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PURPOSE In x-ray radiography, the commonly used antiscatter grid for enhancing image quality causes artifacts in the form of periodic noises, such as shadows, cutoff, and Moire fringes. Software degridding is traditionally performed via linear or homomorphic filtering in the spectral domain. These methods inevitably result in image blurring, information loss, and distortion, thus hindering detection and assessment of diseases. We seek effective and practical solutions for grid artifact correction based on spatial-domain analysis toward high-quality imaging. METHODS By analyzing the physical process of grid artifact formation, we track down the root of the problem associated with spectral filtering. We propose the grid regression demodulation (GRD). The degridding cost is forged as a functional of the latent x-ray photon image and parametric grid model characterizing grid transmission property. Regularization on the grid spectra is incorporated. We devise optimization algorithms for artifact correction and grid pattern estimation. GRD decouples the partially overlapped spectra of the grid and anatomy, and removes the artifacts independently, thus restoring the underlying clinically relevant data. RESULTS Method efficacy is demonstrated using simulated and real data. GRD effectively preserves image edges, textures, and patterns while removing grid artifacts. For the known ground truth setting, GRD gives a near-perfect correction. For real data, GRD is capable of correcting not only the primary grid artifacts, but also the higher grid harmonic artifacts while keeping image content unaltered, which is unachievable by the other methods. Our method has low residual errors and exhibits a successful demodulation effect without introducing additional artifacts, while ringing or cilia artifacts are present in the others. CONCLUSIONS The proposed method outperforms the prevalent transform techniques for correcting grid artifacts in digital radiography. It is self-sustained and self-adaptive to a range of targets and beam quality. Our approach is advantageous in restoring the latent image while suppressing grid noises. It retrieves the true scale factor of the degridded data, which is unattainable via any spectral filtering techniques. This work unlocks a promising venue to improve and upgrade low-dose medical radiographic imaging technology.

Anti-scatter grids are routinely used to prevent degradation of image quality caused by scattered X-ray beams. However, these grids might cause linear artifacts that represent the shadows of the radiopaque septa. In this paper, we propose a machine learning-based method for grid artifact suppression in radiography. There are two major difficulties in the application of a deep learning technique for the grid-line suppression problem. The first is quantitative shortage of learning data. It is difficult to acquire a sufficient amount of learning data from observed images in consideration of the various situations in which grid lines appear. The second is difficulty determining the target data. It is practically impossible to generate a clean target image for an arbitrary grid-line image. To overcome these problems, we propose a deep convolutional neural network architecture and a learning data construction method. A discrete cosine transform-based band-stop filtering technique and an image synthesizing algorithm were adopted for the learning data construction method. A patch sampling method was employed to overcome the shortage of the amount of learning data. The proposed method enables learning without clean target data and overcomes the weakness of conventional frequency analysis-based methods with regard to the grid-line suppression problem. This method makes it possible to expect complementary performance through the construction of a combined learning data set including observed images and artificially generated grid-line images.

PURPOSE Grid artifacts occur in digital mammography when synchronization between the grid assembly and generator is not achieved, including when malfunctions occur in the grid assembly or generator subsystems. Such artifacts are not explicitly monitored or evaluated by existing mammography quality control programs. In this study, we developed an automated method for quantifying the presence of grid artifacts in two-dimensional (2D) digital mammography images and assessed its utility as a supplement to existing quality control programs. METHODS Four digital mammography systems (Hologic Dimensions 3D 5000) were configured to automatically transfer 2D images to a server where the strength of the grid pattern, γmax , was quantified using a template-matching algorithm and stored in amySQL database. This analysis was performed on both American College of Radiology (ACR) phantom and clinical images. Changes in γmax were compared with image quality and service records to establish preliminary action limits for physicist intervention for each type of image. These action limits were applied around selected service events to evaluate their clinical utility. RESULTS All systems exhibited a gradual increase in γmax in ACR phantom images prior to having identical major components of the generator subsystem replaced, despite the absence of visible gridlines in the images. Retrospective analysis of phantom images suggested that physicists should consider AEC testing when γmax exceeds 0.050 and that clinical image quality may be affected when γmax exceeds 0.060. Eighteen of 19 visible grid artifacts were identified using a threshold γmax value of 0.065 in clinical images. Warning limits that indicate abnormal operation before visible degradation in image quality were also established. These warning limits were 0.046 and 0.041 for the 24 × 29 cm and 18 × 24 cm paddles, respectively. Specific malfunctions in the generator and grid subsystems can be detected by applying these limits. CONCLUSIONS Automated monitoring of γmax provides useful information about the status of digital mammography units without affecting clinical operations. When used with appropriate action limits, this type of monitoring can help physicists identify specific equipment malfunctions before they would be detected by other quality control tests and before they affect clinical images.

The mammography is the first choice of breast cancer screening, which has proven to be the most effective screening method. An antiscatter grid is usually employed to enhance the contrast of image by absorbing unexpected scattered signals. However, the grid pattern casts shadows and grid artifacts, which severely degrade the image quality. To solve the problem, we propose the patch based frequency signal filtering for fast grid artifacts suppressing. As opposed to whole image processing synchronously, the proposed method divides image into a number of blocks for tuning filter simultaneously, which reduces the frequency interference among image blocks and saves computation time by multithread processing. Moreover, for mitigating grid artifacts more precisely, characteristic peak detection is employed in each block automatically, which can accurately identify the location of the antiscatter grid and its motion pattern. Qualitative and quantitative studies were performed on simulation and real machine data to validate the proposed method. The results show great potential for fast suppressing grid artifacts and generating high quality of digital mammography.

In digital radiography imaging, the antiscatter grid is usually employed to obtain clear projected images by absorbing scattered x-ray beams. However, due to the shadow of the stationary antiscatter grid, the grid artifact appears in the obtained x-ray image as stripes if a linear grid is considered. In order to alleviate the grid artifact, band-stop filters (BSFs) can be used and may cause annoying ringing artifact especially for metallic materials. In this paper, in order to reduce the ringing artifact while applying BSFs to alleviate the grid artifact, a spatial prefiltering technique, which is called the gradient-reduction filter, is proposed. The gradient-reduction filter can remove steep edges in the obtained x-ray images especially for metallic materials and can prevent the grid- artifact BSFs from yielding serious ringing artifact. The structure of the proposed filter is similar to that of the Crawford noise-reduction filter. However, the nonlinear function in the proposed filter has decreasing slopes as its input increases contrary to the Crawford filter case. Through extensive experiments for real digital x-ray images, which are obtained from a direct digital radiography detector, we can see the superior ringing-artifact reduction performance comparing to the conventional filtering approaches especially when the metallic materials present in or on the patients.

In digital radiography imaging, the x-ray images are formed based on a multiplicative model, in which the projected image of an object is multiplied by the antiscatter grid shadow. Hence, the formed image is amplitude- modulated by the grid shadow and the resultant modulated terms appear as the grid artifacts. Since the bandwidths of the modulated terms are as wide as that of the projected image, we should employ relatively wide-bandwidth band-stop filters (BSFs) to reduce the grid artifacts. When we apply such BSFs, the object to be recovered is prone to distortion due to the wide filter bandwidth. In this paper, to reduce the signal bandwidth of the grid shadow images in reduction of the grid artifacts, applying BSFs based on the homomorphic operation is proposed by employing the logarithmic function. By taking the logarithm of the formed image, we can separate the multiplicative grid component from both projected image and the exposure of x-rays. Hence, by employing a relatively narrow and fixed-bandwidth BSFs, we can efficiently alleviate the grid artifacts independently of the strength of the grid artifacts comparing to the conventional linear approaches. For real x-ray images, the superior performance of the proposed approach is compared in this paper.

Purpose: Demonstrate the effectiveness of an anti-scatter grid artifact minimization method by removing the grid-line artifacts for three different grids when used with a high resolution CMOS detector. Method: Three different stationary x-ray grids were used with a high resolution CMOS x-ray detector (Dexela 1207, 75 µm pixels, sensitivity area 11.5cm × 6.5cm) to image a simulated artery block phantom (Nuclear Associates, Stenosis/Aneurysm Artery Block 76–705) combined with a frontal head phantom used as the scattering source. The x-ray parameters were 98kVp, 200mA, and 16ms for all grids. With all the three grids, two images were acquired: the first for a scatter-less flat field including the grid and the second of the object with the grid which may still have some scatter transmission. Because scatter has a low spatial frequency distribution, it was represented by an estimated constant value as an initial approximation and subtracted from the image of the object with grid before dividing by an average frame of the grid flat-field with no scatter. The constant value was iteratively changed to minimize residual grid-line artifact. This artifact minimization process was used for all the three grids. Results: Anti-scatter grid lines artifacts were successfully eliminated in all the three final images taken with the three different grids. The image contrast and CNR were also compared before and after the correction, and also compared with those from the image of the object when no grid was used. The corrected images showed an increase in CNR of approximately 28%, 33% and 25% for the three grids, as compared to the images when no grid at all was used. Conclusion: Anti-scatter grid-artifact minimization works effectively irrespective of the specifications of the grid when it is used with a high spatial resolution detector. Partial support from NIH Grant R01-EB002873 and Toshiba Medical Systems Corp.

In order to reduce the grid artifacts, which are caused by using the antiscatter grid in obtaining x-ray digital images, we analyze the effect of applying band-rejection filters (BRFs) in the frequency domain. The property of the grid artifacts is quite different depending on the shape of the object to be recovered in the spatial domain. Furthermore, if the frequency component of the grid artifact is relatively close to that of the object, then simply applying a BRF may seriously distort the object and cause the ringing artifact. In this paper, we propose a hybrid filtering scheme, which can cooperate such different properties in the spatial domain. In order to perform a robust detection of the position of the ringing artifact, we propose the frequency-modulation (FM) model for the extracted signal, which corresponds to a specific grid artifact. An estimation of the position image for the ringing artifact is then conducted based on the slope detection algorithm, which is commonly used as an FM discriminator in the communication area. Numerical result for real x-ray images shows that applying BRFs in the frequency domain in conjunction with the spatial property of the ringing artifact can successfully remove the grid artifact, distorting the object less.

By applying band-rejection filters (BRFs) in the frequency domain, we can efficiently reduce the grid artifacts, which are caused by using the antiscatter grid in obtaining x-ray digital images. However, if the frequency component of the grid artifact is relatively close to that of the object, then simply applying a BRF may seriously distort the object and cause the ringing artifacts. Since the ringing artifacts are quite dependent on the shape of the object to be recovered in the spatial domain, the spatial property of the x-ray image should be considered in applying BRFs. In this paper, we propose an adaptive filtering scheme, which can cooperate such different properties in the spatial domain. In the spatial domain, we compare several approaches, such as the mangnitude, edge, and frequency-modulation (FM) model-based algorithms, to detect the ringing artifact or the grid artifact component. In order to perform a robust detection whether the ringing artifact is strong or not, we employ the FM model for the extracted signal, which corresponds to a specific grid artifact. A detection of the position for the ringing artifact is then conducted based on the slope detection algorithm, which is commonly used as an FM discriminator in the communication area. However, the detected position of the ringing artifact is not accurate. Hence, in order to obtain an accurate detection result, we combine the edge-based approach with the FM model approach. Numerical result for real x-ray images shows that applying BRFs in the frequency domain in conjunction with the spatial property of the ringing artifact can successfully remove the grid artifact, distorting the object less.

In order to reduce the grid artifacts, which are caused by using the antiscatter grid in obtaining x-ray digital images, we analyze the grid artifacts based on the multiplicative image formation model instead of the traditional additive model, and apply filters to suppress the artifact terms. The artifact terms are aliases of the modulated terms from the harmonics of the grid frequency. Hence, several filters are required to efficiently suppress the grid artifacts. However, applying filters also distort the original image that will be recovered. If the distance between the origin and the center frequency of the artifact term is relatively large, then we can suppress the artifact term less distorting the original image. In this paper, by increasing the distances for a given sampling frequency of the image detector, we design antiscatter grids, which are good in terms of efficient grid artifact reduction. In order to design optimal grids, we formulate min-max optimization problems and provide optimal grid frequencies for a fixed grid angle with respect to the sampling direction of the image detector and optimal grid angles for a fixed grid frequency. We then propose using rotated grids with the optimal grid angles in digital radiography imaging. By applying band-rejection filters to the artifact, we can considerably reduce the grid artifacts when comparing to the traditional non-rotated grid case for real x-ray images.

In order to reduce the grid artifacts in the digital radiography with stationary antiscatter grids, artifact frequency detection schemes should be adopted and appropriate band-rejection filters should be applied. However, by rotating the grid angle for certain grid densities and sampling frequencies, the artifact frequencies can be placed near the boundary of the frequency domain, and then the grid artifacts can be easily alleviated by applying a simple low-pass filter without any estimates of the artifact frequencies. In this paper, we propose a grid artifact reduction algorithm, which is based on rotating the grid and applying a low-pass filter. For the sampling frequency 7.194 lines/mm, an optimal rotate angle is arctan(1/2) ≈ 26.57° and the grid density is 102.2 lines/inch. Better performances are obtained for real x-ray images.

Purpose: To provide a template for the comprehensive clinical evaluation of new imaging technologies with initial application to mobile digital radiography (DR). Methods: Four mobile DR devices (GE Optima XR220amx+Flashpad; Carestream DRX Revolution+DRX-1C; Philips Mobile Diagnost wDR; Philips Mobile Diagnost wDR+Skyplate) were evaluated under four categories: 1)Technical specifications: Vendor data were collected and complied into a unified nomenclature. 2)Physical performance: Each unit underwent imaging physics evaluation including MTF, NPS, DQE, and grid artifact analysis. 3)Clinical performance: Fifteen bedside chest radiographs were acquired using each unit. Metadata were stripped and images cropped to 14:17 aspect ratio to ensure vendor anonymity. Six cardiothoracic radiologists scored the randomized images on PACS workstations using five criteria: overall quality, mediastinum noise, rib-lung contrast, lung density, and lung detail. Results were processed using multiple linear regression analysis. 4)Operability performance: A survey was designed and administered to technologists asking questions which captured use, preference, and experiential data for each unit. To avoid impropriety, units were randomly assigned numbers 1–4. Results: Vendor specifications were compiled into a single-page table enabling ready comparative review. All systems had MTF within 20% and NNPS within 27% of the average response at frequencies below 2.5 mm−1. Units [1,2,4] had DQE within 19% of average while unit 3 was 49% below average at high-frequencies. According to grid artifact analysis, the best and worst results were from units [2,3,4,1], respectively. The radiologist study revealed high inter-radiologist variability which limited the number of significant overall results. Survey results uncovered clear technologist biases. In general, technologists value practical over optional features. Conclusion: The compilation of vendor, physicist, radiologist, and technologist data provides an easy means for healthcare professionals to compare different medical equipment options using a data-driven approach. This type of comprehensive assessment now serves as a model for new technology review at our institution. This work was supported by Duke University Health Systems.

Fourier ptychography (FP) is a recently developed imaging approach that facilitates high-resolution imaging beyond the cutoff frequency of the employed optics. In the original FP approach, a periodic LED array is used for sample illumination, and therefore, the scanning pattern is a uniform grid in the Fourier space. Such a uniform sampling scheme leads to 3 major problems for FP, namely: 1) it requires a large number of raw images, 2) it introduces the raster grid artefacts in the reconstruction process, and 3) it requires a high-dynamic-range detector. Here, we investigate scanning sequences and sampling patterns to optimize the FP approach. For most biological samples, signal energy is concentrated at low-frequency region, and as such, we can perform non-uniform Fourier sampling in FP by considering the signal structure. In contrast, conventional ptychography perform uniform sampling over the entire real space. To implement the non-uniform Fourier sampling scheme in FP, we have designed and built an illuminator using LEDs mounted on a 3D-printed plastic case. The advantages of this illuminator are threefold in that: 1) it reduces the number of image acquisitions by at least 50% (68 raw images versus 137 in the original FP setup), 2) it departs from the translational symmetry of sampling to solve the raster grid artifact problem, and 3) it reduces the dynamic range of the captured images 6 fold. The results reported in this paper significantly shortened acquisition time and improved quality of FP reconstructions. It may provide new insights for developing Fourier ptychographic imaging platforms and find important applications in digital pathology.

Antiscatter grids absorb scattered radiation and increase X- ray image contrast. Stationary grids leave line artifacts or create Moire patterns on resized digital images. Various grid designs were investigated to determine relevant physical properties that affect an image. A detection algorithm is based on grid peak determination in the image's averaged 1D Fourier spectrum. Grid artifact removal is based on frequency filtering in a spatial dimension orthogonal to the grid stripes. Different filter design algorithms were investigated to choose the transfer function that maximizes the suppression of grid artifacts with minimal image distortion. Algorithms were tested on synthetic data containing a variety of SNRs and grid spatial inclinations, on radiographic data containing phantoms with and without grids, and on a set of real CR images. Detector and filter performance were optimized by using Intel Signal Processing Library, resulting in a time of about 3 sec to process a 2Kx2.5K CR image on a Pentium II PC> There are no grid artifacts and no image blur revealed on processed images as evaluated by third party technical and medical experts. This automated grid artifact suppression method is built into a new version of Kodak PACS Link Medical Image Manager.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper we describe our design and the manufacturing of a 2D grid artifact of chrome on quartz on a 150 mm X 150 mm X 6.35 mm plate. The design has been agreed upon by a number of Semiconductor Equipment Manufacturers International participants working on a 2D grids calibration task force within the microlithography committee of SEMI. We present the measurement procedures and describe the algorithms used in the measurement process. We have procured a prototype artifact which is expected to be developed into a National Institute of Standards and Technology (NIST) distributed standard reference material once the final design has been agree upon. We will present measurements made at leading industrial sites and develop a traceability chain based on these measurements in combination with NIST based measurements. The artifact has been measured on the NIST linescale interferometer as well as other NIST metrology tools. We will also present the status of the comparisons between these measurements and those performed by the industrial collaborators.

Purpose: Grid artifacts are caused when using the antiscatter grid in obtaining digital x-ray images. In this paper, research on grid artifact reduction techniques is conducted especially for the direct detectors, which are based on amorphous selenium. Methods: In order to analyze and reduce the grid artifacts, the authors consider a multiplicative grid image model and propose a homomorphic filtering technique. For minimal damage due to filters, which are used to suppress the grid artifacts, rotated grids with respect to the sampling direction are employed, and min-max optimization problems for searching optimal grid frequencies and angles for given sampling frequencies are established. The authors then propose algorithms for the grid artifact reduction based on the band-stop filters as well as low-pass filters. Results: The proposed algorithms are experimentally tested for digital x-ray images, which are obtained from direct detectors with the rotated grids, and are compared with other algorithms. It is shown that the proposed algorithms can successfully reduce the grid artifacts for direct detectors. Conclusions: By employing the homomorphic filtering technique, the authors can considerably suppress the strong grid artifacts with relatively narrow-bandwidth filters compared to the normal filtering case. Using rotated grids also significantly reduces the ringing artifact. Furthermore, for specific grid frequencies and angles, the authors can use simple homomorphic low-pass filters in the spatial domain, and thus alleviate the grid artifacts with very low implementation complexity.

Purpose: We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm × 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system. Methods and Materials: In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earth's magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm × 20 cm high resolution fiber-optic prototype consisting of 250 × 250 fibers, each with an input dimension of 0.8 mm × 0.8 mm was constructed. Its performance was tested using several phantoms studies. Results: Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects. Conclusions: The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.

Linear focused grids are commonly used in general radiography and mammography to control scatter. In these applications, if lines would be visible when the grid was stationary, then the grid is moved during the x-ray exposure to blur out grid lines. Presented is a theoretical framework for estimating grid line artifact magnitude and evaluating artifact suppression techniques. The framework takes as parameters the grid pitch, septum thickness, and exposure time, and allows for a variation in grid velocity and in x-ray tube output during the exposure. Grid line artifacts are evaluated for a variety of conditions. These include a stationary grid, a grid moving at a constant velocity with no kV ripple, a grid moving at a constant velocity with large kV ripple, and a grid moving with decreasing velocity and no kV ripple. Also evaluated are grid line artifacts for a novel suppression technique in which the grid moves at a constant velocity and the x-ray exposure waveform is "feathered," i.e., when the x-ray exposure waveform has a soft start and stop. Of practical interest is that it is possible to effectively eliminate grid line artifacts when the grid moves only a short distance with an appropriately "feathered" exposure waveform. This capability permits one to design efficient and compact coarse strip density grid systems.