An MR/MRI compatible core holder with the RF probe immersed in the confining fluid.

An open frame RF probe for high pressure and high temperature MR/MRI measurements was designed, fabricated, and tested. The open frame RF probe was installed inside an MR/MRI compatible metallic core holder, withstanding a maximum pressure and temperature of 5000 psi and 80 °C. The open frame RF probe was tunable for both 1H and 19F resonance frequencies with a 0.2 T static magnetic field. The open frame structure was based on simple pillars of PEEK polymer upon which the RF probe was wound. The RF probe was immersed in the high pressure confining fluid during operation. The open frame structure simplified fabrication of the RF probe and significantly reduced the amount of polymeric materials in the core holder. This minimized the MR background signal detected. Phase encoding MRI methods were employed to map the spin density of a sulfur hexafluoride gas saturating a Berea core plug in the core holder. The SF6 was imaged as a high pressure gas and as a supercritical fluid.

Local T1-T2 distribution measurements in porous media.

A novel slice-selective T1-T2 measurement is proposed to measure spatially resolved T1-T2 distributions. An adiabatic inversion pulse is employed for slice-selection. The slice-selective pulse is able to select a quasi-rectangular slice, on the order of 1 mm, at an arbitrary position within the sample.The method does not employ conventional selective excitation in which selective excitation is often accomplished by rotation of the longitudinal magnetization in the slice of interest into the transverse plane, but rather a subtraction based on CPMG data acquired with and without adiabatic inversion slice selection. T1 weighting is introduced during recovery from the inversion associated with slice selection. The local T1-T2 distributions measured are of similar quality to bulk T1-T2 measurements. The new method can be employed to characterize oil-water mixtures and other fluids in porous media. The method is beneficial when a coarse spatial distribution of the components is of interest.

A high-pressure metallic core holder for magnetic resonance based on Hastelloy-C.

A metallic core holder, fabricated from non-magnetic Hastelloy-C276, has been designed for Magnetic Resonance (MR) and Magnetic Resonance Imaging (MRI) of core plug samples at high pressures and temperatures. Core plug samples, 1.5″ in diameter and 2″ in length, can be tested in the core holder at elevated pressures and temperatures, up to 5000 psi and 80 °C. These are conditions commonly found in petroleum reservoirs. A radio frequency probe, which excites and detects magnetic resonance signals, was placed inside the metal vessel. Proximity to the sample improves the signal to noise ratio of the resulting measurements. The metallic core holder is positioned between the poles of a 0.2 T permanent magnet and subjected to rapidly switched magnetic field gradients as part of the imaging process. This switching induces eddy currents on the conductive core holder, which degrades the magnetic field gradient waveform in the sample space. The low electrical-conductivity of Hastelloy-C276 minimizes the duration and the magnitude of such eddy currents. A recently developed pre-equalization technique was employed to ensure that magnetic field gradient pulses, required for MRI, are near ideal in the sample space. A representative core flooding experiment was undertaken in conjunction with MR/MRI measurements.

Local diffusion and diffusion-T2 distribution measurements in porous media.

Slice-selective pulsed field gradient (PFG) and PFG-T2 measurements are developed to measure spatially-resolved molecular diffusion and diffusion-T2 distributions. A spatially selective adiabatic inversion pulse was employed for slice-selection. The slice-selective pulse is able to select a coarse slice, on the order of 1cm, at an arbitrary position in the sample. The new method can be employed to characterize oil-water mixtures in porous media. The new technique has an inherent sensitivity advantage over phase encoding imaging based methods due to signal being localized from a thick slice. The method will be advantageous for magnetic resonance of porous media at low field where sensitivity is problematic. Experimental CPMG data, following PFG diffusion measurement, were compromised by a transient ΔB0(t) field offset. The off resonance effects of ΔB0(t) were examined by simulation. The ΔB0 offset artifact in D-T2 distribution measurements may be avoided by employing real data, instead of magnitude data.

Spatially resolved measurement of rock core porosity.

Density weighted, centric scan, Conical SPRITE MRI techniques are applied in the current work for local porosity measurements in fluid saturated porous media. The methodology is tested on a series of sandstone core samples. These samples vary in both porosity and degree of local heterogeneity due to bedding plane structure. The MRI porosity measurement is in good agreement with traditional gravimetric measurements of porosity. Spatially resolved porosity measurements reveal significant porosity variation in some samples. This novel MRI technique should have applications to the characterization of local porosity in a wide variety of porous media.

A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine.

Sodium density maps acquired with three SPRITE-based methods have been compared in terms of the resulting quantitative information as well as image quality and acquisition times. Consideration of factors relevant for the clinical implementation of SPRITE shows that the Conical-SPRITE variant is preferred because of a 20-fold reduction in acquisition time, slightly improved image quality, and no loss of quantitative information. The acquisition of a 3D data set (32x32x16; FOV=256x256x160 mm) for the quantitative determination of sodium density is demonstrated. In vivo Conical-SPRITE 23Na images of the brain of a healthy volunteer were acquired in 30 min with a resolution of 7.5x7.5x7.5 mm and a signal-to-noise ratio of 23 in cerebrospinal fluid and 17 in brain tissue.

Quantitative density profiling with pure phase encoding and a dedicated 1D gradient.

A new centric scan imaging methodology for density profiling of materials with short transverse relaxation times is presented. This method is shown to be more robust than our previously reported centric scan pure phase encode methodologies. The method is particularly well suited to density imaging of low gyro-magnetic ratio non-proton nuclei through the use of a novel dedicated one-dimensional magnetic field gradient coil. The design and construction of this multi-layer, water cooled, gradient coil is presented. Although of large diameter (7.62 cm) to maximize sample cross section, the gradient coil has an efficiency of several times that offered by conventional designs (6 mT/m/A). The application of these ideas is illustrated with high resolution density-weighted proton (1H) images of hazelnut oil penetration into chocolate, and lithium ion (7Li) penetration into cement paste. The methods described in this paper provide a straightforward and reliable means for imaging a class of samples that, until now, have been very difficult to image.

A novel high temperature 1H NMR imaging probe for combustion studies: the behaviour of both a lit and unlit methane gas jet.

The design of a NMR probe suitable for very high temperature samples is described. The loop gap resonator is water cooled and tuned to 100 MHz for use in a 2.4 T horizontal bore magnet. The probe has been specifically designed for imaging of the combustion process. An experiment is described in this paper which shows the behaviour of a methane gas jet when both lit and unlit. The jet of gas may be observed in its unlit state flowing at up to 2 ms(-1) from a 1 mm diameter orifice using a Single Point Imaging technique. Images of the lit gas show loss of nuclear polarisation within 3 mm of the orifice. A residual amount of un-decomposed gas is visible in the first few millimetres of the flame neck. A computational fluid dynamics model is used to verify the distribution of molecular methane, as well as the temperature of the flame.

Local T2 measurement employing longitudinal Hadamard encoding and adiabatic inversion pulses in porous media.

Band selective adiabatic inversion radio frequency pulses were employed for multi-slice T2 distribution measurements in porous media samples. Multi-slice T2 measurement employing longitudinal Hadamard encoding has an inherent sensitivity advantage over slice-by-slice local T2 measurements. The slice selection process is rendered largely immune to B1 variation by employing hyperbolic secant adiabatic inversion pulses, which simultaneously invert spins in several well-defined slices. While Hadamard encoding is well established for local spectroscopy, the current work is the first use of Hadamard encoding for local T2 measurement.

Mapping B(1)-induced eddy current effects near metallic structures in MR images: a comparison of simulation and experiment.

Magnetic resonance imaging (MRI) in the presence of metallic structures is very common in medical and non-medical fields. Metallic structures cause MRI image distortions by three mechanisms: (1) static field distortion through magnetic susceptibility mismatch, (2) eddy currents induced by switched magnetic field gradients and (3) radio frequency (RF) induced eddy currents. Single point ramped imaging with T1 enhancement (SPRITE) MRI measurements are largely immune to susceptibility and gradient induced eddy current artifacts. As a result, one can isolate the effects of metal objects on the RF field. The RF field affects both the excitation and detection of the magnetic resonance (MR) signal. This is challenging with conventional MRI methods, which cannot readily separate the three effects. RF induced MRI artifacts were investigated experimentally at 2.4 T by analyzing image distortions surrounding two geometrically identical metallic strips of aluminum and lead. The strips were immersed in agar gel doped with contrast agent and imaged employing the conical SPRITE sequence. B1 mapping with pure phase encode SPRITE was employed to measure the B1 field around the strips of metal. The strip geometry was chosen to mimic metal electrodes employed in electrochemistry studies. Simulations are employed to investigate the RF field induced eddy currents in the two metallic strips. The RF simulation results are in good agreement with experimental results. Experimental and simulation results show that the metal has a pronounced effect on the B1 distribution and B1 amplitude in the surrounding space. The electrical conductivity of the metal has a minimal effect.

Single-point magnetic resonance imaging study of water adsorption in pellets of zeolite 4A.

The water uptake process in commercial type particles of zeolite 4A has been studied using a single-point MRI method. True proton density, T1, T2, and T*2 relaxation times were obtained with submillimetric resolution, overcoming the restrictions of short T*2 signals. The molecular mobility in nonequilibrium conditions has been characterized by relaxation time mapping. A clear reduction of the water sorption rate was observed by comparing MRI profiles of a loosely packed bed and gravimetric measurements of spread particles from the same sieved zeolite batch.

Water content profiles with a 1D centric SPRITE acquisition.

The purpose of this work is to develop a rapid MRI method amenable to profiling with minimal or no T(1) relaxation weighting. The behavior of a signal during a centric SPRITE acquisition is analyzed. It is shown that the technique can be made immune to a broad range of T(1) changes. In a properly executed measurement, only T(2)* and proton density parameters define the image intensity. A T(2)* mapping technique can be easily applied, separating T(2)* and proton density contributions to the image. A drying soil sample with low initial water content is experimentally studied as a demonstration of the technique. A characteristic baseline artifact is easily removed from the profiles by a simple operation.

Imaging of heterogeneous materials with a turbo spin echo single-point imaging technique.

A magnetic resonance imaging method is presented for imaging of heterogeneous broad linewidth materials. This method allows for distortionless relaxation weighted imaging by obtaining multiple phase encoded k-space data points with each RF excitation pulse train. The use of this method, turbo spin echo single-point imaging-(turboSPI), leads to decreased imaging times compared to traditional constant-time imaging techniques, as well as the ability to introduce spin-spin relaxation contrast through the use of longer effective echo times. Imaging times in turboSPI are further decreased through the use of low flip angle steady-state excitation. Two-dimensional images of paramagnetic doped agarose phantoms were obtained, demonstrating the contrast and resolution characteristics of the sequence, and a method for both amplitude and phase deconvolution was demonstrated for use in high-resolution turboSPI imaging. Three-dimensional images of a partially water-saturated porous volcanic aggregate (T(2L) approximately 200 ms, Deltanu(1/2) approximately 2500 Hz) contained in a hardened white Portland cement matrix (T(2L) approximately 0.5 ms, Deltanu(1/2) approximately 2500 Hz) and a water-saturated quartz sand (T(2) approximately 300 ms, T(2)(*) approximately 800 microseconds) are shown.

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping.

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.

Relaxation time mapping of short T*2 nuclei with single-point imaging (SPI) methods.

New techniques for quantitative mapping of T1, T2, and T*2 are proposed, based on the single-point imaging (SPI) method, for materials with short nuclear magnetic resonance relaxation times which cannot be imaged with traditional methods. Relaxation times extracted from two-dimensional images of uniform doped agarose phantoms (T*2 approximately 60-210 microseconds) as well as hardened mortar (T*2 approximately 220 microseconds) and polymers (T*2 approximately 20-100 microseconds), using these techniques, agreed with bulk measurements. The method was then applied to a partially dried cylindrical concrete sample (T*2 approximately 115 microseconds).

Diffusion in Fe(II/III) radiation dosimetry gels measured by magnetic resonance imaging.

We analyse the diffusion problem in the traditional Fe(II/III) agarose gel system employed in MRI studies of radiation dosimetry. The diffusion coefficient is measured using an inversion recovery null-point imaging method in a model gel/water phantom. The diffusion coefficient of Fe(III) in 1% agarose gel at pH 1.1 is D = 2.7 +/- 0.3 x 10(-6) cm2 s-1. The diffusion coefficient of Fe(II) is D = 3.3 +/- 0.5 x 10(-6) cm2 s-1. Measurement of the diffusion coefficients permits simulation of the MRI signal intensity from phantoms with model radiation dose distributions. We allow for diffusion of both Fe(II) and Fe(III) in our simulations as well as the effect of both relaxation agents on the local spin-lattice relaxation time T1. We also analyse the effects of the physical penumbra on the diffusion problem.

Concrete/mortar water phase transition studied by single-point MRI methods.

A series of magnetic resonance imaging (MRI) water density and T2* profiles in hardened concrete and mortar samples has been obtained during freezing conditions (-50 degrees C < T < 11 degrees C). The single-point ramped imaging with T1 enhancement (SPRITE) sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 microseconds and T1 < 3.6 ms). The frozen and evaporable water distribution was quantified through a position based study of the profile magnitude. Submillimetric resolution of proton-density and T2*-relaxation parameters as a function of temperature has been achieved.

B(1) mapping with a pure phase encode approach: quantitative density profiling.

In MRI, it is frequently observed that naturally uniform samples do not have uniform image intensities. In many cases this non-uniform image intensity is due to an inhomogeneous B1 field. The 'principle of reciprocity' states that the received signal is proportional to the local magnitude of the applied B1 field per unit current. Inhomogeneity in the B1 field results in signal intensity variations that limit the ability of MRI to yield quantitative information. In this paper a novel method is described for mapping B1 inhomogeneities based on measurement of the B1 field employing centric-scan pure phase encode MRI measurements. The resultant B1 map may be employed to correct related non-uniformities in MR images. The new method is based on acquiring successive images with systematically incremented low flip angle excitation pulses. The local image intensity variation is proportional to B1(2), which ensures high sensitivity to B1 field variations. Pure phase encoding ensures the resultant B1 field maps are free from distortions caused by susceptibility variation, chemical shift and paramagnetic impurities. Hence, the method works well in regions of space that are not accessible to other methods such as in the vicinity of conductive metallic structures, such as the RF probe itself. Quantitative density images result when the centric scan pure phase encode measurement is corrected with a relative or absolute B1 field map. The new technique is simple, reliable and robust.

Lateral diffusion in model membranes is independent of the size of the hydrophobic region of molecules.

We have systematically investigated the probe size and shape dependence of lateral diffusion in model dimyristoyl phosphatidylcholine membranes. Linear hydrophobic polymers, which differ in length by an order of magnitude, were used to explore the effect on the lateral diffusion coefficient of hydrodynamic restrictions in the bilayer interior. The polymers employed are isoprenoid alcohols--citronellol, solanesol, and dolichol. Tracer lateral diffusion coefficients were measured by fluorescence photobleaching recovery. Despite the large difference in lengths, the nitrobenzoxadiazole labelled alcohols all diffuse at the rate of lipid self-diffusion (5.0 x 10(-12) m2 s-1, 29 degrees C) in the liquid crystal phase. Companion measurements in isotropic polymer solution, in gel phase lipid membranes and with nonpolar fluorescent polyaromatic hydrocarbons, show a marked dependence of the lateral diffusion coefficient on the probe molecule size. Our results in the liquid crystal phase are in accord with free area theory which asserts that lateral diffusion in the membrane is restricted by the surface-free area. Probe molecules which are significantly longer than the host phospholipid, seven times longer in the case of dolichol, are still restricted in their lateral motion by the surface properties of the bilayer in the liquid crystal phase. Fluorescence quenching experiments indicate that the nitrobenzoxadiazole label does not reside at the aqueous interface, although it must reside in close proximity according to the diffusion measurements.

Concrete thawing studied by single-point ramped imaging.

A series of two-dimensional images of proton distribution in a hardened concrete sample has been obtained during the thawing process (from -50 degrees C up to 11 degrees C). The SPRITE sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 micros and T1 < 3.6 ms). The relaxation parameters of the sample were determined in order to optimize the time efficiency of the sequence, permitting a 4-scan 64 x 64 acquisition in under 3 min. The image acquisition is fast on the time scale of the temperature evolution of the specimen. The frozen water distribution is quantified through a position based study of the image contrast. A multiple point acquisition method is presented and the signal sensitivity improvement is discussed.