Ring Artifacts Research Articles

基于极坐标变换去除计算机层析图像环形伪影

基于极坐标变换去除计算机层析图像环形伪影

Strategy of computed tomography sinogram inpainting based on sinusoid-like curve decomposition and eigenvector-guided interpolation

Projection incompleteness in x-ray computed tomography (CT) often relates to sparse sampling or detector gaps and leads to degraded reconstructions with severe streak and ring artifacts. To suppress these artifacts, this study develops a new sinogram inpainting strategy based on sinusoid-like curve decomposition and eigenvector-guided interpolation, where each missing sinogram point is considered located within a group of sinusoid-like curves and estimated from eigenvector-guided interpolation to preserve the sinogram texture continuity. The proposed approach is evaluated on real two-dimensional fan-beam CT data, for which the projection incompleteness, due to sparse sampling and symmetric detector gaps, is simulated. A Compute Unified Device Architecture (CUDA)-based parallelization is applied on the operations of sinusoid fittings and interpolations to accelerate the algorithm. A comparative study is then conducted to evaluate the proposed approach with two other inpainting methods and with a compressed sensing iterative reconstruction. Qualitative and quantitative performances demonstrate that the proposed approach can lead to efficient artifact suppression and less structure blurring.

X-ray micro-CT 이미지 내 패임 및 동심원상 화상결함 제거를 위한 이미지 보정 기법

X-ray micro-CT를 이용한 지반재료 내부 미세구조 및 공극구조의 정밀한 이미지 처리는 종종 이미지 내에 원천적으로 포함되는 화상결함으로 인해 제약된다. 본 논문에서는 X-ray micro-CT 이미지에 가장 일반적으로 나타나는 화상결함인 패임(영상 외곽과 중심부의 명암 차이) 및 동심원상(영상 중심으로부터 방사방향으로 연속적으로 나타나는 원)을 제거할 수 있는 이미지 보정 기법을 제시한다. 결함 제거는 좌표 변환법, 정규화 및 2차원 푸리에 변환에 의한 저역 통과 필터링 기법의 순차적 적용을 통해 이루어진다. 이미지 처리 기법의 효과를 다공성 현무암의 CT 이미지에서 화상결함들을 제거하고 이진화 후 적층하여 3차원 공극 구조를 추출하는 과정을 통해 설명하였다. 패임 및 동심원상 결함을 제거한 이미지와 원본 이미지의 비교 결과 결함 제거는 대상 재료 공극률의 과대평가를 방지할 수 있으며, 따라서 화상결함의 적절한 보정은 X-ray CT의 지반재료 적용 시 필수적인 과정으로 판단된다. High quality X-ray computed microtomography (micro-CT) imaging of internal microstructures and pore space in geomaterials is often hampered by some inherent noises embedded in the images. In this paper, we introduce image calibration techniques for removing the most common noises in X-ray micro-CT, cupping (brightness difference between the periphery and central regions) and ring artifacts (consecutive concentric circles emanating from the origin). The artifacts removal sequentially applies coordinate transformation, normalization, and low-pass filtering in 2D Fourier spectrum to raw CT-images. The applicability and performance of the techniques are showcased by describing extraction of 3D pore structures from micro-CT images of porous basalt using artifacts reductions, binarization, and volume stacking. Comparisions between calibrated and raw images indicate that the artifacts removal allows us to avoid the overestimation of porosity of imaged materials, and proper calibration of the artifacts plays a crucial role in using X-ray CT for geomaterials.

A pair of noisy/blurry patches-based PSF estimation and channel-dependent deblurring

A camera shake of the relative motion between cameras and original scenes often occurs during a shoot, resulting in blurred images. This paper presents a method of restoring blurred images based on patch-based PSF (Point Spread Function) estimation and channel-dependent image deblurring. Initially, a pair of noisy and blurred images is acquired by consecutive shooting under short and long exposures to more accurately estimate the 2D trajectory formed by the camera shake, which is also known as PSF. From the noisy image, one patch with strong edges is extracted using gradient operators and the corresponding blurry patch is found within the blurry image. The boundaries of both patches are padded in a specified tile pattern to prevent the appearance of vertical or horizontal lines due to boundary discontinuity. Tikhonov regularization is then iteratively conducted on Gaussian pyramid images with adaptive noise thresholding to get the optimized PSF. Finally, the color conversion of the blurred image into a luminance and two chrominance images is done to apply the channel dependent image deblurring where the spatial resolution of the chrominance images is reduced based on the natural gradient distribution to abandon the valueless data regardless of the restoration quality. Moreover, another artificially blurred image made by the convolution of the known Gaussian PSF and the noisy image is used to reduce undesirable ringing-artifacts and increase the sharpness. The experimental results show that the proposed method can provide more improved PSFs and restored images accompanied by the reduction of computing time and ring artifacts.

High-quality 3D correction of ring and radiant artifacts in flat panel detector-based cone beam volume CT imaging

The use of an x-ray flat panel detector is increasingly becoming popular in 3D cone beam volume CT machines. Due to the deficient semiconductor array manufacturing process, the cone beam projection data are often corrupted by different types of abnormalities, which cause severe ring and radiant artifacts in a cone beam reconstruction image, and as a result, the diagnostic image quality is degraded. In this paper, a novel technique is presented for the correction of error in the 2D cone beam projections due to abnormalities often observed in 2D x-ray flat panel detectors. Template images are derived from the responses of the detector pixels using their statistical properties and then an effective non-causal derivative-based detection algorithm in 2D space is presented for the detection of defective and mis-calibrated detector elements separately. An image inpainting-based 3D correction scheme is proposed for the estimation of responses of defective detector elements, and the responses of the mis-calibrated detector elements are corrected using the normalization technique. For real-time implementation, a simplification of the proposed off-line method is also suggested. Finally, the proposed algorithms are tested using different real cone beam volume CT images and the experimental results demonstrate that the proposed methods can effectively remove ring and radiant artifacts from cone beam volume CT images compared to other reported techniques in the literature.

Comparison of ring artifact removal methods using flat panel detector based CT images

BackgroundRing artifacts are the concentric rings superimposed on the tomographic images often caused by the defective and insufficient calibrated detector elements as well as by the damaged scintillator crystals of the flat panel detector. It may be also generated by objects attenuating X-rays very differently in different projection direction. Ring artifact reduction techniques so far reported in the literature can be broadly classified into two groups. One category of the approaches is based on the sinogram processing also known as the pre-processing techniques and the other category of techniques perform processing on the 2-D reconstructed images, recognized as the post-processing techniques in the literature. The strength and weakness of these categories of approaches are yet to be explored from a common platform.MethodIn this paper, a comparative study of the two categories of ring artifact reduction techniques basically designed for the multi-slice CT instruments is presented from a common platform. For comparison, two representative algorithms from each of the two categories are selected from the published literature. A very recently reported state-of-the-art sinogram domain ring artifact correction method that classifies the ring artifacts according to their strength and then corrects the artifacts using class adaptive correction schemes is also included in this comparative study. The first sinogram domain correction method uses a wavelet based technique to detect the corrupted pixels and then using a simple linear interpolation technique estimates the responses of the bad pixels. The second sinogram based correction method performs all the filtering operations in the transform domain, i.e., in the wavelet and Fourier domain. On the other hand, the two post-processing based correction techniques actually operate on the polar transform domain of the reconstructed CT images. The first method extracts the ring artifact template vector using a homogeneity test and then corrects the CT images by subtracting the artifact template vector from the uncorrected images. The second post-processing based correction technique performs median and mean filtering on the reconstructed images to produce the corrected images.ResultsThe performances of the comparing algorithms have been tested by using both quantitative and perceptual measures. For quantitative analysis, two different numerical performance indices are chosen. On the other hand, different types of artifact patterns, e.g., single/band ring, artifacts from defective and mis-calibrated detector elements, rings in highly structural object and also in hard object, rings from different flat-panel detectors are analyzed to perceptually investigate the strength and weakness of the five methods. An investigation has been also carried out to compare the efficacy of these algorithms in correcting the volume images from a cone beam CT with the parameters determined from one particular slice. Finally, the capability of each correction technique in retaining the image information (e.g., small object at the iso-center) accurately in the corrected CT image has been also tested.ConclusionsThe results show that the performances of the algorithms are limited and none is fully suitable for correcting different types of ring artifacts without introducing processing distortion to the image structure. To achieve the diagnostic quality of the corrected slices a combination of the two approaches (sinogram- and post-processing) can be used. Also the comparing methods are not suitable for correcting the volume images from a cone beam flat-panel detector based CT.

Extrinsic Versus Intrinsic Uniformity Correction for -Cameras

SPECT produces nuclear medicine images using a 3-dimensional diagnostic tool that eliminates the superimposition of adjacent structures, thus providing improved disease localization. Another method of uniformity correction-to use the complete capabilities of this tool-is discussed and evaluated in this article. The conventional method of intrinsic uniformity correction accounts only for nonuniformities within the γ-camera, excluding the collimator. If SPECT image quality is related to overall camera performance, then using an extrinsic uniformity correction method rather than an intrinsic method will improve image quality. SPECT uniformity images were obtained using a SPECT phantom with application of intrinsic and extrinsic uniformity correction tables with 2 different γ-imaging systems. The image results were qualitatively assessed. Even with acceptable nonuniformity analyses, significant ring artifacts within the intrinsic uniformity-corrected images are observed, whereas the artifacts are considerably less significant with the extrinsic uniformity correction and disappear completely in some of these images. Extrinsic uniformity correction may significantly improve the overall image quality by taking into account nonuniformities that arise from the collimator. This method will result in fewer image artifacts and improved image quality, thereby improving patient care.

High-quality non-blind image deconvolution with adaptive regularization

Non-blind image deconvolution is a process that obtains a sharp latent image from a blurred image when a point spread function (PSF) is known. However, ringing and noise amplification are inevitable artifacts in image deconvolution since perfect PSF estimation is impossible. The conventional regularization to reduce these artifacts cannot preserve image details in the deconvolved image when PSF estimation error is large, so strong regularization is needed. We propose a non-blind image deconvolution method which preserves image details, while suppressing ringing and noise artifacts by controlling regularization strength according to local characteristics of the image. In addition, the proposed method is performed fast with fast Fourier transforms so that it can be a practical solution to image deblurring problems. From experimental results, we have verified that the proposed method restored the sharp latent image with significantly reduced artifacts and it was performed fast compared to other non-blind image deconvolution methods.

Adaptive image interpolation by cardinal splines in piecewise constant tension

The cardinal spline in tension is modified to allow for different tensions in different sampling intervals. Varying the tension in proportion to an index of sharp change in image brightness, we obtain image interpolation results with less ringing artifacts compared to those by the cubic spline interpolation.

WE‐A‐301‐11: Low Dose Multi‐Slice Helical CT Reconstruction Using Tight Frame Regularization

Purpose: Multi‐slice helical CT has been widely used in CT diagnostic imaging due to its large coverage and short acquisition time. However, the associated high x‐ray radiation dose has become a major concern. Two straightforward ways to reduce the dose is to lower the mAs level and use high pitch. Yet, the traditional linear interpolation (LI) based filtered backprojection (FBP) reconstruction method leads to highly degraded images containing obvious image noise and ring artifacts around the edges under these situations. The aim of this work is to propose a novel iterative reconstruction algorithm for low dose helical CT. Methods: In our method, the helical scanning geometry and the cone angle in the multi‐slice CT are accurately modeled by proper computation of the forward projection matrix. CT images are reconstructed by minimizing a cost function consisting of a data fidelity term and a regularization term which requires that the solution has a sparse representation under a piecewise linear Tight‐frame (TF) basis. We tested our algorithm on a digital NCAT phantom at thorax region with two different high pitches (2:1 and 2.5:1) and low mAs level. The conventional LI‐based FBP algorithm was also studied for comparison purposes. Results: The CT images reconstructed by LI‐based FBP method contain obvious noise and ring artifacts around the edges, while the images by our TF‐based method show better quality with less noise and smooth edges. Compared with the LI‐based FBP method, our method enhances the signal‐to‐noise ratio by a factor of about 3.5 and also leads to higher edge cross‐correlation coefficients. Conclusions: The TF‐based iterative reconstruction algorithm outperforms the LI‐based FBP method in effectively removing the image noise due to low mAs level and ring artifacts of edges due to high pitches in the cases studied, demonstrating its promising applications for helical CT reconstruction.

SU-E-J-10: Panoramic Cone Beam Computed Tomography (CBCT)

Purpose: Cone-beam computed tomography (CBCT) is the main imaging tool for image-guided radiotherapy but its functionalities is limited by small imaging volume and restricted image position (imaged at the central instead of treatment position for lateral targets to avoid collisions). In this paper we present the concept of “panoramic CBCT” which can image patients at the treatment position with a volume as large as practically needed. Methods: This novel “panoramic CBCT” technique scans the target panoramically for different imager positions, stitch together the panoramic projection images of the same gantry angle to form a larger projection image, and perform CBCT reconstruction using the stitched projection images. To study the potential reconstruction artifacts of this imaging technique, we calculated cone-beam projections of the MCAT thorax phantom for one imager in three adjacent positions. Different gaps and exposure levels were introduced between adjacent imager positions to simulate imperfect stitching. Simultaneous algebraic reconstruction technique (SART) was used to reconstruct CBCT images using the stitched projection images for half-scan (180-degree+cone-anlge/2 gantry rotation). As a gold standard, CBCT reconstruction using projection images big enough to encompass the target were also performed for full scan (360-degree gantry rotation). Results: Incomplete reconstruction with artifacts was observed for reconstruction using projections from the central imager position only. When stitching is perfect, complete reconstruction was obtained from the stitched projection images with quality similar to the gold standard. Streak and ring artifacts were observed when stitching is imperfect and the severity of artifacts increases with the gap size and exposure level difference. Conclusions: Since half-scan can be achieved for most treatment positions without collisions, the proposed “panoramic CBCT” can image tumors of any location for patients of any size at the treatment position. Good image stitching algorithms are needed to eliminate the reconstruction artifacts from imperfect stitching.

Industrial computed tomography system for aerospace applications: development and characterisation

In this paper we report on the development and characterisation of an industrial computed tomography (ICT) system to image larger aerospace components and assemblies with sizes up to φ1000 mm, 10 m height and 2000 kg weight. The system consists of a 450 kV X-ray source, a linear detector array and a 6-axes computer numerical control (CNC) object manipulator. Mechanical as well as radiographic alignments were carried out to achieve 10 μm accuracy in mechanical motion of the objects under study. Rotate-only and translate-rotate scanning geometries were implemented for different sizes of object. Bi-directional scanning was implemented in translate-rotate geometry to reduce the scanning time by half. Corrections for centre of rotation error, ring artifacts, streak artifacts, unequal ray spacing error and cupping (beam-hardening) artifacts are implemented. A filtered back projection (FBP) algorithm is implemented for image reconstruction. The modulation transfer function (MTF) of the system is estimated using a 160 mm-diameter Perspex cylinder. The system resolution is found to be 350 μm. Details on the scanning of a few components are presented. 3D isovalue surface generation is implemented as an additional feature to aid better defect evaluation.

SU-E-T-95: Imaging Protocol Investigations with a Fan-Beam Optical CT Scanner for 3D Dosimetry

Purpose: To investigate data acquisition protocol choices and their effects on image noise for a prototype fan-beam optical CT scanner intended for evaluating 3D dosimeters. Methods: The prototype scans dosimeters submerged in a refractive index-matching bath. For all tests, uniform samples of scattering solution (synthetic polymer in water) were housed in cylindrical containers (1-litre, 95 mm diameter) and scanned in a water bath. for reference data (I_o), options evaluated were: i) a projection through the bath, ii) an average projection through a “blank” (water-filled container), and iii) an entire sinogram through a “blank”. Scan parameters considered were: i) temporal delay between I_o and I scans, ii) number of projections per rotation, iii) number of samples averaged per projection, and iv) vertical displacement between I_o and I slices. Images were reconstructed using filtered backprojection. Relative noise (st.dev./mean) in a consistent region of interest was used to quantify image quality. Results: Using a reference scan through a “blank” as opposed to one through only a water bath eliminated major ring artefacts. Furthermore, using a full I_o sinogram through a “blank” instead of an averaged profile considerably reduced noise (22.4% to 3.1%). Minimal increases in noise were seen when delaying scans (3% to 5%, one hour later). Increases in noise were seen with fewer than 360 projections per rotation; with > 360 projections, no significant reductions in noise were seen (up to 1800). No benefit was found by acquiring multiple samples per projection. Significant noise increase was seen when I_o and I slices were displaced by even 1 mm (2.6% to 15.5%). Conclusions: Noise reduction is optimized when comparing scan sinograms ray-by-ray to full sinograms of a “blank”. Main sources of image noise were attributed to variations in the surface quality of the containers used. Funding from the Canadian Institutes of Health Research and the University of Victoria supported portions of this work.

Image Deconvolution With Multi-Stage Convex Relaxation and Its Perceptual Evaluation

This paper proposes a new image deconvolution method using multi-stage convex relaxation, and presents a metric for perceptual evaluation of deconvolution results. Recent work in image deconvolution addresses the deconvolution problem via minimization with non-convex regularization. Since all regularization terms in the objective function are non-convex, this problem can be well modeled and solved by multi-stage convex relaxation. This method, adopted from machine learning, iteratively refines the convex relaxation formulation using concave duality. The newly proposed deconvolution method has outstanding performance in noise removal and artifact control. A new metric, transduced contrast-to-distortion ratio (TCDR), is proposed based on a human vision system (HVS) model that simulates human responses to visual contrasts. It is sensitive to ringing and boundary artifacts, and very efficient to compute. We conduct comprehensive perceptual evaluation of image deconvolution using visual signal-to-noise ratio (VSNR) and TCDR. Experimental results of both synthetic and real data demonstrate that our method indeed improves the visual quality of deconvolution results with low distortions and artifacts.

Classification of ring artifacts for their effective removal using type adaptive correction schemes

High resolution tomographic images acquired with a digital X-ray detector are often degraded by the so called ring artifacts. In this paper, a detail analysis including the classification, detection and correction of these ring artifacts is presented. At first, a novel idea for classifying rings into two categories, namely type I and type II rings, is proposed based on their statistical characteristics. The defective detector elements and the dusty scintillator screens result in type I ring and the mis-calibrated detector elements lead to type II ring. Unlike conventional approaches, we emphasize here on the separate detection and correction schemes for each type of rings for their effective removal. For the detection of type I ring, the histogram of the responses of the detector elements is used and a modified fast image inpainting algorithm is adopted to correct the responses of the defective pixels. On the other hand, to detect the type II ring, first a simple filtering scheme is presented based on the fast Fourier transform (FFT) to smooth the sum curve derived form the type I ring corrected projection data. The difference between the sum curve and its smoothed version is then used to detect their positions. Then, to remove the constant bias suffered by the responses of the mis-calibrated detector elements with view angle, an estimated dc shift is subtracted from them. The performance of the proposed algorithm is evaluated using real micro-CT images and is compared with three recently reported algorithms. Simulation results demonstrate superior performance of the proposed technique as compared to the techniques reported in the literature.

Partial ring artifact on cardiac CT: image presentation and clinical implication

In computed tomography (CT), the term "artifact" is applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object. A partial ring artifact in cardiac CT has a unique pattern and has not been previously reported in real clinical practice. In this report, we will demonstrate the images and animations of a cardiac CT which is affected by a partial ring artifact due to a broken detector module. The different characteristics of ring artifacts on routine and cardiac CT, and the implications for clinical interpretation will also be described. In summary, when a partial ring artifact is noted incidentally on cardiac CT, we suggest interpreting the images by reviewing reconstructed images from different RR-intervals to avoid unnecessary repeated scans and additional radiation exposure.

Suppression of ring artefacts when tomographing anisotropically attenuating samples

There are many objects for which the attenuation varies significantly as they are rotated during computerized X-ray tomography, for example plate samples. This can lead to significant ring artefacts in the subsequent tomographic reconstructions. In this paper a new method is presented that can successfully suppress such ring artefacts and is applicable to both parallel and cone-beam geometries. Rapid correction is achieved via an analytical formula which involves only a matrix-vector multiplication, for which the matrix is known and depends on a regularization parameter. The efficacy of the method is demonstrated for a paleontological sample (calcified shark cartilage) and a carbon-carbon composite/Ti-SiC metal matrix composite test sample.

Novel Characterization of Microdrops and Microbubbles in Emulsions and Foams Using Atomic Force Microscopy

The nature of the interface of drops or bubbles and the dynamic interactions between them often mediate or control macroscopic behavior in the formulation and processing of emulsions and foams in solvent extraction, froth flotation, food, personal care products, and microfluidics as well as in many biological processes. Characterization of these interfaces is often complicated due to the small size of the drops and bubbles that may range from the micrometer scale to hundreds of micrometers. We report the direct measurement of the surface or interfacial tension of drops or bubbles in aqueous solutions as a function of the concentration and type of surfactant, using atomic force microscopy (AFM) and a recently developed nanoneedle AFM cantilever. We also demonstrate the viability of imaging drops or bubbles of this size in both tapping and contact imaging modes through a systematic study of parameters, including cantilever spring constant, tip geometry, imaging force, and feedback settings as well as the AFM manufacturer. The imaging study demonstrates the viability of using AFM to visualize complex structures at the oil-water or air-water interface as well as how concentric ring artifacts observed in the literature are the result of earlier AFM instrument limitations.

X-ray CT 스캔을 이용한 사질토 간극비 측정

본 연구에서는 산업 전분야에 걸쳐 다양하게 활용되고 있는 X-ray CT의 지반분야 적용을 위해 X-ray CT를 이용한 사질토의 간극비 측정에 관한 연구를 수행하였다. 이를 위해 고분해능 X-ray CT 시스템의 적용에 문제가 되는 빔하드닝(Beam hardening) 및 동심원상 화상결함(Ring artifact) 등에 대한 문제점 지적 및 해결방안을 제시하였다. 또한 X-ray CT를 이용한 간극비 산정을 위한 알고리즘 및 프로그램을 개발하였으며, 이를 이용하여 주문진 표준사의 간극비를 측정하여 신뢰성을 검토하였다. In this study, a new methodology by using the X-ray CT scan is proposed for estimating void ratio of sandy soil. The general problems in high resolution X-ray CT scan such as beam hardening and ring artifact had been successfully settled up using thin metal plate filter and some calibration process. In order to calculate the void ratio of sand from its CT image, the procedures and algorithm for CT image processing are developed. CT scan tests for Joomunjin sand are carried out to verify its applicability to void ratio testing.

Image restoration through regularization based on error energy minimization

AbstractImage restoration has been an important field of image processing for decades. Various methods of restoration have been studied. Meanwhile, regularization presents a very general methodology for image restoration. However, more regularization will remove more noise but will also lose more image detail, and vice versa. In this article, a novel image restoration algorithm is presented, which is derived from regularization by minimizing error energy of noise and ring artifact. Thus, this proposed algorithm is not only immune to noise but also ringing artifact, where a designed high‐pass filter is used in the algorithm proposed here such as the difference of a delta function and a blurring function or an edge operator like a Laplacian operator. This proposed algorithm can achieve high‐resolution images by being implemented with a 1D noiseless signal to reveal its ability of generating high frequencies beyond the diffraction limit and with a 2D computer‐generated noisy image to show the ability of being immune to noise as well as with a 94‐GHz millimeter‐wave image to validate its reliability. © 2010 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 20, 308–315, 2010