Water suppression with B0 field gradient homospoil pulses in high-resolution NMR spectroscopy

Proton MRS and MRSI of the brain without water suppression

Water suppression (WS) techniques have played a vital role in the commencement and development of in vivo proton magnetic resonance spectroscopy (MRS, including spectroscopic imaging - MRSI). WS not only made in vivo proton MRS functionally available but also made its applications conveniently accessible, and it has become an indispensable tool in most of the routine applications of in vivo proton MR spectroscopy. On the other hand, WS brought forth some challenges. Therefore, various techniques of proton MRS without WS have been developed since the pioneering work in the late 1990s. After more than one and a half decades of advances in both hardware and software, non-water-suppressed proton MRS is coming to the stage of maturity and seeing increasing application in biomedical research and clinical diagnosis. In this article, we will review progress in the technical development and applications of proton MRS without WS.

Correction of gradient-induced phase errors in radial MRI

PURPOSE

To correct gradient-induced phase errors in radial MRI.

METHODS

Gradient-induced eddy currents affect the MRI data acquisition by gradient delays and phase errors that may lead to severe image artifacts for non-Cartesian imaging scenarios such as radial trajectories. While gradient delays are dealt with by respective shifts of the acquisition window during radial image acquisition, this work introduces a simple method for quantifying and correcting phase errors from the actual data prior to image reconstruction. For a given gradient system, the approach yields a specific phase error per gradient that can be used for correcting the raw data.

RESULTS

Phantom studies at 9.4 T demonstrated marked improvements in radial image quality. It could be shown that the phase correction is not compromised by data undersampling. Moreover, the selective correction of gradient-induced phase errors retained the phase information caused by different concentrations of a paramagnetic contrast agent.

CONCLUSION

The proposed method does not require additional reference measurements and separately corrects for phase errors induced by eddy currents, while retaining the residual phase of the object which may carry physiologic information.

High-throughput NMR assessment of the tertiary structure of food allergens

Background

In vitro component-resolved diagnosis of food allergy requires purified allergens that have to meet high standards of quality. These include the authentication of their conformation, which is relevant for the recognition by specific IgE antibodies from allergic patients. Therefore, highly sensitive and reliable screening methods for the analysis of proteins/allergens are required to assess their structural integrity. In the present study one-dimensional 1H Nuclear Magnetic Resonance (1D 1H-NMR) analysis was adopted for the assessment of overall structural and dynamic properties and authentication of a set of relevant food allergens, including non-specific lipid transfer proteins from apple, peach and hazelnut, 7/8S seed storage globulins from hazelnut and peanut, 11S seed storage globulins from hazelnut and peanut, caseins from cows' and goats' milk and tropomyosin from shrimp.

Methodology/Principal Findings

Two sets of 1D 1H-NMR experiments, using 700 MHz and 600 MHz instruments at 298 K were carried out to determine the presence and the extent of tertiary structure. Structural similarity among members of the individual allergen families was also assessed and changes under thermal stress investigated. The nuclear magnetic resonance (NMR) results were compared with structural information available either from the literature, Protein Data Bank entries, or derived from molecular models.

Conclusions/Significance

1D ¹H-NMR analysis of food allergens allowed their classification into molecules with rigid, extended and ordered tertiary structures, molecules without a rigid tertiary structure and molecules which displayed both features. Differences in thermal stability were also detected. In summary, 1D ¹H-NMR gives insights into molecular fold of proteins and offers an independent method for assessing structural properties of proteins.

Water modeled signal removal and data quantification in localized MR spectroscopy using a time-scale postacquistion method

We have previously shown the continuous wavelet transform (CWT), a signal-processing tool, which is based upon an iterative algorithm using a lorentzian signal model, to be useful as a postacquisition water suppression technique. To further exploit this tool we show its usefulness in accurately quantifying the signal metabolites after water removal. However, due to the static field inhomogeneities, eddy currents, and "radiation damping," the water signal and the metabolites may no longer have a lorentzian lineshape. Therefore, another signal model must be used. As the CWT is a flexible method, we have developed a new algorithm using a gaussian model and found that it fits the signal components, especially the water resonance, better than the lorentzian model in most cases. A new framework, which uses the two models, is proposed. The framework iteratively extracts each resonance, starting by the water peak, from the raw signal and adjusts its envelope to both the lorentzian and the gaussian models. The model giving the best fit is selected. As a consequence, the small signals originating from metabolites when selecting, removing, and quantifying the dominant water resonance from the raw time domain signal are preserved and an accurate estimation of their concentrations is obtained. This is demonstrated by analyzing (1H) magnetic resonance spectroscopy unsuppressed water data collected from a phantom with known concentrations at two different field strengths and data collected from normal volunteers using two different localization methods.

Empirical compensation function for eddy current effects in pulsed field gradient nuclear magnetic resonance experiments

An empirical compensation function for the correction of eddy current effects in the Stejskal-Tanner pulsed-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) experiments has been established. Eddy currents may arise as a result of the application of sharp and strong gradient pulses and may cause severe distortion of the NMR signals. In this method, the length of one gradient pulse is altered to compensate for the eddy current effects. The compensation is considered to be ideal when the position and the phase of the spin-echo maximum obtained from an aqueous solution of poly(ethylene glycol) (PEG) is the same in the presence and absence of a gradient pulse in the PGSE pulse sequence. We first characterized the functional dependence of the length of the required compensation on the three principal variables in the PGSE experiment: the gradient strength, the duration of the gradient pulse, and the interval between the two gradient pulses. Subsequently, we derived a model which successfully describes the general relationship between these variables and the size of the induced eddy current. The parameters extracted from fitting the model to the experimental compensation data may be used to predict the correct compensation for any combination of the three principal variables.

Measurement of the T2 relaxation time of ethanol and cerebral metabolites, in vivo

The SPACE volume selection technique was combined with a spin-echo sequence to measure the transverse relaxation time of the resonances of ethanol and cerebral metabolites in the dog brain, in vivo. The method was extended to measure brain metabolite T2 values in the rat using 1H NMR microspectroscopy. The T2 decays for the resonances of the metabolites N-acetylaspartate, creatine/phosphocreatine, and choline/phosphorylcholine were found to be biexponential with long T2 components of 490, 260, and 350 ms for the dog and 490, 220, and 355 ms for the rat brain, respectively. The existence of a second T2 component may originate from J-coupled nonresolved metabolite resonances. The relaxation decay for the ethanol triplet could be fitted to a single exponential giving a T2 relaxation time of 335 ms. However, given the large errors in the measurement of ethanol peak intensities at short echo times because of overlapping lipid signal and the effects of J-modulation, a biexponential decay with a long T2 component of 335 ms cannot be ruled out. Ambiguities regarding the reported partial detection of the 1H NMR signal of ethanol in the brain are discussed.

A homonuclear shift correlated and spatially localized spectroscopy using stimulated echoes

A new method which combines localized high-resolution proton NMR spectroscopy with two-dimensional correlated spectroscopy using stimulated echoes is presented. Stimulated-echo correlated spectroscopy (STECSY) is a straightforward extension of the STEAM method. Experiments with phantoms illustrate the efficacy of STECSY. An in situ application on rat adipose tissue demonstrates that STECSY is a helpful tool with which to select and assign resonances in complex 1H NMR spectra.

The unequivocal determination of lactic acid using a one-dimensional zero-quantum coherence-transfer technique

A method based on zero-quantum coherence transfer for spectral editing of metabolites in aqueous solution in a one-dimensional experiment is described. Water suppression factors of approximately 8000 were achieved by the use of pulsed B0 field gradients and excellent editing was obtained in a single acquisition. The methyl resonances of both lactate and acetaldehyde were readily observed in a mouse brain homogenate by generating zero-quantum coherence using Gaussian pulses for selective excitation of the respective CH resonances.