Cone-beam X-ray Computed Tomography Research Articles

Research on the model of mandibular alveolar bone defect in rabbits

Objectives: The purpose of this study was to create an animal model of a mandibular alveolar bone defect without compromising the animal’s well-being. Materials and methods: A total of 24 New Zealand white rabbits underwent surgery to create mandibular alveolar bone defects. The animals were sacrificed at 2, 4, 6, 8, 10, and 12 weeks post-surgery. To assess bone regeneration at the surgical site, radiography, dental cone-beam computed tomography (CT), and histological examination using Hematoxylin and Eosin staining were performed on the skull. Results: A straightforward and easily executable method was devised to create the rabbit mandibular alveolar defect model. After 10 weeks, complete soft tissue and bone regeneration were observed. X-ray and cone-beam CT evaluations demonstrated a progressive increase in bone density from weeks 2 to 12. Histological examination revealed that the alveolar bone structure was formed incrementally at the surgical site. The bone and connective tissue had filled the defect after 8 weeks. Conclusion: The creation of a model of mandibular alveolar bone defects in rabbits is a straightforward process that can be used to assess the regeneration of alveolar bone at the defect site. This animal model can serve as the foundation for tests to evaluate the capacity of biomaterials to regenerate the alveolar bone Key words: mandibular alveolar bone, Alveolar bone defects, animal models, bone regenerative medicine, tissue engineering.

X-Ray Scatter Correction by Deep Semi-supervised Learning of Simulated Projections with Beam-hole Array

In nondestructive testing and evaluation using cone-beam X-ray computed tomography (CBCT), X-ray scattering is a major cause of degrading the quality of CT images. To correct the negative effects of scattering, recent studies focused on the deep convolutional neural network (DCNN) for fast and accurate correction, and they trained the DCNNs using training data obtained by X-ray simulation. However, such approaches have a drawback where accurately simulating the physical behavior of Xrays requires considerable computational time. Moreover, it is unclear whether the simulated training data are effective to real measured data. To address these problems, we propose a new scatter correction method based on deep semi-supervised learning that can be applied to real measurements. Our training data are incomplete transmission images in which primary X-ray intensities are obtained for a part of pixels. We introduce a data augmentation method based on X-ray radiology, which allows us to significantly reduce training data in our semi-supervised learning. Through experiments using simulated data, this paper shows that our semi-supervised method using a few training data works as effectively as a baseline fully supervised method.

Fourier method for 3-dimensional data fusion of X-ray Computed Tomography and ultrasound

X-ray Computed Tomography (CT) is essential for nondestructive inspection of many manufactured components but is susceptible to various forms of artifacts and noise. In particular, multi-detector row cone-beam CT systems can suffer from so called “cone-beam” artifacts and partial volume effects, particularly on planar edges at the periphery of the field of view in the cone angle dimension. Uni-directional ultrasonic testing methods generally have extremely accurate in-plane depth resolution but poorer lateral resolution due to physical and geometric constraints. This paper presents a novel technique called Computed Tomographic Fusion (CT-F) which uses three-dimensional Fourier filtering to combine x-ray cone-beam CT reconstructions and ultrasound data in the frequency space. The result is a single image with improved accuracy and quality. CT-F mitigates artifacts while allowing for rapid, accurate characterization of large three-dimensional CT volumes. Using simulations and experiments, we demonstrate artifact reduction and edge contrast improvement in volumetric reconstructions.

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.

Verifying the measurement accuracy for X-ray cone-beam CT scans of objects smaller than 5 mm diameter

Dimensional metrology has become an important and well-established application of industrial X-ray computed tomography (CT). Currently there is an increasing need to use CT-based metrology for objects scanned at very high resolution (< 1 μm voxel size). In order to validate the measurement accuracy of a CT scanner, calibrated length standards are required that match the range of field-of-view (FOV) sizes offered by the scanner. While there are now many lengths standards available for FOV diameters of 50 mm and above [1], so far there are few or no standards that are suitable for a small FOV diameter, and at the same time meet the requirements of guideline VDI/VDE 2617/2630 [2], dealing with CT-based metrology. In order to fit a FOV diameter of less than 5 mm, a length standard was designed in accordance with that guideline, manufactured from a support material and ruby spheres, and tactile measurements were performed with a ZEISS F25 coordinate measurement machine. CT metrology was performed using a ZEISS METROTOM 800, and the results are compared.

Evaluation of Scanning Parameters Based on Image Entropy for Dimensional Computed Tomography Metrology

Many types of artifacts appear in X-ray computed tomography (CT) volume data, which influence measurement quality of industrial cone beam X-ray CT. Most of those artifacts are associated to CT scanning parameters; therefore, a good scanning parameter setting can weaken the influence to improve measurement accuracy. This paper presents a simulation method for evaluating CT scanning parameters for dimensional metrology. The method can aid CT metrology to achieve high measurement accuracy. In the method, image entropy is used as a criterion to evaluate the quality of CT volume data. For entropy calculation of CT volume data, a detailed description about bin width and entropy zone is given. The relationship between entropy values of CT volume data and error parameters of CT metrology is shown and discussed. By use of this method, mainly we focus on specimen orientation evaluation, and some other typical scanning parameters are used to evaluate the proposed method. Two typical specimens are used to evaluate the performance of the proposed method.

SU‐E‐I‐08: Investigation of Deconvolution Methods for Blocker‐Based CBCT Scatter Estimation

Purpose:To investigate whether deconvolution methods can improve the scatter estimation under different blurring and noise conditions for blocker‐based scatter correction methods for cone‐beam X‐ray computed tomography (CBCT).Methods:An “ideal” projection image with scatter was first simulated for blocker‐based CBCT data acquisition by assuming no blurring effect and no noise. The ideal image was then convolved with long‐tail point spread functions (PSF) with different widths to mimic the blurring effect from the finite focal spot and detector response. Different levels of noise were also added. Three deconvolutionMethods:1) inverse filtering; 2) Wiener; and 3) Richardson‐Lucy, were used to recover the scatter signal in the blocked region. The root mean square error (RMSE) of estimated scatter serves as a quantitative measure for the performance of different methods under different blurring and noise conditions.Results:Due to the blurring effect, the scatter signal in the blocked region is contaminated by the primary signal in the unblocked region. The direct use of the signal in the blocked region to estimate scatter (“direct method”) leads to large RMSE values, which increase with the increased width of PSF and increased noise. The inverse filtering is very sensitive to noise and practically useless. The Wiener and Richardson‐Lucy deconvolution methods significantly improve scatter estimation compared to the direct method. For a typical medium PSF and medium noise condition, both methods (∼20 RMSE) can achieve 4‐fold improvement over the direct method (∼80 RMSE). The Wiener method deals better with large noise and Richardson‐Lucy works better on wide PSF.Conclusion:We investigated several deconvolution methods to recover the scatter signal in the blocked region for blocker‐based scatter correction for CBCT. Our simulation results demonstrate that Wiener and Richardson‐Lucy deconvolution can significantly improve the scatter estimation compared to the direct method.

Improved proton computed tomography by dual modality image reconstruction

Proton computed tomography (CT) is a promising image modality for improving the stopping power estimates and dose calculations for particle therapy. However, the finite range of about 33 cm of water of most commercial proton therapy systems limits the sites that can be scanned from a full 360° rotation. In this paper the authors propose a method to overcome the problem using a dual modality reconstruction (DMR) combining the proton data with a cone-beam x-ray prior. A Catphan 600 phantom was scanned using a cone beam x-ray CT scanner. A digital replica of the phantom was created in the Monte Carlo code Geant4 and a 360° proton CT scan was simulated, storing the entrance and exit position and momentum vector of every proton. Proton CT images were reconstructed using a varying number of angles from the scan. The proton CT images were reconstructed using a constrained nonlinear conjugate gradient algorithm, minimizing total variation and the x-ray CT prior while remaining consistent with the proton projection data. The proton histories were reconstructed along curved cubic-spline paths. The spatial resolution of the cone beam CT prior was retained for the fully sampled case and the 90° interval case, with the MTF = 0.5 (modulation transfer function) ranging from 5.22 to 5.65 linepairs/cm. In the 45° interval case, the MTF = 0.5 dropped to 3.91 linepairs/cm For the fully sampled DMR, the maximal root mean square (RMS) error was 0.006 in units of relative stopping power. For the limited angle cases the maximal RMS error was 0.18, an almost five-fold improvement over the cone beam CT estimate. Dual modality reconstruction yields the high spatial resolution of cone beam x-ray CT while maintaining the improved stopping power estimation of proton CT. In the case of limited angles, the use of prior image proton CT greatly improves the resolution and stopping power estimate, but does not fully achieve the quality of a 360° proton CT scan.

Feature-based analysis for quality assessment of x-ray computed tomography measurements

This paper presents an approach to assess the quality of the data extracted with computed tomography (CT) measuring systems to perform geometrical evaluations. The approach consists in analyzing the error features introduced by the CT measuring system during the extraction operation. The analysis of the features is performed qualitatively (using graphical analysis tools) and/or quantitatively (by means of the root-mean-square deviation parameter of the error features). The approach was used to analyze four sets of measurements performed with an industrial x-ray cone beam CT measuring system. Three test parts were used in the experiments: a high accuracy manufacturing multi-wave standard, a calibrated step cylinder and a calibrated production part. The results demonstrate the usefulness of the approach to gain knowledge on CT measuring processes and improve the quality of CT geometrical evaluations. Advantages and limitations of the approach are discussed.

High resolution cone beam X-ray computed tomography of 3D-microstructures of cast Al-alloys

X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterisation of materials. XCT systems with cone beam geometry, micro- or nano-focus tubes and matrix detectors are increasingly used in research and non-destructive testing. Spatial resolutions down to 1μm can be reached with such XCT-systems for heterogeneities in metals with high absorption contrast. High resolution cone beam XCT is applied to five different Al-alloys: AlMg5Si7, AlCu4Mg1, AlZn6Mg2Cu2, AlZn8Mg2Cu2 and AlSi12Ni1. Up to four different types of inhomogeneities are segmented in one alloy using voxel sizes between (0.4μm)3 and (2.3μm)3. Target metallography and elemental analysis by energy dispersive X-ray analysis are used to identify the inhomogeneities. The possibilities and restrictions of XCT applied to Al-alloys are discussed.AlMg5Si7 XCT-data with a voxel size of (0.4μm)3 show inhomogeneities with brighter grey-values than the Al-matrix identified as elongated Fe-aluminides, and those with lower grey-values identified as pores and Mg2Si-particles with a “Chinese script-like” structure. Higher-absorbing interdendritic Al–Al2Cu-eutectic regions appear brighter than the Al-dendrites in the CT-data of AlCu4Mg1 with (1.1μm)³/voxel, whereas pores>4μm appear darker than the Al-matrix. The size and the 3D-structure of the α-Al dendrite arms with a diameter of 50–100μm are determined in samples from chill cast billets of AlCu4Mg1 and AlZn6Mg2Cu2 alloys. The irregular interdendritic regions containing eutectic segregations with Cu- and Zn-rich phases are >5μm wide. Equally absorbing primary equi-axed Al3(Sc, Zr) particles >5μm are distinguished in the centres of the dendrites by the level of sphericity values. The distribution of Ni- and Fe-aluminides in a squeeze cast AlSi12Ni1-alloy is imaged with (0.4μm)³/voxel, but the Si-phase cannot be segmented.

Application of cone beam CT 3D images to cephalometric analysis

Orthodontic diagnosis is currently based primarily on two-dimensional (2D) measurements. The development of maxillofacial cone beam X-ray computed tomography (CBCT) has promoted the use of three-dimensional (3D) measurements for orthodontic diagnosis. This technology is also expected to generate new analytic methods aiming at establishing optimal occlusion according to the individual's maxillary shape and function. However, during the transitional stage from 2D to 3D measurements, it is necessary to relate 3D data to existing data analysis methods (e.g., lateral cephalogram) for the assessment of skeletal and dental relationship. Our objectives were to develop a new cephalometric analysis method based on 3D data obtained by CBCT from four aspects: (1) measurement of magnification of cephalogram, (2) assessment of the dimensional accuracy of CBCT images, (3) development of a method of cephalometric analysis using CBCT images and (4) comparison of cephalometric analytical values obtained from CBCT images with those obtained from existing cephalograms. The results demonstrated that our analytical method facilitated cephalometric analysis of CBCT images and therefore allowed direct comparison of CBCT images with existing cephalograms. Cephalometric analysis using CBCT images resulted in a smaller range of inter-operator measurement variability than that using conventional cephalograms.

Modulator design for x‐ray scatter correction using primary modulation: Material selection

An optimal material selection for primary modulator is proposed in order to minimize beam hardening of the modulator in x-ray cone-beam computed tomography (CBCT). Recently, a measurement-based scatter correction method using primary modulation has been developed and experimentally verified. In the practical implementation, beam hardening of the modulator blocker is a limiting factor because it causes inconsistency in the primary signal and therefore degrades the accuracy of scatter correction. This inconsistency can be purposely assigned to the effective transmission factor of the modulator whose variation as a function of object filtration represents the magnitude of beam hardening of the modulator. In this work, the authors show that the variation reaches a minimum when the K-edge of the modulator material is near the mean energy of the system spectrum. Accordingly, an optimal material selection can be carried out in three steps. First, estimate and evaluate the polychromatic spectrum for a given x-ray system including both source and detector; second, calculate the mean energy of the spectrum and decide the candidate materials whose K-edge energies are near the mean energy; third, select the optimal material from the candidates after considering both the magnitude of beam hardening and the physical and chemical properties. A tabletop x-ray CBCT system operated at 120 kVp is used to validate the material selection method in both simulations and experiments, from which the optimal material for this x-ray system is then chosen. With the transmission factor initially being 0.905 and 0.818, simulations show that erbium provides the least amount of variation as a function of object filtrations (maximum variations are 2.2% and 4.3%, respectively, only one-third of that for copper). With different combinations of aluminum and copper filtrations (simulating a range of object thicknesses), measured overall variations are 2.5%, 1.0%, and 8.6% for 25.4 microm of copper, erbium, and tungsten, respectively. With and without 300 microm of copper in the beam, the measured variations for 25.4 microm of copper, erbium, and tungsten, 1 mm of aluminum, as well as 406 microm of copper, are 1.8%, 0.2%, 5.5%, 1.9%, and 7.5%, respectively. The spatial variation in the effective transmission factor of the modulator blocker due to beam hardening caused by the modulator itself reaches a minimum when the K-edge of the modulator material is near the mean energy of the spectrum. An optimal modulator material selection using the K-edge discontinuity is therefore proposed.

Sci-Fri AM: YIS-04: Scatter correction of cone beam optical computed tomography for polymer gel dosimetry.

Optical computed tomography (OptCT) may become the preferred scanning method for gel dosimetry dose validations, due to its high sensitivity and relatively low cost. Cone beam computed tomography (CBCT) arrangements are advantageous because of reduced scan time. However, CBCT arrangements are more sensitive to errors associated with scatter than other CT configurations. Unfortunately in polymer gel dosimetry this problem is amplified as the primary mode of beam attenuation is through scatter. Thus, managing and reducing the effects of scatter remains an important challenge for cone beam OptCT. In this work we examine two schemes for reducing the effects of scatter in the Vista cone beam OptCT system. First, we employed a pair of anti-scatter polarizing planes to reduce the magnitude of stray light reaching the camera. Secondly, we implemented a beam stop array (BSA) sampling method -which has been successful in correcting for scatter in X-ray CBCT- to obtain scatter field measurements that are subtracted from CT projections removing veiling glare. While both implementations reduced scatter related artifacts, the BSA technique yielded greater improvement without obvious image degradation. Comparative studies between absorbing dye standards and colloidal scattering standards with the same spectrophotometric optical attenuation revealed that application of the BSA technique nullified OptCT measurement disagreements between scattering and absorbing systems. When the BSA scatter correction technique was applied to polymer gel dosimetry 3%3mm agreement rose from 79.2% to 99.82%. These findings underscore the strength of the BSA sampling technique and its utility in cone beam OptCT.

A cone-beam tomography system with a reduced size planar detector: A backprojection-filtration reconstruction algorithm as well as numerical and practical experiments

In a traditional cone-beam computed tomography (CT) system, the cost of product and computation is very high. In this paper, we develop a transversely truncated cone-beam X-ray CT system with a reduced-size detector positioned off-center, in which X-ray beams only cover half of the object. The existing filtered backprojection (FBP) or backprojection-filtration (BPF) algorithms are not directly applicable in this new system. Hence, we develop a BPF-type direct backprojection algorithm. Different from the traditional rebinning methods, our algorithm directly backprojects the pretreated projection data without rebinning. This makes the algorithm compact and computationally more efficient. Because of avoiding interpolation errors of rebinning process, higher spatial resolution is obtained. Finally, some numerical simulations and practical experiments are done to validate the proposed algorithm and compare with rebinning algorithm.

Development and validation of MCNP4C-based Monte Carlo simulator for fan- and cone-beam x-ray CT

An x-ray computed tomography (CT) simulator based on the Monte Carlo N-particle radiation transport computer code (MCNP4C) was developed for simulation of both fan- and cone-beam CT scanners. A user-friendly interface running under Matlab 6.5.1 creates the scanner geometry at different views as MCNP4C's input file. The full simulation of x-ray tube, phantom and detectors with single-slice, multi-slice and flat detector configurations was considered. The simulator was validated through comparison with experimental measurements of different nonuniform phantoms with varying sizes on both a clinical and a small-animal CT scanner. There is good agreement between the simulated and measured projections and reconstructed images. Thereafter, the effects of bow-tie filter, phantom size and septa length on scatter distribution in fan-beam CT were studied in detail. The relative difference between detected total, primary and scatter photons for septa length varying between 0 and 95 mm is 11.2%, 1.9% and 84.1%, respectively, whereas the scatter-to-primary ratio decreases by 83.8%. The developed simulator is a powerful tool for evaluating the effect of physical, geometrical and other design parameters on scanner performance and image quality in addition to offering a versatile tool for investigating potential artefacts and correction schemes when using CT-based attenuation correction on dual-modality PET/CT units.

The measurement of proton stopping power using proton-cone-beam computed tomography

A cone-beam computed tomography (CT) system utilizing a proton beam has been developed and tested. The cone beam is produced by scattering a 160 MeV proton beam with a modifier that results in a signal in the detector system, which decreases monotonically with depth in the medium. The detector system consists of a Gd2O2S:Tb intensifying screen viewed by a cooled CCD camera. The Feldkamp-Davis-Kress cone-beam reconstruction algorithm is applied to the projection data to obtain the CT voxel data representing proton stopping power. The system described is capable of reconstructing data over a 16 × 16 × 16 cm3 volume into 512 × 512 × 512 voxels. A spatial and contrast resolution phantom was scanned to determine the performance of the system. Spatial resolution is significantly degraded by multiple Coulomb scattering effects. Comparison of the reconstructed proton CT values with x-ray CT derived proton stopping powers shows that there may be some advantage to obtaining stopping powers directly with proton CT. The system described suggests a possible practical method of obtaining this measurement in vivo.