Passive imaging of collisional orogens: a review of a decade of geophysical studies in the Pyrénées

This contribution reviews the challenges of imaging collisional orogens, focusing on the example of the Pyrenean domain. Indeed, important progresses have been accomplished regarding our understanding of the architecture of this mountain range over the last decades, thanks to the development of innovative passive imaging techniques, relying on a more thorough exploitation of the information in seismic signals, as well as new seismic acquisitions. New tomographic images provide evidence for continental subduction of Iberian crust beneath the western and central Pyrénées, but not beneath the eastern Pyrénées. Relics of a Cretaceous hyper-extended and segmented rift are found within the North Pyrenean Zone, where the imaged crust is thinner (10–25 km). This zone of thinned crust coincides with a band of positive Bouguer anomalies that is absent in the Eastern Pyrénées. Overall, the new tomographic images provide further support to the idea that the Pyrénées result from the inversion of hyperextended segmented rift systems.

Macular Hole Detection Using a New Hybrid Method: Using Multilevel Thresholding and Derivation on Optical Coherence Tomographic Images

Optical coherence tomography (OCT) is a noninvasive imaging test. OCT imaging is analogous to ultrasound imaging, except that it uses light instead of sound. In this type of image, microscopic quality intratissue images are provided. In addition, fast and direct imaging of tissue morphology and reproducibility of results are the advantages of this imaging. Macular holes are a common eye disease that leads to visual impairment. The macular perforation is a rupture in the central part of the retina that, if left untreated, can lead to vision loss. A novel method for detecting macular holes using OCT images based on multilevel thresholding and derivation is proposed in this paper. This is a multistep method, which consists of segmentation, feature extraction, and feature selection. A combination of thresholding and derivation is used to diagnose the macular hole. After feature extraction, the features with useful information are selected and finally the output image of the macular hole is obtained. An open-access data set of 200 images with the size of 224 × 224 pixels from Sankara Nethralaya (SN) Eye Hospital, Chennai, India, is used in the experiments. Experimental results show better-diagnosing results than some recent diagnosing methods.

Material-separating regularizer for multi-energy X-ray tomography

Dual-energy X-ray tomography is considered in a context where the target under imaging consists of two distinct materials. The materials are assumed to be possibly intertwined in space, but at any given location there is only one material present. Further, two X-ray energies are chosen so that there is a clear difference in the spectral dependence of the attenuation coefficients of the two materials. A novel regularizer is presented for the inverse problem of reconstructing separate tomographic images for the two materials. A combination of two things, (a) non-negativity constraint, and (b) penalty term containing the inner product between the two material images, promotes the presence of at most one material in a given pixel. A preconditioned interior point method is derived for the minimization of the regularization functional. Numerical tests with digital phantoms suggest that the new algorithm outperforms the baseline method, Joint Total Variation regularization, in terms of correctly material-characterized pixels. While the method is tested only in a two-dimensional setting with two materials and two energies, the approach readily generalizes to three dimensions and more materials. The number of materials just needs to match the number of energies used in imaging.

Pneumatocele formation following COVID-19 pneumonia. Is there a role for surgical intervention?

COVID-19 mainly causes a lower respiratory tract illness, meaning there has been great interest in the chest and lung radiological findings seen during the course of the disease. Most of this interest has centred around the computed tomographic findings. Most commonly, computed tomographic images report ground-glass opacities but a less common finding, and potential complication associated with COVID-19, is pneumatocele formation. In this case series, we describe the presentation and management of three patients with large pneumatoceles that developed during the recovery phase of COVID-19. A conservative approach is most recommended, with surgical intervention reserved for complicated cases that cause cardiorespiratory compromise.

Experimental Investigation of Progressive Failure Processes Using 3D Acoustic Emission Tomography

In this paper, the potential of 3D acoustic emission (AE) tomography technique in demonstrating fracture development and delineating stress conditions was examined. Brazilian tests and uniaxial compression tests were monitored by 3D AE tomography. AE counts, AE source locations and 3D tomographic images of locally varying velocity distributions were analyzed along with stress and strain measurements. Experimental results revealed two distinct failure processes between Brazilian tests and uniaxial compression tests indicated by differences in AE counts, source locations and the temporal variation of velocity. Furthermore, the development of micro-cracks showed by the results correlated well with theoretical analysis and experimental observations. Additionally, stress patterns, failure modes and final failure planes were indicated by AE locations and velocity tomography. Three-dimensional velocity tomographic images indicated the anisotropy of samples caused by stresses as well. These results confirm the usefulness of AE tomography as a method to monitor stress induced failure and the potential of AE tomography for delineating stress conditions and predicting rock failure.