A study of the use of Overhauser enhancement to assist with needle and catheter placement during interventional MRI

The practicability of using Overhauser enhancement of saline in interventional MRI was investigated. Saline was used as a means of marking the path taken by a fluid-filled cavity, similar to that formed by a needle, catheter, or cannula during interventional MRI procedures. A prototype device was designed and constructed for saturation and propulsion of 0.6 ml of doped liquid. The pertinent Overhauser parameters, such as the obtainable enhancement factor, were measured. Signal enhancement in excess of 10 was demonstrated in practice by acquiring images showing an enhancement of fluid in a catheter tube.     

Dipolar and J encoded DQ MAS spectra under rotational resonance

A two-dimensional (2D) double-quantum (DQ) experiment under rotational resonance (R(2)) conditions is introduced for evaluating dipolar couplings in rotating solids. The contributions from the R(2)-recoupled dipolar interaction and the J coupling can be conveniently separated in the resulting 2D R(2)-DQ spectrum, so that the unknown dipolar coupling can readily be extracted, provided that the values of the involved J coupling constants are known. Since the measured parameters are integral intensity ratios between suitably chosen absorption peaks in the 2D spectrum, the proposed method is characterized by a reduced sensitivity to relaxation parameters. The effect of rotor-modulated terms, including chemical shift anisotropy, is efficiently averaged out by synchronizing the excitation/reconversion time with the rotor period. All of these features are demonstrated theoretically by the example of two model systems, namely, isolated spin-pairs and a three-spin system. The results of the theoretical models are applied to both (13)C and (1)H nuclei to extract dipolar couplings in uniformly (13)C labeled L-alanine and a crosslinked natural rubber.     

Tagging of the Magnetization with the Transition Zones of 360° Rotations Generated by a Tandem of Two Adiabatic DANTE Inversion Sequences

A new technique is presented for generating myocardial tagging using the signal intensity minima of the transition zones between the bands of 0° and 360° rotations, induced by a tandem of two adiabatic delays alternating with nutations for tailored excitation (DANTE) inversion sequences. With this approach, the underlying matrix corresponds to magnetization that has experienced 0° or 360° rotations. The DANTE sequences were implemented from adiabatic parent pulses for insensitivity of the underlying matrix to B₁ inhomogeneity. The performance of the proposed tagging technique is demonstrated theoretically with computer simulations and experimentally on phantom and on the canine heart, using a surface coil for both RF transmission and signal reception. The simulations and the experimental data demonstrated uniform grid contrast and sharp tagging profiles over a twofold variation of the B₁ field magnitude.     

A new time-domain frequency-selective quantification algorithm

In this paper a new time-domain frequency-selective quantification algorithm is presented. Frequency-selective quantification refers to a method that analyzes spectral components in a selected frequency region, ignoring all the other components outside. The algorithm, referred to as MeFreS (Metropolis Frequency-Selective), is based on rank minimization of an opportune Hankel matrix. The minimization procedure is satisfied by the down-hill simplex method, implemented with the simulated annealing method. MeFreS does not use any preprocessing step or filter to suppress nuisance peaks, but the signal model function is directly fitted. In this manner, neither inherent signal distortions nor estimation biases to be corrected occur. The algorithm was tested with Monte Carlo simulations. A comparison with VARPRO and AMARESw algorithms was carried out. Finally, two samples of known content from NMR data were quantified.     

Development of NMR instrumentation to achieve excitation of large bandwidths in high-resolution spectra at high field

A prototype 2.5-mm (1)H high-resolution probe for an 18.8-T (800 MHz) nuclear magnetic resonance spectrometer has been designed, together with a dedicated amplifier capable of delivering up to 1 kW of power. This probe permits a 90 degrees pulse length of 2 mus to be achieved at 300 W, corresponding to an excitation bandwidth of +/-125 kHz. Probe performances were tested on samples commonly used for this purpose as well as on protein and paramagnetic model compound samples. It is shown that this probe is useful for a wide range of applications at high magnetic field, especially in the study of systems characterized by very broad and far-shifted resonances and in experiments that require high-power radiofrequency irradiation.     

H NMR Characterization of the Product from Single Solid-Phase Resin Beads Using Capillary NMR Flow Probes

A capillary NMR flow probe was designed to generate high-resolution ¹H NMR spectra at 600 MHz from the cleaved product of individual 160-μm Tentagel combinatorial chemistry beads. By injecting a dissolved sample sandwiched between an immiscible, perfluorinated organic liquid directly into the probe, NMR spectra of the product cleaved from single beads were acquired in just 1 h of spectrometer time without diffusional dilution. Sample handling efficiency on the single bead scale was comparable to that obtained with a bulk sample. Using the relative intensity of the DMSO-d₅H versus the analyte signals in a fully relaxed CPMG spectrum, the amount of product cleaved from a single bead was determined to be 540±170 pmol in one of the samples. Following the NMR data collection, the samples were examined with electrospray ionization mass spectrometry to provide additional structural information. By coupling with microliter-volume fluidic capabilities, the capillary flow probe described here will enable multidimensional characterization of single solid-phase resin products in an online manner.     

Probe--sample coupling in the magnetic resonance force microscope

The magnetic resonance force microscope (MRFM) provides a route to achieving scanned probe magnetic resonance imaging with extremely high spatial resolution. Achieving this capability will require understanding the force exerted on a microscopic magnetic probe by a spatially extended sample over which the probe is scanned. Here we present a detailed analysis of this interaction between probe and sample. We focus on understanding the situation where the micromagnet mounted on the mechanical resonator generates a very inhomogeneous magnetic field and is scanned over a sample with at least one spatial dimension much larger than that of the micromagnet. This situation differs quite significantly from the conditions under which most MRFM experiments have been carried out where the sample is mounted on the mechanical resonator and placed in a rather weak magnetic field gradient. In addition to the concept of a sensitive slice (the spatial region where the magnetic resonance condition is met) it is valuable to map the forces exerted on the probe by spins at various locations; this leads to the concept of the force slice (the region in which spins exert force on the resonator). Results of this analysis, obtained both analytically and numerically, will be qualitatively compared with an initial experimental finding from an EPR-MRFM experiment carried out on DPPH at 4 K.     

Picoliter (1)H NMR spectroscopy

In this study, a 267-microm-diameter solenoid transceiver is used to acquire localized (1)H NMR spectra and the measured signal-to-noise ratio (SNR) at 500 MHz is shown to be within 20--30% of theoretical limits formulated by considering only its resistive losses. This is illustrated using a 100-microm-diameter globule of triacylglycerols (approximately 900mM) that may be an oocyte precursor in young Xenopus laevis frogs and a water sample containing choline at a concentration often found in live mammalian cells (approximately 33 mM). In chemical shift imaging (CSI) experiments performed using a few thousand total scans, the choline methyl line is shown to have an acceptable SNR in resolved volume elements containing only 50 pL of sample, and localized spectra are resolved from just 5 pL in the Xenopus globule. These findings demonstrate the feasibility of performing (1)H NMR on picoliter-scale sample volumes in biological cells and tissues and illustrate how the achieved SNR in spectroscopic images can be predicted with reasonable accuracy at microscopic spatial resolutions.     

Analytical Planar Mixing Transfer Functions for Two Coupled Spin-1 Nuclei

Analytical coherence transfer functions are presented for spin systems consisting of two spins 1 under planar mixing conditions. Compared to isotropic mixing experiments, larger transfer amplitudes are found and differences in the multiplet patterns are discussed.     

Improved Lanczos algorithms for blackbox MRS data quantitation

Magnetic resonance spectroscopy (MRS) has been shown to be a potentially important medical diagnostic tool. The success of MRS depends on the quantitative data analysis, i.e., the interpretation of the signal in terms of relevant physical parameters, such as frequencies, decay constants, and amplitudes. A variety of time-domain algorithms to extract parameters have been developed. On the one hand, there are so-called blackbox methods. Minimal user interaction and limited incorporation of prior knowledge are inherent to this type of method. On the other hand, interactive methods exist that are iterative, require user involvement, and allow inclusion of prior knowledge. We focus on blackbox methods. The computationally most intensive part of these blackbox methods is the computation of the singular value decomposition (SVD) of a Hankel matrix. Our goal is to reduce the needed computational time without affecting the accuracy of the parameters of interest. To this end, algorithms based on the Lanczos method are suitable because the main computation at each step, a matrix-vector product, can be efficiently performed by means of the fast Fourier transform exploiting the structure of the involved matrix. We compare the performance in terms of accuracy and efficiency of four algorithms: the classical SVD algorithm based on the QR decomposition, the Lanczos algorithm, the Lanczos algorithm with partial reorthogonalization, and the implicitly restarted Lanczos algorithm. Extensive simulation studies show that the latter two algorithms perform best.     

A two-dimensional artifact from asynchronous decoupling

Many heteronuclear NMR experiments employ decoupling to collapse the heteronuclear multiplet, using decoupling schemes with a periodic phase modulation like WALTZ, MLEV, or GARP. Because of the periodic nature of these schemes, cycling sidebands are generated, whose intensity can be strongly reduced by decoupling asynchronously. We show that the most common implementation of asynchronous decoupling on modern spectrometers is such that the cycling sidebands are subjected to a periodic modulation. For multidimensional experiments, this results in ridges that can seriously compromise the quality of the spectrum. Based on our model, the artifact in a 2D [(1)H]-(15)N NOE equilibrium experiment is simulated and it is shown that the artifact can be prevented by using synchronous decoupling.     

An open magnet utilizing ferro-refraction current magnification

Ferro-refraction is the field magnification that is obtained when a current segment is near a high magnetic permeable boundary. It is shown that ferro-refraction may be used in the design of magnets for NMR or MRI to increase the efficiency of these magnets. The field may be modeled analytically with the Biot--Savart law and the inclusion of mirror image currents. Ferro-refraction is particularly useful in the design of monohedral magnets, magnets producing a remote homogeneous region which have the magnetic sources arranged to one side. These magnets have also been called planar magnets. Two designs for a monohedral magnet which produce good agreement between experimental and analytic results are presented.     

Linewidth-Resolved N HSQC, a Simple 3D Method to Measure N Relaxation Times from T1 and T2 Linewidths

A three-dimensional approach for measuring ¹⁵N relaxation times is described. Instead of selecting particular values for the relaxation period, in the proposed method the relaxation period is incremented periodically in order to create a 3D spectrum. This additional frequency domain of the transformed spectrum contains the relaxation time information in the T₁ and T₂ linewidths, and thus the longitudinal and transverse ¹⁵N relaxation times can be measured without determination of 2D cross peak volumes/intensities and subsequent curve fitting procedures.     

Determination of membrane Peptide orientation by 1H-detected 2H NMR spectroscopy

We demonstrate the application of the proton inverse detected deuteron (PRIDE) NMR technique to the measurement of the orientation of membrane-bound peptides with enhanced sensitivity. Gramicidin D, a transmembrane peptide, and ovispirin, a surface-bound peptide, were used as model systems. The peptides were 2H-labeled by 1H/2H exchange and oriented uniaxially on glass plates. The directly detected 2H spectra of both peptides showed only a strong D(2)O signal and no large quadrupolar splittings. In contrast, the PRIDE spectrum of gramicidin exhibited quadrupolar splittings as large as 281 kHz, consistent with its transmembrane orientation. Moreover, the large D(2)O signal in the directly detected 2H spectra was cleanly suppressed in the PRIDE spectrum. For ovispirin, the 1H indirectly detected 2H spectrum revealed a 104 kHz splitting and a zero-frequency peak. The former reflects the in-plane orientation of most of the helix axis, while the latter results from residues with a magic-angle orientation of the N-D bonds. These are consistent with previous 15N NMR results on ovispirin. The combination of PRIDE and exchange labeling provides an economical and sensitive method of studying membrane peptide orientations in lipid bilayers without the influence of D(2)O and with the ability to detect N-D bonds at the magic angle from the bilayer normal.     

A subspace time-domain algorithm for automated NMR spectral normalization

Recently, two methods have been proposed for quantitatively comparing NMR spectra of control and treated samples, in order to examine the possible occurring variations in cell metabolism and/or structure in response to numerous physical, chemical, and biological agents. These methods are the maximum superposition normalization algorithm (MaSNAl) and the minimum rank normalization algorithm (MiRaNAl). In this paper a new subspace-based time-domain normalization algorithm, denoted by SuTdNAl (subspace time-domain normalization algorithm), is presented. By the determination of the intersection of the column spaces of two Hankel matrices, the common signal poles and further on the components having proportionally varying amplitudes are detected. The method has the advantage that it is computationally less intensive than the MaSNAl and the MiRaNAl. Furthermore, no approximate estimate of the normalization factor is required. The algorithm was tested by Monte Carlo simulations on a set of simulation signals. It was shown that the SuTdNAl has a statistical performance similar to that of the MiRaNAl, which itself is an improvement over the MaSNAl. Furthermore, two samples of known contents are compared with the MiRaNAl, the SuTdNAl, and an older method using a standard. Finally, the SuTdNAl is tested on a realistic simulation example derived from an in vitro measurement on cells.     

Circularly Polarized RF Magnetic Fields for Spin-1 NQR

The low sensitivity of nuclear quadrupole resonance (NQR) of powders is due, in part, to the inability to efficiently excite and detect nuclei at all crystal orientations. Here we describe the use of circularly polarized RF magnetic fields for excitation followed by detection of the resultant circular RF magnetization in I=1 NQR to increase the fraction of nuclei excited and detected. We show that the technique can greatly improve the effective RF field homogeneity and increase the largest signal amplitude by a factor of 1.72. In favorable cases, the resulting circularly polarized NQR signal can be separated from linearly polarized magnetoacoustic and piezoelectric ringing artifacts that occur in some NQR materials detection applications.     

Optimization of residual water signal removal by HLSVD on simulated short echo time proton MR spectra of the human brain

Suppression of the residual water signal from proton magnetic resonance (MR) spectra recorded in human brain is a prerequisite to an accurate quantification of cerebral metabolites. Several postacquisition methods of residual water signal suppression have been reported but none of them provide a complete elimination of the residual water signal, thereby preventing reliable quantification of brain metabolites. In the present study, the elimination of the residual water signal by the Hankel Lanczos singular value decomposition method has been evaluated and optimized to provide fast automated processing of spectra. Model free induction decays, reproducing the proton signal acquired in human brain localized MR spectroscopy at short echo times (e.g., 20 ms), have been generated. The optimal parameters in terms of number of components and dimension of the Hankel data matrix allowing complete elimination of the residual water signal are reported.     

NMR Relaxation and Diffusion Measurements on Iron(III)-Doped Kaolin Clay

Kaolin clay samples were mixed with various amounts of Fe₂O₃ powder. The influence of this magnetic impurity on NMR relaxation and diffusion measurements on the water in this porous material was investigated. The NMR relaxation measurements showed a nearly mono-exponential decay, leading to the conclusion that the pore size distribution of the clay samples is either narrow and/or that the pores are interconnected very well. Both the longitudinal and the transverse relaxation rate depend linearly on the concentration of the Fe₂O₃ impurity. The NMR diffusion measurements revealed that the Fe₂O₃ causes internal magnetic field gradients that largely exceed the maximum external gradient that could be applied by our NMR apparatus (0.3 T/m). Additional SQUID measurements yielded the magnetization and magnetic susceptibility of the samples at the magnetic field strength used in the NMR measurements (0.8 T). A theoretical estimate of the internal magnetic field gradients leads to the conclusion that the water in the porous clay samples cannot be described by the commonly observed motional averaging regime. Probably an intermediate or a localization regime is induced by the large internal gradients, which are estimated to be on the order of 1 to 10 T/m in the pore volume and may exceed 1000 T/m at the pore surface.     

A General-Purpose Finite-Volume Advection Scheme for Continuous and Discontinuous Fields on Unstructured Grids

We present a new general-purpose advection scheme for unstructured meshes based on the use of a variation of the interface-tracking flux formulation recently put forward by O. Ubbink and R. I. Issa (J. Comput. Phys.153, 26 (1999)), in combination with an extended version of the flux-limited advection scheme of J. Thuburn (J. Comput. Phys.123, 74 (1996)), for continuous fields. Thus, along with a high-order mode for continuous fields, the new scheme presented here includes optional integrated interface-tracking modes for discontinuous fields. In all modes, the method is conservative, monotonic, and compatible. It is also highly shape preserving. The scheme works on unstructured meshes composed of any kind of connectivity element, including triangular and quadrilateral elements in two dimensions and tetrahedral and hexahedral elements in three dimensions. The scheme is finite-volume based and is applicable to control-volume finite-element and edge-based node-centered computations. An explicit–implicit extension to the continuous-field scheme is provided only to allow for computations in which the local Courant number exceeds unity. The transition from the explicit mode to the implicit mode is performed locally and in a continuous fashion, providing a smooth hybrid explicit–implicit calculation. Results for a variety of test problems utilizing the continuous and discontinuous advection schemes are presented.     

A Multiphase Godunov Method for Compressible Multifluid and Multiphase Flows

We propose a new model and a solution method for two-phase compressible flows. The model involves six equations obtained from conservation principles applied to each phase, completed by a seventh equation for the evolution of the volume fraction. This equation is necessary to close the overall system. The model is valid for fluid mixtures, as well as for pure fluids. The system of partial differential equations is hyperbolic. Hyperbolicity is obtained because each phase is considered to be compressible. Two difficulties arise for the solution: one of the equations is written in non-conservative form; non-conservative terms exist in the momentum and energy equations. We propose robust and accurate discretisation of these terms. The method solves the same system at each mesh point with the same algorithm. It allows the simulation of interface problems between pure fluids as well as multiphase mixtures. Several test cases where fluids have compressible behavior are shown as well as some other test problems where one of the phases is incompressible. The method provides reliable results, is able to compute strong shock waves, and deals with complex equations of state.