The Gabor wavelet is widely used to simulate the receptive fields of simple cells in the low–level visual cortex (such as V1, V2, and V3) of the human visual system. Based on this, end-to-end encoding models have achieved advanced encoding results in the low–level visual cortex. However, most current end-to-end encoding models are lightweight models with relatively simple structures and few parameters. This limitation may cause the models to perform poorly in processing detailed features of different frequencies and directions in complex Gabor feature maps. In this paper, a novel visual coding model GaborNeXt based on Gabor features is proposed. The model utilizes ConvNeXt convolutional layers to group independent convolutional kernels for convolutional operations and concatenates the outputs of each group to enhance nonlinear expressive power. We conducted experiments on the NSD (Natural Scenes Dataset) and the results demonstrate that our model outperforms the baseline models in encoding accuracy across several the low-level visual cortex. Additionally, we compared the effects of various Gabor convolutional layer kernel sizes on model performance through ablation experiments and found that using larger convolutional kernels in the Gabor convolutional layer has a positive impact on the model's performance.
Ring artifacts are one of the most common artifacts in all types of computed tomography (CT) images and are usually caused by inconsistent response of detector pixels to X-rays. Effective removal of ring artifacts can greatly improve the quality of CT images and improve the accuracy of later diagnosis and analysis, which is a necessary step in CT image reconstruction. Therefore, the ring artifact removal method (also known as "ring artifact correction") was systematically reviewed. Firstly, the formation mechanism and performance of ring artifacts were introduced. Secondly, the ring artifact correction methods of hardware correction, software correction and deep learning correction are introduced in turn, and the principle, development process, advantages and disadvantages of each type of method are analyzed. Finally, the advantages and disadvantages of the existing ring artifact removal methods are summarized, and the solutions are prospected.
Ptychography is a powerful technique that combines scanning transmission X-ray microscopy (STXM) and coherent diffraction imaging (CDI). Thanks to the fluctuation of light and the development of phase retrieval algorithms, ptychography has achieved ultra-high imaging resolution. However, iterative solutions are time consuming. A cascaded residual network is used to recover the amplitude and phase information of the sample directly from the received diffraction patterns. With the powerful coding and decoding capabilities of the residual blocks, the correspondence between diffraction patterns and sample distribution information can be learned by the network. Experimental results show that the use of cascaded residual blocks enhances the reconstructive capability of the network.
KEYWORDS: Image processing, Data processing, Denoising, Relative intensity noise, Data transmission, X-rays, Signal to noise ratio, Signal attenuation, Medical imaging
Photon counting detector (PCD) is a hot topic at present. Compared with traditional energy integral detector, it has the potential of high spatial resolution, high sensitivity and low dose, which can effectively promote medical imaging diagnosis. However, when PCD is counting X-ray photons, the photon number of each energy bin is relatively small. Additionally, charge-sharing response and pulse superposition effect will also affect the photon count rate, resulting in serious noise and affecting the imaging quality. In this paper, a photon-counting denoising algorithm based on subspace decomposition is proposed. According to the similarity between the data of different bins and the self-similarity of the data, this paper constructs sparse representation by subspace decomposition method and uses block matching algorithm to suppress noise. In simulation experiments, we carried out spectral computed tomography imaging experiments with the three-dimensional phantom of a digital mice based on PCD, and denoised the data by different algorithms. The quantitative results show that our method improves peak signal-to-noise ratio by 2.21dB compared with block-matching and 3D filtering when photon flux is 4×103 , which verifies the potential of the proposed algorithm in medical imaging.
With the development of deep neural networks (DNN), building visual decoding models based on functional magnetic resonance imaging (fMRI) to simulate the visual system of the human brain and studying visual mechanisms have become a research hotspot. Although existing visual decoding models built using DNNs have achieved a certain accuracy, most models ignore the differences between different voxels. Among them, the BRNN-based category decoding model uses the bidirectional long short term memory (LSTM) network to simulate the visual bidirectional information flow, which improves the decoding accuracy, but it uses the voxels of each brain area as an overall input model. Therefore, we embed the channel attention module, the Squeeze-and-Excitation Networks (SENet), into the LSTM network to construct an LSTM-SENet vision that introduces an attention mechanism The decoding model allows the model to learn by itself and assign different weights to each voxel, focusing on important voxels, thereby improving the classification accuracy of natural images. The experimental results show that our method improves the accuracy of (three-level) category decoding than other methods, and the results further verify the effectiveness of building a visual decoding model based on the visual mechanism.
Computed Tomography (CT) is one of the essential techniques for non-destructive testing. The acquisition of accurate reconstructed images is the basis for the subsequent analytical processing tasks. This paper proposes a convolutional neural network-based CT reconstruction algorithm to generate reconstructed CT images directly from sinogram by the feature coding and decoding capability. The reconstruction of abdominal scanning data is carried out by this method, and the results show that we can quickly obtain corresponding reconstruction results. During the network training, we designed different data pre-processing methods. We analysed the role of each module in the network by visualizing the output features of each module. Finally, the role of different modules in feature extraction and image generation is further analysed. We found that the conversion from projection to image can be effectively achieved using only convolution operations. It is essential for reconstructing CT images using deep learning techniques.
A markerless projection drift alignment approach for X-ray nanotomography is presented. Drifts in projection from different angles are aligned by applying offsets calculated between successive images after acquisition time division, taking the advantage of the fact that the shorter the time, the less the drift. Involving neither iteration nor parameter selection, it can combine a number of existing image registration techniques and could be adopted for other tomographic imaging techniques. The application of this algorithm has been demonstrated in a laboratory X-ray nanotomography system using single photon detection, in which a standard Siemens star resolution target is initially captured for 2D evaluation and a bamboo stick is used for 3D imaging, leading to sharper image without blur and a much higher resolution.
Due to the limitation of scanning conditions and the emerging clinical application of local imaging, local tomography has become a research hotspot. Traditionally, completing the projection data through interpolation or spatial transformation iteration is popular to overcome the truncation artifact, such as iterative reconstruction algorithms. Recently, deep learning networks have been incorporated in dealing with such problems. Instead of global filtering on truncated data, the proposed work focuses on the full feature extraction using U-net as well as the usage of the redundancy between projection sinogram, by performing extrapolation of truncated projection data through data learning and then filter back projection local reconstruction with high efficiency. During the learning process, 3439 projections are selected as complete projection data , and the corresponding truncated data is simulated according to the actual truncation situation. Then, 150 of the truncated data are randomly selected as the test samples, while the rest 3289 of those as the training samples in the U-net. The output sinogram data is compared with the original complete data by calculating the L2 loss function of both. And the Adam optimizer is used to continuously optimize the parameters of the network. RMSE and NMAD are used to quantitatively evaluate the reconstruction effect. Experimental results show that the proposed method based on truncated data extrapolation network can obviously suppress the ring artifacts and compared with images directly reconstructed using truncated projection data, the RMSE is reduced by an average of 43.185%, and the NMAD is reduced by 44.24%.
Low dose computed tomography (LDCT) has attracted considerable interest in medical imaging fields. Reducing tube current intensity and decrease the exposure time are the two main ways in clinic applications. Nevertheless, the resulting statistical noise will seriously degrade CT image quality for diagnosis. To make full use of the original projection data as well as further improving the small dataset processing ability of U-net, this study aimed to investigate a low dose X-ray CT image denoising method via U-net in projection domain (PDI U-net). Meanwhile, in view of avoiding the excessive smoothing of the small structures, the inception module is introduced in the encoding stage of the network. And different convolution kernel operations of 1×1, 3×3, and 5×5 are used in parallel to obtain multi-scale image features, increasing the depth and width of the network while reducing the parameters. Furthermore, the shortcut connection is utilized to transfer the low-level area local detail information to the high-level area. By merging it with the global information of the high-level area, the proposed network can maintain the image details while de-noising. The experimental results show that the method proposed can significantly improve the image quality with clear feature edges and close visual appearance to the reference high dose CT images. Compared with LDCT and Residual Encoder-Decoder Convolutional Neural Network (RED-CNN), the peak signal to noise radio (PSNR) is improved by 9.02dB and 2.74dB, and the structural similarity (SSIM) is improved by 0.43 and 0.07, respectively.
Computed tomography (CT) has been extensively used in nondestructive testing, medical diagnosis, etc. In the field of modern medicine, metal implants are widely used in people's daily life, and the serious artifacts in CT reconstruction images caused by metal implants cannot be ignored. Sinogram contains the most realistic projection information of patients. Processing in the sinogram domain directly can make the effective information maximum extent preserved. In this paper, we propose a novel method based on full convolutional network (FCN) for metal artifact reduction in the sinogram domain. The networks we introduced use the complete sinogram data to learn a mapping function to correct the metal-corrupted sinogram data. The network takes the metal-corrupted sinogram as the input and takes the artifact-free sinogram as the target. Compared with the existing deep learning-based CT artifact reduction methods, our work just uses the sinogram information to correct the metal artifacts. The proposed network can process images of different sizes. Our initial results on a simulated dataset to demonstrate the potential effectiveness of this new approach to suppressing artifacts.
To investigate the effects of x ray tube setting on image quality in industrial computed tomography, an experimental characterization with constant tube powers has been reported in this paper. A series of CT scans for a QRM Medium-Contrast-Phantom were performed with a constant tube power of 40W and other scanning parameters, varying tube voltages from 80kV to 125kV and tube currents from 320μA to 500μA. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measured on the reconstructed images indicated that increasing the tube voltage can improve the SNR, as well as the CNR in high density areas. While in low density regions in the phantom, higher CNR resulted from lower voltage or higher tube current. Furthermore, a custom-made aluminum cylinder is scanned several times for the assessment of the CT spatial resolution, similarly keeping a constant tube power and variable tube voltages and currents. According to the obtained modulation transfer function (MTF)1/10 values, defined as the spatial frequency corresponding to a contrast loss of 10 %, it is found that using the same tube power, the tube voltage has a greater impact on improving the CT spatial resolution.
Limited-angle computed tomography (CT) image reconstruction is a challenging reconstruction problem in the fields of CT. With the development of deep learning, the generative adversarial network (GAN) perform well in image restoration by approximating the distribution of training sample data. In this paper, we proposed an effective GAN-based inpainting method to restore the missing sinogram data for limited-angle scanning. To estimate the missing data, we design the generator and discriminator of the patch-GAN and train the network to learn the data distribution of the sinogram. We obtain the reconstructed image from the restored sinogram by filtered back projection and simultaneous algebraic reconstruction technique with total variation. Experimental results show that serious artifacts caused by missing projection data can be reduced by the proposed method, and it is hopeful to solve the reconstruction problem of 60° limited scanning angle.
The improvement of computed tomography (CT) image resolution is beneficial to the subsequent medical diagnosis, but it is usually limited by the scanning devices and great expense. Convolutional neural network (CNN)- based methods have achieved promising ability in super-resolution. However, existing methods mainly focus on the super-resolution of reconstructed image and do not fully explored the approach of super-resolution from projectiondomain. In this paper, we studied the characteristic of projection and proposed a CNN-based super-resolution method to establish the mapping relationship of low- and high-resolution projection. The network label is high-resolution projection and the input is its corresponding interpolation data after down sampling. FDK algorithm is utilized for three-dimensional image reconstruction and one slice of reconstruction image is taken as an example to evaluate the performance of the proposed method. Qualitative and quantitative results show that the proposed method is potential to improve the resolution of projection and enables the reconstructed image with higher quality.
Metal objects inside the field of view would introduce severe artifacts in x-ray CT images, which would severely degrade the quality of CT data and bring huge difficulties for subsequent image processing and analysis. Correction of metal artifacts has become a hot and difficult issue in X-ray CT. In recent years, deep learning has rapidly gained attention for employment on image processing. In this study, we introduce a Fully Convolutional Networks (FCNs) into the MAR in image domain. The network reduces metal artifacts by learning an end-to-end mapping of images from metal-corrupted CT images to their corresponding artifact-free ground truth. The network takes the metal-corrupted CT images as the input and takes the artifact-free images as the target. The convolution layers extract features from the input images and map them to the target images, and the deconvolution layers use these features to build the predicted outputs. Experimental results demonstrate that the proposed method can well reduce metal artifacts of CT images, and take a shorter time to process the images than traditional method.
In X-ray computed tomography (CT), variability in tube voltage and current setting may affect the image quality. Based on an industrial X-ray micro-CT scanner, this paper will investigate the impact of the X-ray tube setting on image quality of the projection images as well as the reconstruction results with various voltage and current choices in the CT experiments. Fresh corn is initially selected as an experimental sample in 6 different series of measurements. We set the tube current at 130μA, 200μA, 270μA while keeping the tube voltage and other acquisition parameters constant, and then keep the tube current constant while varying the tube voltage at 70kV and 100kV, respectively. For evaluation both the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) are calculated as image quality criteria for each set of the projected images and reconstructed images. The results indicate that increasing the tube current and voltage can both improve the SNR and CNR. Furthermore, the tube voltage has more impact on the improvements. At the same time, the variations on image quality of reconstruction images keeps the same pace with that of the projection images. The reliability of the conclusion will be further explored experimentally using aircraft blades in CT nondestructive testing.
Histopathological grading of breast cancer is an important tumor-related prognostic factor and plays an important role in breast cancer prognosis analysis. Nowadays, histopathological grading of breast cancer is mainly identified by pathological images and radiologists cannot differentiate the histopathological grade directly from digital mammograms. In this paper, we propose to discriminate the histopathological grades directly based on digital mammograms, which is noninvasive and convenient. End-to-end training Convolutional Neural Network (CNN) is firstly designed to extract semantic features directly from raw image data. Considering the scarce annotated mammograms data and large size of tumor region, a light and deep network with less training parameters is modified to prevent overfitting. Results demonstrate that our proposed network is superior to other CNN models and traditional classifier based on hand-crafted features.
X-ray cone-beam computed tomography, featuring high precision and fast-imaging speed, has been widely used in industrial non-destructive testing applications for the three dimensional visualization of internal structures. Due to mechanical imperfections, geometric calibrations are imperative to high quality image reconstruction. Currently, the twoball phantom-based calibration procedures exploiting the projection trajectories of the phantoms are the most commonly used approach for the estimation of the geometrical parameters and the calibration of CT system. However, an additional scan needs to be performed, even after each object acquisition when lack of system reproducibility, leading to multiplied calibration times. The emphasis of this paper is to optimize the process of acquisition in cone-beam CT imaging with minimal time, based on the understanding of the determination of the ball position in typical phantom-based geometric calibration algorithms. An applicable condition of the calibration algorithm for simultaneously scanning objects and calibration phantoms is proposed and demonstrated, which is that the minimum projection value of the scanned object needs to be at least 100 counts higher than those of the calibration phantom, with consideration of the system noise. The CT experiments are based on a laboratory industrial cone-beam CT system with a micro-focus x-ray tube (Thales Hawkeye 130) and a flat panel detector (Thales Pixium RF4343). Objects imaged are chosen with a wide projection value range, from low-Z watermelon seeds and high-Z materials, including a standard Micro CT Bar Pattern Phantom (QRM) for image quality assessment. In these experiments, objects, as well as two-ball phantoms, both placed in the field of view without overlapping in the vertical direction, are projected over 360 degrees, instead of scanning the calibration phantoms separately. Hence, the true geometrical relationship is resolved utilizing the two-ball algorithm. Both simulation and experimental results confirm that the calculated geometrical parameters will not be affected by the objects as long as their projection value difference meeting the requirements above. And the reconstruction image quality is almost the same with those by an independent calibration. Compared to the traditional application of the phantombased geometrical calibration method, the novel approach presented in this paper has obvious advantages from an imaging perspective, saving acquisition time and eliminating the undesired influence from the operation staff for the same cost.
Object detection is a computer vision problem which caught a large amount of attention. But the candidate boundingboxes extracted from only image features may end up with false-detection due to the semantic gap between the top-down and the bottom up information. In this paper, we propose a novel method for generating object bounding-boxes proposals using the combination of eye fixation point, saliency detection and edges. The new method obtains a fixation orientated Gaussian map, optimizes the map through single-layer cellular automata, and derives bounding-boxes from the optimized map on three levels. Then we score the boxes by combining all the information above, and choose the box with the highest score to be the final box. We perform an evaluation of our method by comparing with previous state-ofthe art approaches on the challenging POET datasets, the images of which are chosen from PASCAL VOC 2012. Our method outperforms them on small scale objects while comparable to them in general.
The circle-plus-line trajectory satisfies the exact reconstruction data sufficiency condition, which can be applied in C-arm X-ray Computed Tomography (CT) system to increase reconstruction image quality in a large cone angle. The m-line reconstruction algorithm is adopted for this trajectory. The selection of the direction of m-lines is quite flexible and the m-line algorithm needs less data for accurate reconstruction compared with FDK-type algorithms. However, the computation complexity of the algorithm is very large to obtain efficient serial processing calculations. The reconstruction speed has become an important issue which limits its practical applications. Therefore, the acceleration of the algorithm has great meanings. Compared with other hardware accelerations, the graphics processing unit (GPU) has become the mainstream in the CT image reconstruction. GPU acceleration has achieved a better acceleration effect in FDK-type algorithms. But the implementation of the m-line algorithm’s acceleration for the circle-plus-line trajectory is different from the FDK algorithm. The parallelism of the circular-plus-line algorithm needs to be analyzed to design the appropriate acceleration strategy. The implementation can be divided into the following steps. First, selecting m-lines to cover the entire object to be rebuilt; second, calculating differentiated back projection of the point on the m-lines; third, performing Hilbert filtering along the m-line direction; finally, the m-line reconstruction results need to be three-dimensional-resembled and then obtain the Cartesian coordinate reconstruction results. In this paper, we design the reasonable GPU acceleration strategies for each step to improve the reconstruction speed as much as possible. The main contribution is to design an appropriate acceleration strategy for the circle-plus-line trajectory m-line reconstruction algorithm. Sheep-Logan phantom is used to simulate the experiment on a single K20 GPU. The development environment trajectory, using CPU and the paper’s GPU acceleration strategy, respectively. The experimental results show considerable reconstruction image quality, and the reconstruction acceleration ratio can reach 620 times.
X-ray computed tomography (CT) has been extensively applied in industrial non-destructive testing (NDT). However, in practical applications, the X-ray beam polychromaticity often results in beam hardening problems for image reconstruction. The beam hardening artifacts, which manifested as cupping, streaks and flares, not only debase the image quality, but also disturb the subsequent analyses. Unfortunately, conventional CT scanning requires that the scanned object is completely covered by the field of view (FOV), the state-of-art beam hardening correction methods only consider the ideal scanning configuration, and often suffer problems for interior tomography due to the projection truncation. Aiming at this problem, this paper proposed a beam hardening correction method based on radon inversion transform for interior tomography. Experimental results show that, compared to the conventional correction algorithms, the proposed approach has achieved excellent performance in both beam hardening artifacts reduction and truncation artifacts suppression. Therefore, the presented method has vitally theoretic and practicable meaning in artifacts correction of industrial CT.
With the development of technology, the traditional X-ray CT can’t meet the modern medical and industry needs for component distinguish and identification. This is due to the inconsistency of X-ray imaging system and reconstruction algorithm. In the current CT systems, X-ray spectrum produced by X-ray source is continuous in energy range determined by tube voltage and energy filter, and the attenuation coefficient of object is varied with the X-ray energy. So the distribution of X-ray energy spectrum plays an important role for beam-hardening correction, dual energy CT image reconstruction or dose calculation. However, due to high ill-condition and ill-posed feature of system equations of transmission measurement data, statistical fluctuations of X ray quantum and noise pollution, it is very hard to get stable and accurate spectrum estimation using existing methods. In this paper, a model-based X-ray energy spectrum estimation method from CT scanning data with energy spectrum filter is proposed. First, transmission measurement data were accurately acquired by CT scan and measurement using phantoms with different energy spectrum filter. Second, a physical meaningful X-ray tube spectrum model was established with weighted gaussian functions and priori information such as continuity of bremsstrahlung and specificity of characteristic emission and estimation information of average attenuation coefficient. The parameter in model was optimized to get the best estimation result for filtered spectrum. Finally, the original energy spectrum was reconstructed from filtered spectrum estimation with filter priori information. Experimental results demonstrate that the stability and accuracy of X ray energy spectrum estimation using the proposed method are improved significantly.
The sparse scanning imaging methods for x-ray CT is a promising approach to speed up scanning or reduce radiation dose to patients. The major problem for sparse parallel projections is hard to reconstruct high quality image. It suffers severe streak artifacts in reconstruction if the popular filtered back projection (FBP) method is employed. Although several total variation (TV) regularization based algorithms have been developed for sparse-view CT imaging, they still face challenges in both time consumption and computational complexity when the objective image is large. In this paper, a CT reconstruction algorithm, which is named INNG-TV (iterative next-neighbor regridding-total variation), based on extrapolation in frequency is proposed to improve the performance. We first convert data, which is sampled from parallel beam CT, into frequency domain by Fourier transform and linear interpolation. In the following process of iteration, the known data of projection in Fourier space keep constant, whereas the unknown data are estimated by INNG extrapolation. At the same time, prior knowledge and constrained optimization, such as non-negativity constraint and total variation regularization, are introduced to image reconstruction in image space. The numerical simulation results show that the proposed method has better performance in reconstruction quality than ART-TV (algebraic reconstruction technique-total variation). The proposed method not only demonstrates its superiority in time consumption, but also offers outstanding reconstruction quality for sparse-view scan, which makes it significant to sparse-view CT imaging.
KEYWORDS: X-ray computed tomography, X-rays, Optical filters, Signal attenuation, Photons, Metals, Monte Carlo methods, Aluminum, Mass attenuation coefficient, Copper
Beam hardening artifact is common in X-ray computed tomography (X-CT). Using the metal sheet as a filter to preferentially attenuate low-energy photons is a simple and effective way for beam hardening artifact correction. However, generally it requires a large quantity of experiments to compare the filter material and thickness, which is lack of guidance of theory. In this paper, a novel filter design method for beam hardening correction, especially for middle energy X-CT, is presented. First, the spectrum of X-ray source under a certain tube voltage is estimated by Monte Carlo (MC) simulation or other simulation methods. Next, according to the X-ray mass attenuation coefficients of the object material, the energy range to be retained can be roughly determined in which the attenuation coefficients change slowly. Then, the spectra filtering performance with different filter materials and thicknesses can be calculated using the X-ray mass attenuation coefficients of each filter material and the simulated primitive spectrum. After that, the mean energy ratio (MER) of post-filter mean energy to pre-filter mean energy is obtained. Finally, based on the spectrum filtering performance and MER of the metal material, a suitable filter strategy is easily selected. Experimental results show that, the proposed method is simple and effective on beam hardening correction as well as increasing the image quality.
KEYWORDS: X-ray computed tomography, X-rays, Optical filters, Signal attenuation, Photons, Metals, Monte Carlo methods, Aluminum, Mass attenuation coefficient, Copper
Beam hardening artifact is common in X-ray computed tomography (X-CT). Using the metal sheet as a filter to preferentially attenuate low-energy photons is a simple and effective way for beam hardening artifact correction. However, generally it requires a large quantity of experiments to compare the filter material and thickness, which is lack of guidance of theory. In this paper, a novel filter design method for beam hardening correction, especially for middle energy X-CT, is presented. First, the spectrum of X-ray source under a certain tube voltage is estimated by Monte Carlo (MC) simulation or other simulation methods. Next, according to the X-ray mass attenuation coefficients of the object material, the energy range to be retained can be roughly determined in which the attenuation coefficients change slowly. Then, the spectra filtering performance with different filter materials and thicknesses can be calculated using the X-ray mass attenuation coefficients of each filter material and the simulated primitive spectrum. After that, the mean energy ratio (MER) of post-filter mean energy to pre-filter mean energy is obtained. Finally, based on the spectrum filtering performance and MER of the metal material, a suitable filter strategy is easily selected. Experimental results show that, the proposed method is simple and effective on beam hardening correction as well as increasing the image quality.
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