Aleš Mohorič

Magnetic Resonance Imaging System Based on Earth's Magnetic Field

Abstract This article describes both the setup and the use of a system for magnetic resonance imaging (MRI) in the Earths magnetic field. Phase instability caused by temporal fluctuations of Earths field can be successfully improved by using a reference signal from a separate Earths field nuclear magnetic resonance (NMR) spectrometer/magnetometer. In imaging, it is important to correctly determine the phase of the NMR signal. A reference signal of a fixed‐frequency oscillator cannot be used since the Larmor frequency changes with time, following temporal fluctuations of Earths magnetic field. The reference signal frequency and phase, provided by a separate NMR spectrometer, change in the same way as Earths field, creating thereby, a stable rotating frame of reference for the measured signal. In principle, excellent homogeneity of the magnetic field enables scanning of very large volume samples. Reduction in S/N ratio due to the weak magnetic field can be partly compensated by the receiving coil design and shielding of electromagnetic pick‐up in audio frequency (AF) range. The smallest voxel examined so far is on the order of 50 mm3. Unlike in the case of strong magnetic fields, detection and processing of low frequency signal are less demanding for the electronics. The techniques used and the results of measurements are briefly presented.

Diffusion and flow in a porous structure by the gradient spin echo spectral analysis

The frequency analysis of relation between the NMR gradient spin echo method and the correlation of molecular motion throws a new light upon the measurement of molecular transport in porous media by magnetic resonance. The spectral analysis provides, in some other way, a known ffiffiffiffiffiffi Dt p early time dependence of attenuation or the pulse gradient spin echo sequence, and at intermediate times, it gives a not-known Dpt þ dð1 � expð� t=trÞÞ: When the displacements are getting larger than the size of compartments, the spin echo is levelling into a time-independent asymptote. In the system of packed poly-dispersed beds, the spin echo measurement of flow dispersion perpendicular to flows confirms the predicted spin echo decay. It demonstrates a clear distinction between different time regimes of signal decay, from which different properties of the porous structure can be revealed. The results gives almost identical long-time dispersion coefficient, D 0 ¼ Dp; for different flows, but the shortening of the dispersion correlation time tr with the increase of interstitial velocity. In combination with the modulated gradient sequence, the method extends the measuring range of spin echo over multi-pore length scale, and opens a new way to provide information about important properties of porous media like average pore size, the interconnectivity and the tortuosity. r 2001 Elsevier Science B.V. All rights reserved.

Autocorrelation spectra of an air-fluidized granular system measured by NMR

The power spectrum of displacement fluctuation of beads in the air-fluidized granular system is measured by a novel NMR technique of modulated gradient spin-echo. The results of measurement together with the related spectrum of the velocity fluctuation autocorrelation function fit well to an empiric formula based on to the model of bead caging between nearest neighbours; the cage breaks up after a few collisions \cite{Menon1}. The fit yields the characteristic collision time, the size of bead caging and the diffusion-like constant for different degrees of system fluidization. The resulting mean squared displacement increases proportionally to the second power of time in the short-time ballistic regime and increases linearly with time in the long-time diffusion regime as already confirmed by other experiments and simulations.

The elimination of magnetic susceptibility artifacts in the micro-image of liquid-solid interfaces: internal gradient modulation by the CPMG RF train

Distortions of magnetic resonance images near solid-liquid interface appear as the result of the restriction to spin self-diffusion in the proximity of impermeable boundary as well as of a susceptibility difference. The spectral analysis of spin echo enables to resolve, in a simple way, how various RF-gradient pulse sequences reduce the effect of the internal magnetic field induced by the susceptibility difference at interfaces. The 1D diffusion-weighted imaging of water in the narrow notch tested efficiency of some sequence. The notch was milled in a piece of Plexiglas. The method can be used to distinguish the susceptibility effect from the effects of applied gradients when investigating the transport of fluid through a porous structure.

Effects of off-resonance spins on the performance of the modulated gradient spin echo sequence

Translational molecular dynamics in various materials can also be studied by diffusion spectra. These can be measured by a constant gradient variant of the modulated gradient spin echo (MGSE) sequence which is composed of a CPMG RF pulse train superimposed to a constant magnetic field gradient. The application of the RF train makes the effective gradient oscillating thus enabling measurements of diffusion spectra in a wide range of frequencies. However, seemingly straightforward implementation of the MGSE sequence proved to be complicated and can give overestimated results for diffusion if not interpreted correctly. In this study, unrestricted diffusion in water and other characteristic materials was analyzed by the MGSE sequence in the frequency range 50-3000Hz using a 6T/m diffusion probe. First, it was shown that the MGSE echo train acquired from the entire sample decays faster than the train acquired only from a narrow band at zero frequency of the sample. Then, it was shown that the decay rate is dependent on the bands off-resonance characterized by the ratio Δω0/ω1 and that with higher off-resonances the decay is faster. The faster decay therefore corresponds to a higher diffusion coefficient if the diffusion is calculated using standard Stejskal-Tanner formula. The result can be explained by complex coherence pathways contributing to the MGSE echo signals when |Δω0|/ω1>0. In a magnetic field gradient, all the pathways are more diffusion attenuated than the direct coherence pathway and therefore decay faster, which leads to an overestimation of the diffusion coefficient. A solution to this problem was found in an efficient off-resonance signal reduction by using only zero frequency filtered MGSE echo train signals.

Velocity autocorrelation spectra in molten polymers measured by NMR modulated gradient spin-echo

The segmental dynamics in molten linear polymers is studied by the NMR method of modulated gradient spin-echo, which directly probes a spectrum of molecular velocity autocorrelation function. Diffusion spectra of mono-disperse poly(isoprene-1.4) with different molecular masses, measured in the frequency range 0.1–10 kHz at a temperature of , have a form similar to the spectrum of Rouse chain dynamics, which implicates the tube-Rouse motion as the dominant dynamic process in this frequency range. The scaling of the center-of-mass diffusion coefficient, given from the fitting parameters, changes from into at around Kuhn steps, which is less than predicted by theory and simulations, while the correlation times of the tube-Rouse mode do not follow the anticipated scaling.

Self-diffusion in nanopores studied by the NMR pulse gradient spin echo

NMR pulse gradient spin echo is the most efficient method for non-invasive elucidation of molecular transport in heterogeneous media. With a proper interpretation of experimental data, the method can also be applied to investigate molecular self-diffusion in pores small enough that the characteristic diffusion times are much shorter than time, needed to build up the spin phase structure by the pulse of magnetic field gradient. This is demonstrated by the analysis of restricted self-diffusion measurement of water molecules trapped in a polyamide membrane. The results are presented as a distribution of spin-relaxation rates and pore sizes in this nanoporous system that also present the method in its true colors as a useful tool in the bio-nanotechology.

Study of translational dynamics in molten polymer by variation of gradient pulse-width of PGSE

Pulsed gradient spin echo is a method of measuring molecular translation. Changing Δ makes it sensitive to diffusion spectrum. Spin translation effects the buildup of phase structure during the application of gradient pulses as well. The time scale of the self-diffusion measurement shortens if this is taken into account. The method of diffusion spectrometry with variable δ is also less sensitive to artifacts caused by spin relaxation and internal gradient fields. Here the method is demonstrated in the case of diffusion spectrometry of molten polyethylene. The results confirm a model of constraint release in a system of entangled polymer chains as a sort of tube Rouse motion.

Pulse Gradient Spin Echo Measurement of Flow Dynamics in a Porous Structure: NMR Spectral Analysis of Motional Correlations

NMR measurement of spin migrations by the spin-echo [1] and magnetic resonance microscopy [2] have important implication on understanding of the molecular transport in porous media. Among various approaches, the analysis of self-diffusion and flow through a porous structure by Modulated Gradient Spin Echo method (MGSE) demonstrated that a properly shaped gradient sequence can be is a powerful non-invasive probe for study of molecular dynamics by measuring the low frequency features of the velocity self-correlation function (VCF) [3, 4]. The method provides new information that might be relevant to a wide range of scientific, technological and medical inquiries such as oil reservoir appraisal and management, aquifers behaviour, distillation and filtration processes, heterogeneous catalyst bed design and performance, ion channelling through membranes, cell migration in biological processes etc.