Magnetic Resonance Imaging Velocimetry Research Articles

MRI hydrodynamic characterization of an ambr15® bioreactor

This work presents the first use of magnetic resonance imaging (MRI) velocimetry to study the flow field inside an ambr® microscale bioreactor. Scale-down bioreactor systems have become popular in order to meet the key biopharmaceutical development goals of reduced costs, accelerated process development timelines and increased productivity. Scalable cell growth characteristics have been reported for the 15 mL ambr®; however, the vessel configuration is dissimilar to that of large scale bioreactors. This results in different fluid dynamics and flow properties, which will influence both mixing and mass transfer. Investigations into the hydrodynamics of the ambr® system and its variation throughout a cell culture run are therefore of interest. MRI provides a non-invasive method for the highly accurate visualisation of these hydrodynamics. Through the use of MRI velocimetry it is shown that the asymmetric vessel geometry and positioning of the impeller and gas sparge tube appear to reduce vortex formation and create flow profiles more typical of larger scale, baffled, cylindrical bioreactors. Volumetric changes and impeller pumping direction are found to have a greater influence on the flow pattern than impeller speed and aeration. At increased suspension densities, dead zones and surface effects are seen.

Comparison of Four-Dimensional Flow Magnetic Resonance Imaging and Particle Image Velocimetry to Quantify Velocity and Turbulence Parameters

Although recent advances of four-dimensional (4D) flow magnetic resonance imaging (MRI) has introduced a new way to measure Reynolds stress tensor (RST) in turbulent flows, its measurement accuracy and possible bias have remained to be revealed. The purpose of this study was to compare the turbulent flow measurement of 4D flow MRI and particle image velocimetry (PIV) in terms of velocity and turbulence quantification. Two difference flow rates of 10 and 20 L/min through a 50% stenosis were measured with both PIV and 4D flow MRI. Not only velocity through the stenosis but also the turbulence parameters such as turbulence kinetic energy and turbulence production were quantitatively compared. Results shows that 4D flow MRI velocity measurement well agreed with the that of PIV, showing the linear regression slopes of two methods are 0.94 and 0.89, respectively. Although turbulence mapping of 4D flow MRI was qualitatively agreed with that of PIV, the quantitative comparison shows that the 4D flow MRI overestimates RST showing the linear regression slopes of 1.44 and 1.66, respectively. In this study, we demonstrate that the 4D flow MRI visualize and quantify not only flow velocity and also turbulence tensor. However, further optimization of 4D flow MRI for better accuracy might be remained.

A mini-module with built-in spacers for high-throughput ultrafiltration

Ultrafiltration membrane modules suffer from a permeate flow decrease arising during filtration and caused by concentration polarization and fouling in, e.g., fermentation broth purification. Such performance losses are frequently mitigated by manipulating the hydrodynamic conditions at the membrane–fluid interface using, e.g., mesh spacers acting as static mixers.This additional element increases manufacturing complexity while improving mass transport in general, yet accepting their known disadvantages such as less transport in dead zones. However, the shape of such spacers is limited to the design of commercially available spacer geometries. Here, we present a methodology to design an industrially relevant mini-module with an optimized built-in 3D spacer structure in a flat-sheet ultrafiltration membrane module to eliminate the spacer as a separate part. Therefore, the built-in structures have been conceptually implemented through an in-silico design in compliance with the specifications for an injection molding process. Ten built-in structures were investigated in a digital twin of the mini-module by 3D-CFD simulations to select two options, which were then compared to the empty feed channel regarding mass transfer. Subsequently, the simulated flux increase was experimentally verified during bovine serum albumin (BSA) filtration. The new built-in sinusoidal corrugation outperforms conventional mesh spacer inlays by up to 30% higher permeation rates. The origin of these improvements is correlated to the flow characteristics inside the mini-module as visualized online and in-situ by low-field and high-field magnetic resonance imaging velocimetry (flow-MRI) during pure water permeation.

The liquid regime of waxy oils suspensions: A magnetic resonance velocimetry analysis

In view of better understanding the properties of waxy crude oils of great practical importance, we study the rheological behavior evolution of model waxy suspensions during transient shear tests, and its variation with concentration in a wide range. Macroscopic tests show a strong destructuring process with the increasing imposed shear rate (starting from rest), while negligible restructuring is observed during shear rate decrease. MRI (Magnetic Resonance Imaging) velocimetry of the suspension during the same tests allow to observe that, in most of the concentration range tested, the material flows only in a limited thickness close to the inner cylinder, and the thickness of this shear-band decreases for increasing concentration but marginally evolves as the apparent shear rate is widely increased then decreased. This means that the extent of the liquid region is governed by the initial start-up flow conditions, and the apparent viscosity variations during the next steps (at other shear rate values) are not associated with an increase of the flowing region but with the evolution of the structure in the liquid region. For the specific case of the lowest wax content analyzed, the thickness of the unsheared region decreases considerably after shearing at a high shear rate. In this case the torque balance in the sample is capable of overcoming the yield stress of the unsheared region. We also show that despite very different (initial) elastic moduli and yield stresses in the solid regime (a factor of several orders of magnitude over the whole range of concentration), we essentially have the same power-law behavior of the fluid in the shear-band, even when the shear-band thickness is close to the particle size.

Wake volume of injected bubbles in fluidized beds: A magnetic resonance imaging velocimetry study

Rapid magnetic resonance imaging (MRI) was used to determine the volume of wakes of bubbles injected into incipiently fluidized beds. MRI was used to generate 2D maps of particle velocity surrounding bubbles with a temporal resolution of 18 ms. The bubble rise velocity, ub, was determined from the change in bubble position in successive maps, and pixels of particles which had an upward velocity greater than or equal to 0.8 ub were considered to be in the “wake” if they were below the bubble and in the “crown” if they were above the bubble. Results show that (i) the wake volume is much larger than the crown volume and (ii) the ratio of the wake volume to bubble volume increases as the bubble rises in the bed but decreases with increasing bubble volume. The influence of two bubbles interacting on bubble wake volume is also investigated. Comparison with geometric theory for wake volume exposes flaws in this theory for fully developed bubbles, bubbles near the distributor and interacting bubbles.

Dynamic Behavior of Dilute Bentonite Suspensions under Different Chemical Conditions Studied via Magnetic Resonance Imaging Velocimetry

This study investigates dilute aqueous suspensions of bentonite particles using magnetic resonance imaging (MRI) velocimetry. Four different chemical conditions are tested to investigate the influence of pH and type of monovalent electrolyte on the local rheological behavior of bentonite suspensions. The results indicate the shear banding in a dilute suspension of 0.1 vol.% solid due to the formation of a continuous three-dimensional particle network under a certain chemical environment (i.e., pH 4 in 1 × 10−2 M KNO3). This network is responsible for the existence of the yield stress in that dilute suspension. Structural changes induced by modification of suspensions’ chemistry are examined via scanning electron microscopy. A previously established method based on processing the torques acquired via conventional rheometric measurement is also applied as an alternative way to recover local flow information. Within the shear rate range covered by our MRI velocimetry, the results of both methods show good agreement. This study suggests that the existence of a master curve (or global flow curve) for dilute suspensions is dependent on the bentonite particle organization, which is influenced by the suspension chemistry and the previous flow history.

Validation and reproducibility of cardiovascular 4D-flow MRI from two vendors using 2 × 2 parallel imaging acceleration in pulsatile flow phantom and invivo with and without respiratory gating.

Background4D-flow magnetic resonance imaging (MRI) is increasingly used.PurposeTo validate 4D-flow sequences in phantom and in vivo, comparing volume flow and kinetic energy (KE) head-to-head, with and without respiratory gating.Material and MethodsAchieva dStream (Philips Healthcare) and MAGNETOM Aera (Siemens Healthcare) 1.5-T scanners were used. Phantom validation measured pulsatile, three-dimensional flow with 4D-flow MRI and laser particle imaging velocimetry (PIV) as reference standard. Ten healthy participants underwent three cardiac MRI examinations each, consisting of cine-imaging, 2D-flow (aorta, pulmonary artery), and 2 × 2 accelerated 4D-flow with (Resp+) and without (Resp−) respiratory gating. Examinations were acquired consecutively on both scanners and one examination repeated within two weeks. Volume flow in the great vessels was compared between 2D- and 4D-flow. KE were calculated for all time phases and voxels in the left ventricle.ResultsPhantom results showed high accuracy and precision for both scanners. In vivo, higher accuracy and precision (P < 0.001) was found for volume flow for the Aera prototype with Resp+ (–3.7 ± 10.4 mL, r = 0.89) compared to the Achieva product sequence (–17.8 ± 18.6 mL, r = 0.56). 4D-flow Resp− on Aera had somewhat larger bias (–9.3 ± 9.6 mL, r = 0.90) compared to Resp+ (P = 0.005). KE measurements showed larger differences between scanners on the same day compared to the same scanner at different days.ConclusionSequence-specific in vivo validation of 4D-flow is needed before clinical use. 4D-flow with the Aera prototype sequence with a clinically acceptable acquisition time (<10 min) showed acceptable bias in healthy controls to be considered for clinical use. Intra-individual KE comparisons should use the same sequence.

Fluid flow in a porous medium with transverse permeability discontinuity

Magnetic Resonance Imaging (MRI) velocimetry methods were used to study fully developed axially symmetric fluid flow in a model porous medium of cylindrical symmetry with a transverse permeability discontinuity. Spatial mapping of fluid ow resulted in radial velocity profiles. High spatial resolution of these profiles allowed the estimating of the slip in velocities at the boundary with a permeability discontinuity zone in a sample. The profiles were compared to theoretical velocity fields for a fully developed axially symmetric flow in a cylinder derived from the Joseph and Beavers and the Brinkman models. Velocity fields were also computed using pore-scale lattice Boltzmann Modelling (LBM) where the assumption about the boundary could be omitted. Both approaches gave a good agreement between theory and experiment though LBM velocity fields followed experiment more closely. This work shows great promise for MRI velocimetry methods in addressing the boundary behavior of fluids in opaque heterogeneous porous media.

Hemodynamic forces in the left and right ventricles of the human heart using 4D flow magnetic resonance imaging: Phantom validation, reproducibility, sensitivity to respiratory gating and free analysis software.

PurposeTo investigate the accuracy, reproducibility and sensitivity to respiratory gating, field strength and ventricle segmentation of hemodynamic force quantification in the left and right ventricles of the heart (LV and RV) using 4D-flow magnetic resonance imaging (MRI), and to provide free hemodynamic force analysis software.Materials and methodsA pulsatile flow phantom was imaged using 4D flow MRI and laser-based particle image velocimetry (PIV). Cardiac 4D flow MRI was performed in healthy volunteers at 1.5T (n = 23). Reproducibility was investigated using MR scanners from two different vendors on the same day (n = 8). Subsets of volunteers were also imaged without respiratory gating (n = 17), at 3T on the same day (n = 6), and 1–12 days later on the same scanner (n = 9, median 6 days). Agreement was measured using the intraclass correlation coefficient (ICC).ResultsPhantom validation showed good accuracy for both scanners (Scanner 1: bias -14±9%, y = 0.82x+0.08, R2 = 0.96, Scanner 2: bias -12±8%, y = 0.99x-0.08, R2 = 1.00). Force reproducibility was strong in the LV (0.09±0.07 vs 0.09±0.07 N, bias 0.00±0.04 N, ICC = 0.87) and RV (0.09±0.06 vs 0.09±0.05 N, bias 0.00±0.03, ICC = 0.83). Strong to very strong agreement was found for scans with and without respiratory gating (LV/RV: ICC = 0.94/0.95), scans on different days (ICC = 0.92/0.87), and 1.5T and 3T scans (ICC = 0.93/0.94).ConclusionSoftware for quantification of hemodynamic forces in 4D-flow MRI was developed, and results show high accuracy and strong to very strong reproducibility for both the LV and RV, supporting its use for research and clinical investigations. The software including source code is released freely for research.

3D MRI velocimetry of non-transparent 3D-printed staggered herringbone mixers

In microfluidics confocal microscopy and PIV are well-suited instruments to analyze laminar and complex fluid dynamics. However, these analysis methods require transparent devices made of e.g. glass or silicon rubber. This prerequisite of transparency precludes investigations of complex geometries generated by modern 3D printing techniques. Yet, 3D printing enables the engineer to free-form fabricate chemical engineering devices of any complex fluidic architecture. Based on a 3D printed meandering channel reactor with incorporated staggered herringbone structure we exploit the limits of 3D flow magnetic resonance imaging (MRI) for the analysis of fluid dynamics on a sub-millimeter scale. We visualize the 3D flow field, in particular the secondary eddy flows and derive shear rate maps. CFD simulations of the virtual shadow are in agreement with our experimental findings and proof that flow MRI gives reliable experimental access to non-transparent flow geometries.

Heat girdling does not affect xylem integrity: an invivo magnetic resonance imaging study in the tomato peduncle.

Heat girdling is a method to estimate the relative contribution of phloem vs xylem water flow to fruit growth. The heat girdling process is assumed to destroy all living tissues, including the phloem, without affecting xylem conductivity. However, to date, the assumption that xylem is not affected by heat girdling remains unproven. In this study, we used invivo magnetic resonance imaging (MRI) velocimetry to test if heat girdling can cause xylem vessels to embolize or affect xylem water flow characteristics in the peduncle of tomato (Solanum lycopersicum cv Dirk). Anatomical and MRI data indicated that, at the site of girdling, all living tissues were disrupted, but that the functionality of the xylem remained unchanged. MRI velocimetry showed that the volume flow through the secondary xylem was not impeded by heat girdling in either the short or the long term (up to 91h after girdling). This study provides support for the hypothesis that in the tomato peduncle the integrity and functionality of the xylem remain unaffected by heat girdling. It therefore confirms the validity of the heat girdling technique as a means to estimate relative contributions of xylem and phloem water flow to fruit growth.

Quantitative exploitation of PFG NMR and MRI velocimetry data for the rheological study of yield stress fluid flows at macro- and micro-scales in complex geometries

We explore the use of magnetic resonance imaging (MRI) velocimetry and pulsed field gradient nuclear magnetic resonance (PFG NMR) data for studying the flow characteristics of yield stress fluids through model pores (a succession of ducts of different diameters) or real porous media (bead packings). We propose different methods for the quantitative analysis of the velocity field, aimed at getting a deep understanding of the different flow regimes (solid and liquid) which typically take place in such fluids and at seeing how the transition from one to the other occurs in space or in time. Our approach exemplifies interdependences between PFG NMR data and local flow features and how the statistical velocity distribution function obtained by this way can be used and/or processed for extracting quantitative information concerning critical flow characteristics at a local scale. This provides a solid framework of analysis of flows through porous media with pores much smaller than the resolution of MR velocimetry.

Understanding the operation and preparation of diesel particulate filters using a multi-faceted nuclear magnetic resonance approach

In the present study, a range of magnetic resonance techniques have been applied to investigate two aspects associated with the preparation and operation of diesel particulate filters (DPFs). First, magnetic resonance techniques are used to investigate the evolution of the water distribution within a DPF during a drying process, typical of that associated with the preparation of a catalyzed DPF. For comparison, the drying characteristics of a flow-through monolith were also studied. In the flow-through monolith, which has impermeable walls, the drying front was observed to propagate from the very start of drying. Conversely drying in the DPF was characterized by uniform, axial drying up to a critical point, after which a drying front moved progressively through the DPF until it was fully dried. An explanation of this two-stage process, by taking into consideration the permeability of the channel walls, is suggested. Second, magnetic resonance imaging velocimetry was used to investigate the flow through a DPF. It is demonstrated that magnetic resonance imaging velocimetry is able to quantitatively and non-invasively characterize the flow fields in two DPF substrates, with asymmetric and symmetric channel geometries, respectively.

Rapid Multiphase Flow Dynamics Mapped by Single‐Shot MRI Velocimetry

A new, fast magnetic resonance imaging (MRI) method is described and applied to map flow fields in systems with internal velocities rapidly varying along the streamlines. While conventional MRI techniques encode the velocity information in a preparatory period prior to the imaging acquisition module, our technique repeatedly refreshes the velocity encoding during a single-shot imaging sequence. In this way, the maximum acceleration responsible for velocity variation of the molecules is increased by up to two orders of magnitude compared to standard procedures. Besides being compatible with high acceleration, this pulse sequence is suited to acquiring in a single scan the multiple velocity images required to construct a full velocity vector map. The power of this new methodology is demonstrated by following the internal dynamics of toluene droplets levitating in a counterflow of water during mass transfer of acetone from the water phase into the drop in the presence of surface-active impurities. The dramatic reduction in measurement time allows visualization for the first time of the important impact of even small concentrations of acetone on accumulation of surfactants at the drop's surface.

Sieve tube geometry in relation to phloem flow.

Sieve elements are one of the least understood cell types in plants. Translocation velocities and volume flow to supply sinks with photoassimilates greatly depend on the geometry of the microfluidic sieve tube system and especially on the anatomy of sieve plates and sieve plate pores. Several models for phloem translocation have been developed, but appropriate data on the geometry of pores, plates, sieve elements, and flow parameters are lacking. We developed a method to clear cells from cytoplasmic constituents to image cell walls by scanning electron microscopy. This method allows high-resolution measurements of sieve element and sieve plate geometries. Sieve tube-specific conductivity and its reduction by callose deposition after injury was calculated for green bean (Phaseolus vulgaris), bamboo (Phyllostachys nuda), squash (Cucurbita maxima), castor bean (Ricinus communis), and tomato (Solanum lycopersicum). Phloem sap velocity measurements by magnetic resonance imaging velocimetry indicate that higher conductivity is not accompanied by a higher velocity. Studies on the temporal development of callose show that small sieve plate pores might be occluded by callose within minutes, but plants containing sieve tubes with large pores need additional mechanisms.

Shear banding and yield stress in soft glassy materials

Shear localization is a generic feature of flows in yield stress fluids and soft glassy materials but is incompletely understood. In the classical picture of yield stress fluids, shear banding happens because of a stress heterogeneity. Using recent developments in magnetic resonance imaging velocimetry, we show here for a colloidal gel that even in a homogeneous stress situation shear banding occurs, and that the width of the flowing band is uniquely determined by the macroscopically imposed shear rate rather than the stress. We present a simple physical model for flow of the gel showing that shear banding (localization) is a flow instability that is intrinsic to the material, and confirm the model predictions for our system using rheology and light scattering.

Shear Thickening of Cornstarch Suspensions as a Reentrant Jamming Transition

We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits shear thickening. From magnetic resonance imaging velocimetry and classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.

Verification of shear-thinning LB simulations in complex geometries

The flow of rheologically complex materials (e.g. shear-thinning fluids) through complex pore structures is a fundamental process in many industrial applications, yet is poorly understood. To this end we apply 3D lattice Boltzmann simulation techniques to provide a pore-scale description of such flow processes. Simulation results are compared directly to corresponding spatially resolved magnetic resonance imaging velocimetry data, providing an extremely stringent and thorough check on the accuracy of the simulation methodology. Excellent agreement is produced for a range of shear-thinning fluids. Heterogeneity in the flow field was observed to increase as the fluids became more shear-thinning, and is quantified.

MRI-Based Multiscale Models for the Hemodynamic and Structural Evaluation of Surgically Reconstructed Aortic Arches:

The surgical reconstruction of the aortic arch is necessary in pediatric patients suffering from different types of congenital heart malformations, in particular, coarctation of the aorta. Among the reconstruction techniques used in surgical practice end-to-end anastomosis (E/E), Gore-tex graft interposition (GGI) and Gore-tex patch graft aortoplasty (GPGA) are compared in this study with a control model, employing a computational fluid-structure-interaction scheme. This study analyzes the impact of introducing synthetic materials on aortic hemodynamics and wall mechanics. Three-dimensional (3D) geometries of a porcine aortic arch were derived from magnetic resonance imaging (MRI) images. Inlet conditions were derived from MRI velocimetry. A multiscale approach was used for the imposition of outlet conditions, wherein a lumped parameter net provided an active afterload. Evidence was found that ring-like repairs increased blood velocity, whereas GPGA limited it. Vortex presence was greater and longer lasting in GGI. The highest power losses corresponded to GPGA. GGI had an intermediate effect, while E/E dissipated only slightly more than the control case. Wall stresses peak in a longitudinal strip on the subject's left side of the vessel, particularly in the frontal area. There was a concentration of stress at the suture lines. All surgical techniques performed equally well in restoring physiological pressures.

Measurement of flow field in biofilm reactors by 3‐D magnetic resonance imaging

Abstract3‐D Magnetic resonance imaging (MRI) was used to measure the flow field of water in a packed‐bed column containing Serratia sp. biofilm supported on polyurethane foam, and subsequently to follow a reaction which precipitates lanthanum phosphate on the biofilm. Sensitizing the MR image contrast to the fluid flow along the axis of the bioreactor provided better image‐contrast between the foam and fluid compared to that based on MR signal intensity alone. After reaction, that same “velocity contrast” effectively defined the difference between blocked and unblocked regions by distinguishing between regions of flow and no flow. Data acquired during progressive blockage of reactors challenged at two different flow rates accord with reactor theory; thus, the faster flow rate replenished the reactants uniformly, whereas at the slower flow rate the reactants were concentration limited. MRI velocimetry was used to generate data that can be used to model reactors where the efficiency is progressively compromised by blockage due to precipitation. © 2005 American Institute of Chemical Engineers AIChE J, 2005