True Resolution Research Articles

Indirect detector for ultra-high-speed X-ray micro-imaging with increased sensitivity to near-ultraviolet scintillator emission.

Ultra-high-speed synchrotron-based hard X-ray (i.e. above 10 keV) imaging is gaining a growing interest in a number of scientific domains for tracking non-repeatable dynamic phenomena at spatio-temporal microscales. This work describes an optimized indirect X-ray imaging microscope designed to achieve high performance at micrometre pixel size and megahertz acquisition speed. The entire detector optical arrangement has an improved sensitivity within the near-ultraviolet (NUV) part of the emitted spectrum (i.e. 310-430 nm wavelength). When combined with a single-crystal fast-decay scintillator, such as LYSO:Ce (Lu2-xYxSiO5:Ce), it exploits the potential of the NUV light-emitting scintillators. The indirect arrangement of the detector makes it suitable for high-dose applications that require high-energy illumination. This allows for synchrotron single-bunch hard X-ray imaging to be performed with improved true spatial resolution, as herein exemplified through pulsed wire explosion and superheated near-nozzle gasoline injection experiments at a pixel size of 3.2 µm, acquisition rates up to 1.4 MHz and effective exposure time down to 60 ps.

(Invited) New Analytical Method of Near-Field Spectroscopy – hBN Polaritons in Heteronanotubes

Near-field spectroscopy is a promising approach for materials nanocharacterization, with a hope to reach beyond trivial materials’ imaging with a high spatial resolution. In a typical method, one uses the spectral modulation of the image spatial scales, determined from such a set of features of near-field scanning maps as a wave pattern or clear wave reflections (if not merely taking the spectral modulation of the image intensity). Applicability of these methods is rather limited and more sophisticated analytical technique would be desirable.Most severe restriction of abovementioned spectral techniques happens when the material under study possesses spatial non-uniformities much smaller than the excitation wavelength. Then, the patterns are highly influenced by the wave diffraction and not reliable to detect spectral information (in other words, the wave patterns are distorted by interference and excitation localization, which prohibits determining intrinsic wave parameters as a function of excitation).We have developed a new analytical technique which allows true sub-wavelength resolution, since the image domain required for the wave pattern analysis could be substantially smaller than a wavelength. The method has been used for studying hBN polaritons in heteronanotubes, the new material developed in Prof. Maruyama group at University of Tokyo. Acknowledgement: This work has been partially supported by NSF DMR-2011839 (MRSEC) and DMR-2039351 (2DCC MIP).

Impact of undersampling on preclinical lung T2* mapping with 3D radial UTE MRI at 7 T

Lung diseases are almost invariably heterogeneous and progressive, making it imperative to capture temporally and spatially explicit information to understand the disease initiation and progression. Imaging the lung with MRI—particularly in the preclinical setting—has historically been challenging because of relatively low lung tissue density, rapid cardiac and respiratory motion, and rapid transverse (T2*) relaxation. These limitations can largely be mitigated using ultrashort-echo-time (UTE) sequences, which are intrinsically robust to motion and avoid significant T2* decay. A significant disadvantage of common radial UTE sequences is that they require inefficient, center-out k-space sampling, resulting in long acquisition times relative to conventional Cartesian sequences. Therefore, pulmonary images acquired with radial UTE are often undersampled to reduce acquisition time. However, undersampling reduces image SNR, introduces image artifacts, and degrades true image resolution. The level of undersampling is further increased if offline gating techniques like retrospective gating are employed, because only a portion (∼40–50%) of the data is used in the final image reconstruction. Here, we explore the impact of undersampling on SNR and T2* mapping in mouse lung imaging using simulation and in-vivo data. Increased scatter in both metrics was noticeable at around 50% sampling. Parenchymal apparent SNR only decreased slightly (average decrease ∼ 1.4) with as little as 10% sampling. Apparent T2* remained similar across undersampling levels, but it became significantly increased (p < 0.05) below 80% sampling. These trends suggest that undersampling can generate quantifiable, but moderate changes in the apparent value of T2*. Moreover, these approaches to assess the impact of undersampling are straightforward to implement and can readily be expanded to assess the quantitative impact of other MR acquisition and reconstruction parameters.

58‐4: Late‐News Paper: Dynamic MTF Measurements of Gaming Monitors

Recent gaming monitors claim high refresh rates of ≥240 Hz with response times ≤1 ms. However, these temporal metrics do not clearly reveal their true spatiotemporal resolution, which depends on the velocity at which the images are displayed. This is the first study to demonstrate the line‐based dynamic modulation transfer function (MTF) measurement method for comparing the performances of different gaming monitors. A single line was scrolled on the monitors, and a small region of the screen was captured with a high‐speed camera during one display refresh period. The dynamic MTF results elucidate the dependence of spatiotemporal resolution characteristics on the scroll speed, refresh rate, and response time of the monitors.

Abstract 5496: Multi-omic spatial analysis of the tumor microenvironment in gliomas

Abstract Introduction: The diverse tumor environment of high-grade glioma, which remains refractory to treatment, demands new innovative, multi-omic approaches to characterize tumor heterogeneity and expression profiles associated with progression and therapeutic response. Approach: We have applied an integrated, multi-omic approach using paired spatial in situ sequencing (Xenium) and protein profiling (PhenoCycler® Fusion) of glioma tissue to identify distinct glioma cellular phenotypes, activation states and metabolic pathways at true single cell resolution. Our multi-omic approach used hundreds of phenotypic RNA target probes and &amp;gt;50 antibodies in an unbiased single-cell analysis of primary, recurrent, and IDH1 mutant/wild type tumors with a range of heterogeneity. Summary: We were able to distinguish differentially regulated genes and pathways between aggregated primary and recurrent GBM, including cell cycle pathways being upregulated in primary vs recurrent GBM glial cell clusters and a down regulation of ERBB4 signaling in vascular cell clusters in primary GBMs. Our data also suggests that SOX11, known to be involved in tumorigenesis, is &amp;gt;2-fold upregulated in annotated tumor cells in primary compared to recurrent tumors. We were also able to quantitatively localize the expression of Epidermal Growth Factor Receptor, variant III (EGFRvIII) to specific tumor-cell sub-types, which is important given its association with therapeutic resistance. Further, we have been able to establish the cellular neighborhoods and cell-cell and receptor-ligand relationships within the tumors. Our spatial protein analysis identified distinct tumor and immune phenotypes with varying abundance of myeloid and lymphoid populations in IDH1 wt and mt tumors. Moreover, spatial proximity analyses and cellular neighborhood analyses revealed differences in higher order organizational landscapes that may contribute to the differential outcomes across wt and mt tumor subtypes. Conclusion: Multiomic spatial analysis enables deeper characterization of the glioma cellular and functional landscape to broaden our understanding of the key TME features that contribute to disease pathogenesis and prognoses. Our study provides an analytical framework to combine RNA and protein-based spatial data for a holistic investigation into a variety of glioma subtypes and aid in the identification of novel biomarkers, spatial neighborhoods, and functional states that drive glioma progression. Citation Format: Dmytro Klymyshyn, Vaibhav Jain, Lauren Whaley, Emily Hocke, Bassem Ben Cheikh, Alan Smith, Karen Abramson, Nadine Nelson, Diane Satterfield, Elizabeth Thomas, Giselle Lopez, Seetha Hariharan, Michael Brown, Niyati Jhaveri, Roger McLendon, David Ashley, Matthew Waitkus, Simon Gregory. Multi-omic spatial analysis of the tumor microenvironment in gliomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5496.

Reusable HNO Sensors Derived from Cu Cyclam: A DFT Study on the Mechanistic Origin of High Reactivity and Favorable Conformation Changes and Potential Improvements.

Nitroxyl (HNO) exhibits unique favorable properties in regulating biological and pharmacological activities. However, currently, there is only one Cu-based HNO sensor that can be recycled for reusable detection, which is a Cu cyclam derivative with a mixed thia/aza ligand. To elucidate the missing mechanistic origin of its high HNO reactivity and subsequent favorable conformation change toward a stable CuI product that is critical to be oxidized back by the physiological O2 level for HNO detection again, a density functional theory (DFT) computational study was performed. It not only reproduced experimental structural and reaction properties but also, more importantly, revealed an unknown role of the coordination atom in high reactivity. Its conformation change mechanism was found to not follow the previously proposed one but involve a novel favorable rotation pathway. Several newly designed complexes incorporating beneficial effects of coordination atoms and substituents to further enhance HNO reactivity while maintaining or even improving favorable conformation changes for reusable HNO detection were computationally validated. These novel results will facilitate the future development of reusable HNO sensors for true spatiotemporal resolution and repeated detection.

Development of a modular system to provide confidence in porosity analysis of additively manufactured components using x-ray computed tomography

X-ray computed tomography (XCT) offers a promising non-destructive method to assess high value components that are additively manufactured (AM) for space-based imaging. However, AM components can be often challenging to measure and the true resolution of the XCT system used is both non-trivial to determine and may change locally. To solve this, we used high precision micro-machining to manufacture a cylindrical reference pin with internal holes. This pin can then be inserted into any component via subtractive machining, prior to the XCT process. A pre-existing AM flexure is modified to allow our modular system to be implemented. This allows XCT scanning and porosity analysis of similar components (similar geometry and manufacturing process) to be refined and adjusted based on the known internal micro-machined hole size. Analysis of the XCT volumetric data is implemented using a Python script developed for Avizo 2022.1, to compare and suggest the ideal threshold grey value (GV). The plugin threshold comparison is semi-automatic and 15 times faster than a manual comparison. Study findings showed how different calibrated micro-machined hole sizes (30 μm–120 μm) needed different thresholding values (188 GV–195 GV). Challenges and future studies related to traceability of the suggested method are discussed.

Single-shot local measurement of terahertz correlated second harmonic generation in laser air plasma filaments.

We present a single-shot detection method of terahertz correlated second harmonic generation in plasma-based sources by directly mixing an optical probe into femtosecond laser-induced plasma filaments in air. The single-shot second harmonic trace is obtained by measuring a second harmonic generation on a conventional CCD with a spatiotemporally distorted probe beam. The system shows a spectrometer resolution of 22 fs/pixel on the CCD and a true resolution on the order of the probe pulse duration. With considerable THz peak electric field strength, this formalism can open the door to single-shot THz detection without bandwidth limitations.

Fusion of altimetry-derived model and ship-borne data in preparation of high-resolution marine gravity determination

SUMMARY Satellite altimetry provides major data sources for marine gravity recovery, and typical altimetry derived models, for example, DTU21 and SS V32.1, were usually released with 1 arcmin × 1 arcmin gridding interval. Their true resolution is much lower than the nominal ∼2 km level. By contrast, the in situ ship-borne measurements are considered to have better short-wavelength resolution. In this paper, we aim to propose a new method to fully utilize satellite altimetry data and ship-borne measurements, namely the frequency-domain fusion method, and give certain analysis of new method along with two spatial-domain fusion methods. Comprehensive analysis is focus on four aspects: gravity signals in fusion images, numerical verifications, power spectra, as well as coherence analysis. Initial evaluation indicates that, first, the frequency-domain fusion method has advantage in flexibility, since it can autonomously select dominant bands to fuse different data sets. Secondly, the new method retains medium-long wavelength signals from altimetry-derived model and effectively incorporate dominant short-wavelength signals of in-situ measurements, while the spatial-domain methods are essentially full-wavelength fusion and inevitably diminish the role of satellite altimetry. To some extent, the new method maximize the positive contribution of satellite altimetry measurements and efficiently exploit the benefits of ship-borne data. Finally, verification experiments were similarly designed in three regions with different amount and ratio of ship-borne data to thoroughly evaluate various methods. In two regions with gridded and dense along-cruise ship-borne data, the average accuracy of this frequency-domain fusion results is improved by 0.346 and 0.613 mGal, respectively. In a region with sparse ship-borne data, we still recommend using spatial-domain fusion methods since the new method is unable to align ship-borne data with model grid. It is concluded from the above analysis that the new method effectively incorporates the short-wavelength signals from ship-borne data into the altimetry-derived gravity field model, and it is significant that the new method maximizes the application of advantageous bands from different data sources.

High-resolution aerosol data from the top 3.8 kyr of the East Greenland Ice coring Project (EGRIP) ice core

Abstract. Here we present the high-resolution continuous flow analysis (CFA) data from the top 479 m of the East Greenland Ice coring Project (EGRIP) ice core covering the past 3.8 kyr. The data consist of 1 mm depth-resolution profiles of calcium, sodium, ammonium, nitrate, and electrolytic conductivity as well as decadal averages of these profiles. The nominally 1 mm data represent an oversampling of the record as the true resolution is limited by the analytical setup to approximately 1 cm. Alongside the data we provide a description of the measurement setup, procedures, the relevant references for the specific methods as well as an assessment of the precision of the measurements, the sample-to-depth assignment, and the depth and temporal resolution of the data set. The error in absolute depth assignment of the data may be on the order of 2 cm; however, relative depth offsets between the records of the individual species are only on the order of 1 mm. The presented data have sub-annual resolution over the entire depth range and have already formed part of the data for an annually layer-counted timescale for the EGRIP ice core used to improve and revise the multi-core Greenland ice-core chronology (GICC05) to a new version, GICC21 (Sinnl et al., 2022). The data are available in full 1 mm resolution and decadal averages on PANGAEA (https://doi.org/10.1594/PANGAEA.945293, Erhardt et al., 2022b).

Optimization and characterization of toroidal foci for super-resolution fluorescence microscopy: tutorial

Single-molecule localization microscopy (SMLM) has become an essential tool to investigate phenomena at the nanoscale. Among the different SMLM approaches, methods that interrogate the molecular position with an intensity minimum, such as minimal emission fluxes (MINFLUX) or the more recent raster scanning a minimum of light (RASTMIN), stand out for reaching true molecular resolution. To implement these methods, the phase of the excitation beam needs to be modulated to obtain a focus with a central minimum, i.e., a so-called toroidal or doughnut-shaped focus. In this tutorial, we explain the basis and experimental tricks to generate and optimize such beams, particularly in raster-scanning microscopes.

Estimating the true spatial resolution of satellite altimeter-derived gravity field models with shipborne data in the South China Sea

Estimating the true spatial resolution of satellite altimeter-derived gravity field models with shipborne data in the South China Sea

Nondestructive thermographic detection of internal defects using pixel-pattern based laser excitation and photothermal super resolution reconstruction

In this work, we present a novel approach to photothermal super resolution based thermographic resolution of internal defects using two-dimensional pixel pattern-based active photothermal laser heating in conjunction with subsequent numerical reconstruction to achieve a high-resolution reconstruction of internal defect structures. With the proposed adoption of pixelated patterns generated using laser coupled high-power DLP projector technology the complexity for achieving true two-dimensional super resolution can be dramatically reduced taking a crucial step forward towards widespread practical viability. Furthermore, based on the latest developments in high-power DLP projectors, we present their first application for structured pulsed thermographic inspection of macroscopic metal samples. In addition, a forward solution to the underlying inverse problem is proposed along with an appropriate heuristic to find the regularization parameters necessary for the numerical inversion in a laboratory setting. This allows the generation of synthetic measurement data, opening the door for the application of machine learning based methods for future improvements towards full automation of the method. Finally, the proposed method is experimentally validated and shown to outperform several established conventional thermographic testing techniques while conservatively improving the required measurement times by a factor of 8 compared to currently available photothermal super resolution techniques.

Impact of Beam Shape on Print Accuracy in Digital Light Processing Additive Manufacture.

Photopolymerization-based additive manufacturing requires selectively exposing a feedstock resin to ultraviolet (UV) light, which in digital light processing is achieved either using a digital micromirror device or a digital mask. The minimum tolerances and resolution for a multilayer process are separate for resolution through the Z-axis, looking through the thickness of a printed part, and resolution in the XY-axes, in the plane of the printed layer. The former depends wholly on the rate of attenuation of the incident UV light through the material relative to the mechanical motion of the build layer, while the latter is determined by a two-dimensional pattern of irradiance on the resin formed by the digital micromirror device or the digital mask. The size or the spacing of elements or pixels of this digital mask is frequently given by manufacturers as the "resolution" of the device, however, in practice the achievable resolution is first determined by the beam distribution from each pixel. The beam distribution is, as standard, modeled as a two-parameter Gaussian distribution but the key parameters of peak intensity and standard deviation of the beam are hidden to the user and difficult to measure directly. The ability of models based on the Gaussian distribution to correctly predict the polymerization of printed features in the microscale is also typically poor. In this study, we demonstrate an alternative model of beam distribution based on a heavy-tailed Lorentzian model, which is able to more accurately predict small build areas for both positive and negative features. We show a simple calibration method to derive the key space parameters of the beam distribution from measurements of a single-layer printed model. We propose that the standard Gaussian model is insufficient to accurately predict a print outcome as it neglects higher-order terms, such as beam skew and kurtosis, and in particular failing to account for the relatively heavy tails of the beam distribution. Our results demonstrate how the amendments to the beam distribution can avoid errors in microchannel formation, and better estimates of the true XY-axes resolution of the printer. The results can be used as the basis for voxel-based models of print solidification that allow software prediction of the photopolymerization process.

Improving the performance of web-streaming by super-resolution upscaling techniques

In recent years, we have seen significant progress in advanced image upscaling techniques, sometimes called super-resolution, ML-based, or AI-based upscaling. Such algorithms are now available not only in form of specialized software but also in drivers and SDKs supplied with modern graphics cards. Upscaling functions in NVIDIA Maxine SDK is one of the recent examples. However, to take advantage of this functionality in video streaming applications, one needs to (a) quantify the impacts of super-resolution techniques on the perceived visual quality, (b) implement video rendering incorporating super-resolution upscaling techniques, and (c) implement new bitrate+resolution adaptation algorithms in streaming players, enabling such players to deliver better quality of experience or better efficiency (e.g. reduce bandwidth usage) or both. Towards this end, in this paper, we propose several techniques that may be helpful to the implementation community. First, we offer a model quantifying the impacts of super resolution upscaling on the perceived quality. Our model is based on the Westerink-Roufs model connecting the true resolution of images/videos to perceived quality, with several additional parameters added, allowing its tuning to specific implementations of super-resolution techniques. We verify this model by using several recent datasets including MOS scores measured for several conventional up-scaling and super-resolution algorithms. Then, we propose an improved adaptation logic for video streaming players, considering video bitrates, encoded video resolutions, player size, and the upscaling method. This improved logic relies on our modified Westerink-Roufs model to predict perceived quality and suggests choices of renditions that would deliver the best quality for given display and upscaling method characteristics. Finally, we study the impacts of the proposed techniques and show that they can deliver practically appreciable results in terms of the expected QoE improvements and bandwidth savings.

Does the slanted-edge method provide the true value of spatial frequency response?

The slanted-edge method is commonly used for measuring the spatial frequency response (SFR) of digital imaging systems. To investigate the intrinsic capability of the slanted-edge method, this study formulated a procedure for calculating the SFR and proposed a numerical method for evaluating the accuracy of the estimated SFR relative to the theoretical value. It is shown that the slanted-edge method yields an accurate value of the SFR under ideal conditions and is a rational and consistent tool for determining the true resolution characteristics. However, under practical conditions, this includes some estimation errors. Based on this analysis, it is possible to identify the parameter that influences the estimation accuracy in the SFR computation.

Electrochemical additive manufacturing (ECAM): A new approach to fabricate metal nanostructures

Nanoscale printing is growing rapidly as a feasible fabrication method for a wide range of applications, including electronics and nanooptics, sensors, nanorobots, and energy storage. In today’s cutting-edge technologies, the constantly reducing dimension necessitates the production of complex conductive structures with nanometre resolution., ECAM methods can produce impurity-free metallic conductors with excellent electrical and mechanical properties; however, the true nanoscale resolution remains very challenging. Here, silver nanostructures are synthesized by an ECAM process and investigated as a substrate for surface-enhanced Raman scattering (SERS) detection of aspirin. Various characterizations such as XRD, TEM, HR-TEM, FE-SEM, EDX-mapping, optical, ultraviolet–visible, and Raman spectroscopy are used to analyse ECAM-printed nanostructured silver substrates. A remarkable enhancement factor of ∼ 107 is realized in the silver nanostructured substrates with the detected limit of 10-8 mol. ECAM printed silver nanostructured substrates can thus be used as viable substrates for SERS applications.

Description and comparison of 21st century thermosphere data

The quality and distribution in time and space of available atmospheric observations are crucial for the accuracy of semi-empirical thermosphere models. However, datasets can be inconsistent, and their qualities and resolutions are often unequal. The main thermospheric density datasets of this century are briefly described and then compared to each other when possible in order to quantify differences. Total mass densities used in the comparisons include all high-resolution CHAMP, GRACE and GOCE data, Swarm A, daily-mean Stella, global daily mean TLE densities, and the SET HASDM density database. The temperature data from TIMED-SABER are also reviewed.The recently updated daily-mean TLE densities (TLE2021) are 2–10 % smaller on average than the previous version (TLE2015). The differences are not constant offsets per altitude level, but fluctuations of up to 5 % are present. Compared to HASDM densities for 6 altitudes from 250 to 675 km, TLE2021 is 15–20 % smaller at 250 km, and then the difference diminishes with altitude to reach the same average value at 575 km. These mean differences also fluctuate by a few percent on time scales of months, to 10 % over half a solar cycle at 575 km. The TLE2021 and HASDM densities are larger than the accelerometer-inferred CHAMP, GRACE and GOCE densities and average offsets are 10–15 % and 10–20 %, respectively. The comparison to Swarm-A and Swarm-B showed mean offsets of 10 % and less, with significant positive trends seen in the comparison with HASDM. Finally, largest differences are found for Stella and HASDM at 800 km, up to 45 % with strong semiannual variations.This study clearly shows that the available density data cannot be simply assimilated or combined without first accurately calibrating the data. The HASDM database is a valuable asset due to its considerable coverage in space and time, but its uncertainty and true resolution are not well understood and are still being evaluated. Data compatibility requires employing physically accurate and harmonized aerodynamic force models in the density derivation procedure, which is presently not achieved. The accuracy of the procedure, independent of the quality of the instrument (GNSS receiver, ground-based orbit determination, or accelerometer), inevitably decreases with altitude due to weakening of the drag signal to noise ratio.The TIMED-SABER instrument provides measurements of pressure and temperature in the lower thermosphere. SABER temperature uncertainty is well-known. The SABER dataset now exceeds twenty years and has been continuously operating that entire time. It was ingested in NRLMSIS 2.0 and comparisons show the much-improved fit in comparison with NRLMSISE-00. The lower thermosphere temperatures significantly modify density at higher altitudes, and its measurement is essential for modeling and assessment.

Compact and versatile neutron imaging detector with sub-4μm spatial resolution based on a single-crystal thin-film scintillator.

A large and increasing number of scientific domains pushes for high neutron imaging resolution achieved in reasonable times. Here we present the principle, design and performance of a detector based on infinity corrected optics combined with a crystalline Gd3Ga5O12 : Eu scintillator, which provides an isotropic sub-4 µm true resolution. The exposure times are only of a few minutes per image. This is made possible also by the uniquely intense cold neutron flux available at the imaging beamline NeXT-Grenoble. These comparatively rapid acquisitions are compatible with multiple high quality tomographic acquisitions, opening new venues for in-operando testing, as briefly exemplified here.

Characterization of mesenteric and portal hemodynamics using 4D flow MRI: the effects of meals and diurnal variation.

To determine the variability of blood flow measurements using 4D flow MRI in the portal and mesenteric circulations and to characterize the effects of meal ingestion, time of day, and between-day (diurnal) variations on portal and mesenteric hemodynamics. In this IRB-approved and HIPAA-compliant study, 7 healthy and 7 portal hypertension patients imaged. MRI exams were conducted at 3T using a 32-channel body coil with large volumetric coverage and 1.25-mm isotropic true spatial resolution. Blood flow was quantified (L/min) in the hepatic and splanchnic vasculature. The first MR scan was performed after at least 8h of fasting. Subsequently, subjects ingested 574mL EnSure Plus® orally. A second acquisition was started 20min after the meal ingestion. A third scan was performed before lunch and a fourth acquisition took place 20min after lunch. A fifth scan was performed around 4pm. Finally, subjects returned one week later for a repeat morning visit, with identical conditions as the first visit. In healthy controls significant increase in blood flow was seen in the PV, SMV, SMA, HA, and SCAo in response to breakfast but only the SCAo, SMA, SMV, and PV had a significant response to lunch. In general, patients with cirrhosis showed reduced response to meals compared to that in healthy controls. Additionally, PV flow in patients had the highest value in the afternoon. Effects of meal ingestion, time of day, and between-day variations were characterized using Radial 4D flow MRI in patients with cirrhosis and healthy controls.