3D simulation of magneto-mechanical coupling in MRI scanners using high order FEM and POD

Magnetic Resonance Imaging (MRI) scanners have become an essential tool in the medi-cal industry due to their ability to produce high resolution images of the human body. To generate an image of the body, MRI scanners combine strong static magnetic fields with transient gradient magnetic fields. The interaction of these magnetic fields with the con-ducting components present in superconducting MRI scanners gives rise to an important problem in the design of new MRI scanners. The transient magnetic fields give rise to the appearance of eddy currents in conducting components. These eddy currents, in turn, result in electromagnetic stresses, which cause the conducting components to deform and vibrate. The vibrations are undesirable as they lead to a deterioration in image quality (with image artefacts) and to the generation of noise, which can cause patient discomfort. The eddy currents, in addition, lead to heat being dissipated and deposited into the cryo-stat, which is filled with helium in order to maintain the coils in a superconducting state. This deposition of heat can cause helium boil off and potentially result in a costly magnet quench. Understanding the mechanisms involved in the generation of these vibrations and the heat being deposited into the cryostat are, therefore, key for a successful MRI scanner design. This involves the solution of a coupled magneto-mechanical problem, which is the focus of this work.In this thesis, a new computational methodology for the solution of three-dimensional (3D) magneto-mechanical coupled problems with application to MRI scanner design is presented. To achieve this, first an accurate mathematical description of the magneto-mechanical coupling is presented, which is based on a Lagrangian formulation and the assumption of small displacements. Then, the problem is linearised using an AC-DC splitting of the fields, and a variational formulation for the solution of the linearised prob-lem in a time-harmonic setting is presented. The problem is then discretised using high order finite elements, where a combination of hierarchical H1 and H(curl) basis func-tions is used. An efficient staggered algorithm for the solution of the coupled system is proposed, which combines the DC and AC stages and makes use of preconditioned iter-ative solvers when appropriate. This finite element methodology is then applied to a set of challenging academic and industrially relevant problems in order to demonstrate its accuracy and efficiency.This finite element methodology results in the accurate and efficient solution of the magneto-mechanical problem of interest. However, in the design stage of a new MRI scanner, this coupled problem must be solved repeatedly for varying model parameters such as frequency or material properties. Thus, even if an efficient finite element solver is available for the solution of the coupled problem, the need for these repeated simulations result in a bottleneck in terms of computational cost, which leads to an increase in design time and its associated financial implications. Therefore, in order to optimise this process, the application of Reduced Order Modelling (ROM) techniques is considered. A ROM based on the Proper Orthogonal Decomposition (POD) method is presented and applied to a series of challenging MRI configurations. The accuracy and efficiency of this ROM is demonstrated by performing comparisons against the full order or high fidelity finite element software, showing great performance in terms of computational speed-up, which has major benefits in the optimisation of the design process of new MRI scanners

Nocardiosis at a London teaching hospital: Be aware and beware of what is rare

Aims To review all laboratory-confirmed cases of nocardiosis at a tertiary referral hospital over an extended period (2000–2018; 216 months) with regard to microbiological and epidemiological characteristics, risk factors, clinical management, morbidity and mortality. Methods The medical records and microbiological data of all laboratory-confirmed cases of nocardiosis, identified by culture (with reference laboratory confirmation) or identified in a reference laboratory only, were included and analysed retrospectively. Results 18 cases of nocardiosis were identified; 72% (n = 13) were male; all were UK resident. Median age at presentation was 56 years (range 6–83 years). Most had underlying pathology or risk factors including cancer in 39% (n = 7) and immunosuppression in 33% (n = 6). Alcohol and acid fast bacilli (AAFB) microscopy performed in 8/18 cases was negative. Routine 48-hour bacterial culture of 18 isolates was positive in 15; 3 culture-negative specimens were subsequently confirmed positive in a reference laboratory. Four patterns of clinical presentation were observed: cerebral 39% (n = 7), disseminated 28% (n = 5), pulmonary 17% (n = 3), and isolated cutaneous/articular (both n = 1). In addition one case of bacteraemia was noted. Nocardia farcinica accounted for half (n = 9) of all nocardia species identified. 55% (n = 10) required surgical intervention. One co-trimoxazole resistant isolate was identified. Morbidity and mortality were high: 78% (n = 14) required critical care. More than half of patients (55%; n = 10) died from refractory infection, including all of those with disseminated disease (n = 5). Conclusions Nocardia spp should never be regarded as a contaminant or commensal organism in clinical specimens. Correlation of clinical and radiology findings plus risk factors are imperative for nocardiosis to be considered in the differential diagnosis in order to guide appropriate laboratory processing of specimens. Although rare, recognition of nocardiosis is important because of its high mortality. Routine 48-hour bacterial culture does not always identify Nocardia spp and isolates should also be sent to a reference laboratory

Nocardia infection following bone marrow transplantation

It is very well known that bone marrow (BM) microvasculature may possess a crucial role in the maintenance of homeostasis of BM due to mutual interactions between BM microvascular system and other physiological functions including haematopoiesis and osteogenesis. Chemotherapy and radiotherapy are known as main approaches for cancer treatment and also are known as the main cause of damage to the BM microvascular system. However, despite the importance of BM microvasculature in orchestrating various biological functions, less attention has been drawn to address the underlying mechanisms for the damage and to explore cellular and molecular mechanisms by which the recovery/regeneration of chemotherapy- and/or radiotherapy-induced BM microvascular system damage can occur. Therefore, in this review we firstly discuss the ultra-/structure and biological characteristics of BM microvascular system (sinusoids). Secondly, potential contribution of BM sinusoids is discussed in pathophysiological circumstances (bone remodelling, haematopoiesis, cancer bone metastasis, and haematological cancers). Thirdly, we address previous preclinical and clinical studies regarding chemotherapy- and irradiation-induced BM microvasculature damage. Finally, potential cellular and molecular mechanisms are discussed for the recovery/regeneration of damaged BM microvascular system, including the potential roles of endothelial progenitor cells, haematopoietic stem/progenitor cells, and stimulation of VEGF/VEGFR and Ang-1/Tie-2 signalling pathways

Mixing and Phytoplankton Growth in an Upwelling System

Previous studies focused on understanding the role of physical drivers on phytoplankton bloom formation mainly used indirect estimates of turbulent mixing. Here we use weekly observations of microstructure turbulence, dissolved inorganic nutrients, chlorophyll a concentration and primary production carried out in the Ria de Vigo (NW Iberian upwelling system) between March 2017 and May 2018 to investigate the relationship between turbulent mixing and phytoplankton growth at different temporal scales. In order to interpret our results, we used the theoretical framework described by the Critical Turbulent Hypothesis (CTH). According to this conceptual model if turbulence is low enough, the depth of the layer where mixing is active can be shallower than the mixed-layer depth, and phytoplankton may receive enough light to bloom. Our results showed that the coupling between turbulent mixing and phytoplankton growth in this system occurs at seasonal, but also at shorter time scales. In agreement with the CTH, higher phytoplankton growth rates were observed when mixing was low during spring-summer transitional and upwelling periods, whereas low values were described during periods of high mixing (fall-winter transitional and downwelling). However, low mixing conditions were not enough to ensure phytoplankton growth, as low phytoplankton growth was also found under these circumstances. Wavelet spectral analysis revealed that turbulent mixing and phytoplankton growth were also related at shorter time scales. The higher coherence between both variables was found in spring-summer at the ~16-30 d period and in fall-winter at the ~16-90 d period. These results suggest that mixing could act as a control factor on phytoplankton growth over the seasonal cycle, and could be also involved in the formation of occasional short-lived phytoplankton blooms.APHY

Different profiles of advanced heart failure among patients with and without diabetes mellitus. Findings from the EPICTER study.

This work aims to compare the characteristics of advanced heart failure (HF) in patients with and without type 2 diabetes mellitus (DM) and to determine the relevance of variables used to define advanced HF. This cross-sectional, multicenter study included patients hospitalized for HF. They were classified into four groups according to presence/absence of advanced HF, determined based on general and cardiac criteria, and presence/absence of DM. To analyze the importance of variables, we grew a random forest algorithm (RF) based on mortality at six months. A total of 3153 patients were included. The prevalence of advanced HF among patients with DM was 24% compared to 23% among those without DM (p=0.53). Patients with advanced HF and DM had more comorbidity related to cardiovascular and renal diseases; their prognosis was the poorest (log-rank <0.0001) though the adjusted hazard ratio by group in the Cox regression analysis was not significant. The variables that were significantly related to mortality were the number of comorbidities (p=0.005) and systolic blood pressure (p=0.024). The RF showed that general criteria were more important for defining advanced HF than cardiac criteria. Patients with advanced HF and DM were characterized by DM in progression with macro and microvascular complications. The outcomes among advanced HF patients were poor; patients with advanced HF and DM had the poorest outcomes. General criteria were the most important to establish accurately a definition of advanced HF, being decisive the evidence of disease progression in patients with DM