CACTUS: a computational framework for generating realistic white matter microstructure substrates.

Monte-Carlo diffusion simulations are a powerful tool for validating tissue microstructure models by generating synthetic diffusion-weighted magnetic resonance images (DW-MRI) in controlled environments. This is fundamental for understanding the link between micrometre-scale tissue properties and DW-MRI signals measured at the millimetre-scale, optimizing acquisition protocols to target microstructure properties of interest, and exploring the robustness and accuracy of estimation methods. However, accurate simulations require substrates that reflect the main microstructural features of the studied tissue. To address this challenge, we introduce a novel computational workflow, CACTUS (Computational Axonal Configurator for Tailored and Ultradense Substrates), for generating synthetic white matter substrates. Our approach allows constructing substrates with higher packing density than existing methods, up to 95% intra-axonal volume fraction, and larger voxel sizes of up to 500µm3 with rich fibre complexity. CACTUS generates bundles with angular dispersion, bundle crossings, and variations along the fibres of their inner and outer radii and g-ratio. We achieve this by introducing a novel global cost function and a fibre radial growth approach that allows substrates to match predefined targeted characteristics and mirror those reported in histological studies. CACTUS improves the development of complex synthetic substrates, paving the way for future applications in microstructure imaging.

Cellular Exchange Imaging (CEXI): Evaluation of a diffusion model including water exchange in cells using numerical phantoms of permeable spheres.

PURPOSE: Biophysical models of diffusion MRI have been developed to characterize microstructure in various tissues, but existing models are not suitable for tissue composed of permeable spherical cells. In this study we introduce Cellular Exchange Imaging (CEXI), a model tailored for permeable spherical cells, and compares its performance to a related Ball & Sphere (BS) model that neglects permeability. METHODS: We generated DW-MRI signals using Monte-Carlo simulations with a PGSE sequence in numerical substrates made of spherical cells and their extracellular space for a range of membrane permeability. From these signals, the properties of the substrates were inferred using both BS and CEXI models. RESULTS: CEXI outperformed the impermeable model by providing more stable estimates cell size and intracellular volume fraction that were diffusion time-independent. Notably, CEXI accurately estimated the exchange time for low to moderate permeability levels previously reported in other studies ( κ < 25 µ m / s $$ \kappa <25\kern0.3em \mu \mathrm{m}/\mathrm{s} $$ ). However, in highly permeable substrates ( κ = 50 µ m / s $$ \kappa =50\kern0.3em \mu \mathrm{m}/\mathrm{s} $$ ), the estimated parameters were less stable, particularly the diffusion coefficients. CONCLUSION: This study highlights the importance of modeling the exchange time to accurately quantify microstructure properties in permeable cellular substrates. Future studies should evaluate CEXI in clinical applications such as lymph nodes, investigate exchange time as a potential biomarker of tumor severity, and develop more appropriate tissue models that account for anisotropic diffusion and highly permeable membranes.

[Left-sided native valve endocarditis by coagulase-negative staphylococci: an emerging disease]. / Endocarditis en válvulas nativas izquierdas por estafilococos coagulasa negativos: una entidad en alza.

OBJECTIVES: To describe the epidemiological, clinical, and prognostic characteristics of patients with left-sided native valve endocarditis (LNVE) caused by coagulase-negative staphylococci (CoNS). PATIENTS AND METHOD: Prospective multicenter study of endocarditis cases reported in the Andalusian Cohort for the Study of Cardiovascular Infections between 1984 and 2005. RESULTS: Among 470 cases of LNVE, 39 (8.3%) were caused by CoNS, a number indicating a 30% increase in the incidence of this infection over the last decade. The mean age of affected patients was 58.32 +/- 15 years and 27 (69.2%) were men. Twenty-one patients (53.8%) had previous known valve disease and half the episodes were considered nosocomial (90% of them from vascular procedures). Median time interval from the onset of symptoms to diagnosis was 14 days (range: 1-120). Renal failure (21 cases, 53.8%), intracardiac damage (11 cases, 28.2%), and central nervous system involvement (10 cases, 25.6%) were the most frequent complications. There were only 3 cases (7.7%) of septic shock. Surgery was performed in 18 patients (46.2%). Nine patients (23.1%) died, overall. Factors associated with higher mortality in the univariate analysis were acute renal failure (P = 0.023), left-sided ventricular failure (P = 0.047), and time prior to diagnosis less than 21 days (P = 0.018). As compared to LNVE due to other microorganisms, the patients were older (P = 0.018), had experienced previous nosocomial manipulation as the source of bacteremia (P < 0.001), and developed acute renal failure more frequently (P = 0.001). Mortality of LNVE due to CoNS was lower than mortality in Staphylococcus aureus infection, but higher than in Streptococcus viridans infection. CONCLUSIONS: Left-sided native valve endocarditis due to CoNS is now increasing because of the ageing of the population. This implies more frequent invasive procedures (mainly vascular) as a consequence of the concomitant disease. Nonetheless, the mortality associated with LNVE due to CoNS does not seem to be greater than infection caused by other pathogens.

Masa cervical posterior después de una meningitis aséptica / Posterior cervical mass following aseptic meningitis

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