Indirect detection of nuclear magnetic resonance via geometrically induced long-range dipolar fields

Dynamic nuclear magnetic resonance field sensing with part-per-trillion resolution

High-field magnets of up to tens of teslas in strength advance applications in physics, chemistry and the life sciences. However, progress in generating such high fields has not been matched by corresponding advances in magnetic field measurement. Based mostly on nuclear magnetic resonance, dynamic high-field magnetometry is currently limited to resolutions in the nanotesla range. Here we report a concerted approach involving tailored materials, magnetostatics and detection electronics to enhance the resolution of nuclear magnetic resonance sensing by three orders of magnitude. The relative sensitivity thus achieved amounts to 1 part per trillion (10-12). To exemplify this capability we demonstrate the direct detection and relaxometry of nuclear polarization and real-time recording of dynamic susceptibility effects related to human heart function. Enhanced high-field magnetometry will generally permit a fresh look at magnetic phenomena that scale with field strength. It also promises to facilitate the development and operation of high-field magnets.

Flat pancake distant dipolar fields for enhancement of intermolecular multiple-quantum coherence signals

Intermolecular multiple-quantum coherences (iMQCs) originated from distant dipolar field (DDF) possess some appealing unique properties for magnetic resonance imaging (MRI). DDF is usually induced with continuous wave (i.e., sine- or square-wave) magnetization modulation in the whole sample. In this article, a spatially localized and enhanced DDF was optimally tailored in a thin slice with an adiabatic inversion pulse. Evidence was provided to show that careful tailoring of the spatially localized DDF can generate highly efficient iMQC signals, with more than two-fold enhancement compared to the conventional sine-wave magnetization modulation method, and 1.5 times of that with the square-wave modulation under the similar condition. Theoretical predictions, simulation results, and experimental verifications agree well with each other. Practical implementation of this approach for efficient iMQC MRI was explored.

High-resolution MR spectroscopy via intermolecular double-quantum coherences in inhomogeneous B0 and B1 fields

Inhomogeneity in static field B0 and/or RF field B1 is inevitable under some circumstances. In this work, a method based on intermolecular double-quantum coherences is employed for high-resolution 1D MR spectroscopy via 2D acquisition under such a condition. High-resolution information on chemical shifts, multiplet patterns, J coupling constants and relative peak areas can be retained in the resulting 1D projected spectra, as shown with results from a narrow-bore NMR spectrometer and a whole-body clinical scanner.

Intermolecular zero-quantum coherence NMR spectroscopy in the presence of local dipole fields

NMR experiments detecting intermolecular zero-quantum coherences (iZQCs) allow for observation of homogeneous line shapes under inhomogeneous magnetic fields. Local dipole fields impair the refocusing capacity of such experiments and render the available theoretical description of signal evolution invalid. In this article, the impact of local dipole fields on two-dimensional iZQC spectroscopy experiments was assessed by performing extensive numerical simulations, which solved the nonlinear Bloch equations for a binary solution in a magnetization array of 64(3) spatial points. Local dipole fields were simulated using spherical volumes with different magnetic susceptibility values corresponding to either a glass sphere or an air inclusion with a diameter of 100 microm. The local field resulted in a broadened distribution of difference frequencies between locally interacting spins and led to the dominating effect of decreasing the amplitude of the solute peak, before line broadening was observed in the spectra. From simulations using a magnetic field strength of 17.6 T, the smallest ratio of sample to inclusion volume that still allowed for observation of the solute peak was determined to be eta(limit)=215 and eta(limit)=392 for glass and air inclusions, respectively. Experimental data acquired with a 100 microm diameter glass sphere embedded in agar gel yielded a value of eta(limit)=252 and confirmed the order of magnitude obtained from the simulations. From these data, it was concluded that iZQC spectroscopy is possible as long as the relative volume occupied by air inclusions does not exceed the order of 0.1% of the sample volume. This limit, in contrast to the previous speculations, strongly excludes materials or tissues with high density of strong inhomogeneities from the investigation by iZQC spectroscopy.

Indirect detection of NMR via geometry-dependent dipolar fields, revisited

We explore the dipolar interactions between two separate nuclear spin ensembles in a mixture containing oil and water. Here we expand initial results [C.A. Meriles, W. Dong, J. Magn. Reson. 181 (2006) 331.] to the case in which both systems have the shape of flat, stacked disks. We find that-in spite of the strong inhomogeneity of the coupling dipolar field-the signal encoded in one of the components can be made approximately proportional to the magnetization in the other. This allows us to use one of these systems as a 'sensor' to indirectly reconstruct the resonance spectrum or to determine the relaxation time of the 'sample' system. In the regime in which dipolar interactions are sufficiently strong, our method can be set to scale-up weaker signals in a non-linear fashion, which, potentially, could allow one to introduce contrast or to improve detection sensitivity of less magnetized samples.

Diffraction-like phenomena in a periodic magnetization distribution at 1.5 T using the distant dipolar field (DDF)

In the CRAZED experiment (COSY revamped by asymmetric Z-gradient echo detection, Warren et al.), a spatially anisotropic magnetization distribution is created by application of a magnetic field gradient (strength G, duration tau) which in turn generates a response called the distant dipolar field (DDF). The DDF is a source of intermolecular multiple-quantum coherences (iMQC) which contain information on the distance d=pi/(gammaGtau) between pairs of dipolar-coupled spins. Diffraction-like phenomena may result for periodically structured samples. In this study, we report the observation of diffraction owing to the DDF at 1.5 T using a clinical whole-body tomograph. Based on the semi-classical treatment of the problem by Robyr and Bowtell, diffraction conditions were obtained for a CRAZED-type pulse sequence that selects iMQC of order N. The predicted distinct difference in N=2 and N not equal2 coherences, i.e., a dominant continuous course as a function of tau (N=2) and prominent diffraction peaks otherwise, could be verified in CRAZED experiments in a periodically structured sample selecting coherence orders N=2 and N=3. The diffractive signal component contains information on the geometric structure of the sample. Applications of this technique may permit the detection of changes in composition and geometry of periodic structures.