Effects of nongaussian diffusion on "isotropic diffusion measurements'': an ex-vivo microimaging and simulation study

Designing novel diffusion-weighted pulse sequences to probe tissue microstructure beyond the conventional Stejskal-Tanner family is currently of broad interest. One such technique, multidimensional diffusion MRI, has been recently proposed to afford model-free decomposition of diffusion signal kurtosis into terms originating from either ensemble variance of isotropic diffusivity or microscopic diffusion anisotropy. This ability rests on the assumption that diffusion can be described as a sum of multiple Gaussian compartments, but this is often not strictly fulfilled. The effects of nongaussian diffusion on single shot isotropic diffusion sequences were first considered in detail by de Swiet and Mitra in 1996. They showed theoretically that anisotropic compartments lead to anisotropic time dependence of the diffusion tensors, which causes the measured isotropic diffusivity to depend on gradient frame orientation. Here we show how such deviations from the multiple Gaussian compartments assumption conflates orientation dispersion with ensemble variance in isotropic diffusivity. Second, we consider additional contributions to the apparent variance in isotropic diffusivity arising due to intracompartmental kurtosis. These will likewise depend on gradient frame orientation. We illustrate the potential importance of these confounds with analytical expressions, numerical simulations in simple model geometries, and microimaging experiments in fixed spinal cord using isotropic diffusion encoding waveforms with 7.5 ms duration and 3000 mT/m maximum amplitude.Comment: 26 pages, 9 figures. Appearing in J. Magn. Reso

Evaluering af turdagbøger

I specialet "Evaluering af Turdagbøger" foretages en undersøgelse af turdagbøgers præcision og pålidelighed som turdataindsamlingsmetode under danske forhold.I specialet gennemføres et forsøg hvorigennem der tilvejebringes turdata fra 20 familiers færden i bil i en 12-dages periode. Sideløbende har familierne en GPS positionslogger installeret i deres bil.Med udgangspunkt i disse datasæt foretages evalueringen af de anvendte turdagbøger, og der stilles forslag til udvikling af en semiautomatisk GPS baseret turrapport

MP-PCA denoising of fMRI time-series data can lead to artificial activation "spreading"

MP-PCA denoising has become the method of choice for denoising in MRI since it provides an objective threshold to separate the desired signal from unwanted thermal noise components. In rodents, thermal noise in the coils is an important source of noise that can reduce the accuracy of activation mapping in fMRI. Further confounding this problem, vendor data often contains zero-filling and other effects that may violate MP-PCA assumptions. Here, we develop an approach to denoise vendor data and assess activation "spreading" caused by MP-PCA denoising in rodent task-based fMRI data. Data was obtained from N = 3 mice using conventional multislice and ultrafast acquisitions (1 s and 50 ms temporal resolution, respectively), during visual stimulation. MP-PCA denoising produced SNR gains of 64% and 39% and Fourier spectral amplitude (FSA) increases in BOLD maps of 9% and 7% for multislice and ultrafast data, respectively, when using a small [2 2] denoising window. Larger windows provided higher SNR and FSA gains with increased spatial extent of activation that may or may not represent real activation. Simulations showed that MP-PCA denoising causes activation "spreading" with an increase in false positive rate and smoother functional maps due to local "bleeding" of principal components, and that the optimal denoising window for improved specificity of functional mapping, based on Dice score calculations, depends on the data's tSNR and functional CNR. This "spreading" effect applies also to another recently proposed low-rank denoising method (NORDIC). Our results bode well for dramatically enhancing spatial and/or temporal resolution in future fMRI work, while taking into account the sensitivity/specificity trade-offs of low-rank denoising methods

Proton pump inhibitor use and risk of adverse cardiovascular events in aspirin treated patients with first time myocardial infarction: nationwide propensity score matched study

Objective To examine the effect of proton pump inhibitors on adverse cardiovascular events in aspirin treated patients with first time myocardial infarction

Initiation and Persistence with Warfarin Therapy in Atrial Fibrillation According to Ethnicity

The aim of this study was to investigate initiation of and persistence with warfarin treatment in patients with atrial fibrillation (AF) according to ethnicity. Patients hospitalized with first-time AF from 1997 to 2009, prescription claims of warfarin and country of birth were identified by individual-level linkage of nationwide administrative agencies. Cox proportional hazards models were used to estimate the relationship between covariates affecting initiation and non-persistence with warfarin treatment. A total of 151,537 patients were included in the study and 5,061(3.3%) were of non-Danish origin. CHADS2 score distribution varied substantially according to ethnicity, the proportion of patients with CHADS2 score ≥1 being 79.2, 78.1, 65.9, and 46.0% for patients of Danish, Western, Eastern, and African origin, respectively. 79,239(52.4%) of all patients initiated treatment with warfarin at some point in time. Multivariable Cox proportional hazard analyses indicated patients of Eastern and African origin were less likely to initiate warfarin therapy (HR 0.75; 95% CI 0.69–0.82 and HR 0.58; 95% CI 0.44–0.76, respectively). Patients of Eastern origin were more likely to interrupt treatment (HR 1.23; 95% CI 1.02–1.47; for all patients; HR 1.62; 95% CI 1.22–2.16; for patients with CHADS2 score >1). African origin was associated with a trend to interrupt treatment (HR 1.44; 95% CI 0.46–4.47; for patients with CHADS2 score >1). Initiation of and persistence with warfarin in AF patients is lower among patients of Eastern and African origin compared to patients of Danish and Western origin, despite equal access to health care and medication. Future studies should address, beyond ethnicity, all possible driving factors of (non)initiation and persistence with treatment in general. This will be particularly interesting in light of the new generation of anticoagulants, which might render different adherence to treatment

Effects of depth and overgrowth of ephemeral macroalgae on a remote subtidal NE Atlantic eelgrass (Zostera marina) community

We conducted a short-term field sampling complemented with time integrating stable isotope analysis to holistically investigate status and ecological interactions in a remote NE Atlantic Zostera marina meadow. We found high nutrient water concentrations, large biomass of fast-growing, ephemeral macroalgae, low abundance, and biodiversity of epifauna and a food web with thornback ray (Raja clavata) as intermediate and cod (Gadus morhua) as top predator. We observed no variation with increasing depth (3.5-11 m) except for decreasing shoot density and biomass of Zostera and macroalgae. Our results indicate that the Finnoya Zostera ecosystem is eutrophicated. During the past three to four decades, nutrients from aquaculture have steadily increased to reach 75% of anthmpogenic input while the coastal top predator cod has decreased by 50%. We conclude that bottom-up regulation is a predominant driver of change since top-down regulation is generally weak in low density and exposed Zostera ecosystems such as Finnoya.Peer reviewe

Trends in one-year outcomes of dialysis-requiring acute kidney injury in Denmark 2005-2012:a population-based nationwide study

Dialysis-requiring acute kidney injury (AKI) is associated with substantial mortality and risk of end-stage renal disease (ESRD). Despite considerable growth in incidence of severe AKI, information pertaining to trends in outcomes remains limited. We evaluated time trends in one year risks of ESRD and death in patients with dialysis-requiring AKI over an eight year period in Denmark.In a retrospective nationwide study based on national registers, all adults requiring acute renal replacement therapy between 2005 and 2012 were identified. Patients with preceding ESRD were excluded. Through individual-level cross-referencing of administrative registries, information pertaining to comorbidity, preceding surgical interventions, and concurrent other organ failure and sepsis was ascertained. Comparisons of period-specific one year odds ratios for ESRD and death were calculated in a multiple logistic regression model.A total of 13,819 patients with dialysis-requiring AKI were included in the study. Within one year, 1,017 (7.4%) patients were registered with ESRD, and 7,908 (57.2%) patients died. The one-year rate of ESRD decreased from 9.0% between 2005 and 2006 to 6.1% between 2011 and 2012. Simultaneously, the one-year mortality rate decreased from 58.2% between 2005 and 2006 to 57.5% between 2011 and 2012. Consequently, the adjusted odds ratios for the period 2011-2012 (with the period 2005-2006 as reference) were 0.75 (0.60-0.95, p = 0.015) and 0.87 (95% CI 0.78-0.97, p = 0.010) for ESRD and death, respectively.In a nationwide retrospective study on time trends in one year outcomes following dialysis-requiring AKI, risk of all-cause mortality and ESRD decreased over a period of 8 years