A large-scale field experiment on salt marsh construction

Salt marshes provide important natural habitats, mitigate effects of subsidence and sea level rise and help in coastal defence. Commissioned by the municipality of Delfzijl, the EcoShape consortium obtained the opportunity to carry out a large-scale field experiment on salt marsh construction as part of their Living Lab for MUD

Evidence for 'critical slowing down' in seagrass:a stress gradient experiment at the southern limit of its range

The theory of critical slowing down, i.e. the increasing recovery times of complex systems close to tipping points, has been proposed as an early warning signal for collapse. Empirical evidence for the reality of such warning signals is still rare in ecology. We studied this on Zostera noltii intertidal seagrass meadows at their southern range limit, the Banc d'Arguin, Mauritania. We analyse the environmental covariates of recovery rates using structural equation modelling (SEM), based on an experiment in which we assessed whether recovery after disturbances (i.e. seagrass & infauna removal) depends on stress intensity (increasing with elevation) and disturbance patch size (1 m(2) vs. 9 m(2)). The SEM analyses revealed that higher biofilm density and sediment accretion best explained seagrass recovery rates. Experimental disturbances were followed by slow rates of recovery, regrowth occurring mainly in the coolest months of the year. Macrofauna recolonisation lagged behind seagrass recovery. Overall, the recovery rate was six times slower in the high intertidal zone than in the low zone. The large disturbances in the low zone recovered faster than the small ones in the high zone. This provides empirical evidence for critical slowing down with increasing desiccation stress in an intertidal seagrass system

Quantitative nuclear imaging for dosimetry in radioembolization

Intra-arterial microsphere radioembolization is an increasingly applied technique for treatment of unresectable liver tumors. During radioembolization, microspheres (diameter 30 μm) loaded with a high-energy beta-emitting radionuclide, such as yttrium-90 (Y-90) and holmium-166 (Ho-166), are instilled in the hepatic artery via a catheter. The majority of microspheres will accumulate in and around the liver tumors, because these are almost exclusively perfused by arterial blood, whereas the healthy liver tissue mainly depends on the portal vein for its blood supply. Prior to treatment, a scout dose of technetium-99m macro-aggregated albumin (Tc-99m-MAA) particles, which serve as microsphere-surrogates and aim to predict the in vivo distribution of the treatment dose of microspheres, is administered to the patient for treatment planning. Image-based assessment of both the scout dose and the treatment dose distribution (for therapy evaluation) can be performed by the nuclear imaging techniques Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). In this thesis, quantitative nuclear imaging methods are developed, evaluated, and exploited to enable accurate assessment of the in vivo Y-90 and Ho-166 microsphere distribution for dosimetry in radioembolization. It was found with phantom experiments and Monte Carlo simulations that Y-90 and Ho-166 gamma-camera images are heavily affected by photon scatter, photon attenuation, and penetration of high-energy photons through the collimator septa. Extensive correction for all of these image degrading effects is required to enable reconstruction of quantitatively accurate Y-90 and Ho-166 SPECT images. However, hindered by the broad range and continuous nature of the photon energy spectra, only approximate correction models are included in current state-of-the-art clinical Y-90 and Ho-166 SPECT reconstruction algorithms. In contrast, model-based correction techniques for scatter, random, and attenuation effects are clinically available for Y-90 PET imaging. Consequently, state-of-the-art clinical Y-90 PET images were found to be superior to state-of-the-art clinical Bremsstrahlung SPECT images for assessment of the microsphere distribution after Y-90 radioembolization. Motivated by the cost-effectiveness and wide availability of SPECT cameras, a new Y-90 Bremsstrahlung SPECT reconstruction algorithm was developed, incorporating a fast Monte Carlo simulator for correction of the image degrading effects. The Monte Carlo-based reconstruction approach substantially improved the quantitative accuracy of Y-90 Bremsstrahlung SPECT images, which may be used as an alternative to Y-90 PET images for post-treatment dosimetry in Y-90 radioembolization. The Monte Carlo-based reconstruction approach also improved the quantitative accuracy of Ho-166 SPECT images, as was demonstrated in phantom experiments. Furthermore, it was shown in a clinical Ho-166 radioembolization study that the in vivo distribution of both a scout dose and a treatment dose of Ho-166 microspheres can be accurately quantified withSPECT. A scout dose of Ho-166 microspheres was found to be a significantly better predictor for the mean radiation absorbed dose to the lungs, which is an important contra-indication for radioembolization, than a scout dose of Tc-99m-MAA microsphere-surrogates. These results indicate that treatment planning in radioembolization is improved by using the same quantifiable microspheres for scout dose and treatment

Artificial intelligence for prostate MRI: open datasets, available applications, and grand challenges

Artificial intelligence (AI) for prostate magnetic resonance imaging (MRI) is starting to play a clinical role for prostate cancer (PCa) patients. AI-assisted reading is feasible, allowing workflow reduction. A total of 3,369 multi-vendor prostate MRI cases are available in open datasets, acquired from 2003 to 2021 in Europe or USA at 3 T (n = 3,018; 89.6%) or 1.5 T (n = 296; 8.8%), 346 cases scanned with endorectal coil (10.3%), 3,023 (89.7%) with phased-array surface coils; 412 collected for anatomical segmentation tasks, 3,096 for PCa detection/classification; for 2,240 cases lesions delineation is available and 56 cases have matching histopathologic images; for 2,620 cases the PSA level is provided; the total size of all open datasets amounts to approximately 253 GB. Of note, quality of annotations provided per dataset highly differ and attention must be paid when using these datasets (e.g., data overlap). Seven grand challenges and commercial applications from eleven vendors are here considered. Few small studies provided prospective validation. More work is needed, in particular validation on large-scale multi-institutional, well-curated public datasets to test general applicability. Moreover, AI needs to be explored for clinical stages other than detection/characterization (e.g., follow-up, prognosis, interventions, and focal treatment)

The influence of self-absorption on PET and PET/CT shielding requirements

The high energy (511 keV) annihilation photons used in positron emission tomography (PET) imaging generally require a substantial amount of lead to protect personnel and the general public from ionizing radiation. A cost-effective design of the PET facility that ensures radiation does not exceed formal dose limits requires accurate estimation of the necessary PET shielding. The American Association of Physicists in Medicine (AAPM) Task Group 108 recently published broad beam transmission factors based on Monte Carlo calculations of 511 keV photons. In this work, an extension to the AAPM model is presented, based on Monte Carlo simulations including the effects of self-absorption on the photon energy spectrum. Monte Carlo calculations were performed using MCNPX. The photon energy spectrum after self-absorption was computed by simulating a normal 18FDG activity distribution in an anthropomorphic phantom. This spectrum was used to calculate the dose rate transmission factors for various wall thicknesses of lead, concrete, and iron. The method was validated by measurement and corresponding simulation of the transmission factors of an 18FDG source in air and in PMMA. Furthermore, a method to generate 3D area dose rate maps of PET facilities incorporating the calculated transmission tables is presented and applied to several shielding situations. The calculated self-absorption correction factor and the broad beam transmission factors resulting from Monte Carlo simulations of a monoenergetic point source emitting 511 keV photons were in excellent agreement with the results of the AAPM publication (0.66 vs 0.64 and R2 = 0.999, respectively). However, when all radiation physics, i.e., also the effect of self-absorption on the photon energy spectrum, is included in the Monte Carlo calculations, a substantial reduction in required shielding material was found. For example, including all radiation physics leads to 13.3 mm of lead required to obtain a typical transmission factor of 0.1, instead of 16.0 mm of lead when the AAPM data including only the self-absorption correction factor are used. These findings were confirmed by the experimental measurements. The transmission factors produced in this work can be applied in the same manner as those estimated by AAPM to allow for a cost-effective design of PET and PET/CT facilities without violating radiation safety regulations. Taking into account the effect of self-absorption on the photon energy spectrum results in more accurate and cost-effective shielding requirement estimation

Where does the salt marsh start? Field-based evidence for the lack of a transitional area between a gradually sloping intertidal flat and salt marsh

Salt marshes are vegetated ecosystems between land and sea, hosting unique plant and animal communities, contributing to crucial habitats for birds and providing numerous other ecosystem services. They form a sustainablenature-based coastal protection, and its protective value increases with the width of the system. Salt marshes and their adjoining tidal flats are often studied separately. At present, there is a lack of studies that an array of abiotic and biotic variables that potentially affect successful vegetation establishment.Unfavourable soil conditions may limit establishment as well as bioturbation of infauna. This is related to the marsh expansion potential and could aid in salt marsh restoration and marsh growth stimulation projects. In a naturally developing salt marsh, we sampled the elevation gradient from the tidal flat to the low marsh for abundance of benthos was highest landwards of the transition zone, in the pioneer zone. Distinct meiobenthic groups occurred in the different zones along the tidal flat to low marsh gradient, but macrobenthos was largely absent from the muddy soil. In the sparsely vegetated transition zone, the abundance of salt-marsh seeds was low, similar to the tidal flat. It suggeststhat, even though a seed source was in proximity, seed availability in spring was insufficient to achieve lateral marsh expansion. Clustering and nMDS analyses showed that an identifiable transition zone was lacking. The transition zone resembled the bare tidal flat in terms of its abiotic and biotic conditions. This was mainly driven by significant changes in soil oxygenation and seed availability