Monte-Carlo-Based Estimation of the X-ray Energy Spectrum for CT Artifact Reduction

Beam hardening and scattering effects can seriously degrade image quality in polychromatic X-ray CT imaging. In recent years, polychromatic image reconstruction techniques and scatter estimation using Monte Carlo simulation have been developed to compensate for beam hardening and scattering CT artifacts, respectively. Both techniques require knowledge of the X-ray tube energy spectrum. In this work, Monte Carlo simulations were used to calculate the X-ray energy spectrum of FleXCT, a novel prototype industrial micro-CT scanner, enabling beam hardening and scatter reduction for CT experiments. Both source and detector were completely modeled by Monte Carlo simulation. In order to validate the energy spectra obtained via Monte Carlo simulation, they were compared with energy spectra obtained via a second method. Here, energy spectra were calculated from empirical measurements using a step wedge sample, in combination with the Maximum Likelihood Expectation Maximization (MLEM) method. Good correlation was achieved between both approaches, confirming the correct modeling of the FleXCT system by Monte Carlo simulation. After validation of the modeled FleXCT system through comparing the X-ray spectra for different tube voltages inside the detector, we calculated the X-ray spectrum of the FleXCT X-ray tube, independent of the flat panel detector response, which is a prerequisite for beam hardening and scattering CT artifacts.

High-throughput microCT scanning of small specimens: preparation, packing, parameters and post-processing

High-resolution X-ray microcomputed tomography, or microCT (μCT), enables the digital imaging of whole objects in three dimensions. The power of μCT to visualize internal features without disarticulation makes it particularly valuable for the study of museum collections, which house millions of physical specimens documenting the spatio-temporal patterns of life. Despite the potential for comparative analyses, most μCT studies include limited numbers of museum specimens, due to the challenges of digitizing numerous individuals within a project scope. Here we describe a method for high-throughput μCT scanning of hundreds of small (< 2 cm) specimens in a single container, followed by individual labelling and archival storage. We also explore the effects of various packing materials and multiple specimens per capsule to minimize sample movement that can degrade image quality, and hence μCT investment. We demonstrate this protocol on vertebrate fossils from Queensland Museum, Australia, as part of an effort to track community responses to climate change over evolutionary time. This system can be easily modified for other types of wet and dry material amenable to X-ray attenuation, including geological, botanical and zoological samples, providing greater access to large-scale phenotypic data and adding value to global collections.

Distortion matrix approach for ultrasound imaging of random scattering media

Focusing waves inside inhomogeneous media is a fundamental problem for imaging. Spatial variations of wave velocity can strongly distort propagating wave fronts and degrade image quality. Adaptive focusing can compensate for such aberration but is only effective over a restricted field of view. Here, we introduce a full-field approach to wave imaging based on the concept of the distortion matrix. This operator essentially connects any focal point inside the medium with the distortion that a wave front, emitted from that point, experiences due to heterogeneities. A time-reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each sensor and image voxel. Phase aberrations can then be unscrambled for any point, providing a full-field image of the medium with diffraction-limited resolution. Importantly, this process is particularly efficient in random scattering media, where traditional approaches such as adaptive focusing fail. Here, we first present an experimental proof of concept on a tissue-mimicking phantom and then, apply the method to in vivo imaging of human soft tissues. While introduced here in the context of acoustics, this approach can also be extended to optical microscopy, radar, or seismic imaging.

LED flicker measurement: Challenges, considerations, and updates from IEEE P2020 working group

The introduction of pulse width modulated LED lighting in automotive applications has created the phenomenon of LED flicker. In essence, LED flicker is an imaging artifact, whereby a light source will appear to flicker when image by a camera system, even though the light will appear constant to a human observer. The implications of LED flicker vary, depending on the imaging application. In some cases, it can simply degrade image quality by causing annoying flicker to a human observer. However, LED flicker has the potential to significantly impact the performance of critical autonomous driving functions. In this paper, the root cause of LED flicker is reviewed, and its impact on automotive use cases is explored. Guidelines on measurement and assessment of LED flicker are also provided.

High-throughput microCT scanning of small specimens: preparation, packing, parameters and post-processing

High-resolution X-ray microcomputed tomography, or microCT (μCT), enables the digital imaging of whole objects in three dimensions. The power of μCT to visualise internal features without disarticulation makes it particularly valuable for the study of museum collections, which house millions of physical specimens documenting the spatio-temporal patterns of life. Despite its potential for comparative analyses, most μCT studies include limited numbers of museum specimens, due to the challenges of digitising numerous individuals within a project scope. Here we describe a method for high-throughput μCT scanning of hundreds of small (< 2 cm) specimens in a single container, followed by individual labelling and archival storage. We also explore the effects of various packing materials and multiple specimens per capsule to minimize sample movement that can degrade image quality, and hence μCT investment. We demonstrate this protocol on vertebrate fossils from Queensland Museum, Australia, as part of an effort to track community responses to climate change over evolutionary time. This system can be easily modified for other types of wet and dry material amenable to X-ray attenuation, including geological, botanical and zoological samples, providing greater access to large-scale phenotypic data and adding value to global collections.

Image Recovery with Data Missing in the Presence of Salt-and-Pepper Noise

In this paper, an image recovery problem under the case of salt-and-pepper noise and data missing that degrade image quality is addressed if they are not effectively handled, where the salt-and-pepper noise as the impulsive noise is remodeled as a sparse signal due to its impulsiveness and the data missing pattern, denoted by a sparse vector, contains only zeros and ones to formulate the data missing. In particular, the salt-and-pepper noise and data missing are reformatted by their sparsity, respectively. The wavelet and framelet domains are explored to sparsely represent the image in order to accurately reconstruct the clean image. From the reformulations conducted and to recover the image, under one optimization framework, a joint estimation is developed to perform the image recovery, the salt-and-pepper noise suppression, and the missing patter estimation. To solve the optimization problem, two efficient solvers are developed to obtain the joint estimation solution, and they are based on the alternating direction method of multipliers (ADMM) and accelerated proximal gradient (APG). Finally, numerical studies verify that the joint estimation algorithm outperforms the state-of-the-art approaches in terms of both objective and subjective evaluation standards.

An Analytical Method for Reducing Metal Artifacts in X-Ray CT Images

Medical CT imaging often encounters metallic implants or some metal interventional therapy apparatus. These metallic objects can produce metal artifacts in reconstruction images, which severely degrade image quality. In this paper, we analyze the difference between polychromatic projection data and Radon transform data and develop an analytical method to reduce metal artifacts. Approximate features of metal artifacts can be obtained by a simplified energy spectrum function of x-ray beam. The developed method can reduce most artifacts, and preserve more original details. It does not require prior knowledge of x-ray energy spectrum and original projection data, avoiding iterative calculation and saving reconstruction time. Simulation experimental results show that the method can greatly remove metal artifacts.

3D Strain Imaging Using a Rectilinear 2D Array

Under mechanical compression, tissue movements are inherently three-dimensional. 2-D strain imaging can suffer from decorrelation noise caused by out-of-plane tissue movement in elevation. With 3-D strain imaging, all tissue movements can be estimated and compensated, hence minimizing out-of-plane decorrelation noise. Promising 3-D strain imaging results have been shown using 1 -D arrays with mechanical translation in elevation. However, the relatively large slice thickness and mechanical translation can degrade image quality. Using 2-D arrays, an improved elevational resolution can be achieved with electronic focusing. Furthermore, scanning with 2-D arrays is also done electronically, which eliminates the need for mechanical translation. In this paper, we demonstrate the feasibility of 3-D strain imaging using a 4 cm × 4 cm ultrasonic sparse rectilinear 2-D array operating at 5MHz. The signal processing combinations of 2-D or 3-D beamforming followed by 2-D or 3-D strain imaging are studied and compared to each other to evaluate the performance of our 3-D strain imaging system. 3-D beamforming followed by 3-D strain imaging showed best performance in all experiments.

Human Factors Assessment of ISO 9241-7, “Requirements for Displays with Reflections”

This work evaluated the recently-published ISO 9241-7 “Ergonomic requirements for office work with visual display terminals (VDTs) - Part 7: Requirements for display with reflections” technical standard in terms of perceived image quality judgments for CRT displays. The effects of five illumination conditions and two screen contrast polarities on image quality were assessed for seven CRT/anti-reflection filter configurations. Participants judged the image quality of the displays after reading text passages on the screen. Image quality judgments then were compared to ISO 9241-7 compliance classifications, as well as to two metrics inherent to the standard: screen image luminance ratio and specular reflection luminance ratio. The findings of this work (along with Kempic, Olacsi, and Beaton, 1998) contribute to a human factors justification of ISO 9241-7 and point up several shortcomings in this international standard. In particular, the findings indicate that specular reflections from CRTs degrade image quality more than do diffuse reflections, and, therefore, the importance of specular reflections is understated in the ISO 9241-7 standard.

Human Factors Assessment of ISO 9241-7, “Requirements for Displays with Reflections.”

This work evaluated the recently-published ISO 9241-7 “Ergonomic requirements for office work with visual display terminals (VDTs) - Part 7: Requirements for display with reflections” technical standard in terms of the readability of text passages presented on CRT displays. The effects of five illumination conditions and two screen contrast polarities on Tinker Reading Test scores (i.e., reading times and errors) were assessed for seven CRT/anti-reflection filter configurations. The readability scores were compared to ISO 9241-7 compliance classifications obtained for the seven CRT displays, as well as to two contrast metrics underlying the ISO 9241-7 compliance classifications: screen image luminance ratio and specular reflection luminance ratio. The findings show that only the specular reflection luminance ratio for large-area, negative polarity correlated with reading times. The present findings, along with those in a companion work (Olacsi, Kempic, and Beaton, 1998), contribute to the understanding of CRT viewability in glare environments and point out some shortcomings of ISO 9241-7. In particular, the findings indicate that specular reflections from CRTs degrade image quality more than do diffuse reflections, and, therefore, the importance of specular reflections is underemphasized in the ISO 9241-7 standard.