Imaging the velocity profiles in tubeless siphon flow by NMR microscopy

Measurement with microscopic MRI and simulation of flow in different aneurysm models

PURPOSE

The impact and the development of aneurysms depend to a significant degree on the exchange of liquid between the regular vessel and the pathological extension. A better understanding of this process will lead to improved prediction capabilities. The aim of the current study was to investigate fluid-exchange in aneurysm models of different complexities by combining microscopic magnetic resonance measurements with numerical simulations. In order to evaluate the accuracy and applicability of these methods, the fluid-exchange process between the unaltered vessel lumen and the aneurysm phantoms was analyzed quantitatively using high spatial resolution.

METHODS

Magnetic resonance flow imaging was used to visualize fluid-exchange in two different models produced with a 3D printer. One model of an aneurysm was based on histological findings. The flow distribution in the different models was measured on a microscopic scale using time of flight magnetic resonance imaging. The whole experiment was simulated using fast graphics processing unit-based numerical simulations. The obtained simulation results were compared qualitatively and quantitatively with the magnetic resonance imaging measurements, taking into account flow and spin-lattice relaxation.

RESULTS

The results of both presented methods compared well for the used aneurysm models and the chosen flow distributions. The results from the fluid-exchange analysis showed comparable characteristics concerning measurement and simulation. Similar symmetry behavior was observed. Based on these results, the amount of fluid-exchange was calculated. Depending on the geometry of the models, 7% to 45% of the liquid was exchanged per second.

CONCLUSIONS

The result of the numerical simulations coincides well with the experimentally determined velocity field. The rate of fluid-exchange between vessel and aneurysm was well-predicted. Hence, the results obtained by simulation could be validated by the experiment. The observed deviations can be caused by the noise in the measurement and by the limited resolution of the simulation. The resulting differences are small enough to allow reliable predictions of the flow distribution in vessels with stents and for pulsed blood flow.

High-resolution MRI velocimetry compared with numerical simulations

Alterations of the blood flow are associated with various cardiovascular diseases. Precise knowledge of the velocity distribution is therefore important for understanding these diseases and predicting the effect of different medical intervention schemes. The goal of this work is to estimate the precision with which the velocity field can be measured and predicted by studying two simple model geometries with NMR micro imaging and computational fluid dynamics. For these initial experiments, we use water as an ideal test medium. The phantoms consist of tubes simulating a straight blood vessel and a step between two tubes of different diameters, which can be seen as a minimal model of the situation behind a stenosis. For both models, we compare the experimental data with the numerical prediction, using the experimental boundary conditions. For the simpler model, we also compare the data to the analytical solution. As an additional validation, we determine the divergence of the velocity field and verify that it vanishes within the experimental uncertainties. We discuss the resulting precision of the simulation and the outlook for extending this approach to the analysis of specific cases of arteriovascular problems.

Fast magnetic resonance imaging and velocimetry for liquids under high flow rates

We here demonstrate the use of NMR velocity imaging techniques to measure flow in a free falling jet of water at speeds up to and on the order of 1m/s. In particular, we show how to adapt the RARE imaging method, based on a CPMG multiple rf pulse train, so that the real and imaginary parts of the signal may be suitably acquired, enabling pulsed gradient spin echo encoding for flow. We term this method "soft-pulse-quadrature-cycled PGSE-RARE" or SPQC-PGSE-RARE. We further demonstrate the use of a one-dimensional (slice selective) imaging method which takes advantage of the cylindrical symmetry of the flow, and considerably shortens the image acquisition time.

Magnetic resonance microscopy: recent advances and applications

Magnetic resonance microscopy is receiving increased attention as more researchers in the biological sciences are turning to non-invasive imaging to characterize development, perturbations, phenotypes and pathologies in model organisms ranging from amphibian embryos to adult rodents and even plants. The limits of spatial resolution are being explored as hardware improvements address the need for increased sensitivity. Recent developments include in vivo cell tracking, restricted diffusion imaging, functional magnetic resonance microscopy and three-dimensional mouse atlases. Important applications are also being developed outside biology in the fields of fluid mechanics, geology and chemistry.

Time-dependence of nuclear magnetic resonance quadrupole interactions for polymers under shear

We consider the problem of performing NMR spectroscopy under conditions of flow, a central issue in Rheo-NMR. By way of example, the effects of rotational motion on the deuterium NMR spectrum are considered for Couette cell experiments involving deformation of polymers under shearing conditions. The polymer was modelled as a power law fluid and for each streamline, the spin Hamiltonian evolved to allow for flow reorientation. The gap-integral spectra are compared with the 'ideal' spectra for a polymer under shear, but without reorientation. It is found that flow does affect the shape of the deuterium spectra, as well as slightly perturbing the splittings.

Convection-compensating PGSE experiment incorporating excitation-sculpting water suppression (CONVEX)

We present a new diffusion experiment which provides simultaneous suppression of an on-resonance solvent peak and compensation for convection. The experiment, which we name CONVEX, exploits similarities between two functionally different pulse sequences to enable the same sequence to be used simultaneously for two different purposes. The CONVEX pulse sequence combines a double-echo PGSE with double excitation-sculpting water suppression, using unequal gradient pulse-pair amplitudes (g1 and g2) and unequal diffusion intervals (Delta1 and Delta2). Convection compensation is achieved by setting g1:g2 = Delta2:Delta1. The new experiment provides the spectral quality, flat baseline, and water-suppression power characteristic of excitation-sculpting experiments, combined with excellent compensation for convection. The resulting Stejskal-Tanner plots are linear over a greater range of signal attenuation than in the absence of water suppression. Possible applications include protein NMR; NMR of cellular or colloidal systems; and the monitoring of technological processes.