CT scan.

Fast estimation of first-order scattering in a medical x-ray computed tomography scanner using a ray-tracing technique.

This study describes a deterministic method for simulating the first-order scattering in a medical computed tomography scanner. The method was developed based on a physics model of x-ray photon interactions with matter and a ray tracing technique. The results from simulated scattering were compared to the ones from an actual scattering measurement. Two phantoms with homogeneous and heterogeneous material distributions were used in the scattering simulation and measurement. It was found that the simulated scatter profile was in agreement with the measurement result, with an average difference of 25% or less. Finally, tomographic images with artifacts caused by scatter were corrected based on the simulated scatter profiles. The image quality improved significantly.

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

Iodine is commonly used as a contrast agent in nonmedical science and engineering, for example, to visualize Darcy flow in porous geological media using X-ray computed tomography (CT). Undesirable beam hardening artifacts occur when a polychromatic X-ray source is used, which makes the quantitative analysis of CT images difficult. To optimize the chemistry of a contrast agent in terms of the beam hardening reduction, we performed computer simulations and generated synthetic CT images of a homogeneous cylindrical sand-pack (diameter, 28 or 56 mm; porosity, 39 vol.% saturated with aqueous suspensions of heavy elements assuming the use of a polychromatic medical CT scanner. The degree of cupping derived from the beam hardening was assessed using the reconstructed CT images to find the chemistry of the suspension that induced the least cupping. The results showed that (i) the degree of cupping depended on the position of the K absorption edge of the heavy element relative to peak of the polychromatic incident X-ray spectrum, (ii) (53)I was not an ideal contrast agent because it causes marked cupping, and (iii) a single element much heavier than (53)I ((64)Gd to (79)Au) reduced the cupping artifact significantly, and a four-heavy-element mixture of elements from (64)Gd to (79)Au reduced the artifact most significantly.

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 (Na6H2W12O40) 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.

On the use of X-ray computed tomography for determining wood properties: a review1This article is a contribution to the series The Role of Sensors in the New Forest Products Industry and Bioeconomy.

In several processes of the forest products industry, an in-depth knowledge of log and board internal features is required and their determination needs fast scanning systems. One of the possible technologies is X-ray computed tomography (CT) technology. Our paper reviews applications of this technology in wood density measurements, in wood moisture content monitoring, and in locating internal log features that include pith, sapwood, heartwood, knots, and other defects. Annual growth ring measurements are more problematic to be detected on CT images because of the low spatial resolution of the images used. For log feature identification, our review shows that the feed-forward back-propagation artificial neural network is the most efficient CT image processing method. There are also some studies attempting to reconstruct three-dimensional log or board images from two-dimensional CT images. Several industrial prototypes have been developed because medical CT scanners were shown to be inappropriate for the wood industry. Because of the high cost of X-ray CT scanner equipment, other types of inexpensive sensors should also be investigated, such as electric resistivity tomography and microwaves. It also appears that the best approach uses various different sensors, each of them having its own strengths and weaknesses.