Beam-hardening Artifact Reduction Research Articles

Potential of photon counting computed tomography derived spectral reconstructions to reduce beam-hardening artifacts in chest CT

PurposeAim of the recent study is to point out a method to optimize quality of CT scans in oncological patients with port systems. This study investigates the potential of photon counting computed tomography (PCCT) for reduction of beam hardening artifacts caused by port-implants in chest imaging by means of spectral reconstructions. MethodIn this retrospective single-center study, 8 ROIs for 19 spectral reconstructions (polyenergetic imaging, monoenergetic reconstructions from 40 to 190 keV as well as iodine maps and virtual non contrast (VNC)) of 49 patients with pectoral port systems undergoing PCCT of the chest for staging of oncologic disease were measured. Mean values and standard deviation (SD) Hounsfield unit measurements of port-chamber associated hypo- and hyperdense artifacts, bilateral muscles and vessels has been carried out. Also, a structured assessment of artifacts and imaging findings was performed by two radiologists. ResultsA significant association of keV with iodine contrast as well as artifact intensity was noted (all p < 0.001). In qualitative assessment, utilization of 120 keV monoenergetic reconstructions could reduce severe and pronounced artifacts completely, as compared to lower keV reconstructions (p < 0.001). Regarding imaging findings, no significant difference between monoenergetic reconstructions was noted (all p > 0.05). In cases with very high iodine concentrations in the subclavian vein, image distortions were noted at 40 keV images (p < 0.01). ConclusionsThe present study demonstrates that PCCT derived spectral reconstructions can be used in oncological imaging of the thorax to reduce port-derived beam-hardening artefacts. When evaluating image data sets within a staging, it can be particularly helpful to consider the 120 keV VMIs, in which the artefacts are comparatively low.

3D Printing of Tooth Impressions Based on Multi-Detector Computed Tomography Images Combined with Beam Hardening Artifact Reduction in Metal Structures

We investigated the role of metal artifact reduction by taking 3D print impressions using 3D data of Computed Tomography (CT) images based on the algorithm applied. We manufactured a phantom of a human mandible tooth made of gypsum and nickel alloy to measure the metal artifacts. CT images were obtained by changing the phantom tube voltage and tube current. The signal intensity of the image generated by the metal artifacts before and after the iterative metal artifact reduction algorithm (iMAR) was measured. A 3D printing process was performed after converting the images, before and after iMAR application, into STL files using InVesalius version 3.1.1 by selecting the conditions that minimized the effect of the artifact. Regarding metal artifacts, the Hounsfield unit (HU) value showed low as the tube voltage increased. The iMAR-applied images acquired under the same conditions showed a significantly lower HU. The artifacts, in the form of flashes, persisted in the 3D-printed product of the image not subjected to iMAR, but were largely removed in the 3D-printed product following iMAR application. In this study, the application of iMAR and data acquired using high tube voltage eliminated a significant portion of the metal artifacts, resulting in an impression shape that was consistent with the human body.

Photon Counting Detectors for X-Ray Imaging With Emphasis on CT

X-ray imaging is a widely used imaging modality in the medical diagnostic field due to its availability, low cost, high spatial resolution, and fast image acquisition. X-ray photons in standard X-ray sources are polychromatic. Detectors that allow to extract the “color” information of the individual X-rays can lead to contrast enhancement, improved material identification or reduction of beam hardening artifacts at the system level, if we compare them with the widely spread energy integrating detectors. Today, in the field of computed tomography (CT), prototypes of clinical grade systems based on spectral photon counting detectors are currently available for clinical research from different companies. One of the key system components in that development is the X-ray photon detector. This article reviews the photon detection hardware, from the conversion of X-rays into electrical signals to the pulse processing electronics. A review of available photon counting application specific integrated circuits (ASICs) for spectroscopic X-ray imaging is presented with emphasis on the CT medical imaging application.

A Two-Stage Approach for Beam Hardening Artifact Reduction in Low-Dose Dental CBCT

This article presents a two-stage method for beam hardening artifact correction of dental cone beam computerized tomography (CBCT). The proposed artifact reduction method is designed to improve the quality of maxillofacial imaging, where soft tissue details are not required. Compared to standard CT, the additional difficulty of dental CBCT comes from the problems caused by offset detector, FOV truncation, and low signal-to-noise ratio due to low X-ray irradiation. To address these problems, the proposed method primarily performs a sinogram adjustment in the direction of enhancing data consistency, considering the situation according to the FOV truncation and offset detector. This sinogram correction algorithm significantly reduces beam hardening artifacts caused by high-density materials such as teeth, bones, and metal implants, while tending to amplify special types of noise. To suppress such noise, a deep convolutional neural network is complementarily used, where CT images adjusted by the sinogram correction are used as the input of the neural network. Numerous experiments validate that the proposed method successfully reduces beam hardening artifacts and, in particular, has the advantage of improving the image quality of teeth, associated with maxillofacial CBCT imaging.

Reduction of beam hardening artifacts on real C-arm CT data using polychromatic statistical image reconstruction

PurposeThis work aims at the compensation of beam hardening artifacts by the means of an extended three-dimensional polychromatic statistical reconstruction to be applied for flat panel cone-beam CT. MethodsWe implemented this reconstruction technique as being introduced by Elbakri et al. (2002) [1] for a multi-GPU system, assuming the underlying object consists of several well-defined materials. Furthermore, we assume one voxel can only contain an overlap of at most two materials, depending on its density value. Given the X-ray spectrum, the procedure enables to reconstruct the energy-dependent attenuation values of the volume. ResultsWe evaluated the method by using flat-panel cone-beam CT measurements of structures containing small metal objects and clinical head scan data. In comparison with the water-corrected filtered backprojection, as well as a maximum likelihood reconstruction with a consistency-based beam hardening correction, our method features clearly reduced beam hardening artifacts and a more accurate shape of metal objects. ConclusionsOur multi-GPU implementation of the polychromatic reconstruction, which does not require any image pre-segmentation, clearly outperforms the standard reconstructions of objects, with respect to beam hardening even in the presence of metal objects inside the volume. However, remaining artifacts, caused mainly by the limited dynamic range of the detector, may have to be addressed in future work.

Calibration-free beam hardening reduction in x-ray CBCT using the epipolar consistency condition and physical constraints.

The beam hardening effect is a typical source of artifacts in x-ray cone beam computed tomography (CBCT). It causes streaks in reconstructions and corrupted Hounsfield units toward the center of objects, widely known as cupping artifacts. We present a novel efficient projection data-based method for reduction of beam-hardening artifacts and incorporate physical constraints on the shape of the compensation functions. The method is calibration-free and requires no additional knowledge of the scanning setup. The mathematical model of the beam hardening effect caused by a single material is analyzed. We show that the effect of beam hardening on the resulting functions on the line integral measurements are monotonous and concave functions of the ideal data. This holds irrespective of any limiting assumptions on the energy dependency of the material, the detector response or properties of the x-ray source. A regression model for the beam hardening effect respecting these theoretical restrictions is proposed. Subsequently, we present an efficient method to estimate the parameters of this model directly in projection domain using an epipolar consistency condition. Computational efficiency is achieved by exploiting the linearity of an intermediate function in the formulation of our optimization problem. Our evaluation shows that the proposed physically constrained ECC algorithm is effective even in challenging measured data scenarios with additional sources of inconsistency. The combination of mathematical consistency condition and a compensation model that is based on the properties of x-ray physics enables us to improve image quality of measured data retrospectively and to decrease the need for calibration in a data-driven manner.

Review of Clinical Applications for Virtual Monoenergetic Dual-Energy CT.

In this article, the authors discuss the technical background and summarize the current body of literature regarding virtual monoenergetic (VM) images derived from dual-energy CT data, which can be reconstructed between 40 and 200 keV. Substantially improved iodine attenuation at lower kiloelectron volt levels and reduced beam-hardening artifacts at higher kiloelectron volt levels have been demonstrated from all major manufacturers of dual-energy CT units. Improved contrast attenuation with VM imaging at lower kiloelectron volt levels enables better delineation and diagnostic accuracy in the detection of various vascular or oncologic abnormalities. Low-kiloelectron-volt VM imaging may be useful for salvaging CT studies with suboptimal contrast material delivery or providing additional information on the arterial vasculature obtained from venous phase acquisitions. For patients with renal impairment, substantial reductions in the use of iodinated contrast material can be achieved by using lower-energy VM imaging. The authors recommend routine reconstruction of VM images at 50 keV when using dual-energy CT to exploit the increased contrast properties. For reduction of beam-hardening artifacts, VM imaging at 120 keV is useful for the initial assessment.

A projection-based sparse-view virtual monochromatic computed tomography method based on a compressed-sensing algorithm

Computed tomography (CT) images obtained at different monochromatic X-ray beam energies can be synthesized from conventional dual-energy CT scans. This approach to synthesizing monochromatic CT images is based on basis material decomposition and the knowledge of attenuation of basis materials. The main benefits of virtual monochromatic CT (VMCT) images include reduction of beam-hardening artifacts and provision of accurate atteuation measurements. Despite the VMCT's benefits, main concerns in the use of VMCT in clinics may be high radiation dose the patient is exposed to. In this study, we investigated a projection-based sparse-view VMCT method in an attempt to overcome these difficulties. We performed a computational simulation and evaluated the feasibility of using the VMCT method in sparse-view CT. Two polychromatic data sets of 90 projections, far less than what is required by the Nyquist sampling theory, were simulated at 80 kVp and 140 kVp and used to synthesize VMCT images at a monochromatic energy range of 40–140 keV . VMCT image characteristics were quantitatively evaluated in terms of intensity profile, the contrast-to-noise ratio, and the signal-to-noise ratio. Our results indicate that the CS-based algorithm produced high-quality sparse-view CT images, and thereby the proposed VMCT method yielded CT image results of improved beam-hardening artifacts and quantitative measurements.

Direct quantitative material decomposition employing grating-based X-ray phase-contrast CT

Dual-energy CT has opened up a new level of quantitative X-ray imaging for many diagnostic applications. The energy dependence of the X-ray attenuation is the key to quantitative material decomposition of the volume under investigation. This material decomposition allows the calculation of virtual native images in contrast enhanced angiography, virtual monoenergetic images for beam-hardening artifact reduction and quantitative material maps, among others. These visualizations have been proven beneficial for various diagnostic questions. Here, we demonstrate a new method of ‘virtual dual-energy CT’ employing grating-based phase-contrast for quantitative material decomposition. Analogue to the measurement at two different energies, the applied phase-contrast measurement approach yields dual information in form of a phase-shift and an attenuation image. Based on these two image channels, all known dual-energy applications can be demonstrated with our technique. While still in a preclinical state, the method features the important advantages of direct access to the electron density via the phase image, simultaneous availability of the conventional attenuation image at the full energy spectrum and therefore inherently registered image channels. The transfer of this signal extraction approach to phase-contrast data multiplies the diagnostic information gained within a single CT acquisition. The method is demonstrated with a phantom consisting of exemplary solid and fluid materials as well as a chicken heart with an iodine filled tube simulating a vessel. For this first demonstration all measurements have been conducted at a compact laser-undulator synchrotron X-ray source with a tunable X-ray energy and a narrow spectral bandwidth, to validate the quantitativeness of the processing approach.

Third generation dual-energy CT with 80/150 Sn kV for head and neck tumor imaging.

Dual-energy CT (DECT) provides additional image datasets which enable improved tumor delineation or reduction of beam hardening artifacts in patients with head and neck squamous cell carcinoma (SCC). To assess radiation dose and image quality of third-generation DECT of the head and neck in comparison to single-energy CT (SECT). Thirty patients with SCC who underwent both SECT (reference tube voltage 120 kVp) and DECT (80/150 Sn kVp) of the head and neck region for staging were retrospectively selected. Attenuation measurements of the sternomastoid muscle, internal jugular vein, submandibular gland and tongue were compared. Image noise was assessed at five anatomic levels. Subjective image quality was evaluated by two radiologists in consensus. CTDIvol was 55% lower with DECT (4.2 vs. 9.3 mGy; P = 0.002). Median image noise was equal or lower in DECT at all levels (nasopharynx: 3.9 vs. 5.8, P < 0.0001; floor of mouth: 3.6 vs. 4.5, P = 0.0002; arytenoids: 3.6 vs. 3.1, P = 0.096; lower thyroid: 4.4 vs. 5.7, P = 0.002; arch of aorta: 5.6 vs. 6.5, P = 0.001). Attenuation was significantly lower in DECT ( P < 0.05). Subjective image analysis revealed that DECT is equal or superior to SECT with regard to overall image quality (nasopharynx: 5 vs. 5, P = 1; floor of mouth: 5 vs. 5, P = 0.0041; arytenoids: 5 vs. 5, P = 0.6; lower thyroid: 5 vs. 3, P < 0.0001; arch of aorta: 5 vs. 4, P < 0.0001). Head and neck imaging with third-generation DECT can reduce radiation dose by half compared to SECT, while maintaining excellent image quality.

Simultaneous characterization of electron density and effective atomic number for radiotherapy planning using stoichiometric calibration method and dual energy algorithms.

Relative electron densities of body tissues (ρe) for radiotherapy treatment planning are normally obtained by CT scanning of tissue substitute materials (TSMs) and producing a Hounsfield Unit-ρe calibration curve. Aiming for more accurate, simultaneous characterization of ρe and effective atomic number (Zeff) of real tissues, an in-house phantom (including 10 water solutions plus composite cork as TSMs) was constructed and scanned at 4kVps. Dual-energy algorithms were applied to 80-140 and 100-140kVp combination scans, for better differentiation of tissues with same attenuation coefficient at 120kVp but different ρe and Zeff. Stoichiometric calibration and closeness of the ρe of the 11 TSMs to real tissues (≤ 0.5%) resulted in smaller ρe calculation discrepancies, compared to studies with commercial phantoms (p < 0.024). Applying an energy subtraction algorithm further mitigated errors by spectral separation and reduction of beam hardening artifacts and noise, reducing the mean and standard deviation of the absolute difference of ρe at 80-140kVp (p < 0.003) and 100-140 kVp (p < 0.0001) scans, compared to 120 kVp scan, respectively. Moreover, a parametrization algorithm decreased the Zeff discrepancy from real tissues at 80-140 kVp scans; for thyroid, the residual error was ≤ 0.18 units of Zeff (vs. 0.2 with the Gammex 467 phantom from a previous study). These results further suggest that a dual-energy algorithm in combination with stoichiometry can decrease errors in calculation of the ρe of real tissues to ameliorate inhomogeneity for dose calculation in radiotherapy treatment planning, especially when the energy spectrum of the X-ray tube of the CT machine is not available.

Influence of Artifact Reduction Tools in Micro–computed Tomography Images for Endodontic Research

IntroductionMicro–computed tomography (μCT) is an imaging modality of growing application in endodontic research because of its nondestructive technology that enables visualization at the micrometer level. In the presence of high-density material, images acquired with μCT may present artifacts. Thus, the aim of this study was to evaluate subjectively and objectively the influence of artifact reduction tools available for μCT image reconstruction to determine whether their use can influence endodontic research outcomes. MethodsTen teeth were scanned in a μCT device, and these images were reconstructed by applying 13 protocols of artifact reduction tools, combining ring artifact reduction (RAR) and beam-hardening artifact reduction (BAR). Images were assessed subjectively (observer's preference) and objectively (root canal surface area and volume). Observer's preference for RAR and BAR protocols was tested by χ2. Analysis of variance was used to compare volumes and surface area of root canals on different protocols. Intraobserver and interobserver reproducibility was calculated by the weighted kappa test. ResultsThere was no preference for a particular protocol for BAR (P = .91) or RAR (P = .80). There were no statistically significant differences for volume (P = .999) or surface area (P = .972) of root canals for all protocols. ConclusionsTo evaluate root canal volume and root canal surface area, artifact reduction tools applied to μCT images can be used according to the observer's visual preference without influence on objective image analysis.

Technical Note: A model-based sinogram correction for beam hardening artifact reduction in CT.

This study aims to propose a physics-based method of reducing beam-hardening artifacts induced by high-attenuation materials such as metal stents or other metallic implants. The proposed approach consists of deriving a sinogram inconsistency formula representing the energy dependence of the attenuation coefficient of high-attenuation materials. This inconsistency formula more accurately represents the inconsistencies of the sinogram than that of a previously reported formula (called the MAC-BC method). This is achieved by considering the properties of the high-attenuation materials, which include the materials' shapes and locations and their effects on the incident X-ray spectrum, including their attenuation coefficients. Numerical simulation and phantom experiment demonstrate that the modeling error of MAC-BC method are nearly completely removed by means of the proposed method. The proposed method reduces beam-hardening artifacts arising from high-attenuation materials by relaxing the assumptions of the MAC-BC method. In doing so, it outperforms the original MAC-BC method. Further research is required to address other potential sources of metal artifacts, such as photon starvation, scattering, and noise.

Dual Energy Computed Tomography Applications for the Evaluation of the Spine.

Capturing the energy-dependent x-ray attenuation of different tissues, dual-energy computed tomography offers multiple benefits in the imaging of the spine, such as bone and iodinated contrast removal, monosodium urate imaging, and robust reduction of beam-hardening artifacts. The emerging new applications of this technique include bone marrow imaging in acute trauma and myeloinfiltrative disorders, improved bone density determination, and noninvasive assessment of spinal gout.

TH-CD-202-02: A Preliminary Study Evaluating Beam-Hardening Artifact Reduction On CT Direct Electron-Density Images

Purpose: A prototype reconstruction algorithm that can provide direct electron density (ED) images from single energy CT scans is being currently developed by Siemens Healthcare GmbH. This feature can eliminate the need for kV specific calibration curve for radiation treatemnt planning. An added benefit is that beam-hardening artifacts are also reduced on direct-ED images due to the underlying material decomposition. This study is to quantitatively analyze the reduction of beam-hardening artifacts on direct-ED images and suggest additional clinical usages. Methods: HU and direct-ED images were reconstructed on a head phantom scanned on a Siemens Definition AS CT scanner at five tube potentials of 70kV, 80kV, 100kV, 120kV and 140kV respectively. From these images, mean, standard deviation (SD), and local NPS were calculated for regions of interest (ROI) of same locations and sizes. A complete analysis of beam-hardening artifact reduction and image quality improvement was conducted. Results: Along with the increase of tube potentials, ROI means and SDs decrease on both HU and direct-ED images. The mean value differences between HU and direct-ED images are up to 8% with absolute value of 2.9. Compared to that on HU images, the SDs are lower on direct-ED images, and the differences are up to 26%. Interestingly, the local NPS calculated from direct-ED images shows consistent values in the low spatial frequency domain for images acquired from all tube potential settings, while varied dramatically on HU images. This also confirms the beam -hardening artifact reduction on ED images. Conclusions: The low SDs on direct-ED images and relative consistent NPS values in the low spatial frequency domain indicate a reduction of beam-hardening artifacts. The direct-ED image has the potential to assist in more accurate organ contouring, and is a better fit for the desired purpose of CT simulations for radiotherapy.

Value of spectral CT imaging for improving beam-hardening artifact of myocardium

Objective To evaluate beam-hardening (BH) artifact reduction of myocardium in coronary computed tomography angiography (CCTA) with single-source dual-energy CT. Methods Thirty patients received CCTA on single-source dual-energy CT with findings of coronary artery stenosis<50% were enrolled in this study prospectively. Scanning mode was gemstone spectral imaging (GSI) , single-source instantaneous (0.5 ms) kVp (140 kVp and 80 kVp) switch. The original images acquired by scanning were reconstructed into monochromatic energy (60,70,80,90,100,110,120,130,140 keV) left vertical short-axis images via 40% ASIR and the polychromatic left vertical short-axis images were conventionally reconstructed. CT values were measured across multiple segments (basal anterior, basal lateral, basal inferior, basal septal, mid anterior, mid lateral, mid inferior, mid septal, apical anterior, apical lateral, apical inferior, apical septal and apex) of left ventricle wall at varying monochromatic energy levels and polychromatic images, and then the left ventricular myocardial average CT value and BH objective value were calculated retrospectively: BH1= CT value of mid inferior wall-CT value of basal inferior wall, BH2= CT value of mid septal wall-CT value of mid inferior wall. BH subjective rating were evaluated by two radiologists independently. Single sample t test was used to compare the difference of myocardial CT values among 13 segments with the left ventricular myocardial average CT value on polychromatic images; Kruskal-Wallis test was used to compare the difference of CT values among thirteen different segments of myocardium on fixed monochromatic energy images; Wilcoxon rank test was used to compare the difference of BH objective value and subjective rating between monochromatic images with polychromatic images. Results On polychromatic images, the mean myocardial CT value was (73±25) HU, the CT value of basic inferior[ (58±23) HU], basic septal[ (85±21) HU], mid septal[ (89±24) HU], apical anterior[ (64±23) HU]and apex [ (61±24) HU] were different from the mean myocardial CT value (t value were -3.76,2.89,3.50, -2.30, -2.86,P all 0.05). The differences of CT value of different myocardial segments had statistical significance at 60 to 80 keV images (P all 0.05) , suggesting that the non-uniformity of CT value among different segments was improved. On polychromatic images,BH1 M (P25,P75) was 11(6,28),BH2 M (P25,P75) was 19(1,29) HU. BH1 was improved on 90 to 140 keV images while BH2 was improved on 70 to 140 keV images, the difference had statistical significance compared with the polychromatic images(P all<0.05). BH1,BH2 decreased with the increasing of monochromatic energy level on 60 to 100 keV images, then increased a little on 110,120 keV images, and hit bottom on 130 keV images with the value of 5.20,0.34 HU, finally exist a slight increase on 140 keV images again. On polychromatic images,BH1,BH2 subjective rating M (P25,P75) both were 1(1,2), BH1 subjective rating was improved on 70 to 140 keV images while BH2 subjective rating was improved on 90 to 140 keV, the difference had statistical significance compared with the polychromatic images(P all<0.05). Conclusion Compared with the polychromatic images,monochromatic energy images of CCTA with dual-energy CT resulted in significant BH artifact reduction and improvements in the uniformity in the myocardium, and 130 keV is the optimal Monochromatic energy. Key words: Coronary artery; Myocardium; Tomography, X-ray computed; Spectral CT imaging

Beam hardening artifact reduction using dual energy computed tomography: implications for myocardial perfusion studies.

Myocardial computed tomography perfusion (CTP) using conventional single energy (SE) imaging is influenced by the presence of beam hardening artifacts (BHA), occasionally resembling perfusion defects and commonly observed at the left ventricular posterobasal wall (PB). We therefore sought to explore the ability of dual energy (DE) CTP to attenuate the presence of BHA. Consecutive patients without history of coronary artery disease who were referred for computed tomography coronary angiography (CTCA) due to atypical chest pain and a normal stress-rest SPECT and had absence or mild coronary atherosclerosis constituted the study population. The study group was acquired using DE and the control group using SE imaging. Demographical characteristics were similar between groups, as well as the heart rate and the effective radiation dose. Myocardial signal density (SD) levels were evaluated in 280 basal segments among the DE group (140 PB segments for each energy level from 40 to 100 keV; and 140 reference segments), and in 40 basal segments (at the same locations) among the SE group. Among the DE group, myocardial SD levels and myocardial SD ratio evaluated at the reference segment were higher at low energy levels, with significantly lower SD levels at increasing energy levels. Myocardial signal-to-noise ratio was not significantly influenced by the energy level applied, although 70 keV was identified as the energy level with the best overall signal-to-noise ratio. Significant differences were identified between the PB segment and the reference segment among the lower energy levels, whereas at ≥70 keV myocardial SD levels were similar. Compared to DE reconstructions at the best energy level (70 keV), SE acquisitions showed no significant differences overall regarding myocardial SD levels among the reference segments. BHA that influence the assessment of myocardial perfusion can be attenuated using DE at 70 keV or higher.

A Study of Pb-Rich Dendrites in a Near-Eutectic 63Sn-37Pb Solder Microstructure via Laboratory-Scale Micro X-ray Computed Tomography (μXCT)

A key challenge in the application of laboratory-scale x-ray computed tomography to the study of metallic alloys is achieving sufficient feature contrast and resolution for the segmentation between solid phases of similar composition and density at spatial length scales suitable for microstructure quantification. In the microelectronic packaging, value exists in the nondestructive evaluation of solder system microstructures resulting from varying compositional and processing factors. A near-eutectic 63Sn-37Pb butt-joint on copper was studied with a custom laboratory-scale microresolved x-ray computed tomography scanner with the goal of quantifying three-dimensional (3D) microstructural constituents resulting from a reproducible reflow process. The 3D character of lead-rich dendrites resulting from non-equilibrium solidification was revealed. The quantification of the dendrite microstructure was made possible through a combination of data acquisition, data processing, and data segmentation techniques. The scanning parameter selection, with respect to the characterization task, is discussed. Data acquisition and processing methods which were determined to be beneficial for 3D microstructure characterization are detailed. A beam-hardening artifact reduction algorithm is provided, without which microstructure quantification would not have been possible. The segmentation of the dendrite features, performed using a semi-automatic 3D region growth method, is described. The segmented solder volume enabled quantitative description of the 3D dendrite microstructure and void content.

Beam hardening artifact reduction in microfocus computed tomography for improved quantitative coal characterization

Microfocus computed tomography (μCT) is a non-destructive technique allowing visualization of the internal microstructure of objects. This technique can be used to quantify the maceral content and to define the cleat system within coal samples. However, a number of artifacts intrinsic to the technique hamper the optimal quantitative use of μCT. One of the most important artifacts is beam hardening, which generates streaks on the images near dense inclusions such as ankerite or pyrite and makes the border of the sample appear denser than in reality. These artifacts can be minimized by using a dual-energy method in which two sets of scans, taken at a different energy level, are combined. In this study existing dual-energy methods are combined into a new technique and adapted to be more ideally suitable for coal samples. The method is calibrated with an artificial sample made of a Plexiglas cylinder in which two aluminum rods are built-in. Plexiglas and aluminum were selected since their X-ray attenuation properties are similar to those of vitrinite and dense phases, such as ankerite, respectively. The method is tested on a coal sample bearing relatively high percentages of iron minerals. The corrected and uncorrected images from one representative slice through the sample are compared with reflected light-microscopy images from the corresponding polished surface. The results show that the applied approach creates images in which the border does not appear denser than in reality and in which the artificial streaks near dense inclusions are removed. In addition, size measurements of the ankerite veins and grains in the sample are much more accurate based on the corrected images. It is therefore assumed that the proposed dual-energy algorithm minimizes the beam hardening artifact. However, the corrected images have a lower signal-to-noise ratio than the uncorrected images. Consequently this approach magnifies to some extent other artifacts (noise, scatter and partial volume effect) which are also present in the uncorrected images. Some solutions to correct for these artifacts and to improve the developed dual-energy method are proposed.