Integrated RF probe for in vivo multinuclear spectroscopy and functional imaging of rat brain using an 11.7 Tesla 89 mm bore vertical microimager

Remote RF excitation for small-bore MR imager at 15.2 T

We report a method for remote excitation of the RF signal for preclinical-equivalent ultra high field Magnetic Resonance Imaging (MRI). A parallel-plate waveguide together with a bio-inspired surface coil were used to perform remote excitation experiments to acquire images with a small-bore MR imager at 15.2 T. The imager bore size limits the RF coil transmitter dimensions, so the Gielis super-formula was used to design an RF coil with small dimensions. Electromagnetic simulations of the principal mode were run to study the waveguide filled with air and loaded with a saline solution-filled tube. Radiation patterns were also computed in a semi-anechoic chamber for the same scenarios as above. A saline solution-filled spherical phantom and a formaldehyde-fixed mouse phantom were used to acquire images. Radiation patterns showed an omnidirectional distribution with no side lobes, and a very smooth behaviour with almost no loss of information in the saline solution-filled tube and without. The theoretical wave impedance was calculated and compared with simulated results showing an excellent correspondence. Spherical phantom image data and simulation results of B₁ were contrasted and showed an important correlation. Ex vivo mouse images were of high quality and exhibited clear delineation of anatomical structures. These imaging results are in very good agreement with the simulations. Numerical, theoretical and experimental results validate this approach, using a bio-inspired surface coil with a simple waveguide for preclinical small-bore MRI at ultra high field.

A volume birdcage coil with an adjustable sliding tuner ring for neuroimaging in high field vertical magnets: ex and in vivo applications at 21.1T

A tunable 900 MHz transmit/receive volume coil was constructed for ¹H MR imaging of biological samples in a 21.1 T vertical bore magnet. To accommodate a diverse range of specimen and RF loads at such a high frequency, a sliding-ring adaptation of a low-pass birdcage was implemented through simultaneous alteration of distributed capacitance. To make efficient use of the constrained space inside the vertical bore, a modular probe design was implemented with a bottom-adjustable tuning and matching apparatus. The sliding ring coil displays good homogeneity and sufficient tuning range for different samples of various dimensions representing large span of RF loads. High resolution in vivo and ex vivo images of large rats (up to 350 g), mice and human postmortem tissues were obtained to demonstrate coil functionality and to provide examples of potential applications at 21.1 T.

Spectral editing for in vivo ¹³C magnetic resonance spectroscopy

In vivo detection of carboxylic/amide carbons is a promising technique for studying cerebral metabolism and neurotransmission due to the very low RF power required for proton decoupling. In the carboxylic/amide region, however, there is severe spectral overlap between acetate C1 and glutamate C5, complicating studies that use acetate as an astroglia-specific substrate. There are no known in vivo MRS techniques that can spectrally resolve acetate C1 and glutamate C5 singlets. In this study, we propose to spectrally separate acetate C1 and glutamate C5 by a two-step J-editing technique after introducing homonuclear (13)C-(13)C scalar coupling between carboxylic/amide carbons and aliphatic carbons. By infusing [1,2-(13)C(2)]acetate instead of [1-(13)C]acetate the acetate doublet can be spectrally edited because of the large separation between acetate C2 and glutamate C4 in the aliphatic region. This technique can be applied to studying acetate transport and metabolism in brain in the carboxylic/amide region without spectral interference.

In vivo detection of intermediate metabolic products of [1-(13) C]ethanol in the brain using (13) C MRS

In this study, in vivo (13) C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1-(13) C]ethanol. After [1-(13) C]ethanol had been administered systemically to rats, (13) C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. (13) C-labeled bicarbonate HCO 3- (161.0 ppm) was also detected. Saturating acetaldehyde C1 at 207.0 ppm was found to have no effect on the ethanol C1 (57.7 ppm) signal intensity after extensive signal averaging, providing direct in vivo evidence that direct metabolism of alcohol by brain tissue is minimal. To compare the labeling of brain metabolites by ethanol with labeling by glucose, in vivo time course data were acquired during intravenous co-infusion of [1-(13) C]ethanol and [(13) C(6) ]-D-glucose. In contrast with labeling by [(13) C(6) ]-D-glucose, which produced doublets of carboxylic/amide carbons with a J coupling constant of 51 Hz, the simultaneously detected glutamate and glutamine singlets were labeled by [1-(13) C]ethanol. As (13) C labels originating from ethanol enter the brain after being converted into [1-(13) C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered (13) C-labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo (13) C MRS of rat brain following intragastric administration of (13) C-labeled ethanol is demonstrated.

Windowed stochastic proton decoupling for in vivo (13)C magnetic resonance spectroscopy with reduced RF power deposition

PURPOSE

To propose a strategy for reducing radiofrequency (RF) power deposition by stochastic proton decoupling based on Rayleigh's theorem.

MATERIALS AND METHODS

Rayleigh's theorem was used to remove frequency components of stochastic decoupling over the 3.90-6.83 ppm range. [2-(13)C] or [2,5-(13) C(2) ]glucose was infused intravenously to anesthetized rats. (13)C labeling of brain metabolites was detected in the carboxylic/amide spectral region at 11.7 T using either the original stochastic decoupling method developed by Ernst or the proposed windowed stochastic decoupling method.

RESULTS

By restricting frequency components of stochastic decoupling to 1.91-3.90 ppm and 6.83-7.60 ppm spectral regions decoupling power deposition was reduced by ≈50%. The proposed windowed stochastic decoupling scheme is experimentally demonstrated for in vivo (13)C MRS of rat brain at 11.7 T.

CONCLUSION

The large reduction in decoupling power deposition makes it feasible to perform stochastic proton decoupling at very high magnetic fields for human brain (13)C MRS studies.

Advanced theory of driven birdcage resonator with losses for biomedical magnetic resonance imaging and spectroscopy

A complete time-dependent physics theory of symmetric unperturbed driven hybrid birdcage resonator was developed for general application. In particular, the theory can be applied for radiofrequency (RF) coil engineering, computer simulations of coil-sample interaction, etc. Explicit time dependence is evaluated for different forms of driving voltage. The major steps of the solution development are shown and appropriate explanations are given. Green's functions and spectral density formula were developed for any form of periodic driving voltage. The concept of distributed power losses based on transmission line theory is developed for evaluation of local losses of a coil. Three major types of power losses are estimated as equivalent series resistances in the circuit of the birdcage resonator. Values of generated resistances in legs and end-rings are estimated. An application of the theory is shown for many practical cases. Experimental curve of B(1) field polarization dependence is measured for eight-sections birdcage coil. It was shown that the steady-state driven resonance frequencies do not depend on damping factor unlike the free oscillation (transient) frequencies. An equivalent active resistance is generated due to interaction of RF electromagnetic field with a sample. Resistance of the conductor (enhanced by skin effect), Eddy currents and dielectric losses are the major types of losses which contribute to the values of generated resistances. A biomedical sample for magnetic resonance imaging and spectroscopy is the source of the both Eddy current and dielectric losses of a coil. As demonstrated by the theory, Eddy current loss is the major effect of coil shielding.

Simultaneous detection of cerebral metabolism of different substrates by in vivo ¹³C isotopomer MRS

In this report a new method is introduced for simultaneous detection of the metabolism of two ¹³C-labeled subtracts in brain in vivo. We recognized and experimentally demonstrated that when a ¹³C-labeled substrate generates [1,2-¹³C₂]acetylCoA ([1-¹³C]acetylCoA) only, glutamate C5, glutamine C5 and apsartate C4 doublets (singlets) are formed exclusively, regardless of the number of turns of the tricarboxylic acid cycle. We utilized the large one-bond ¹³C-¹³C homonuclear J coupling between a carboxylic/amide carbon and an aliphatic carbon (~50 Hz) and demonstrated that it is feasible to simultaneously detect the labeling of brain metabolites by two different substrates using different isotopomer signals of the same carbon atom. Uniformly labeled glucose was used to generate the doublets and a second substrate ([2-¹³C] lactate or [1,3-¹³C₂]β-hydroxybutyrate or [1-¹³C] acetate) was used to generate the singlets. It was shown that contribution to cerebral metabolism from different substrates can be simultaneously measured in vivo.

Slice with angulated non-parallel boundaries

Adiabatic pulses are widely used for spatial localization in magnetic resonance spectroscopy because of their high immunity to RF inhomogeneity and excellent slice profiles. Since non-rectangular volume is often preferred in localized spectroscopy, we propose a scheme for selecting a trapezoidal slice using adiabatic π pulses. In this scheme, a time-varying gradient orthogonal to a stationary slice selection gradient is used to change the boundaries of the slice profile from parallel to non-parallel. Numerical simulation results for the transverse and longitudinal magnetization using different RF and gradient waveforms are presented for non-parallel slice selection. Phantom imaging and in vivo(1)H MRS of rat brain using non-parallel slices are demonstrated.

High fidelity magnetic resonance imaging by frequency sweep encoding and Fourier decoding

Using a RF pulse with linear frequency sweep and a simultaneous encoding gradient, magnetization is sequentially excited accompanied by a quadratic phase profile. This quadratic dependence of magnetization phase on position dephases magnetization away from its vertices, allowing direct spatial encoding and image formation in the time domain. In this work, we show that Fourier decoding or least square fitting in combination with frequency sweep spatial encoding schemes can generate high fidelity images and we also extend spatial encoding to include nonlinear frequency sweep. Application to in vivo multiscan susceptibility-weighted imaging is demonstrated. Our results show that Fourier-decoded, spatially encoded images compare favorably with conventional high resolution images while preserving the unique features of sequential excitation.

Detection of reduced GABA synthesis following inhibition of GABA transaminase using in vivo magnetic resonance signal of [13C]GABA C1

Previous in vivo magnetic resonance spectroscopy (MRS) studies of gamma-aminobutyric acid (GABA) synthesis have relied on (13)C label incorporation into GABA C2 from [1-(13)C] or [1,6-(13)C(2)]glucose. In this study, the [(13)C]GABA C1 signal at 182.3 ppm in the carboxylic/amide spectral region of localized in vivo (13)C spectra was detected. GABA-transaminase of rat brain was inhibited by administration of gabaculine after pre-labeling of GABA C1 and its metabolic precursors with exogenous [2,5-(13)C(2)]glucose. A subsequent isotope chase experiment was performed by infusing unlabeled glucose, which revealed a markedly slow change in the labeling of GABA C1 accompanying the blockade of the GABA shunt. This slow labeling of GABA at elevated GABA concentration was attributed to the relatively small intercompartmental GABA-glutamine cycling flux that constitutes the main route of (13)C label loss during the isotope chase. Because this study showed that using low RF power broadband stochastic proton decoupling is feasible at very high field strength, it has important implications for the development of carboxylic/amide (13)C MRS methods to study brain metabolism and neurotransmission in human subjects at high magnetic fields.

Measuring N-acetylaspartate synthesis in vivo using proton magnetic resonance spectroscopy

N-Acetylaspartate (NAA) is an important marker of neuronal function and viability that can be measured using magnetic resonance spectroscopy (MRS). In this paper, we proposed a method to measure NAA synthesis using proton MRS with infusion of uniformly (13)C-labeled glucose, and demonstrated its feasibility in an in vivo study of the rat brain. The rate of (13)C-label incorporation into the acetyl group of NAA was measured using a localized, long echo-time proton MRS method. Signals from the (13)C satellites of the main NAA methyl protons at 2.02 ppm were continuously monitored for 10h. Quantification of the data based on a linear kinetic model showed that NAA synthesis rate in isoflurane-anesthetized rats was 0.19+/-0.02 micromol/g/h (mean+/-standard deviation, n=12).

Inverse polarization transfer for detecting in vivo 13C magnetization transfer effect of specific enzyme reactions in 1H spectra

The wide chemical shift dispersion and long T(1) of (13)C have allowed determination of in vivo magnetization transfer effects caused by aspartate aminotransferase and lactate dehydrogenase reactions using (13)C magnetic resonance spectroscopy. In this report, we demonstrate that these effects can be observed in the proton spectra by transferring the equilibrium magnetization of (13)C via the one-bond scalar coupling between (13)C and (1)H using an inverse insensitive nuclei enhanced by polarization transfer-based heteronuclear polarization transfer method. This inverse method allows a combination of the advantages of the long (13)C T(1) for maximum magnetization transfer and the high sensitivity of proton detection. The feasibility of this in vivo inverse polarization transfer approach was evaluated for detecting the (13)C magnetization transfer effect of aspartate aminotransferase and lactate dehydrogenase reactions from a 72.5-microl voxel in the rat brain at 11.7 T.

In vivo 13C saturation transfer effect of the lactate dehydrogenase reaction

Lactate dehydrogenase (LDH, EC 1.1.1.27) catalyzes an exchange reaction between pyruvate and lactate. It is demonstrated here that this reaction is sufficiently fast to cause a significant magnetization (saturation) transfer effect when the 13C resonance of pyruvate is saturated by a continuous-wave (CW) RF pulse. Infusion of [2-(13)C]glucose was used to allow labeling of pyruvate C2 at 207.9 ppm to determine the pseudo first-order rate constant of the unidirectional lactate-->pyruvate flux in vivo. During systemic administration of GABAA receptor antagonist bicuculline, this pseudo first-order rate constant was determined to be 0.08+/-0.01 s-1 (mean+/-SD, N=4) in halothane-anesthetized adult rat brains. In 9L and C6 rat glioma models, the 13C saturation transfer effect of the LDH reaction was also detected in vivo. Our results demonstrate that the 13C magnetization transfer effect of the LDH reaction may be useful as a novel marker for utilizing noninvasive in vivo MRS to study many physiological and pathological conditions, such as cancer.

Relayed (13)C magnetization transfer: detection of malate dehydrogenase reaction in vivo

Malate dehydrogenase catalyzes rapid interconversion between dilute metabolites oxaloacetate and malate. Both oxaloacetate and malate are below the detection threshold of in vivo MRS. Oxaloacetate is also in rapid exchange with aspartate catalyzed by aspartate aminotransferase, the latter metabolite is observable in vivo using (13)C MRS. We hypothesized that the rapid turnover of oxaloacetate can effectively relay perturbation of magnetization between malate and aspartate. Here, we report indirect observation of the malate dehydrogenase reaction by saturating malate C2 resonance at 71.2 ppm and detecting a reduced aspartate C2 signal at 53.2 ppm due to relayed magnetization transfer via oxaloacetate C2 at 201.3 ppm. Using this strategy the rate of the cerebral malate dehydrogenase reaction was determined to be 9+/-2 micromol/g wet weight/min (means+/-SD, n=5) at 11.7 Tesla in anesthetized adult rats infused with [1,6-(13)C(2)]glucose.

In vivo dynamic turnover of cerebral 13C isotopomers from [U-13C]glucose

An INEPT-based (13)C MRS method and a cost-effective and widely available 11.7 Tesla 89-mm bore vertical magnet were used to detect dynamic (13)C isotopomer turnover from intravenously infused [U-(13)C]glucose in a 211 microL voxel located in the adult rat brain. The INEPT-based (1)H-->(13)C polarization transfer method is mostly adiabatic and therefore minimizes signal loss due to B(1) inhomogeneity of the surface coils used. High quality and reproducible data were acquired as a result of combined use of outer volume suppression, ISIS, and the single-shot three-dimensional localization scheme built in the INEPT pulse sequence. Isotopomer patterns of both glutamate C4 at 34.00 ppm and glutamine C4 at 31.38 ppm are dominated first by a doublet originated from labeling at C4 and C5 but not at C3 (with (1)J(C4C5) = 51 Hz) and then by a quartet originated from labeling at C3, C4, and C5 (with (1)J(C3C4) = 35 Hz). A lag in the transition of glutamine C4 pattern from doublet-dominance to quartet dominance as compared to glutamate C4 was observed, which provides an independent verification of the precursor-product relationship between neuronal glutamate and glial glutamine and a significant intercompartmental cerebral glutamate-glutamine cycle between neurons and glial cells.

Increased oxygen consumption in the somatosensory cortex of alpha-chloralose anesthetized rats during forepaw stimulation determined using MRS at 11.7 Tesla

The significance of changes in cerebral oxygen consumption in focally activated brain tissue is still controversial. Since the rate of cerebral oxygen consumption is tightly coupled to that of tricarboxylic acid cycle which can be measured from the turnover kinetics of [4-(13)C]glutamate using in vivo (1)H{(13)C} magnetic resonance spectroscopy, changes in tricarboxylic acid cycle flux rate were assessed in primary somatosensory cortex of alpha-chloralose anesthetized rats during electrical forepaw stimulation. With markedly improved (1)H{(13)C} magnetic resonance spectroscopy technique and the use of high magnetic field strength of 11.7 T accessible to the current study, [4-(13)C]glutamate at 2.35 ppm was spectrally resolved from overlapping resonances of [4-(13)C]glutamine at 2.46 ppm and [2-(13)C]GABA at 2.28 ppm as well as the more distal [3-(13)C]glutamate and [3-(13)C]glutamine. The results showed a significantly increased V(TCA) in focally activated primary somatosensory cortex during forepaw stimulation, corresponding to approximately 51 +/- 27% (n = 6, mean +/- SD) increase in cerebral oxygen consumption rate. Considering the high efficiency in producing adenosine triphosphate by oxidative metabolism of glucose, the results demonstrate that aerobic oxidative metabolism provides the majority of energy required for cerebral focal activation in alpha-chloralose anesthetized rats subjected to forepaw stimulation.

Theoretical analysis of carbon-13 magnetization transfer for in vivo exchange between alpha-ketoglutarate and glutamate

Many enzymes catalyze fast exchange between a small pool and a large pool in vivo. For example, aspartate aminotransferase catalyzes fast exchanges between alpha-ketoglutarate and glutamate and between oxaloacetate and aspartate, which can be detected using in vivo(13)C MRS while saturating alpha-carbons of the keto acids. Unlike in the traditional saturation transfer experiments studied using (31)P MRS, the tricarboxylic acid cycle intermediates alpha-ketoglutarate and oxaloacetate are below the detection limit of in vivo NMR. In this work, a theoretical analysis of the saturation transfer between alpha-ketoglutarate and glutamate catalyzed by aspartate aminotransferase was presented to examine the requirements for complete saturation of the rapidly turning over alpha-ketoglutarate pool without affecting the longitudinal magnetization of glutamate. The fast turnover of the small alpha-ketoglutarate pool also allows a quasi-steady-state approximation of its dynamic longitudinal relaxation. The theoretical analysis provides a useful guide for designing experimental methods to characterize saturation transfer processes associated with fast turning over small pools in vivo.

Single-shot echo-planar functional magnetic resonance imaging of representations of the fore- and hindpaws in the somatosensory cortex of rats using an 11.7T microimager

Most of functional magnetic resonance imaging (fMRI) experiments have been performed on horizontal bore magnets. Here, we present practical aspects of fMRI based on single-shot, spin-echo echo-planar imaging (EPI) using a widely available, cost effective 89 mm bore vertical 11.7 T microimager. It was demonstrated that reproducible, high-quality fMRI data can be obtained from alpha-chloralose anesthetized adult rat brain. Both coronal and the more extended horizontal EPI images were acquired to measure blood oxygenation level dependent (BOLD) responses to electrical stimulation of fore- and hindpaws. The BOLD patterns observed match the known representations of fore- and hindpaws in the somatosensory cortex in rats. Preliminary results on BOLD signal enhancement using aminophylline are also presented.