Reduced data acquisition time in multi-dimensional NMR spectroscopy using multiple-coil probes

Multiplexing experiments in NMR and multi-nuclear MRI

Multiplexing NMR experiments by direct detection of multiple free induction decays (FIDs) in a single experiment offers a dramatic increase in the spectral information content and often yields significant improvement in sensitivity per unit time. Experiments with multi-FID detection have been designed with both homonuclear and multinuclear acquisition, and the advent of multiple receivers on commercial spectrometers opens up new possibilities for recording spectra from different nuclear species in parallel. Here we provide an extensive overview of such techniques, designed for applications in liquid- and solid-state NMR as well as in hyperpolarized samples. A brief overview of multinuclear MRI is also provided, to stimulate cross fertilization of ideas between the two areas of research (NMR and MRI). It is shown how such techniques enable the design of experiments that allow structure elucidation of small molecules from a single measurement. Likewise, in biomolecular NMR experiments multi-FID detection allows complete resonance assignment in proteins. Probes with multiple RF microcoils routed to multiple NMR receivers provide an alternative way of increasing the throughput of modern NMR systems, effectively reducing the cost of NMR analysis and increasing the information content at the same time. Solid-state NMR experiments have also benefited immensely from both parallel and sequential multi-FID detection in a variety of multi-dimensional pulse schemes. We are confident that multi-FID detection will become an essential component of future NMR methodologies, effectively increasing the sensitivity and information content of NMR measurements.

Portable NMR with Parallelism

Portable NMR combining a permanent magnet and a complementary metal-oxide-semiconductor (CMOS) integrated circuit has recently emerged to offer the long desired online, on-demand, or in situ NMR analysis of small molecules for chemistry and biology. Here we take this cutting-edge technology to the next level by introducing parallelism to a state-of-the-art portable NMR platform to accelerate its experimental throughput, where NMR is notorious for inherently low throughput. With multiple (N) samples inside a single magnet, we perform simultaneous NMR analyses using a single silicon electronic chip, going beyond the traditional single-sample-per-magnet paradigm. We execute the parallel analyses via either time-interleaving or magnetic resonance imaging (MRI). In the time-interleaving method, the N samples occupy N separate NMR coils: we connect these N NMR coils to the single silicon chip one after another and repeat these sequential NMR scans. This time-interleaving is an effective parallelization, given a long recovery time of a single NMR scan. To demonstrate this time-interleaved parallelism, we use N = 2 for high-resolution multidimensional spectroscopy such as J-coupling resolved free induction decay spectroscopy and correlation spectroscopy (COSY) with the field homogeneity carefully optimized (<0.16 ppm) and N = 4 for multidimensional relaxometry such as diffusion-edited T₂ mapping and T₁-T₂ correlation mapping, expediting the throughput by 2-4 times. In the MRI technique, the N samples (N = 18 in our demonstration) share 1 NMR coil connected to the single silicon chip and are imaged all at once multiple times, which reveals the relaxation time of all N samples simultaneously. This imaging-based approach accelerates the relaxation time measurement by 4.5 times, and it could be by 18 times if the signal-to-noise were not limited. Overall, this work demonstrates the first portable high-resolution multidimensional NMR with throughput-accelerating parallelism.

Low Rank Enhanced Matrix Recovery of Hybrid Time and Frequency Data in Fast Magnetic Resonance Spectroscopy

GOAL

The two dimensional magnetic resonance spectroscopy (MRS) possesses many important applications in bioengineering but suffers from long acquisition duration. Non-uniform sampling has been applied to the spatiotemporally encoded ultrafast MRS, but results in missing data in the hybrid time and frequency plane. An approach is proposed to recover this missing signal, of which enables high quality spectrum reconstruction. M ethods: The natural exponential characteristic of MRS is exploited to recover the hybrid time and frequency signal. The reconstruction issue is formulated as a low rank enhanced Hankel matrix completion problem and is solved by a fast numerical algorithm.

RESULTS

Experiments on synthetic and real MRS data show that the proposed method provides faithful spectrum reconstruction, and outperforms the state-of-the-art compressed sensing approach on recovering low-intensity spectral peaks and robustness to different sampling patterns. C onclusion: The exponential signal property serves as an useful tool to model the time-domain MRS signals and even allows missing data recovery. The proposed method has been shown to reconstruct high quality MRS spectra from non-uniformly sampled data in the hybrid time and frequency plane.

SIGNIFICANCE

Low-intensity signal reconstruction is generally challenging in biological MRS and we provide a solution to this problem. The proposed method may be extended to recover signals that generally can be modeled as a sum of exponential functions in biomedical engineering applications, e.g., signal enhancement, feature extraction, and fast sampling.

Synchronized and concurrent experiments in Moving Tube NMR: using separate sample volumes for different pulse sequences

This study presents a new application of sample shuttling with a long NMR tube (Moving Tube NMR, MT-NMR) as a method for collecting different experiments synchronously or even concurrently using separate sample regions. Synchronized experiments were performed using an automated shuttling apparatus to move different sample regions into the coil between transients such that each experiment was collected using a separate, specific sample segment. Additionally, a 2D NOESY spectrum and a double quantum filtered COSY (DQCOSY) spectrum were collected concurrently by shuttling between two different sample regions during the NOESY mixing time. These applications of the Moving Tube technique show that it is a useful platform for compounded data acquisition to optimize spectrometer time by minimizing measurement times and avoiding problems arising from instrument and sample instabilities. Furthermore, collecting a DQCOSY during a 2D NOESY mixing time opens a wide array of possibilities, as this principle can be applied to collect any experiment during a NOESY mixing time provided that the mixing period is longer than the sum of the sample shuttling time plus a complete scan of the intermittent experiment. While this methodology relies on the use of a long sample tube, it does not require excessive sample volumes, as two milliliters is enough to constitute multiple sample regions.

Fast multidimensional localized parallel NMR spectroscopy for the analysis of samples

A parallel localized spectroscopy (PALSY) method is presented to speed up the acquisition of multidimensional NMR (nD) spectra. The sample is virtually divided into a discrete number of nonoverlapping slices that relax independently during consecutive scans of the experiment, affording a substantial reduction in the interscan relaxation delay and the total experiment time. PALSY was tested for the acquisition of three experiments 2D COSY, 2D DQF-COSY and 2D TQF-COSY in parallel, affording a time-saving factor of 3-4. Some unique advantages are that the achievable resolution in any dimension is not compromised in any way: it uses conventional NMR data processing, it is not prone to generate spectral artifacts, and once calibrated, it can be used routinely with these and other combinations of NMR spectra.

A single-chip array of NMR receivers

We present the first single-chip array of integrated NMR receivers for parallel spectroscopy and imaging. The array, optimized for operation at 300 MHz, is composed of eight separate channels, with each channel consisting of a detection coil, a tuning capacitor, a low noise amplifier and a 50 ohm buffer. As all the integrated electronics are placed underneath the reception coils, the array is densely packed. Each single-channel reception coil has a diameter of 500 microm, resulting in a total active area of 1 mm by 2 mm for the array. The (1)H time-domain spin sensitivity of a single channel is approximately 1x10(15) spins/square root(Hz).

Development of a dual cell, flow-injection sample holder, and NMR probe for comparative ligand-binding studies

NMR based ligand screening is becoming increasingly important for the very early stages of drug discovery. We have proposed a method that makes highly efficient use of a single sample of a scarce target, or one with poor or limited solubility, to screen an entire compound library. This comparative method is based on immobilizing the target for the screening procedure. In order to support the method, a dual cell, flow injection probe with a single receiver coil has been constructed. The flow injection probe has been mated to a single high performance pump and sample handling system to enable the automated analysis of large numbers of compound mixes for binding to the target. The probe, having an 8 mm 1H/2H dual tuned coil and triple axis gradients, is easily shimmed and yields NMR spectra of comparable quality to a standard 5 mm high-resolution probe. The lineshape in the presence of a solid support is identical to that in glass NMR tubes in a 5 mm probe. Control spectra of each cell are identical and well separated, while ligand binding in a complex mixture can be readily detected in 20-30 min, thus paving the way for use of the probe for actual drug discovery efforts.

Spectral unraveling by space-selective Hadamard spectroscopy

Spectral unraveling by space-selective Hadamard spectroscopy (SUSHY) enables recording of NMR spectra of multiple samples loaded in multiple sample tubes in a modified spinner turbine and a regular 5mm liquids NMR probe equipped with a tri-axis pulsed field gradient coil. The individual spectrum from each sample is extracted by adding and subtracting data that are simultaneously obtained from all the tubes based on the principles of spatially resolved Hadamard spectroscopy. The well-known Hadamard spectroscopy has been applied for spatial selection and the method utilizes standard configuration of NMR instrument hardware. The SUSHY method can be easily incorporated in multi-dimensional multi-tube NMR experiments. This method combines the excitation multiplexing, natural advantage of FTNMR, and sample multiplexing and offers high-throughput by reducing the total experimental time by up to a factor of four in a 4-tube mode.

Comparison of the performance of round and rectangular wire in small solenoids for high-field NMR

This paper considers the effects of conductor geometry on the performance of small solenoidal coils for high-field NMR. First, a simple analytical model is presented for investigating the effects of conductor geometry on the current distribution in such coils. The model was used to derive optimum parameters for coils constructed from wire with either rectangular or circular cross-sections as a function of the length-to-diameter ratio. Second, a commercial software package utilizing full three-dimensional finite-element solutions to Maxwell's equations was used to confirm the basic findings of the simple analytical model, and also to compare simulated S/N estimations with experimental NMR spectra acquired with 2.5 mm and 1.0 mm-diameter solenoid coils: reasonable agreement was found. Third, as a demonstration of the usefulness of such coils for mass-limited samples, multidimensional experiments were performed at 750 MHz on approximately 4.7 nmol (41 microg) of PF1061, a protein from Pyrococcus furiosus.

Monitoring of fluid motion in a micromixer by dynamic NMR microscopy

The velocity distribution of liquid flowing in a commercial micromixer has been determined directly by using pulsed-field gradient NMR. Velocity maps with a spatial resolution of 29 microm x 43 microm were obtained by combining standard imaging gradient units with a homebuilt rectangular surface coil matching the mixer geometry. The technique provides access to mixers and reactors of arbitrary shape regardless of optical transparency. Local heterogeneities in the signal intensity and the velocity pattern were found and serve to investigate the quality and functionality of a micromixer, revealing clogging and inhomogeneous flow distributions.