Simple mobile single-sided NMR apparatus with a relatively homogeneous B0 distribution

This work presents a simple design for a mobile single-sided nuclear magnetic resonance (NMR) apparatus with a relatively homogeneous static magnetic field (B₀) distribution. In the proposed design, the B₀ magnetic field of the apparatus is synthesized using only two permanent magnet blocks, i.e., a cube (main) magnet and a small shim magnet placed above the main magnet. The magnetic flux of the shim magnet partially cancels out that of the main magnet, subsequently creating a smooth B₀ profile above the shim magnet where low-resolution NMR experiments are performed. Compared with many previously published designs, this straightforward design simplifies the construction of the apparatus and simultaneously generates a B₀ field parallel to the apparatus surface, allowing the use of a simple loop-type radiofrequency (RF) coil. Additionally, an apparatus prototype is constructed according to the proposed design. Weighing only 1.8 kg, the constructed apparatus has a compact structure and can be held in the palm of a hand. The apparatus generates a B₀ strength of about 0.0746 T. Within a B₀ field deviation of 3 mT, the region with a relatively homogeneous B₀ distribution extends to about 11 mm above the shim magnet. The proposed apparatus can detect a clear Hahn echo or Carr-Purcell-Meiboom-Gill (CPMG) echoes of a pencil eraser block or a bottle of oil placed on the apparatus in 5 s with signal averaging using an RF transmitter power of only 19 W; the detection range of the apparatus exceeds 6 mm. The strength of the residual static magnetic field gradient of the apparatus is roughly estimated at 0.58 T/m. Applying different CPMG echo spacings in this residual static gradient leads to various transverse relaxation time (T₂) contrasts for liquids with distinct viscosities such as water and oil. Two nondestructive inspection applications of the apparatus, including correlating the concentrations of magnetic nanoparticle solutions with their measured transverse relaxation rates (R₂) and monitoring the outgassing from an opened bottle of oxygen-supersaturated water by measuring its longitudinal relaxation rate (R₁), are also demonstrated.     

The interpretation of multi-exponential water proton transverse relaxation in the human and porcine eye lens

We report results of 1H NMR transverse relaxation experiments on human and porcine eye lenses. Several authors have reported that transverse relaxation is not mono-exponential when observed by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and have interpreted the results by postulating the presence of "pools" of water molecules in different binding environments that do not exchange rapidly on the NMR timescale. We have compared CPMG data for intact lenses with results for lens homogenates and have combined a CPMG spectroscopic pulse train with NMR micro-imaging to study the nature of the transverse relaxation process in human and porcine lenses. Fast exchange of water protons with the lens proteins (crystallins) leads to an enhanced transverse relaxation rate that varies linearly with protein concentration. At the resolution of NMR micro-imaging the transverse relaxation process is mono-exponential. The results show that the multi-exponential CPMG data observed spectroscopically for whole lenses reflect spatial variations in crystallin content through the lens rather than the presence of distinct "bound" and "free" water pools.     

Novel NMR techniques for porous media research

Diffusion in porous media has been used as a probe of pore geometry in various NMR techniques. We will examine the effect of time-dependent diffusion in CPMG by showing that the diffusion time in CPMG is approximately the echo time, even in grossly inhomogeneous magnetic fields. Extension of the diffusion time in modified CPMG sequences is discussed. Diffusion in the susceptibility-contrast induced internal field is discussed as a means to probe pore size and pore shape. Finally, we present the general concept of two-dimensional relaxation-type experiments for study of molecules, fluids, materials and their dynamics that are characterized by spin relaxation and diffusion.     

The modulation of coupled relaxation in porous media.

This work investigates the effects of modulation of the transverse and longitudinal relaxation of the surface-fluid/pore-fluid spin system in porous media. Important new NMR well logging applications identify pore fluids by varying the CPMG T(2) pulse spacing to discriminate on the basis of fluid diffusivities in applied and local static magnetic field gradients. However, anomalous laboratory CPMG T(2) results have been reported repeatedly over 25 years for various porous media filled with a single fluid. In relatively large pores, at near bulk conditions, the transverse relaxation of diffusing molecular spins should be proportional to the square of the CPMG pulse spacing tau, the susceptibility contrast at the pore wall and the applied gradient. Observed is a markedly linear tau dependence that saturates at a plateau for large tau. The effect is not quadratic in applied gradient or susceptibility. For large pores, the tau dependence and the saturation value are proportional to the surface-to-volume ratio of the pores. This is in distinct contrast to the behavior observed by Borgia, Brown and Fantazzini for systems with much smaller pores at higher magnetic fields. The large-pore anomalous behaviors can be explained as a modulation of the exchange between surface-fluid and pore-fluid spins, such as observed by Luz and Meiboom in 1963 for water enriched with quadrupolar 17O. Scalar coupling of the solid-surface spins to the fluid-surface spins was postulated by Kleinberg, Kenyon and Mitra as a dominant relaxation mechanism for the surface fluid. The CPMG tau effect can be described as the modulation of the exchange coupling by the CPMG pi pulses, which mix the magnetizations between the exchanging, strongly coupled spin systems of the pore-fluid and the surface-fluid, which is, in turn, weakly coupled by scalar or pseudo-scalar interactions to the fast-relaxing solid surface.     

Inductive shielding of NMR phase noise

We report on a solution to the problem of phase noise in nuclear magnetic resonance (NMR) experiments. Phase noise refers to the variation in the phases of NMR signals from successive acquisitions due to an unstable applied field. Such a situation exists in high-field resistive Bitter magnets and, for sufficiently long timescales, can cause serious signal degradation upon signal averaging. An inductive shield, formed by a highly conducting metal tube placed around the sample and along the applied field, provides screening of the AC components of the applied field and thereby retains phase coherence over long periods. Although simple in principle there are technical difficulties for practical implementation of this method. We present demonstrations of the utility of this approach. In particular, we show a significant extension of the effective transverse coherence time of the ¹³C resonance in doubly ¹³C-labeled glycerol in a resistive Bitter magnet. This was accomplished through the use of a highly conducting aluminum shield, cooled to 4 K with liquid helium.     

Categories of coherence pathways for the CPMG sequence

The CPMG sequence has been extremely useful for efficient measurements of NMR signal, spin-spin relaxation, and diffusion, particularly in inhomogeneous magnetic fields, such as when samples are outside the magnet and RF coil. Due to the inaccuracy of the pulses and the off-resonance effects, the CPMG echoes have contributions from the Hahn echo as well as signals that are similar to stimulated echoes. The systematic understanding of the CPMG pulse sequence requires decomposing the magnetization dynamics into different coherence pathways. In this paper, we describe a method to classify the CPMG coherence pathways and illustrate the nature of these types of pathways. This classification shows that direct echo and stimulated echoes are the major contribution to the CPMG signal. It also provides a clear understanding of the effect of restricted diffusion in porous media.     

The influence of chemical and diffusive exchange on water proton transverse relaxation in plant tissues

Transverse water proton relaxation in parenchyma tissue of courgette, onion and apple shows a dependence on CPMG pulse spacing characteristic of each tissue. An analysis of this dependence suggests that transverse relaxation in these tissues is caused by various combinations of fast proton exchange between water and cell biopolymers (or solutes) and diffusion through internally generated magnetic field gradients. Diffusion between intra- and extracellular water compartments also averages the water proton signal to an extent that depends on cell morphology and membrane permeability and this is calculated using a two-compartment model. No recourse need be made to popular concepts such as exchange between free and "bound" water. The implications of our results for NMR image contrast are discussed.     

Relaxometry of insensitive nuclei: optimizing dissolution dynamic nuclear polarization

We report measurements of spin-lattice relaxation of carbon-13 as a function of the magnetic field ('relaxometry') in view of optimizing dissolution-DNP. The sample is temporarily lifted into the stray field above a high-resolution magnet using a simple and inexpensive 'shuttle'. The signals of arbitrary molecules can be observed at high field with high-resolution and sensitivity. During the dissolution process and subsequent 'voyage' from the polarizer to the NMR magnet, relaxation is accelerated by paramagnetic polarizing agents, but it can be quenched by using scavengers.     

High-field QCPMG NMR of large quadrupolar patterns using resistive magnets

Spectroscopy in a high magnetic field reduces second-order quadrupolar shift while increasing chemical shift. It changes the scale between quadrupolar and chemical shift of half-integer quadrupolar spins. The application of QCPMG multiple echo for acquiring large quadrupolar pattern under the high magnetic field of a 25 T resistive magnet is presented for acquiring large quadrupolar patterns. It shows that temporal field fluctuations and spatial homogeneity of the Keck magnet at the NHMFL contribute about ±20 ppm in line broadening. NMR patterns which have breadths of hundreds to thousands of kilohertz can be efficiently recorded using a combination of QCPMG and magnetic field stepping with negligible hindrance from the inhomogeneity and field fluctuations of powered magnets.     

Practical aspects involved in the design and set up of a 0.15 T, 6-coil resistive magnet, whole body NMR imaging facility

Many technical and logistical questions must be addressed when planning the installation of an NMR imaging system. These considerations become particularly significant when the facility is being established within an existing medical center complex. This paper presents a report on the practical aspects and experience obtained in siting a 6-coil 0.15 T resistive magnet system. The topics discussed include: floor loading; ferromagnetic environment; the effect of iron on the magnet field strength and homogeneity characteristics; shimming procedures; temperature stability requirements; rf shielding; and effects of the magnetic field on common medical instrumentation and magnetic media. It was found that the field shift as a function of the distance of a steel mass from the center of the magnet exhibited an (1/r)^5.2±0.5 to (1/r)^4.2±0.3 dependence for axial and radial positions respectively which, as expected, is somewhat weaker than the (1/r)⁶ dependence expected by point dipole approximations. Field distortions caused by the presence of ferromagnetic material in radial positions may be essentially fully compensated with first order transverse shim coils (most conveniently, the x and y imaging gradient coils could be used). Axially distributed material requires, in addition to first order z-gradient correction, higher order axial shim compensation. The temperature stability of the magnet system over the scan period must be better than 0.2°C to insure that temperature-induced field fluctuations are less than the intrinsic static inhomogeneity: and, ideally, below 0.01°C to reduce these fluctuations to less than those caused by power supply instability.     

Field gradient CPMG applied on postmortem muscles

As a new approach, Carr-Purcell-Meiboom-Gill (CPMG) experiments were performed in vitro on porcine muscles (n = 10) during the period from 15 min to 85 min postmortem and again at 24 h postmortem in the absence (G = 0) and the presence of an external field gradient (G = 0.5*10(-3) T/m), which was applied throughout the CPMG sequence. The experiments were performed on low-field nuclear magnetic resonance (NMR) equipment (0.47 T). Due to the inclusion of different pre-slaughter treatments (adrenaline treatment and pre-slaughter exercise/electrical stunning), the muscles could be divided into (I) a group (n = 5) characterized by a reduced decrease in pH postmortem and a high water-holding capacity and (II) a group (n = 5) characterized by an increased rate of pH decrease postmortem and a low water-holding capacity. Distributed analysis of the CPMG data revealed two major relaxation populations with relaxation times about 30-40 and 200-500 ms, respectively, and comparison of data obtained with G = 0 and G = 0.5*10(-3) T/m revealed effects of the external gradient on the relaxation time of both the two relaxation populations, which implies that both diffusion and relaxation contributes to the relaxation of the two populations. At 24 h postmortem the effect of the external field gradient on the relaxation time was significantly affected by muscle group (I vs. II), which reveals local differences in water diffusion in the two meat qualities. Finally, the discriminatory power with regard to muscle group (I vs. II) was investigated for data acquired with G = 0 and G = F = 0.5*10(-3) T/m, and both the two types of data were found highly suitable for separation of muscles according to meat quality.     

Transverse spin relaxation in liquid 129Xe in the presence of large dipolar fields

Using spin-echo NMR techniques we study the transverse spin relaxation of hyperpolarized liquid 129Xe in a spherical cell. We observe an instability of the transverse magnetization due to dipolar fields produced by liquid 129Xe, and find that imperfections in the pi pulses of the spin-echo sequence suppress this instability. A simple perturbative model of this effect is in good agreement with the data. We obtain a transverse spin relaxation time of 1300 sec in liquid 129Xe, and discuss applications of hyperpolarized liquid 129Xe as a sensitive magnetic gradiometer and for a permanent electric dipole moment search.     

Resin dynamics contributes to the NMR line broadening of organic molecules grafted onto a polystyrene resin

Despite the use of high resolution magic angle spinning NMR, the NMR linewidth of anchored molecules on the commonly used Merrifield solid phase resins remains larger than that of the corresponding molecules in solution. We investigate the different mechanisms that might be at the origin of this line broadening. Experimentally, we use the CPMG method to determine the (15)N relaxation times of a tethered tripeptide and show that the slow resin dynamics significantly contributes to the transverse relaxation.     

2D NMR技术在石油测井中的应用

近几年,2D NMR技术得到迅速发展,特别是在核磁共振测井领域. 该文将主要介绍2D NMR技术的脉冲序列、弛豫原理以及2D NMR技术在石油测井中应用. 2D NMR技术是在梯度场的作用下,利用一系列回波时间间隔不同的CPMG脉冲进行测量,利用二维的数学反演得到2D NMR. 2D NMR技术可以直接测量自扩散系数、弛豫时间、原油粘度、含油饱和度、可动水饱和度、孔隙度、渗透率等地层流体性质和岩石物性参数. 从2D NMR谱上,可以直观的区分油、气、水,判断储层润湿性,确定内部磁场梯度等. 2D NMR技术为识别流体类型提供了新方法.     

Magnetic relaxation dispersion probe

The magnetic field dependence of nuclear spin-lattice relaxation rates provides a powerful approach to characterizing intra and intermolecular dynamics. NMR spectrometers that provide extensive magnetic relaxation dispersion profiles may switch magnetic field strengths rapidly by either moving the sample or by changing the current in an electromagnet. If the sample is moved, the polarization and detection fields may be very high, which provides both high sensitivity and resolution. This report summarizes the design of a pneumatic sample transport system for glass sample containers that may be used in either a dual magnet spectrometer or in a single magnet system that exploits the fringe field as the secondary magnetic field.     

Combining PFG and CPMG NMR measurements for separate characterization of oil and water simultaneously present in a heterogeneous system

When analyzingT ₂ relaxation time curves from an ordinary Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information regarding the mobility of the components and on the type of environment experienced by each component. Compared to a standard CPMG experiment, a combination of pulsed field gradient (PFG) methods with the CPMG experiment will increase the amount of information that is obtainable from the nuclear magnetic resonance (NMR) experiment on a system of components differing significantly in molecular mobility. We propose a method for achieving separate measurements of theT ₂ attenuation of two components simultaneously present within a sample, and we believe it to be generally valid for any system in which the components differ significantly in molecular mobility. The two components could be oil and water in porous rock, or fat and water in a biological tissue, where a separation of theT ₂ attenuations for the two components will add insight to the study of the systems. In order to verify the method we made use of a sample containing a mixture of oil and water in two separate bulk phases, and compared the results with PFG-CPMG experiments performed on samples containing oil or water only, respectively. The method was applied to systems containing glass spheres immersed in water and oil, and it was possible to obtain information about the physical environment of the components which otherwise is not easily obtainable. The method presented here is therefore presumably applicable to whole rock cores or tissue samples.     

Relationships between (1)H NMR relaxation data and some technological parameters of meat: a chemometric approach

In this paper chemometrics (ANOVA and PCR) is used to measure unbiased correlations between NMR spin-echo decays of pork M. Longissimus dorsi obtained through Carr-Purcell-Meiboom-Gill (CPMG) experiments at low frequency (20 MHz) and the values of 14 technological parameters commonly used to assess pork meat quality. On the basis of the ANOVA results, it is also found that the CPMG decays of meat cannot be best interpreted with a "discrete" model (i.e., by expanding the decays in a series of a discrete number of exponential components, each with a different transverse relaxation time), but rather with a "continuous" model, by which a continuous distribution of T(2)'s is allowed. The latter model also agrees with literature histological results.     

Experimental validation of a T2rho transverse relaxation model using LASER and CPMG acquisitions

The transverse relaxation rate (R2=1/T2) of many biological tissues are altered by endogenous magnetized particles (i.e., ferritin, deoxyhemoglobin), and may be sensitive to the pathological progression of neurodegenerative disorders associated with altered brain-iron stores. R2 measurements using Carr-Purcell-Meiboom-Gill (CPMG) acquisitions are sensitive to the refocusing pulse interval (2taucp), and have been modeled as a chemical exchange (CE) process, while R2 measurements using a localization by adiabatic selective refocusing (LASER) sequence have an additional relaxation rate contribution that has been modeled as a R2rho process. However, no direct comparison of the R2 measured using these two sequences has been described for a controlled phantom model of magnetized particles. The three main objectives of this study were: (1) to compare the accuracy of R2 relaxation rate predictions from the CE model with experimental data acquired using a conventional CPMG sequence, (2) to compare R2 estimates obtained using LASER and CPMG acquisitions, and (3) to determine whether the CE model, modified to account for R2rho relaxation, adequately describes the R2 measured by LASER for a full range of taucp values. In all cases, our analysis was confined to spherical magnetic particles that satisfied the weak field regime. Three phantoms were produced that contained spherical magnetic particles (10 microm diameter polyamide powders) suspended in Gd-DTPA (1.0, 1.5, and 2.0 mmol/L) doped gel. Mono-exponential R2 measurements were made at 4T as a function of refocusing pulse interval. CPMG measurements of R2 agreed with CE model predictions while significant differences in R2 estimates were observed between LASER and CPMG measurements for short taucp acquisitions. The discrepancy between R2 estimates is shown to be attributable to contrast enhancement in LASER due to T2rho relaxation.     

Exploring the separate NMR responses from crude oil and water in rock cores

In analysis of transverse relaxation time (T ₂) curves in a Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system originating from measurements of oil and water in rock cores, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information to be extracted of the different components contributing to the totalT ₂ relaxation time curve. From the study of rock cores saturated with different amounts of crude oil and water, we show that with an optimised experimental setup it is possible to extract information from the nuclear magnetic resonance response that is not resolved by any other methods. This setup combines pulsed field gradient methods with the CPMG experiment utilizing data from both rock cores and bulk oil and water. Then it becomes feasible to separate the signals from oil and water where the two-dimensional inverse Laplace transform ordinarily seems to fail.     

Diffusion effects on the CPMG relaxation rate in a dipolar field

The diffusion in the magnetic dipolar field around a sphere is considered. The diffusion is restricted to the space between two concentric spheres, where the inner sphere is the source of the magnetic dipolar field. Analytical expressions for the CPMG transverse relaxation rate as well as the free induction decay and the spin echo time evolution are given in the Gaussian approximation. The influence of the inter-echo time is analyzed. The limiting cases of small and large inter-echo times as well as the short and long time behavior are evaluated.