L.D. Hall

Partially restricted diffusion in a permeable sandstone : observations by stimulated echo PFG NMR

We demonstrate a variant, insensitive to eddy current effects, of an alternating pulsed field gradient technique designed to null the effect of background gradients in liquid-saturated porous media, using a 38 mm diameter sample of a natural sandstone. Measurements of the effective diffusivity confirm predictions of a decline as the square root of an effective diffusion time. A value of the ratio S/Vp for the dominant pores is extracted, yielding with T1 a value for the surface relaxivity. We test also a geometry-dependent data collapse recently suggested for a range of diffusion times and wavenumbers. The data agree with a sheet-like pore model for this granular sandstone, and fail to agree with a tube-like model; a pore length scale is also extracted.

Study of flow and hydrodynamic dispersion in a porous medium using pulsed–field–gradient magnetic resonance

This paper reports on the use of pulsed–field gradient (PFG) magnetic resonance (MR) techniques to obtain displacement and velocity spectra of steady–state, saturated flow through a column packed with glass beads. The displacement spectra obtained by PFG MR correspond to travel–distance probability–density functions (PDF) for initial conditions of a concentration impulse in a column with zero concentration. These spectra show strong dispersion–time dependence, and depart from Gaussian–shaped PDFs for short dispersion times. These data provide estimates of the dispersion–time dependence of transverse and longitudinal dispersion coefficients. The longitudinal dispersion coefficient reaches its long–time behaviour more slowly than the transverse coefficient; long–time values obtained from MR data agree well with those calculated using existing empirical correlations. A model based on three components of apparent velocities and dispersion coefficients is sufficient to describe the time dependence of displacement spectra for water flow through the bead column. The short–distance component arise because of convection–dispersion–diffusion processes within the narrow necks between particles. The long–distance component, on the other hand, represents a macroscopic convection–dispersion process. This study shows that PFG MR flow spectroscopy is a simple but potentially useful method for the investigation of flow and hydrodynamic dispersion in porous media, especially for time–dependent phenomena.

Visualisation of changes in regional cerebral blood flow (rCBF) produced by ketamine using long TE gradient-echo sequences: Preliminary results

Autoradiographic studies have shown that low dose ketamine produces increases in regional glucose utilisation and blood flow in the hippocampus, cerebral cortex, and olfactory lobe in the rat brain, probably due to antagonism at the NMDA receptor. Functional MRI using deoxyhaemoglobin contrast can be used to study changes in regional cerebral blood flow (rCBF). Long TE gradient-echo sequences were used to study changes in rCBF produced by low dose ketamine in rats anaesthetised with nitrous oxide, supplemented with either halothane (HAL) or fentanyl/fluanisone/midazolam (FFM) combination. Images from rats in the FFM group showed a 10-14% increase in signal intensity in the hippocampus, cerebral cortex, and olfactory lobe following either a single bolus or a low dose infusion of ketamine (p < .05). These changes were significantly reduced in the HAL group (p < .005). Halothane is known to attenuate the changes in regional glucose utilisation produced by the noncompetitive NMDA antagonist dizocilpine (MK-801), and its effects on ketamine-induced changes in rCBF seen in this study may be due to a similar effect. The potential use of functional MRI in studying the effect of pharmacological interventions on rCBF is discussed.

An investigation of the origins of contrast in NMR spin echo images of plant tissue

The effects that the spatial distribution of water protons and their transverse relaxation times have on the image contrast of spin echo images of courgette was investigated. The T2-weighted image of courgette contains the most anatomical information. The image contrast was explained using a phenomenological theory based on the Bloch equations, which gave an insight into the morphology and microdynamics of water in the plant tissue. The perceived contrast in the spin echo images of courgette, glucose and Sephadex bead solutions can be dramatically altered by keeping all the imaging acquisition parameters constant, such as the recycle and echo time, but reducing the interpulse spacing by introducing a CPMG train of 180 degrees pulses into the middle of the sequence. These changes were interpreted by considering the microenvironment of the water. This work demonstrates that the origin of image contrast in T2-weighted images of plant tissue can be understood using the water proton transverse relaxation theory developed by Hills et al.

Magnetic resonance imaging analysis of cardiac cycle events in diabetic rats: the effect of angiotensin-converting enzyme inhibition

Non‐invasive magnetic resonance imaging (MRI) was used to characterize changes in left and right ventricular cardiac cycles following induction of experimental, streptozotocin (STZ)‐induced, diabetes in male Wistar rats at different ages. The effects of the angiotensin‐converting enzyme (ACE) inhibitor captopril upon such chronic physiological changes were then evaluated, also for the first time. Diabetes was induced at the age of 7 weeks in two experimental groups, of which one group was subsequently maintained on captopril (2 g l−1)‐containing drinking water, and at 10 and 13 weeks in two further groups. The fifth group provided age‐matched controls. All groups (each n = 4 animals) were scanned consistently at 16 weeks, in parallel with timings used in earlier studies that employed this experimental model. Cine magnetic resonance (MR) image acquisition provided transverse sections through both ventricles at twelve time points covering systole and most of diastole. These yielded reconstructions of cardiac anatomy used to derive critical functional indices and their dependence upon time following the triggering electrocardiographic R waves. The left and right ventricular end‐diastolic (EDV), end‐systolic (ESV) and stroke volumes (SV), and ejection fractions (EF) calculated from each, control and experimental, group showed matching values. This confirmed a necessary condition requiring balanced right and left ventricular outputs and further suggested that STZ‐induced diabetes produced physiological changes in both ventricles. Absolute left and right ventricular SVs were significantly altered in all diabetic animals; EDVs and EFs significantly altered in animals diabetic from 7 and 10 but not 13 weeks. When normalized to body weight, left and right ventricular SVs had significantly altered in animals diabetic from 7 and 10 weeks but not 13 weeks. Normalized left ventricular EDVs were also significantly altered in animals diabetic from 7 and 10 weeks. However, normalized right ventricular EDVs were significantly altered only in animals made diabetic from 7 weeks. Diabetic hearts showed major kinetic changes in left and right ventricular contraction (ejection) and relaxation (filling). Both the initial rates of volume change (dV/dt) in both ventricles and the plots of dV/dt values through the cardiac cycle demonstrated more gradual developments of tension during systole and relaxation during diastole. Estimates of the derived left ventricular performance parameters of cardiac output, cardiac power output and stroke work in control animals were comparable with human values when normalized to both body (or cardiac) weight and heart rate. All deteriorated with diabetes. Comparisons of experimental groups diabetic from 7 weeks demonstrated that captopril treatment relieved the alterations in critical volumes, dependence of SV upon EDV, kinetics of systolic contraction and diastolic relaxation and in the derived indicators of ventricular performance. This study represents the first demonstration using non‐invasive MRI of early, chronic changes in diastolic filling and systolic ejection in both the left and the right ventricles and of their amelioration by ACE inhibition following STZ‐induction of diabetes in intact experimental animals.

Two-species chemical-shift imaging using prior knowledge and estimation theory. Application to rock cores

Abstract A scheme is presented whereby NMR chemical-shift imaging of two species can be achieved even when the spectral lines cannot easily be resolved in a normal spectrum. Processing of the data compensates for magnetic field inhomogeneities and also makes optimal use of the available signal-to-noise ratio in order to minimize acquisition time. The method is illustrated here with data acquired from water and dodecane in Hopeman sandstone obtained at a magnetic field strength of 2.0 T. The scheme is also applicable to imaging at very low field strength where the signal-to-noise ratio is poor, and where significant signal decay occurs during the long chemical-shift evolution periods.

DESIGN OF A BIPLANAR GRADIENT COIL USING A GENETIC ALGORITHM

A biplanar z-gradient coil has been designed using a genetic algorithm, and its efficiency for producing a gradient along the axis of a solenoid magnet compared to that of a conventional Maxwell coil set. Coils of 21.8 cm by 20.9 cm area and 10 cm separation give 0.37 m Tm-1 A-1 with standard and maximum deviations of 2.6 and 13.1% of this value over an optimised cuboid region of 12 by 15 by 1.8 cm. The experimentally useable linear volume extends beyond this to 50% of the separation between the planes. Design data are also given for a transverse gradient set.

Magnetic resonance imaging analysis of left ventricular function in normal and spontaneously hypertensive rats

1 We have used magnetic resonance imaging (MRI) to examine acute morphological changes in the left ventricle throughout the cardiac cycle in normal Wistar Kyoto rats (WKY) and also to follow the development of chronic changes in spontaneously hypertensive rats (SHR). This involved the development of MRI and quantitative analysis techniques for characterizing contractile changes during the cardiac cycle. 2 Images of the cardiac anatomy in two age groups (8 and 12 weeks old) of young anaesthetized adult normal WKY and SHR were acquired in planes both parallel and perpendicular to the principal cardiac axis. 3 Complete coverage of the heart by imaging planes was achieved with high time resolution (13 ms), with typically 12 time frames in the cardiac cycle, using a short echo time (5 ms) multislice gradient‐echo imaging sequence. Imaging was synchronized to the R wave of the electrocardiogram. 4 The image slices could be reconstructed into complete geometrically and temporally coherent three‐dimensional data sets. Left ventricular (LV) volumes were thus reconstructed throughout the cardiac cycle by combining transverse cardiac image sections. This volume analysis revealed structural and functional differences between the normal WKY and SHR in both age groups of 8 and 12 weeks. Measurements from the cardiac images were additionally validated against histological measurements. 5 The SHR showed a raised LV end‐systolic volume and a correspondingly poorer ejection fraction as well as LV hypertrophy when compared with the controls. Left ventricular function in the SHR appeared stable between the two age groups. 6 We developed a simple geometrical model of the left ventricle based on a single longitudinal image section and successfully used this to describe some functional parameters of the left ventricle in the WKY and SHR. This geometrical model has the potential to greatly reduce the imaging time needed to study the beating heart in future serial investigations of cardiac physiology in rats. 7 Our experimental and analytical methods together form a powerful set of quantitative techniques which combine both imaging and functional analysis and will be applicable for future studies of chronic physiological changes in animal disease models.

Rapid MRI and velocimetry of cylindrical couette flow

A narrow-gap, temperature-controlled Couette flow rheometer has been developed to study fluid velocities within the annular gap between two concentric cylinders by nuclear magnetic resonance (NMR) imaging and velocimetry. Alternative pulsed-field-gradient-based nuclear magnetic resonance imaging strategies which may be used for measurement of velocity within the Couette flow device have been evaluated. These include two-dimensional (2-D) imaging techniques with acquisition times of several minutes and a one-dimensional (1-D) projection method which exploits the symmetry of the device to reduce overall measurement time to less than 1 min. Velocity measurements made using each technique are presented for a Newtonian fluid undergoing Couette flow at shear rates of approximately 20 and 60 s(-1).

STUDY OF INFILTRATION INTO A HETEROGENEOUS SOIL USING MAGNETIC RESONANCE IMAGING

In this paper, the feasibility of using magnetic resonance imaging (MRI) to study water infiltration into a heterogeneous soil is examined, together with its difficulties and limitations. MRI studies of ponded water infiltration into an undisturbed soil core show that the combination of one- and two-dimensional imaging techniques provides a visual and non-destructive means of monitoring the temporal changes of soil water content and the moisture profile, and the movement of the wetting front. Two-dimensional images show air entrapment in repetitive ponded infiltration experiments. During the early stages of infiltration, one-dimensional images of soil moisture profiles clearly indicate preferential flow phenomena. The observed advance of wetting fronts can be described by a linear relationship between the square root of infiltration time (√t) and the distance of the wetting front from the soil surface. Similarly, the cumulative infiltration is also directly proportional to √t. Furthermore, from the MRI infiltration moisture profiles, it is possible to estimate the parameters that feature in infiltration equations.