Biomedical subjects

Yoav Shrot

Publications and source records attributed to Yoav Shrot.

8 recordsLinked to original sources

PubMed2006

Ultrafast two-dimensional NMR spectroscopy using constant acquisition gradients.

Multidimensional NMR spectroscopy plays an important role in the characterization of molecular structure and dynamics. A new methodology for acquiring this kind of spectra has been recently demonstrated, endowed with the potential to compress arbitrary multidimensional NMR acquisitions into a single scan. This "ultrafast" nD acquisition protocol is based on a spatiotemporal encoding of the indirect-domain spin evolution, followed by a repetitive decoding and re-encoding of the information thus stored employing a train of alternating-sign gradients. Such train of switching gradients extending throughout the course of the data acquisition may pose extreme demands on a magnetic resonance system, particularly when dealing with nonshielded gradients, strong eddy currents, or rapidly relaxing spin systems. Limits to the in vivo applicability of such fast-switching scheme may also arise due to gradient-induced perineural stimulation. The present study describes a new approach to ultrafast nD NMR that reduces the number of gradient switchings during the acquisition period to zero, leading in essence to a constant-gradient acquisition scheme. This approach operates on the basis of a novel spatiotemporal encoding including discrete, temporally overlapping, frequency-shifted pulses. Principles and examples of this new approach are given; sensitivity limitations and signal-enhancing prospects of such constant-gradient acquisitions are also discussed and exemplified.

Algorithms↗

PubMed2005

Symmetric spatial encoding in ultrafast 2D NMR spectroscopy.

Single-scan multidimensional spectroscopy utilizes spatial dimensions for encoding the indirect-domain internal spin interactions. Various strategies have been hitherto demonstrated for fulfilling the encoding needs underlying this methodology; in analogy with their time-domain counterparts all of them have in common the fact that they proceed monotonically-starting at one end of the sample and concluding at the other. The present manuscript discusses another possibility that arises for the case of amplitude-modulated ultrafast nD NMR, whereby the spatial encoding progresses from both ends of the sample simultaneously towards the center. Such symmetric encoding is compatible with continuous or discrete excitations as well as with homonuclear or heteronuclear correlations, and exhibits a number of advantages vis-à-vis the unidirectional encodings that have been used so far: it originates echoes that are free from large first-order phase distortions, and yields nD peaks possessing a purely-absorptive character. It has the added advantage that for a given indirect-domain spectral resolution it can complete its task in half the time required by a conventional monotonic spatial encoding, leading to potentially important gains in sensitivity. The main features underlying this new spatially symmetric encoding protocol are derived, and its advantages are demonstrated with a series of amplitude-modulated homo- and hetero-nuclear 2D ultrafast NMR examples.

Nuclear Magnetic Resonance, Biomolecular↗

PubMed2005

Spatially encoded NMR and the acquisition of 2D magnetic resonance images within a single scan.

An approach that enables the acquisition of multidimensional NMR spectra within a single scan has been recently proposed and demonstrated. The present paper explores the applicability of such ultrafast acquisition schemes toward the collection of two-dimensional magnetic resonance imaging (2D MRI) data. It is shown that ideas enabling the application of these spatially encoded schemes within a spectroscopic setting, can be extended in a straightforward manner to pure imaging. Furthermore, the reliance of the original scheme on a spatial encoding and subsequent decoding of the evolution frequencies endows imaging applications with a greater simplicity and flexibility than their spectroscopic counterparts. The new methodology also offers the possibility of implementing the single-scan acquisition of 2D MRI images using sinusoidal gradients, without having to resort to subsequent interpolation procedures or non-linear sampling of the data. Theoretical derivations on the operational principles and imaging characteristics of a number of sequences based on these ideas are derived, and experimentally validated with a series of 2D MRI results collected on a variety of model phantom samples.

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PubMed2004

Line shape considerations in ultrafast 2D NMR.

We have recently proposed and demonstrated an approach that enables the acquisition of 2D nuclear magnetic resonance (NMR) spectra within a single scan. The approach is based on spatially encoding the spins' evolution along the indirect domain with the aid of a magnetic field gradient, and subsequently decoding this information numerous times over the course of the signal acquisition while spins are subject to a train of gradient echoes. The present paper discusses further considerations pertaining the 2D line shapes arising from this new way of collecting NMR data. Specific issues that are hereby addressed include (i) the effects introduced by fast relaxation onto the spatial encoding process, particularly the line widths and line shapes that will then arise in the frequency domain; (ii) approaches capable of correcting for the mixed-phase kernels resulting in these fast-relaxation cases, corresponding in essence to spatially encoded analogs of the TPPI and hypercomplex time-domain acquisition procedures; (iii) the enveloping characteristics imposed by the use of discrete excitation pulses on the attainable spectral widths along the indirect domain; and (iv) an analysis of the signal-to-noise characteristics of the methodology, with experimental corroborations of theoretical predictions and an illustration of the method's capabilities to analyze protein solutions in the mM-range concentration.

Algorithms↗

PubMed2004

Spatially resolved multidimensional NMR spectroscopy within a single scan.

We have recently demonstrated that the spatial encoding of internal nuclear magnetic resonance (NMR) spin interactions can be exploited to collect multidimensional NMR spectra within a single scan. Such experiments rely on an inhomogeneous spatial excitation of the spins throughout the sample, and lead to indirect-domain peaks via a constructive interference among the spatially resolved spin-packets that are thus created. The shape of the resulting indirect-domain echo peaks approaches a Sinc function when the chemical's distribution is uniform, but will depart from this function otherwise. It is hereby shown that a Fourier analysis of either the diagonal- or the cross-peaks resolved in these single-scan two-dimensional (2D) NMR experiments can in fact provide a weighted spatial distribution of the analyte originating such peak, thus opening up the possibility of completing spatially resolved multidimensional NMR measurements within a fraction of a second. Principles of this new mode of analysis are discussed, and examples where the potential of spatially resolved ultrafast 2D NMR spectroscopy is brought to bear are presented. Potential extensions of this approach to higher dimensions are also briefly addressed.

Algorithms↗

PubMed2004

Ultrafast 2D NMR spectroscopy using a continuous spatial encoding of the spin interactions.

A new protocol for acquiring multidimensional NMR spectra within a single scan is introduced and illustrated. The approach relies on applying a pair of frequency-chirped excitation and storage pulses in combination with echoing magnetic field gradients, in order to impart the kind of linear spatial encoding of the NMR interactions that is required by ultrafast 2D NMR spectroscopy. It is found that when dealing with 2D NMR experiments involving a t1 amplitude-modulation of the spin evolution, such continuous encoding scheme presents a number of advantages over alternatives employing discrete excitation pulses. From an experimental standpoint this is mainly reflected by the use of a single pair of bipolar gradients during the course of the indirect-domain encoding, as opposed to the numerous (and more intense) gradient echoes required so far. In terms of the spectral outcome, main advantages of the continuous spatial encoding scheme are the avoidance of "ghost peaks" and of "enveloping effects" associated to the discrete excitation mode. The principles underlying this new spatial encoding protocol are derived, and its applicability is demonstrated with homo- and heteronuclear 2D ultrafast NMR applications on small molecule and on protein samples.

Carbon Isotopes↗

PubMed2003

Single-scan NMR spectroscopy at arbitrary dimensions.

Multidimensional nuclear magnetic resonance (NMR) provides one of the foremost analytical tools available to elucidate the structure and dynamics of complex molecules in their native states. Executing this kind of experiment generally requires collecting an n-dimensional time-domain signal S, from which the spectrum arises via an appropriate Fourier analysis of its various time variables. This time-domain signal is actually measured directly only along one of the time axes, while the effects introduced by the remaining time variables are monitored via a parametric incrementation of their values throughout independent experiments. Two-dimensional (2D) NMR experiments thus usually require longer acquisition times than unidimensional experiments, 3D NMR is orders-of-magnitude more time consuming than 2D spectroscopy, etc. Very recently, we proposed and demonstrated an approach whereby data acquisition in 2D NMR can be parallelized, enabling the collection of complete 2D spectral sets within a single transient. The present paper discusses the extension of this 2D NMR methodology to an arbitrary number of dimensions. The principles of the ensuing ultrafast n-dimensional NMR approach are described, and a variety of homo- and heteronuclear 3D and 4D NMR spectra collected within a fraction of a second are presented.

Journal Article↗

PubMed2003

Ghost-peak suppression in ultrafast two-dimensional NMR spectroscopy.

Two-dimensional (2D) spectroscopy is central to many contemporary applications of NMR. Recently, we have introduced a new approach whereby 2D NMR spectra can be collected within a single scan. This methodology employs a magnetic field gradient in order to spatially encode the time evolution occurring along the indirect dimension. The discrete nature of the t1 incrementation and its one-to-one correspondence with the spatial encoding, may lead to a number of artifacts. Most notable among these is a periodicity of the spectral peaks that are observed along the indirect axes. The appearance of such 'ghost-peaks', which may sometime coincide with genuine cross-peaks, could hamper a proper interpretation of the spectra. This contribution reviews the origin of such multiple resonances, and proposes a procedure for their elimination based on the acquisition of a small number of complementary scans. Such complementary scans can be simultaneously employed for the sake of phase-cycling out other unwanted signals, and improve the overall indirect-domain spectral resolution. Brief mathematical descriptions of the ghost-peak generation and ghost-peak suppression mechanisms are described, followed by experimental tests on a number of samples using various pulse sequences.

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