RCDN -- Robust X-Corner Detection Algorithm based on Advanced CNN Model

Accurate detection and localization of X-corner on both planar and non-planar patterns is a core step in robotics and machine vision. However, previous works could not make a good balance between accuracy and robustness, which are both crucial criteria to evaluate the detectors performance. To address this problem, in this paper we present a novel detection algorithm which can maintain high sub-pixel precision on inputs under multiple interference, such as lens distortion, extreme poses and noise. The whole algorithm, adopting a coarse-to-fine strategy, contains a X-corner detection network and three post-processing techniques to distinguish the correct corner candidates, as well as a mixed sub-pixel refinement technique and an improved region growth strategy to recover the checkerboard pattern partially visible or occluded automatically. Evaluations on real and synthetic images indicate that the presented algorithm has the higher detection rate, sub-pixel accuracy and robustness than other commonly used methods. Finally, experiments of camera calibration and pose estimation verify it can also get smaller re-projection error in quantitative comparisons to the state-of-the-art.Comment: 15 pages, 8 figures and 4 tables. Unpublished further research and experiments of Checkerboard corner detection network CCDN (arXiv:2302.05097) and application exploration for robust camera calibration (https://ieeexplore.ieee.org/abstract/document/9428389

Novel Bayesian Inference-Based Approach for the Uncertainty Characterization of Zhang's Camera Calibration Method

Camera calibration is necessary for many machine vision applications. The calibration methods are based on linear or non-linear optimization techniques that aim to find the best estimate of the camera parameters. One of the most commonly used methods in computer vision for the calibration of intrinsic camera parameters and lens distortion (interior orientation) is Zhang¿s method. Additionally, the uncertainty of the camera parameters is normally estimated by assuming that their variability can be explained by the images of the different poses of a checkerboard. However, the degree of reliability for both the best parameter values and their associated uncertainties has not yet been verified. Inaccurate estimates of intrinsic and extrinsic parameters during camera calibration may introduce additional biases in post-processing. This is why we propose a novel Bayesian inference-based approach that has allowed us to evaluate the degree of certainty of Zhang¿s camera calibration procedure. For this purpose, the a prioriprobability was assumed to be the one estimated by Zhang, and the intrinsic parameters were recalibrated by Bayesian inversion. The uncertainty of the intrinsic parameters was found to differ from the ones estimated with Zhang¿s method. However, the major source of inaccuracy is caused by the procedure for calculating the extrinsic parameters. The procedure used in the novel Bayesian inference-based approach significantly improves the reliability of the predictions of the image points, as it optimizes the extrinsic parameters.This work was supported by the Madrid Government (Comunidad de Madrid Spain) under the Multiannual Agreement with UC3M ("Fostering Young Doctors Research", APBI-CM-UC3M), and in the context of the VPRICIT (Research and Technological Innovation Regional Programme and by the FEDER/Ministry of Science and Innovation -Agencia Estatal de Investigacion (AEI) of the Government of Spain through the projects PID2022-136468OB-I00 and PID2022-142015OB-I00.Publicad

A Rotating Aperture Mask for Small Telescopes

Observing the dynamic interaction between stars and their close stellar neighbors is key to establishing the stars’ orbits, masses, and other properties. Our ability to visually discriminate nearby stars is limited by the power of our telescopes, posing a challenge to astronomers at small observatories that contribute to binary star surveys. Masks placed at the telescope aperture promise to augment the resolving power of telescopes of all sizes, but many of these masks must be manually and repetitively reoriented about the optical axis to achieve their full benefits. This paper introduces a design concept for a mask rotation mechanism that can be adapted to telescopes of different types and proportions, focusing on an implementation for a Celestron C11 Schmidt–Cassegrain optical tube assembly. Mask concepts were first evaluated using diffraction simulation programs, later manufactured, and finally tested on close double stars using a C11. An electronic rotation mechanism was designed, produced, and evaluated. Results show that applying a properly shaped and oriented mask to a C11 enhances contrast in images of double star systems relative to images captured with the unmasked telescope, and they show that the rotation mechanism accurately and repeatably places masks at target orientations with minimal manual effort. Detail drawings of the mask rotation mechanism and code for the software interface are included

Development and Applications of a Real-time Magnetic Electron Energy Spectrometer for Use with Medical Linear Accelerators

Purpose – This work presents a design for a real-time electron energy spectrometer, and provides data analysis methods and characterization of the real-time system. This system is intended for use with medical linear accelerators (linacs). The goal is 1 Hz acquisition of the energy range 4-25 MeV, reconstructed in 0.1 MeV increments. Methods – Our spectrometer uses a nominal 0.54 T permanent magnet block as the dispersive element and scintillating fibers coupled to a CCD camera as the position sensitive detector. A broad electron beam produced by a linac is collimated by a 6.35 mm dimeter aperture at the entrance to the spectrometer. The collimated beam is dispersed by the magnetic field onto a row of 60 vertical 1 mm x 1 mm square scintillating fibers mounted to a lateral face of the magnet. Detector response functions (DRFs) were created using a simplified physics model of the spectrometer to determine electron trajectories within the magnet block from the entrance aperture to the detector plane. The DRFs were used in an iterative method to transform the fiber signal intensity versus position into an energy spectrum. We made measurements on an Elekta Infinity linac; each available energy (7, 9, 10, 11, 13, 16, 20 MeV) was investigated. Measurements were used to assess setup reproducibility, pinhole mismatch, dose rate effects, temporal stability, and linac detuning. Results – Our reconstruction method was able to reconstruct energy spectra from idealized simulations to within 0.14 MeV ± 0.28 MeV of the ideal FWHM value, and 0.06 MeV ± 0.12 MeV of the ideal most probable energy, Ep0. The measured spectral stability was consistent with the expected linac operating stability. The system achieved a refresh rate of 0.8 Hz during real-time operation. Conclusions – We developed a real-time electron energy spectrometer that measures electron energies from 4 to 25 MeV with a continuous readout rate of 0.8 Hz. The device can be used for assessing linac performance as a routine clinical tool, assist in diagnostic maintenance and repair, or potentially provide a more efficient method for beam tuning and matching

Analysis of contrast-enhanced medical images.

Early detection of human organ diseases is of great importance for the accurate diagnosis and institution of appropriate therapies. This can potentially prevent progression to end-stage disease by detecting precursors that evaluate organ functionality. In addition, it also assists the clinicians for therapy evaluation, tracking diseases progression, and surgery operations. Advances in functional and contrast-enhanced (CE) medical images enabled accurate noninvasive evaluation of organ functionality due to their ability to provide superior anatomical and functional information about the tissue-of-interest. The main objective of this dissertation is to develop a computer-aided diagnostic (CAD) system for analyzing complex data from CE magnetic resonance imaging (MRI). The developed CAD system has been tested in three case studies: (i) early detection of acute renal transplant rejection, (ii) evaluation of myocardial perfusion in patients with ischemic heart disease after heart attack; and (iii), early detection of prostate cancer. However, developing a noninvasive CAD system for the analysis of CE medical images is subject to multiple challenges, including, but are not limited to, image noise and inhomogeneity, nonlinear signal intensity changes of the images over the time course of data acquisition, appearances and shape changes (deformations) of the organ-of-interest during data acquisition, determination of the best features (indexes) that describe the perfusion of a contrast agent (CA) into the tissue. To address these challenges, this dissertation focuses on building new mathematical models and learning techniques that facilitate accurate analysis of CAs perfusion in living organs and include: (i) accurate mathematical models for the segmentation of the object-of-interest, which integrate object shape and appearance features in terms of pixel/voxel-wise image intensities and their spatial interactions; (ii) motion correction techniques that combine both global and local models, which exploit geometric features, rather than image intensities to avoid problems associated with nonlinear intensity variations of the CE images; (iii) fusion of multiple features using the genetic algorithm. The proposed techniques have been integrated into CAD systems that have been tested in, but not limited to, three clinical studies. First, a noninvasive CAD system is proposed for the early and accurate diagnosis of acute renal transplant rejection using dynamic contrast-enhanced MRI (DCE-MRI). Acute rejection–the immunological response of the human immune system to a foreign kidney–is the most sever cause of renal dysfunction among other diagnostic possibilities, including acute tubular necrosis and immune drug toxicity. In the U.S., approximately 17,736 renal transplants are performed annually, and given the limited number of donors, transplanted kidney salvage is an important medical concern. Thus far, biopsy remains the gold standard for the assessment of renal transplant dysfunction, but only as the last resort because of its invasive nature, high cost, and potential morbidity rates. The diagnostic results of the proposed CAD system, based on the analysis of 50 independent in-vivo cases were 96% with a 95% confidence interval. These results clearly demonstrate the promise of the proposed image-based diagnostic CAD system as a supplement to the current technologies, such as nuclear imaging and ultrasonography, to determine the type of kidney dysfunction. Second, a comprehensive CAD system is developed for the characterization of myocardial perfusion and clinical status in heart failure and novel myoregeneration therapy using cardiac first-pass MRI (FP-MRI). Heart failure is considered the most important cause of morbidity and mortality in cardiovascular disease, which affects approximately 6 million U.S. patients annually. Ischemic heart disease is considered the most common underlying cause of heart failure. Therefore, the detection of the heart failure in its earliest forms is essential to prevent its relentless progression to premature death. While current medical studies focus on detecting pathological tissue and assessing contractile function of the diseased heart, this dissertation address the key issue of the effects of the myoregeneration therapy on the associated blood nutrient supply. Quantitative and qualitative assessment in a cohort of 24 perfusion data sets demonstrated the ability of the proposed framework to reveal regional perfusion improvements with therapy, and transmural perfusion differences across the myocardial wall; thus, it can aid in follow-up on treatment for patients undergoing the myoregeneration therapy. Finally, an image-based CAD system for early detection of prostate cancer using DCE-MRI is introduced. Prostate cancer is the most frequently diagnosed malignancy among men and remains the second leading cause of cancer-related death in the USA with more than 238,000 new cases and a mortality rate of about 30,000 in 2013. Therefore, early diagnosis of prostate cancer can improve the effectiveness of treatment and increase the patient’s chance of survival. Currently, needle biopsy is the gold standard for the diagnosis of prostate cancer. However, it is an invasive procedure with high costs and potential morbidity rates. Additionally, it has a higher possibility of producing false positive diagnosis due to relatively small needle biopsy samples. Application of the proposed CAD yield promising results in a cohort of 30 patients that would, in the near future, represent a supplement of the current technologies to determine prostate cancer type. The developed techniques have been compared to the state-of-the-art methods and demonstrated higher accuracy as shown in this dissertation. The proposed models (higher-order spatial interaction models, shape models, motion correction models, and perfusion analysis models) can be used in many of today’s CAD applications for early detection of a variety of diseases and medical conditions, and are expected to notably amplify the accuracy of CAD decisions based on the automated analysis of CE images

Thermofluid topology optimization for cooling channel design

A framework for topology optimization of cooling channels is proposed, which paves the way towards automated design of additively-manufactured cooling channels, required in applications such as the efficient heat management of die casting molds. Combining a selection of pertinent techniques and methods, the proposed density-based approach is strengthened by systematic verification and validation steps, including the body-fitted meshing of an optimized design. Furthermore, this work features applications to simplified, yet industrially-relevant cases, as well as a detailed discussion of the effects of the hyper-parameters of the optimization problem. These enable the reader to acquire a better understanding of the control and regularization mechanisms, which are necessary for a robust development towards complex scenarios

Investigating the role of right parietal cortex in multistable perception using non-invasive brain stimulation

Multistable perception describes the spontaneous fluctuation between two or more perceptual states when sensory input is ambiguous. An example hereof is bistability, which occurs when a stimulus has two competing interpretations that perceptually alternate over time. For instance, in structure- from-motion (SFM) bistable perception, the coherent movement of dots creates the illusion of a rotating sphere, where the direction of movement is uncertain. Another example is binocular rivalry (BR), which occurs when the two eyes are presented with dissimilar visual stimuli in the same retinal space, leading to an alternation of conscious awareness between the two stimuli. Multistable perception has been used to investigate the neural correlates of conscious experience, since an unchanging stimulus leads to a change in awareness, hence dissociating consciousness from sensory processing. Functional magnetic resonance imaging (fMRI) has consistently shown activity of the right intraparietal sulcus (IPS) and right superior parietal lobule (SPL) during perceptual transitions in multistable perception. Previously, transcranial magnetic stimulation (TMS) and in particular inhibitory theta burst stimulation (cTBS) has been used on the IPS to probe its causal role in multistable perception. That endeavour has produced inconsistent results on whether IPS inhibition shortens or lengthens multistable dominance durations. Problematically, the neural effects of cTBS over IPS during multistable perception are unknown, as is indeed the causal role of IPS in mediating perceptual reversals. Chapter 1 cTBS was applied over IPS or over vertex control site, between two sessions of fMRI, to illuminate the changes in neural activity accompanied by IPS cTBS. During the fMRI sessions, participants viewed alternating blocks of a bistable SFM stimulus or a replay condition using depth-cue disambiguated SFM. Behaviourally, it was found that IPS cTBS lengthened dominance durations when comparing pre vs post cTBS as well as when comparing IPS with vertex stimulation. Neurally, IPS cTBS led to a decrease in blood-oxygen-level dependent (BOLD) response in thalamus, foveal V1, right superior parietal lobule and middle frontal gyrus compared to vertex cTBS. Moreover, a decrease of functional connectivity between activity in IPS and ipsilateral hippocampus was observed. The present results suggest that the combined effects of a reduction of sensory processing as well as decoupling between IPS and the memory site hippocampus allows inhibitory TMS over parietal cortex to stabilise the current perceptual content. Together, these results provide a hitherto unreported insight into the brain networks that subserve the resolution of bistable perception and how IPS stimulation modulates them to bring about a behavioural effect. Chapter 2 Next to the IPS, also the more posterior SPL has been indicated as serving a causal role in multistable perception. TMS has been used to modulate bistable dominance durations for both sites, but in opposite directions. This led to the proposal that parietal cortex is fractionated, such that IPS and SPL serve opposing functions. However, neuroimaging evidence also suggests that higher cortical activity, including parietal cortex, is diminished when BR percept switches are either unreported or unreportable. This suggests that parietal regions have no causal role in multistable perception, but are active only as consequence thereof. To resolve this conflict, chapter 2 investigates whether cTBS to the IPS as well as the SPL affects the temporal dynamics of BR using regular button press rivalry as well as no-report and invisible rivalry paradigms. Specifically, it was hypothesised that cTBS would lead to a change in BR dominance when it was visible or unreported, but not when invisible. However, contrary to expectation, not only was it not possible to replicate the previously observed functional fractionation of parietal cortex, but also no difference was found between any cTBS condition. To verify if cTBS had its desired inhibitory effect, also motor-evoked potentials (MEP) were recording prior and following cTBS to primary motor cortex. It was found that cTBS to M1 decreases MEP amplitude. However, this effect did not correlate with the main findings over parietal cortex, leading to the conclusion that cTBS is not an apt neurostimulation technique to answer the present research question. Chapter 3 Relative intensities of steady-state responses (SSRs) over early visual cortex have been reported to correlate with conscious perception in paradigms like BR and have even be used to predict the content of consciousness. However, their causal role in perception remains uninvestigated despite their common use. Are modulations of SSRs mere epiphenomena of perception or do they aid in determining its content? To test this, it was enquired if interference with the SSR by means of transcranial alternating current stimulation (tACS) would affect conscious perception. Sham or real tACS across left and right parieto-occipital cortex was applied at either the same or a different frequency or in and out of phase with an SSR eliciting flicker stimulus, while participants viewed either BR or tried to detect stimuli masked by continuous flash suppression (CFS). It was found that tACS did not differentially affect conscious perception in the forms of BR predominance, CFS detection accuracy, reaction time, or metacognitive sensitivity. Importantly, the present null-findings are supported by Bayesian statistics. In conclusion, the application of tACS at frequencies and phases of stimulus-induced SSRs does not have perceptual effects. The relationship of tACS with SSRs and the possibility that SSRs are epiphenomenal to conscious perception is discussed. Chapter 4 One reason for the difference between findings of studies, which attempted to modulate multistable dominance durations thought cTBS to the IPS, may be that different stimuli were used, dissimilar properties of which modulated the TMS effect direction. To test this, cTBS was applied to the IPS between two sessions of SFM bistable perception (chapter 1), random dot motion BR (chapter 2), as well as checkerboard BR (chapter 3). It was foremost hypothesised that the findings of the first two chapters would be replicated, and moreover that the TMS effect would correlate between stimuli. Contrary to this hypothesis, cTBS neither consistently affected dominance durations in any of the stimuli, nor were effect sizes correlated across participants. This is supported by Bayesian statistics. Baseline dominance durations prior to TMS correlated across the three stimuli, suggesting a common mechanism to resolve multistability. However, the lack of correlation pertaining to the cTBS effect points towards the absence of any cTBS effect. Considering the present results, the small samples and effect sizes of previous studies, as well as recent literature of variable cTBS effects on motor cortex, this chapter concludes that there is good reason to cast general doubt over the ability of parietal cTBS to modulate dominance durations in multistable perception. Chapter 1 pointed towards the importance of multiple brain networks including the IPS in the resolution of multistability. Chapters 2 & 4 by contrast presented with null results that do not allow inference as to the causal role of IPS. Similarly, the use of tACS to modulate SSRs in chapter 3 was not able to demonstrate conclusively whether SSRs have a causal role in multistability. The search for the contribution of IPS by use of cTBS or tACS has been hindered by methodological concerns over whether these methods have an interpretable or even any effect on IPS activity. In summary, the causal role of IPS activity in multistable perception remains elusive

Microstructure evolution and mechanical properties of selective laser melted Ti-6Al-4V

Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys, and in particular Ti-6Al-4V. A thorough understanding of the relationship between the process, microstructure and mechanical properties of the components produced by this technology is however crucial for the establishment of SLM as an alternative manufacturing route. The purpose of the present study is thus to determine the microstructure evolution, crystallographic texture and the mechanical properties of SLM Ti-6Al-4V. The effect of several processing parameters on the density and the microstructure of the SLM samples were initially investigated. It was found that different sets of process parameters can be used to fabricate near fully dense components. It was found that the samples built using the optimised process window consist exclusively of α′ martensitic phase precipitated from prior β columnar grains. It was observed that the β grain solidification is influenced by the laser scan strategy and that the β phase has a strong texture along its grain growth direction. The α′ martensitic laths that originate from the parent β grains precipitate according to the Burgers orientation relationship. It was found that α′ laths clusters from the same β grain have a specific misorientation that minimise the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms. The mechanical properties of as-built and stress relieved SLM Ti-6Al-4V built using the same optimised process parameters were then investigated. It was found that the build orientation affects the tensile properties, and in particular the ductility of the samples. Samples built perpendicularly to the building direction showed higher ductility than those built in the vertical orientation. It was also observed that a stress relief heat treatment was beneficial to the mechanical properties of SLM Ti-6Al-4V. The ductility of the stress relieved samples was indeed higher than those found in the as-built condition. It was found that the predominant fracture mode during tensile testing is inter-granular. In terms of high-cycle fatigue, it was found that SLM Ti-6Al-4V is comparable to HIPed cast Ti-6Al-4V but it has a significantly lower fatigue resistance than that of wrought and annealed alloys. It was observed that porosity and the elongated prior β grain boundaries decrease substantially the fatigue life of the components. Cracks propagate either by fatigue striation or ductile tearing mechanisms. Using alternative laser scan strategies it was possible to control the microstructure of the as-built samples. It was observed that the laser scan vector length influences several microstructural features, such as the width of the prior β grains and the thickness of the α′ laths. It was found that re-melting the same layer has instead little effect on the microstructure. A novel laser scan strategy characterised by much lower laser power and scan speed than those typically used in SLM enabled finally to fabricate SLM Ti-6Al-4V with a microstructure close to that of conventionally manufactured Ti-6Al-4V. This study investigates for the first time the crystallographic texture evolution in Ti-6Al-4V manufactured by SLM. Further, this research presents for the first time the effect of the characteristic microstructure and crystallographic texture on the mechanical properties and fracture of SLM Ti-6Al-4V. Lastly, for the first time this research shows examples of microstructural control during the SLM fabrication of the same alloy using long laser dwell times