Magnetic Resonance Imaging of Gastric Motility in Conscious Rats.

Introduction: Gastrointestinal (GI) magnetic resonance imaging (MRI) enables simultaneous assessment of gastric peristalsis, emptying, and intestinal filling and transit. However, GI MRI in animals typically requires anesthesia, which complicates physiology and confounds interpretation and translation to humans. This study aimed to establish GI MRI in conscious rats, and for the first time, characterize GI motor functions in awake versus anesthetized conditions.

Magnetic Resonance Imaging of Gastric Motility in Conscious Rats.

Introduction: Gastrointestinal (GI) magnetic resonance imaging (MRI) can simultaneously capture gastric peristalsis, emptying, and intestinal filling and transit. Performing GI MRI with animals requires anesthesia, which complicates physiology and confounds interpretation and translation from animals to humans. This study aims to enable MRI in conscious rats, and for the first time, characterize GI motor functions in awake versus anesthetized conditions.

Surface mapping of gastric motor functions using MRI: a comparative study between humans and rats.

The stomach's ability to store, mix, propel, and empty its content requires highly coordinated motor functions. However, current diagnostic tools cannot simultaneously assess these motor processes. This study aimed to use magnetic resonance imaging (MRI) to map multifaceted gastric motor functions, including accommodation, tonic and peristaltic contractions, and emptying, through a single noninvasive experiment for both humans and rats.

Real-time imaging of decompression gas bubble growth in the spinal cord of live rats.

Purpose: To observe the growth and resolution of decompression gas bubbles in the spinal cord of live rats in real time using MRI.

Methods: We constructed an MRI-compatible pressure chamber system to visualize gas bubble dynamics in deep tissues in real time. The system pressurizes and depressurizes rodents inside an MRI scanner and monitors their respiratory rate, heart rate, and body temperature while providing gaseous anesthesia under pressure during the experiments.

Diffeomorphic Surface Modeling for MRI-Based Characterization of Gastric Anatomy and Motility.

Objective: Gastrointestinal magnetic resonance imaging (MRI) provides rich spatiotemporal data about the movement of the food inside the stomach, but does not directly report muscular activity on the stomach wall. Here we describe a novel approach to characterize the motility of the stomach wall that drives the volumetric changes of the ingesta.

Methods: A neural ordinary differential equation was optimized to model a diffeomorphic flow that ascribed the deformation of the stomach wall to a continuous biomechanical process.

Ogden material calibration via magnetic resonance cartography, parameter sensitivity and variational system identification.

Contemporary material characterization techniques that leverage deformation fields and the weak form of the equilibrium equations face challenges in the numerical solution procedure of the inverse characterization problem. As material models and descriptions differ, so too must the approaches for identifying parameters and their corresponding mechanisms. The widely used Ogden material model can be comprised of a chosen number of terms of the same mathematical form, which presents challenges of parsimonious representation, interpretability and stability.

Stereotactic Transcranial Focused Ultrasound Targeting System for Murine Brain Models.

An inexpensive, accurate focused ultrasound stereotactic targeting method guided by pretreatment magnetic resonance imaging (MRI) images for murine brain models is presented. An uncertainty of each sub-component of the stereotactic system was analyzed. The entire system was calibrated using clot phantoms.

Assessing structural and functional response of murine vasculature to acute β-adrenergic stimulation during hypothermic and hyperthermic conditions.

Because of the importance of adrenoreceptors in regulating the cardiovascular (CV) system and the role of the CV system in thermoregulation, understanding the response to these two stressors is of interest. The purpose of this study was to assess changes of arterial geometry and function during thermal and β-adrenergic stress induced in mice and quantified by MRI. Male mice were anesthetized and imaged at 7 T.

Robust high resolution strain imaging by alternating pulsed field gradient stimulated echo imaging (APGSTEi) at 7 Tesla.

A novel displacement-encoding spin-echo-stimulated-echo MRI sequence (APGSTEi) was used to obtain full-volume 3D strain fields in samples of two soft materials, a silicone elastomer and an ovine ligament. The samples were stretched cyclically and imaged synchronously. The multi-slice imaging sequence employed a combination of hard and soft spin-echos with bipolar gradient pulses for spatial encoding and decoding, combined with rapid multi-slice spin echo readouts.

In Vivo MRI Assessment of Blood Flow in Arteries and Veins from Head-to-Toe Across Age and Sex in C57BL/6 Mice.

Although widely used as a preclinical model for studying cardiovascular diseases, there is a scarcity of in vivo hemodynamic measurements of the naïve murine system in multiple arterial and venous locations, from head-to-toe, and across sex and age. The purpose of this study is to quantify cardiovascular hemodynamics in mice at different locations along the vascular tree while evaluating the effects of sex and age. Male and female, adult and aged mice were anesthetized and underwent magnetic resonance imaging.

Anatomical location, sex, and age influence murine arterial circumferential cyclic strain before and during dobutamine infusion.

Background: One of the primary biomechanical factors influencing arterial health is their deformation across the cardiac cycle, or cyclic strain, which is often associated with arterial stiffness. Deleterious changes in the cardiovascular system, e.g.

Cross-sectional areas of deep/core veins are smaller at lower core body temperatures.

The cardiovascular system plays a crucial role in thermoregulation. Deep core veins, due to their large size and role in returning blood to the heart, are an important part of this system. The response of veins to increasing core temperature has not been adequately studied in vivo.

Dispersivity of Bidisperse Packings of Spheres and Evidence for Distinct Random Structures.

The intrinsic longitudinal and transverse dispersivity of bidisperse random packings of spheres with size ratio 5∶1 was determined by pulsed field gradient nuclear magnetic resonance, in the dilute regime where small spheres occupy between 0% and 5% of the packings' volume. Small spheres plugging pores systematically raise the mechanical transverse and longitudinal dispersivity above that of reference packings of monodisperse spheres. NMR-derived porosities, widths of velocity distributions, and dispersivities reveal distinct states of structural disorder above and below a relative sphere concentration n/N=1, where n and N are the number densities of small and large spheres.

Cross-sectional area of the murine aorta linearly increases with increasing core body temperature.

Purpose: The cardiovascular (CV) system plays a vital role in thermoregulation. To date, the response of core vasculature to increasing core temperature has not been adequately studied in vivo. Our objective was to non-invasively quantify the arterial response in murine models due to increases in body temperature, with a focus on core vessels of the torso and investigate whether responses were dependent on sex or age.

In vivo characterization of the murine venous system before and during dobutamine stimulation: implications for preclinical models of venous disease.

Although widely used as a preclinical model for studying venous diseases, there is a scarcity of in vivo characterizations of the naïve murine venous system. Additionally, previous studies on naïve veins (ex vivo) have not included the influence of surrounding structures and biomechanical forces. Using MRI, we noninvasively quantified the cross-sectional area, cyclic strain, and circularity of the venous system in young and old, male and female C57BL/6 mice.

Longitudinal and transverse dispersion in flow through random packings of spheres: a quantitative comparison of experiments, simulations, and models.

Pulsed field gradient nuclear magnetic resonance was used to determine the intrinsic longitudinal and transverse dispersivity of random packings of nearly monodisperse spheres in experiments covering 3.5 orders of magnitude in reduced velocity Pe, from the diffusion dominated regime Pe < 1 to the high velocity regime Pe > 1000. Additionally, using lattice-Boltzmann simulations with tracer tracking, the dispersivities of random packings were determined numerically.

Constant gradient PFG sequence and automated cumulant analysis for quantifying dispersion in flow through porous media.

This paper describes a new variant of established stimulated echo pulse sequences, and an analytical method for determining diffusion or dispersion coefficients for Gaussian or non-Gaussian displacement distributions. The unipolar displacement encoding PFGSTE sequence uses trapezoidal gradient pulses of equal amplitude g and equal ramp rates throughout while sampling positive and negative halves of q-space. Usefully, the equal gradient amplitudes and gradient ramp rates help to reduce the impact of experimental artefacts caused by residual amplifier transients, eddy currents, or ferromagnetic hysteresis in components of the NMR magnet.

Pore-scale mixing and transverse dispersivity of randomly packed monodisperse spheres.

We show that transverse dispersion in flow through randomly packed monodisperse spheres (sphere diameter d) is a velocity-dependent superposition of three separable random processes-diffusion with coefficient D(r), intrinsic mechanical dispersion with dispersivity l(m)=d/33 caused by advection on streamlines, and a newly identified coupled mechanical dispersion with dispersivity l(c)=d/11, which arises by coupled advection and transverse diffusion at the pore scale. The velocity dependence of the transverse dispersivity is derived from first principles. Our analysis is insensitive to details of the pore geometry and is verified by pulsed field gradient NMR experiments which covered 4.

Reversible photorheology in solutions of cetyltrimethylammonium bromide, salicylic acid, and trans-2,4,4'-trihydroxychalcone.

We show photorheology in aqueous solutions of weakly entangled wormlike micelles prepared with cetyltrimethylammonium bromide (CTAB), salicylic acid (HSal), and dilute amounts of the photochromic multistate compound trans-2,4,4'-trihydroxychalcone (Ct). Different chemical species of Ct are associated with different colorations and propensities to reside within or outside CTAB micelles. A light-induced transfer between the intra- and intermicellar space is used to alter the mean length of wormlike micelles and hence the rheological properties of the fluid, studied in steady-state shear flow and in dynamic rheological measurements.

Small angle neutron scattering (SANS and V-SANS) study of asphaltene aggregates in crude oil.

We report small angle neutron scattering (SANS) experiments on two crude oils. Analysis of the high-Q SANS region has probed the asphaltene aggregates in the nanometer length scale. We find that the radius of gyration decreases with increasing temperature.

Intrinsic dispersivity of randomly packed monodisperse spheres.

We determine the intrinsic longitudinal dispersivity l(d) of randomly packed monodisperse spheres by separating the intrinsic stochastic dispersivity l(d) from dispersion by unavoidable sample dependent flow heterogeneities. The measured l(d), scaled by the hydrodynamic radius r(h), coincide with theoretical predictions [Saffman, J. Fluid Mech.

A cumulant analysis for non-Gaussian displacement distributions in Newtonian and non-Newtonian flows through porous media.

We use displacement encoding pulsed field gradient (PFG) nuclear magnetic resonance to measure Fourier components S(q) of flow displacement distributions P(zeta) with mean displacement (zeta) for Newtonian and non-Newtonian flows through rocks and bead packs. Displacement distributions are non-Gaussian; hence, there are finite terms above second order in the cumulant expansion of ln(S(q)). We describe an algorithm for an optimal self-consistent cumulant analysis of data, which can be used to obtain the first three (central) moments of a non-Gaussian P(zeta), with error bars.

Stray field measurements of flow displacement distributions without pulsed field gradients.

The probability distribution P(zeta) of diffusive and advective molecular displacements is determined using a fixed field gradient (FFG) pulse sequence, on fluid flow through a Bentheimer sandstone, in the grossly inhomogeneous stray field of a super-conducting magnet. Two decades of q-space are scanned with stimulated echoes, using the gradient of the stray field and variable encoding times delta. The strength of the gradient permits the use of short encoding times, which is desirable for limiting the distorting effects produced by flow displacements through susceptibility induced field inhomogeneities.

NMR-propagator measurements in porous media in the presence of surface relaxation and internal fields.

We measure the probability distribution P(zeta)--the propagator--of molecular displacements on Stokes flow through a pack of microporous glass beads and a carbonate rock. An optimized sampling of q-space is introduced for the measurement of a P(zeta) and its first moment zeta. Our results delineate and provide an understanding of the experimental regimes where background fields and surface relaxation distort the measured propagators.

An NMR technique for rapid measurement of flow.

We present a one-scan method for determining fluid flow velocity within a few milliseconds in the presence of a static field gradient, and without the need of multiple scans. A few RF-pulses populate a series of coherence pathways, each of which exhibits a phase shift that is proportional to fluid velocity. These coherence pathways produce spin echoes separated in the time domain, thus eliminating the need for phase cycling.