Shading Correction Research Articles

WE‐G‐207‐09: A Practical Bowtie Ring Artifact Correction Algorithm for Cone‐Beam CT

Purpose:Shaded ring artifact in cone‐beam CT (CBCT) is caused by scatter contamination from bowtie‐modulated primary beam and presents a unique ring pattern in CBCT image. Previous shading correction methods, which suppress the artifact as conventional scatter correction in projection domain, are complicated due to the non‐trivial scatter estimation. In this work, we propose a practical and readily implementable algorithm to correct for the severe shaded ring in CBCT without relying on prior information and directly in image domain.Methods:Due to the correlation between the ring pattern and bowtie modulator, an initial bowtie mask was reconstructed from bowtie‐modulated air‐scan projections as if they were acquired from a flat‐field exposure penetrating the bowtie modulator. The shape of the bowtie mask matches well with that of the shaded ring in CBCT, while its intensity needs to be scaled to fully compensate for the shading ring. To find a correct scaling factor, we start from the anatomical knowledge that the same tissue has comparable CT number. This knowledge indicates a sharp peak in histogram of that specific tissue. One way to achieve this goal is to maximize the peak value of the histogram of that tissue. The above concept is formulated as a mathematical optimization problem which is solved using a standard Simplex method. The shaded ring artifact in CBCT is finally corrected for by adding a scaled bowtie mask.Results:The proposed method is evaluated on one pelvis patient. Severe shaded ring artifact is greatly suppressed. Our method reduces the CT number error from >200 HU to be ∼50 HU, and increases the spatial uniformity by 1.3 times.Conclusion:We propose a practical image‐domain algorithm for shaded ring artifact correction in CBCT. It is computationally efficient and does not rely on prior knowledge. It is thus attractive for clinical use.This work is supported by the National Science Foundation of China (NSFC Grant No. 81201091), National High Technology Research and Development Program of China (863 program, Grant No. 2015AA020917), and Fund Project for Excellent Abroad Scholar Personnel in Science and Technology.

Intensity calibration and flat-field correction for fluorescence microscopes.

Standardization in fluorescence microscopy involves calibration of intensity in reproducible units and correction for spatial nonuniformity of illumination (flat-field or shading correction). Both goals can be achieved using concentrated solutions of fluorescent dyes. When a drop of a highly concentrated fluorescent dye is placed between a slide and a coverslip it produces a spatially uniform field, resistant to photobleaching and with reproducible quantum yield; it can be used as a brightness standard for wide-field and confocal microscopes. For wide-field microscopes, calibration can be further extended to absolute molecular units. This can be done by imaging a solution of known concentration and known depth; the latter can be prepared by placing a small spherical lens in a diluted solution of the same fluorophore that is used in the biological specimen.

Shade correction technique for free hand bonded restorations.

The aim of this short communication is to describe an easy technique illustrating how to correct an esthetic mismatch between a natural tooth and a free hand bonded restoration. The proposed technique is based on a case report showing the procedures required to perform a shade correction of an unesthetic anterior composite resin restoration. The clinical case shows an easy and minimally invasive procedure which allows for color correction of anterior free bonded restorations, avoiding the complete removal of a class IV composite resin. This technique is suitable especially for anterior restorations whenever an imperfect color match is detected.

Shading correction for whole slide image using low rank and sparse decomposition.

Many microscopic imaging modalities suffer from the problem of intensity inhomogeneity due to uneven illumination or camera nonlinearity, known as shading artifacts. A typical example of this is the unwanted seam when stitching images to obtain a whole slide image (WSI). Elimination of shading plays an essential role for subsequent image processing such as segmentation, registration, or tracking. In this paper, we propose two new retrospective shading correction algorithms for WSI targeted to two common forms of WSI: multiple image tiles before mosaicking and an already-stitched image. Both methods leverage on recent achievements in matrix rank minimization and sparse signal recovery. We show how the classic shading problem in microscopy can be reformulated as a decomposition problem of low-rank and sparse components, which seeks an optimal separation of the foreground objects of interest and the background illumination field. Additionally, a sparse constraint is introduced in the Fourier domain to ensure the smoothness of the recovered background. Extensive qualitative and quantitative validation on both synthetic and real microscopy images demonstrates superior performance of the proposed methods in shading removal in comparison with a well-established method in ImageJ.

Automatic concrete cracks detection and mapping of terrestrial laser scan data

Terrestrial laser scanning has become one of the standard technologies for object acquisition in surveying engineering. The high spatial resolution of imaging and the excellent capability of measuring the 3D space by laser scanning bear a great potential if combined for both data acquisition and data compilation. Automatic crack detection from concrete surface images is very effective for nondestructive testing. The crack information can be used to decide the appropriate rehabilitation method to fix the cracked structures and prevent any catastrophic failure. In practice, cracks on concrete surfaces are traced manually for diagnosis. On the other hand, automatic crack detection is highly desirable for efficient and objective crack assessment.The current paper submits a method for automatic concrete cracks detection and mapping from the data that was obtained during laser scanning survey. The method of cracks detection and mapping is achieved by three steps, namely the step of shading correction in the original image, step of crack detection and finally step of crack mapping and processing steps. The detected crack is defined in a pixel coordinate system. To remap the crack into the referred coordinate system, a reverse engineering is used. This is achieved by a hybrid concept of terrestrial laser-scanner point clouds and the corresponding camera image, i.e. a conversion from the pixel coordinate system to the terrestrial laser-scanner or global coordinate system. The results of the experiment show that the mean differences between terrestrial laser scan and the total station are about 30.5, 16.4 and 14.3 mms in x, y and z direction, respectively.

Montage Serial Sectioning: Some Finer Aspects of Practice

Serial sectioning as a probe of topological properties of solids has become increasingly important for quantitative materials characterization and provides accurate inputs for microstructure simulations. In recent years, large-area montage serial sectioning has gained traction, due to the high resolution of micrographs it produces and the large volumes it samples. But some finer aspects of the practice dealing with error detection and correction—necessary for accurate reconstruction of a microstructural volume—have not received much attention. This article describes the tools and techniques we developed to (1) correct shading in tiles that make up a montage and (2) detect metallographic errors in serial sectioning. These tools help reconstruct the most accurate three-dimensional microstructures for use in investigating the micromechanics of materials under a variety of conditions. They also help ensure accuracy in stereological measurements, in reconstruction of constituent morphologies for characterization or simulation, and in other applications of montage serial sectioning.

Nonparametric Illumination Correction for Scanned Document Images via Convex Hulls

A scanned image of an opened book page often suffers from various scanning artifacts known as scanning shading and dark borders noises. These artifacts will degrade the qualities of the scanned images and cause many problems to the subsequent process of document image analysis. In this paper, we propose an effective method to rectify these scanning artifacts. Our method comes from two observations: that the shading surface of most scanned book pages is quasi-concave and that the document contents are usually printed on a sheet of plain and bright paper. Based on these observations, a shading image can be accurately extracted via convex hulls-based image reconstruction. The proposed method proves to be surprisingly effective for image shading correction and dark borders removal. It can restore a desired shading-free image and meanwhile yield an illumination surface of high quality. More importantly, the proposed method is nonparametric and thus does not involve any user interactions or parameter fine-tuning. This would make it very appealing to nonexpert users in applications. Extensive experiments based on synthetic and real-scanned document images demonstrate the efficiency of the proposed method.

Full parallax three-dimensional computer generated hologram with occlusion effect using ray casting technique

Holographic display is capable of reconstructing the whole optical wave field of a three-dimensional (3D) scene. It is the only one among all the 3D display techniques that can produce all the depth cues. With the development of computing technology and spatial light modulators, computer generated holograms (CGHs) can now be used to produce dynamic 3D images of synthetic objects. Computation holography becomes highly complicated and demanding when it is employed to produce real 3D images. Here we present a novel algorithm for generating a full parallax 3D CGH with occlusion effect, which is an important property of 3D perception, but has often been neglected in fully computed hologram synthesis. The ray casting technique, which is widely used in computer graphics, is introduced to handle the occlusion issue of CGH computation. Horizontally and vertically distributed rays are projected from each hologram sample to the 3D objects to obtain the complex amplitude distribution. The occlusion issue is handled by performing ray casting calculations to all the hologram samples. The proposed algorithm has no restriction on or approximation to the 3D objects, and hence it can produce reconstructed images with correct shading effect and no visible artifacts. Programmable graphics processing unit (GPU) is used to perform parallel calculation. This is made possible because each hologram sample belongs to an independent operation. To demonstrate the performance of our proposed algorithm, an optical experiment is performed to reconstruct the 3D scene by using a phase-only spatial light modulator. We can easily perceive the accommodation cue by focusing our eyes on different depths of the scene and the motion parallax cue with occlusion effect by moving our eyes around. The experiment result confirms that the CGHs produced by our algorithm can successfully reconstruct 3D images with all the depth cues.

Enhancing automated micrograph‐based evaluation of LPS‐stimulated macrophage spreading

To evaluate macrophage spreading in immunofluorescence images of macrophages for surface protein CD11b and nuclear counterstaining with DAPI, it is necessary to measure the size of the macrophages at different time points after stimulation. Manual evaluation of fluorescent micrographs is usually a time-consuming and error-prone task, with poor reproducibility. Automatic image analysis methods can be used to improve the results. The quality of the analysis with these methods mainly depends on the quality of the image segmentation. A segmentation and quantification scheme based on shading correction, k-means clustering, and fast marching level sets has been developed for the purpose. An initial application of this approach showed that separating touching and overlapping cells in particular suffers severely in the inevitably blurred conditions, leading to partly erroneous measurements of macrophage spreading. An alternative method of segmentation in fluorescent micrographs was therefore investigated and evaluated in this study. The proposed approach uses a methodology that separates foreground objects from background objects on the basis of Boykov's graph cuts. In this process, a rough estimation of background pixels is used for background seeds. To identify foreground seeds, a difference of Gaussian band pass filter based workflow is developed. Information on foreground and background seeds is then used for a gradient magnitude based graph cut resulting in a robust figure-ground separation method. In addition, a fast marching level set approach is used in the post-processing step, which makes it possible to split touching cells by incorporating information about the cell nuclei. An evaluation based on a total of 553 manually labeled macrophages depicted in 21 micrographs showed that the proposed method significantly improves segmentation and splitting performance for fluorescent micrographs of LPS-stimulated macrophages and reduces the rate of error in automated analysis of macrophage spreading in comparison with alternative methods.

Non-uniform illumination correction in infrared images based on a modified fuzzy c-means algorithm

The correction of non-uniform illumination and the elimination of shading artifacts is an important preprocessing task used in a great number of image processing applications such as segmentation, registration or quantitative analysis. Although, a careful and accurate set up of the image acquisition system may degrade the importance of a brightness normalization algorithm, non-uniform illumination appears due to the interaction of objects and light on the scene requires retrospective shading correction. The image formation process and the corresponding shading effects are described by a linear image formation model, which consists of a multiplicative and an additive shading component. In this paper a novel brightness normalization method is proposed to eliminate the non-uniform illumination effects. The method is based on the application of a fuzzy c-means algorithm (FCM) only on the background part of the acquired image, where the objective function is modified to take into account local information of each pixel in the estimation of the multiplicative and the additive shading components. The modified FCM algorithm is iterative, as the standard FCM, and at each iteration the multiplicative and the additive shading components are re-estimated based on the cluster centers and the membership of each pixel in a specific cluster. Brightness correction is performed by the inverse of the image formation model after FCM convergence. Experiments were conducted in a database of both real and artificial infrared images. The experimental results show that the proposed method decreases significantly the non-uniform illumination effects and does not introduce brightness variations if the background is uniform.

LUT 쉐이딩 보정 알고리듬을 이용한 스캐닝 이미지 향상 FPGA 설계 구현

본 논문은 CCD 센서의 스캔 이미지 향상을 위해 쉐이딩 보정 알고리듬을 이용한 FPGA 설계 및 구현에 관한 연구이다. 쉐이딩 보정 기법은 룩업테이블(LUT)을 적용한다. 향상된 이미지 스캔을 위하여 CCD 센서의 모든 픽셀에 대한 히스토그램 최대값 및 최소값을 구하고, 균일한 히스토그램 값을 갖도록 오프셋 데이터의 쉐이딩 보정 LUT를 생성한다. 스캔 과정에서 센서의 출력은 보정된 LUT값으로 변환되고 LUT 변환 처리는 FPGA로 구현하여 실시간처리가 가능하도록 한다. 구현 시스템을 사용하여 실험한 결과 2.4ms 이하의 스캔 타이밍을 충족 할 수 있다. 제안시스템은 낮은 성능의 프로세서 기반에서 저비용 및 실시간 향상된 이미지 스캔이 가능하다. This paper describes FPGA design and realization using the shading correction algorithm for a CCD scan image enhancement. The shading algorithm is used by LUT (Look-up Table). The image enhancement results from that the histogram minimum value and maximum with respect to all pixels of the CCD image should be extracted, and the shading LUT is constructed to keep constant histogram with offset data. The output of sensor be converted to corrected LUT image in preprocessing, and the converting system is realized by FPGA to be enabled to operate in real time. The result of the experimentation for the proposed system is showed to take the scanning time 2.4ms below. The system is presented to be based on a low speed processor system to scan enhanced images in real time and be guaranteed to be low cost.

WE-G-217BCD-10: Shading Correction in Image Domain for Cone-Beam CT Without Prior Information.

Cone-beam CT (CBCT) images contain severe shading artifacts mostly due to scatter. Many algorithms have been proposed to alleviate this problem by data correction on projections. Sophisticated methods have also been designed to further improve the image quality when prior patient information is available. In this work, we develop a novel algorithm for shading correction directly on CBCT images without any prior information. In CBCT, projection errors (mostly scatter) result in dominant low-frequency shading artifacts in image domain. Due to circular scan geometry, these artifacts often show global or local radial patterns. We first convert the uncorrected images into the polar coordinate system. Median filtering and polynomial fitting are applied on the transformed images to accurately estimate the low-frequency bias field of shading angle-by-angle and slice-by-slice. The estimated bias field is then converted back to the Cartesian coordinate system, followed by 3D low-pass filtering to eliminate possible high-frequency components. The shading-corrected images are finally obtained as the uncorrected volume divided by the shading bias field. The proposed algorithm has been evaluated on CBCT images of a pelvis patient and a head patient. Within regions of interest, the average CBCT error is reduced from around 250 HU to 42 and 38 HU, and the spatial non-uniformity error is reduced from above 17.5% to 2.1% and 1.7% for the pelvis and the head patients, respectively. As our method suppresses only low-frequency shading artifacts, patient anatomy and contrast are well retained in the corrected images of both cases. We propose an effective shading correction algorithm on CBCT images, with several advantages compared to existing approaches. The method has a high efficiency since it is deterministic and directly operates on the reconstructed images. It also requires no prior information, which facilitates its clinical use as a standard image correction solution.

Blind retrospective shading correction using a multi-objective minimization criterion

In this paper, a fully automatic blind retrospective shading correction method based mainly on a minimization of a multi-objective criterion is presented. The proposed method assumes that the acquired image has distorted from a multiplicative and an additive shading component and thus can be adequately described by the linear image formation model. The estimation of the shading-free image is based on parametric estimation of the multiplicative and the additive shading component and the consequent application of the inverse image formation model. First of all, an initial estimation of the shading-free image is performed by the minimization of the multi-objective function of an appropriate image criterion. Secondly, the multiplicative and the additive shading components are estimated, based on assumptions about their frequency content and then, they median filtered. Finally, an estimation of a shading-free image is obtained using the above estimations for the components and the application of the inverse image formation model.Qualitative and quantitative experiments were conducted in a variety of image modalities including artificial and real images of finger, retinal images, transmission electron microscopy (TEM) images, arm, palm and hand vein images and thorax X-ray images. In all cases of distorted images, the proposed method had successfully removed the majority of the shading effects and had not distorted the shading-free images satisfying the main goal of retrospective shading correction. The signal to noise ratio (SNR) or equivalently the reciprocal of the coefficient of variations is used as a quantitative measure of the reduction/increase of intensity variations within the objects of the same class after shading correction. In our experiments, for the purpose of evaluation the signal to noise ratio is calculated for two different classes (object and background).

Comparative study of retrospective methods to reduce non-uniform illumination effects to bridge coating

Abstract Digital image processing methods have been actively studied and applied to various infrastructure components including bridge painting assessment. Despite the advantages of image recognition techniques to bridge coating, the results can be greatly affected by the non-uniform illumination of target conditions. It is necessary to first convert all images to a standardized background illumination condition. The effect caused from non-uniform illumination can be minimized by setting lighting conditions properly or capturing additional calibration images. However, capturing additional calibration images is not possible in many practical situations and hence a good method of shading correction is required. This paper addresses the way to correct digital bridge coating images containing non-uniform illumination, which becomes more difficult when it comes to assessing facility surfaces with rust defects. In order to choose the most appropriate one, four representative shading correction methods were selected and tested to identify their suitability to processing bridge coating images. These methods include: (1) image smoothing, (2) homomorphic filtering, (3) morphological filtering, and (4) entropy minimization method. Test results on bridge coating images showed that the entropy minimization method outperformed the other three methods.

Full parallax three-dimensional display with occlusion effect using computer generated hologram

Computational holography becomes highly complicated and demanding when it is employed to produce real three-dimensional (3D) images. Here we present a novel algorithm for generating a full parallax 3D computer generated hologram (CGH) with occlusion effect, which is an important property of 3D perception, but has often been neglected in most CGH related works. The ray casting technique is introduced to handle the occlusion issue. Horizontally and vertically distributed rays are projected from each hologram sample to the 3D objects to obtain the complex amplitude distribution. The proposed algorithm has no restriction on-or approximation to-the 3D objects, and it can produce reconstructed images with correct shading effect and no visible artifacts. An optical experiment is performed to validate our approach, using a phase-only spatial light modulator to optically reconstruct a 3D scene. The experimental result confirmed that the CGHs produced by our algorithm can successfully reconstruct 3D images with full parallax and occlusion effect.

Focus on lasers and imaging

Focus on lasers and imaging

An a priori Shading Correction Technique for Contact Imaging Devices

Digital imaging devices are increasingly used for color measurements in fields such as forensics, the food industry, and ecological and medical research. Especially concerning the skin, in the follow-up of benign or malignant lesions, it is crucial to ensure that the measured colors are accurate and reproducible. Several color calibration methods having been presented over the last few years. However, the choice of illuminant used remains a major source of color misinterpretation, thus, much effort is being spent in trying to evaluate this a posteriori. The device presented overcomes this problem by integrating its own light source, although corrections in lighting heterogeneity are still required. In this paper, we present a lighting modelling technique used for shading correction which improves color consistency (as assessed by ∆E evaluation versus colorimeter), noise filtering, computation time, and memory consumption for this type of device.

Estimating costs for shade matching and shade corrections of fixed partial dentures for dental technicians in Germany: A pilot investigation

Aims. To evaluate the additional operating expenses caused by shade matching by dental technicians in the dental practice and by color changes of fixed partial dentures. Methods. A questionnaire was handed out to visitors of the 2010 ADT dental technician congress in Germany. Thirty-one completed questionnaires were evaluated. Results. Mean costs caused by shade matching and shade correction per month were 1269$ (SD = 1278$; n = 25) and per unit of FPD 9.32$ (SD = 8.89$). Conclusions. An improvement of shade matching, shade communication and reproduction should be made in order to minimize the considerable economic damage for dental laboratories.

Shading correction for on‐board cone‐beam CT in radiation therapy using planning MDCT images

Applications of cone-beam CT (CBCT) to image-guided radiationtherapy (IGRT) are hampered by shading artifacts in the reconstructed images. These artifacts are mainly due to scatter contamination in the projections but also can result from uncorrected beam hardening effects as well as nonlinearities in responses of the amorphous silicon flat panel detectors. While currently, CBCT is mainly used to provide patient geometry information for treatment setup, more demanding applications requiring high-quality CBCT images are under investigation. To tackle these challenges, many CBCT correction algorithms have been proposed; yet, a standard approach still remains unclear. In this work, we propose a shading correction method for CBCT that addresses artifacts from low-frequency projection errors. The method is consistent with the current workflow of radiation therapy. With much smaller inherent scatter signals and more accurate detectors, diagnostic multidetector CT (MDCT) provides high quality CT images that are routinely used for radiation treatment planning. Using the MDCT image as "free" prior information, we first estimate the primary projections in the CBCT scan via forward projection of the spatially registered MDCT data. Since most of the CBCT shading artifacts stem from low-frequency errors in the projections such as scatter, these errors can be accurately estimated by low-pass filtering the difference between the estimated and raw CBCT projections. The error estimates are then subtracted from the raw CBCT projections. Our method is distinct from other published correction methods that use the MDCT image as a prior because it is projection-based and uses limited patient anatomical information from the MDCT image. The merit of CBCT-based treatment monitoring is therefore retained. The proposed method is evaluated using two phantom studies on tabletop systems. On the Catphan 600 phantom, our approach reduces the reconstruction error from 348 Hounsfield unit (HU) without correction to 4 HU around the object center after correction, and from 375 HU to 17 HU in the high-contrast regions. In the selected regions of interest (ROIs), the average image contrast is increased by a factor of 3.3. When noise suppression is implemented, the proposed correction substantially improves the contrast-to-noise ratio (CNR) and therefore the visibility of low-contrast objects, as seen in a more challenging pelvis phantom study. Besides a significant improvement in image uniformity, a low-contrast object of approximately 25 HU, which is otherwise buried in the shading artifacts, can be clearly identified after the proposed correction due to a CNR increase of 3.1. Compared to a kernel-based scatter correction method coupled with an analytical beam hardening correction, our approach also shows an overall improved performance with some residual artifacts. By providing effective shading correction, our approach has the potential to improve the accuracy of more advanced CBCT-based clinical applications for IGRT, such as tumor delineation and dose calculation.

Focus on lasers and imaging

Focus on lasers and imaging