PRESS-based proton single-voxel spectroscopy and spectroscopic imaging with very short echo times using asymmetric RF pulses

High-resolution NMR spectroscopy in inhomogeneous fields

High-resolution NMR spectroscopy, providing information on chemical shifts, J coupling constants, multiplet patterns, and relative peak areas, is a mainstream tool for analysis of molecular structures, conformations, compositions, and dynamics. Generally, a homogeneous magnetic field is a prerequisite for obtaining high-resolution NMR information. Magnetic field inhomogeneity, whether from non-ideal experimental conditions or from intrinsic magnetic susceptibility discontinuities in samples, represents a hurdle for applications of high-resolution NMR. Numerous techniques have been proposed for measuring high-resolution NMR spectra free from the influence of inhomogeneous magnetic fields. Besides developments and improvements in NMR instrumentation, various types of experimental approaches have been established for recovering NMR information in inhomogeneous magnetic fields. Three main types are systematically described in this review. In addition, other high-resolution NMR approaches or data processing methods are also briefly described. All high-resolution NMR approaches covered in this review have individual advantages and disadvantages in practical applications, and no one technique is applicable to all practical circumstances. Hence, they are complementary for high-resolution NMR applications in inhomogeneous fields. The underlying mechanisms of these approaches are presented, together with analyses of their applicability and efficiency.

1H NMR spectroscopy of rat brain in vivo at 14.1Tesla: improvements in quantification of the neurochemical profile

Ultra-short echo-time proton single voxel spectra of rat brain were obtained on a 14.1T 26 cm horizontal bore system. At this field, the fitted linewidth in the brain tissue of adult rats was about 11 Hz. New, separated resonances ascribed to phosphocholine, glycerophosphocholine and N-acetylaspartate were detected for the first time in vivo in the spectral range of 4.2-4.4 ppm. Moreover, improved separation of the resonances of lactate, alanine, gamma-aminobutyrate, glutamate and glutathione was observed. Metabolite concentrations were estimated by fitting in vivo spectra to a linear combination of simulated spectra of individual metabolites and a measured spectrum of macromolecules (LCModel). The calculated concentrations of metabolites were generally in excellent agreement with those obtained at 9.4T. These initial results further indicated that increasing magnetic field strength to 14.1T enhanced spectral resolution in (1)H NMR spectroscopy. This implies that the quantification of the neurochemical profile in rodent brain can be achieved with improved accuracy and precision.

Short-TE localised 1H MRS of the human brain at 3 T: quantification of the metabolite signals using two approaches to account for macromolecular signal contributions

The goal of this study was to validate metabolite quantification at short TE, with particular focus on how to best account for the macromolecular signal contribution. A robust, short-TE PRESS protocol is presented, which allows reliable quantification, in vivo, of metabolite signals at 3 T in human brain. Water suppression was adapted to the experimental conditions at 3 T. Metabolite signal from the parietal white matter was quantified in the time domain using QUEST (jMRUI). The increased macromolecular signal contribution at short TE was dealt with by two approaches, based on either metabolite nulling or initial signal truncation. A detailed comparison of the two approaches was made. The first used a metabolite-nulled signal, measured either individually or averaged over different subjects. The second used the total signal, metabolites and macromolecules, from a single scan. The two approaches gave similar quantification results in terms of metabolite concentrations, but differed in their precision and the number of metabolites quantified reliably. With an average metabolite-nulled baseline, a set of seven metabolites could be reliably quantified in parietal white matter under these experimental conditions: N-acetylaspartate, myo-inositol, glucose, glutamate, glutathione, creatine and choline. When initial signal truncation was used, glucose was removed from this set. The short TE (10-11 ms) facilitated quantification of glutamate. The reliable quantification of N-acetylaspartyl glutamate at 3 T proved very difficult.

Quantitative proton spectroscopic imaging of the neurochemical profile in rat brain with microliter resolution at ultra-short echo times

Proton spectroscopy allows the simultaneous quantification of a high number of metabolite concentrations termed the neurochemical profile. The spin echo full intensity acquired localization (SPECIAL) scheme with an echo time of 2.7 ms was used at 9.4T for excitation of a slab parallel to a home-built quadrature surface coil in conjunction with phase encoding in the two remaining spatial dimensions to yield an effective spatial resolution of 1.7 microL. The absolute concentrations of at least 10 metabolites were calculated from the spectra of individual voxels using LCModel analysis. The calculated concentrations were used for constructing quantitative metabolic maps of the neurochemical profile in normal and pathological rat brain. Summation of individual spectra was used to assess the neurochemical profile of unique brain regions, such as corpus callosum, in rat for the first time. Following focal ischemia in rat pups, imaging the neurochemical profile indicated increased choline groups in the ischemic core and increased glutamine in the penumbra, which is proposed to reflect glutamate excitotoxicity. We conclude that it is feasible to achieve a sensitivity that is sufficient for quantitative mapping of the neurochemical profile at microliter spatial resolution.

Localized short-echo-time proton MR spectroscopy with full signal-intensity acquisition

We developed a short-echo-time (TE) sequence for proton localized spectroscopy by combining a 1D add-subtract scheme with a doubly slice-selective spin-echo (SE) sequence. The sequence preserves the full magnetization available from the selected volume of interest (VOI). By reducing the number of radiofrequency (RF) pulses acting on transverse magnetization, we were able to minimize the TE to the level that is achievable with the stimulated echo acquisition mode (STEAM) technique, and also gained a twofold increase in sensitivity. The use of an adiabatic pulse in the add-subtract localization improved the efficiency of excitation in spatially inhomogeneous RF fields, which are frequently encountered at high magnetic fields. The localization performance and sensitivity gains of this method, which is termed SPin ECho, full Intensity Acquired Localized (SPECIAL) spectroscopy, were demonstrated in vivo in rat brains. In conjunction with spectroscopic imaging, a 2-microl spatial resolution was accomplished with a signal-to-noise ratio (SNR) above 30, which is usually sufficient for reliable quantification of a large number of metabolites (neurochemical profile).

Ultra-short echo time spectroscopic imaging in rats: implications for monitoring lipids in glioma gene therapy

We demonstrate the feasibility of using ultra-short echo time (TE = 2 ms) magnetic resonance spectroscopic imaging (MRSI) to detect intracranial mobile lipids in the rat brain. High-performance outer volume suppression and pre-localization were demonstrated in phantoms and by the total absence of signals arising from extra-cranial lipids in MRSI spectra from control rats. The sequence performance was tested on glioma-bearing BDIX rats. Fast-relaxing lipid signals were spatially varied within a glioma during herpes simplex virus thymidine kinase-mediated gene therapy, demonstrating the potential application of this method.

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Optimized 1H MRS and MRSI methods for the in vivo detection of boronophenylalanine

Boronophenylalanine (BPA) is used as Boron-10 carrier in boron neutron capture therapy, an experimental cancer radiotherapy. Results of quantitative, noninvasive in vivo detection and imaging of BPA in laboratory animals using (1)H NMR are presented for the first time. The purpose of this study was to implement and validate optimized techniques for the efficient detection of BPA. The (1)H NMR signals through which BPA is most readily detected in vivo are those from the aromatic ring of the molecule, which are part of a scalar-coupled spin system. The preferred detection method should therefore be based on a pulse sequence in which the effective TE is as short as possible. Modified versions of LASER (tau(CP) = 4.6 ms, TE = 27.6 ms) and double-echo slice-selective 2D MRSI (TE = 12 ms) were implemented for single-voxel spectroscopy and spectroscopic imaging of BPA, respectively. Chemical shift selective excitation was used for both sequences, based on a pulse that enabled narrow-band excitation without concomitant delay in TE. SI data without water suppression was used for absolute quantitation and for correction of B(0) variations. Experiments were conducted at 4.7 T in phantoms and in mice where the infused BPA was detected in the kidney.

Localized in vivo human 1H MRS at very short echo times

A new point-resolved spectroscopy (PRESS) sequence was developed that allows localized human proton MR spectra to be acquired at echo times (TEs) of 10 ms or less. The method was implemented on a 4 Tesla Varian research console and a clinical 3 Tesla Siemens Trio scanner. Human brain spectra acquired in vivo from the prefrontal cortex at TE=8 ms showed improved signals from coupled resonances (such as glutamate, glutamine, and myo-inositol) compared to spectra acquired at TE=30 ms. These improvements should result in more accurate quantitation of these metabolites.

High-field localized 1H NMR spectroscopy in the anesthetized and in the awake monkey

Localized cerebral in vivo 1H NMR spectroscopy (MRS) was performed in the anesthetized as well as the awake monkey using a novel vertical 7 T/60 cm MR system. The increased sensitivity and spectral dispersion gained at high field enabled the quantification of up to 16 metabolites in 0.1- to 1-ml volumes. Quantification was accomplished by using simulations of 18 metabolite spectra and a macromolecule (MM) background spectrum consisting of 12 components. Major cerebral metabolites (concentrations >3 mM) such as glutamate (Glu), N-acetylaspartate (NAA), creatine (Cr)/phosphocreatine (PCr) and myo-inositol (Ins) were identified with an error below 3%; most other metabolites were quantified with errors in the order of 10%. Metabolite ratios were 1.39:1 for total NAA, 1.38:1 for glutamate (Glu)/glutamine (Gln) and 0.09:1 for cholines (Cho) relative to total Cr. Taurine (Tau) was detectable at concentrations lower than 1 mM, while lactate (Lac) remained below the detection limit. The spectral dispersion was sufficient to separate metabolites of similar spectral patterns, such as Gln and Glu, N-acetylaspartylglutamate (NAAG) and NAA, and PCr-Cr. MRS in the awake monkey required the development and refinement of acquisition and correction strategies to minimize magnetic susceptibility artifacts induced by respiration and movement of the mouth or body. Periods with major motion artifacts were rejected, while a frequency/phase correction was performed on the remaining single spectra before averaging. In resting periods, both spectral amplitude and line width, that is, the voxel shim, were unaffected permitting reliable measurements. The corrected spectra obtained from the awake monkey afforded the reliable detection of 6-10 cerebral metabolites of 1-ml volumes.