Development of a fast method for quantitative measurement of hyperpolarized 129Xe dynamics in mouse brain

A fast method has been established for the precise measurement and quantification of the dynamics of hyperpolarized (HP) xenon‐129 (¹²⁹Xe) in the mouse brain. The key technique is based on repeatedly applying radio frequency (RF) pulses and measuring the decrease of HP ¹²⁹Xe magnetization after the brain Xe concentration has reached a steady state due to continuous HP ¹²⁹Xe ventilation. The signal decrease of the ¹²⁹Xe nuclear magnetic resonance (NMR) signal was well described by a simple theoretical model. The technique made it possible to rapidly evaluate the rate constant α, which is composed of cerebral blood flow (CBF), the partition coefficient of Xe between the tissue and blood (λ_i), and the longitudinal relaxation time (T_1i) of HP ¹²⁹Xe in the brain tissue, without any effect of depolarization by RF pulses and the dynamics in the lung. The technique enabled the precise determination of α as 0.103 ± 0.018 s^‐1 (± SD, n = 5) on healthy mice. To investigate the potential of this method for detecting physiological changes in the brain of a kainic acid (KA) ‐induced mouse model of epilepsy, an attempt was made to follow the time course of α after KA injection. It was found that the α value changes characteristically with time, reflecting the change in the physiological state of the brain induced by KA injection. By measuring CBF using ¹H MRI and ¹²⁹Xe dynamics simultaneously and comparing these results, it was suggested that the reduction of T_1i, in addition to the increase of CBF due to KA‐induced epilepsy, are possible causes of the change in ¹²⁹Xe dynamics. Thus, the present method would be useful to detect a pathophysiological state in the brain and provide a novel tool for future brain study. Copyright © 2011 John Wiley & Sons, Ltd.     

Hyperpolarized 129Xe MRI using isobutene as a new quenching gas

The use of a quenching gas, isobutene, with a low vapor pressure was investigated to enhance the utility of hyperpolarized ¹²⁹Xe (HP Xe) MRI. Xenon mixed with isobutene was hyperpolarized using a home‐built apparatus for continuously producing HP Xe. The isobutene was then readily liquefied and separated almost totally by continuous condensation at about 173 K, because the vapor pressure of isobutene (0.247 kPa) is much lower than that of Xe (157 kPa). Finally, the neat Xe gas was continuously delivered to mice by spontaneous inhalation. The HP Xe MRI was enhanced twofold in polarization level and threefold in signal intensity when isobutene was adopted as the quenching gas instead of N₂. The usefulness of the HP Xe MRI was verified by application to pulmonary functional imaging of spontaneously breathing mice, where the parameters of fractional ventilation (r_a) and gas exchange (f_D) were evaluated, aiming at future extension to preclinical studies. This is the first application of isobutene as a quenching gas for HP Xe MRI.     

MRI of stroke using hyperpolarized 129Xe

Because there is no background signal from xenon in biological tissue, and because inhaled xenon is delivered to the brain by blood flow, we would expect a perfusion deficit, such as is seen in stroke, to reduce the xenon concentration in the region of the deficit. Thermal polarization yields negligible xenon signal relative to hyperpolarized xenon; therefore, hyperpolarized xenon can be used as a tracer of cerebral blood flow. Using a rat permanent right middle cerebral artery occlusion model, we demonstrated that hyperpolarized ¹²⁹Xe MRI is able to detect, in vivo, the hypoperfused area of focal cerebral ischemia, that is the ischemic core area of stroke. To the best of our knowledge, this is the first time that hyperpolarized ¹²⁹Xe MRI has been used to explore normal and abnormal cerebral perfusion. Our study shows a novel application of hyperpolarized ¹²⁹Xe MRI for imaging stroke, and further demonstrates its capacity to serve as a complementary tool to proton MRI for the study of the pathophysiology during brain hypoperfusion. Copyright © 2010 John Wiley & Sons, Ltd.     

Preclinical hyperpolarized 129Xe MRI: ventilation and T2* mapping in mouse lungs at 7 T using multi‐echo flyback UTE

Fast apparent transverse relaxation (short T₂*) is a common obstacle when attempting to perform quantitative ¹H MRI of the lungs. While T₂* times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous ¹²⁹Xe), T₂* can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T₂* of HP ¹²⁹Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi‐echo radial flyback imaging sequence and use it to measure HP ¹²⁹Xe T₂* at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T₁ relaxation outside the animal by using multiple gradient‐refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the ¹²⁹Xe T₂* in vivo for 10 healthy C57Bl/6 J mice and found T₂* ~ 5 ms in the lung airspaces. Interestingly, T₂* was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O₂ content of the inhaled gas mixture. These results are discussed in the context of T₂* relaxation within porous media.     

Regional fractional ventilation mapping in spontaneously breathing mice using hyperpolarized 129Xe MRI

The feasibility of ventilation imaging with hyperpolarized (HP) ¹²⁹Xe MRI has been investigated for quantitative and regional assessment of ventilation in spontaneously breathing mice. The multiple breath ventilation imaging technique was modified to the protocol of spontaneous inhalation of HP ¹²⁹Xe delivered continuously from a ¹²⁹Xe polarizer. A series of ¹²⁹Xe ventilation images was obtained by varying the number of breaths before the ¹²⁹Xe lung imaging. The fractional ventilation, r, was successfully evaluated for spontaneously breathing mice. An attempt was made to detect ventilation dysfunction in the emphysematous mouse lung induced by intratracheal administration of porcine pancreatic elastase (PPE). As a result, the distribution of fractional ventilation could be visualized by the r map. Significant dysfunction of ventilation was quantitatively identified in the PPE‐treated group. The whole‐lung r value of 0.34 ± 0.01 for control mice (N = 4) was significantly reduced, to 0.25 ± 0.07, in PPE‐treated mice (N = 4) (p = 0.038). This study is the first application of multiple breath ventilation imaging to spontaneously breathing mice, and shows that this methodology is sensitive to differences in the pulmonary ventilation. This methodology is expected to improve simplicity as well as noninvasiveness when assessing regional ventilation in small rodents. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative evaluation of pulmonary gas‐exchange function using hyperpolarized 129Xe CEST MRS and MRI

Hyperpolarized ¹²⁹Xe gas MR has been a powerful tool for evaluating pulmonary structure and function due to the extremely high enhancement in spin polarization, the good solubility in the pulmonary parenchyma, and the excellent chemical sensitivity to its surrounding environment. Generally, the quantitative structural and functional information of the lung are evaluated using hyperpolarized ¹²⁹Xe by employing the techniques of chemical shift saturation recovery (CSSR) and xenon polarization transfer contrast (XTC). Hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (Hyper‐CEST) is another method for quantifying the exchange information of hyperpolarized ¹²⁹Xe by using the exchange of xenon signals according to its different chemical shifts, and it has been widely used in biosensor studies in vitro. However, the feasibility of using hyperpolarized ¹²⁹Xe CEST to quantify the pulmonary gas exchange function in vivo is still unclear. In this study, the technique of CEST was used to quantitatively evaluate the gas exchange in the lung globally and regionally via hyperpolarized ¹²⁹Xe MRS and MRI, respectively. A new parameter, the pulmonary apparent gas exchange time constant (T_app), was defined, and it increased from 0.63 s to 0.95 s in chronic obstructive pulmonary disease (COPD) rats (induced by cigarette smoke and lipopolysaccharide exposure) versus the controls with a significant difference (P = 0.001). Additionally, the spatial distribution maps of T_app in COPD rats' pulmonary parenchyma showed a regionally obvious increase compared with healthy rats. These results indicated that hyperpolarized ¹²⁹Xe CEST MR was an effective method for globally and regionally quantifying the pulmonary gas exchange function, which would be helpful in diagnosing lung diseases that are related to gas exchange, such as COPD.     

Reinvestigating hyperpolarized (129)Xe longitudinal relaxation time in the rat brain with noise considerations

The longitudinal relaxation time of hyperpolarized (HP) (129)Xe in the brain is a critical parameter for developing HP (129)Xe brain imaging and spectroscopy and optimizing the pulse sequences, especially in the case of cerebral blood flow measurements. Various studies have produced widely varying estimates of HP (129)Xe T(1) in the rat brain. To make improved measurements of HP (129)Xe T(1) in the rat brain and investigate how low signal-to-noise ratio (SNR) contributes to these discrepancies, we developed a multi-pulse protocol during the washout of (129)Xe from the brain. Afterwards, we applied an SNR threshold theory to both the multi-pulse protocol and an existing two-pulse protocol. The two protocols yielded mean +/- SD HP (129)Xe T(1) values in the rat brain of 15.3 +/- 1.2 and 16.2 +/- 0.9 s, suggesting that the low SNR might be a key reason for the wide range of T(1) values published in the literature, a problem that might be easily alleviated by taking SNR levels into account.     

A protocol for quantifying cardiogenic oscillations in dynamic 129Xe gas exchange spectroscopy: The effects of idiopathic pulmonary fibrosis

The spectral parameters of hyperpolarized ¹²⁹Xe exchanging between airspaces, interstitial barrier, and red blood cells (RBCs) are sensitive to pulmonary pathophysiology. This study sought to evaluate whether the dynamics of ¹²⁹Xe spectroscopy provide additional insight, with particular focus on quantifying cardiogenic oscillations in the RBC resonance. ¹²⁹Xe spectra were dynamically acquired in eight healthy volunteers and nine subjects with idiopathic pulmonary fibrosis (IPF). ¹²⁹Xe FIDs were collected every 20 ms (T_E = 0.932 ms, 512 points, dwell time = 32 μs, flip angle ≈ 20°) during a 16 s breathing maneuver. The FIDs were pre‐processed using the spectral improvement by Fourier thresholding technique (SIFT) and fit in the time domain to determine the airspace, interstitial barrier, and RBC spectral parameters. The RBC and gas resonances were fit to a Lorentzian lineshape, while the barrier was fit to a Voigt lineshape to account for its greater structural heterogeneity. For each complex resonance the amplitude, chemical shift, linewidth(s), and phase were calculated. The time‐averaged spectra confirmed that the RBC to barrier amplitude ratio (RBC:barrier ratio) and RBC chemical shift are both reduced in IPF subjects. Their temporal dynamics showed that all three ¹²⁹Xe resonances are affected by the breathing maneuver. Most notably, several RBC spectral parameters exhibited prominent oscillations at the cardiac frequency, and their peak‐to‐peak variation differed between IPF subjects and healthy volunteers. In the IPF cohort, oscillations were more prominent in the RBC amplitude (16.8 ± 5.2 versus 9.7 ± 2.9%; P = 0.008), chemical shift (0.43 ± 0.33 versus 0.083 ± 0.05 ppm; P < 0.001), and phase (7.7 ± 5.6 versus 1.4 ± 0.8°; P < 0.001). Dynamic ¹²⁹Xe spectroscopy is a simple and sensitive tool that probes the temporal variability of gas exchange and may prove useful in discerning the underlying causes of its impairment.     

Quantitative analysis of hyperpolarized 129Xe ventilation imaging in healthy volunteers and subjects with chronic obstructive pulmonary disease

In this study, hyperpolarized ¹²⁹Xe MR ventilation and ¹H anatomical images were obtained from three subject groups: young healthy volunteers (HVs), subjects with chronic obstructive pulmonary disease (COPD) and age‐matched controls (AMCs). Ventilation images were quantified by two methods: an expert reader‐based ventilation defect score percentage (VDS%) and a semi‐automated segmentation‐based ventilation defect percentage (VDP). Reader‐based values were assigned by two experienced radiologists and resolved by consensus. In the semi‐automated analysis, ¹H anatomical images and ¹²⁹Xe ventilation images were both segmented following registration to obtain the thoracic cavity volume and ventilated volume, respectively, which were then expressed as a ratio to obtain the VDP. Ventilation images were also characterized by generating signal intensity histograms from voxels within the thoracic cavity volume, and heterogeneity was analyzed using the coefficient of variation (CV). The reader‐based VDS% correlated strongly with the semi‐automatically generated VDP (r = 0.97, p < 0.0001) and with CV (r = 0.82, p < 0.0001). Both ¹²⁹Xe ventilation defect scoring metrics readily separated the three groups from one another and correlated significantly with the forced expiratory volume in 1 s (FEV₁) (VDS%: r = –0.78, p = 0.0002; VDP: r = –0.79, p = 0.0003; CV: r = –0.66, p = 0.0059) and other pulmonary function tests. In the healthy subject groups (HVs and AMCs), the prevalence of ventilation defects also increased with age (VDS%: r = 0.61, p = 0.0002; VDP: r = 0.63, p = 0.0002). Moreover, ventilation histograms and their associated CVs distinguished between subjects with COPD with similar ventilation defect scores, but visibly different ventilation patterns. Copyright © 2012 John Wiley & Sons, Ltd.     

Oxygen‐induced frequency shifts in hyperoxia: a significant component of BOLD signal

In comparison to the well‐documented significance of intravascular deoxyhemoglobin (deoxyHgb), the effects of dissolved oxygen on the blood‐oxygen‐level‐dependent (BOLD) signal have not been widely reported. Based on the fact that the prolonged inspiration of high oxygen fraction gas can result in up to a sixfold increase of the baseline tissue oxygenation, the current study focused on the influence of dissolved oxygen on the BOLD signal during hyperoxia. As results, our in vitro study revealed that the r₁ and r₂ (relaxivities) of the oxygen‐treated serum were 0.22 mM^?1 · s^?1and 0.19 mM^?1 · s^?1_, respectively. In an in vivo experiment, hyperoxic respiration induced negative BOLD contrast (i.e. signal decrease) in 18–42% of measured brain regions, voxels with accompanying significant decreases in both the T₂^* (?12.1% to ?19.4%) and T₁ (?5.8% to ?3.3%) relaxation times. In contrast, the T₂^* relaxation time significantly increased (11.2% to 14.0%) for the voxels displaying positive BOLD contrast (in 41–50% of the measured brain), which reflected a hyperoxygenation‐induced reduction in tissue deoxyHgb concentration. These data imply that hyperoxia‐driven BOLD signal changes are primarily determined by the counteracting effects of extravascular oxygen and intravascular deoxyHgb. Oxygen‐induced magnetic susceptibility was further demonstrated by the study of 1 min hypoxia, which induced BOLD signal changes opposite to those under hyperoxia. Vasoconstriction was more common in voxels with negative BOLD contrast than in voxels with positive contrast (% change of blood volume, ?9.8% to ?12.8% versus 2.0% to 2.2%), which further suggests that negative BOLD contrast is mainly evoked by an increase in extravascular oxygen concentration. Conclusively, frequency shifts, which are induced by the accumulation of oxygen molecules and associated magnetic field inhomogeneity, are a significant source of the negative BOLD contrast during hyperoxia. Copyright © 2014 John Wiley & Sons, Ltd.     

Continuous‐wave saturation considerations for efficient xenon depolarization

The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper‐CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper‐CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady‐state for long saturation times, Hyper‐CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper‐CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B₁, corresponding to five times the Xe exchange rate, k_BA (i.e. B₁ = 5 k_BA/γ with the gyromagnetic ratio of ¹²⁹Xe, γ), generates directly and without further optimization 96 % of the maximal Hyper‐CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper‐CEST response simultaneously, we found an optimal saturation pulse strength corresponding to times the Xe exchange rate, i.e. . When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe. Copyright © 2015 John Wiley & Sons, Ltd.     

Single breath‐hold measurement of pulmonary gas exchange and diffusion in humans with hyperpolarized 129Xe MR

Pulmonary diseases usually result in changes of the blood‐gas exchange function in the early stages. Gas exchange across the respiratory membrane and gas diffusion in the alveoli can be quantified using hyperpolarized ¹²⁹Xe MR via chemical shift saturation recovery (CSSR) and diffusion‐weighted imaging (DWI), respectively. Generally, CSSR and DWI data have been collected in separate breaths in humans. Unfortunately, the lung inflation level cannot be the exactly same in different breaths, which causes fluctuations in blood‐gas exchange and pulmonary microstructure. Here we combine CSSR and DWI obtained with compressed sensing, to evaluate the gas diffusion and exchange function within a single breath‐hold in humans. A new parameter, namely the perfusion factor of the respiratory membrane (SVR_d/g), is proposed to evaluate the gas exchange function. Hyperpolarized ¹²⁹Xe MR data are compared with pulmonary function tests and computed tomography examinations in healthy young, age‐matched control, and chronic obstructive pulmonary disease human cohorts. SVR_d/g decreases as the ventilation impairment and emphysema index increase. Our results indicate that the proposed method has the potential to detect the extent of lung parenchyma destruction caused by age and pulmonary diseases, and it would be useful in the early diagnosis of pulmonary diseases in clinical practice.     

Simultaneous assessment of both lung morphometry and gas exchange function within a single breath‐hold by hyperpolarized 129Xe MRI

During the measurement of hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI), the diffusion‐weighted imaging (DWI) technique provides valuable information for the assessment of lung morphometry at the alveolar level, whereas the chemical shift saturation recovery (CSSR) technique can evaluate the gas exchange function of the lungs. To date, the two techniques have only been performed during separate breaths. However, the request for multiple breaths increases the cost and scanning time, limiting clinical application. Moreover, acquisition during separate breath‐holds will increase the measurement error, because of the inconsistent physiological status of the lungs. Here, we present a new method, referred to as diffusion‐weighted chemical shift saturation recovery (DWCSSR), in order to perform both DWI and CSSR within a single breath‐hold. Compared with sequential single‐breath schemes (namely the ‘CSSR + DWI’ scheme and the ‘DWI + CSSR’ scheme), the DWCSSR scheme is able to significantly shorten the breath‐hold time, as well as to obtain high signal‐to‐noise ratio (SNR) signals in both DWI and CSSR data. This scheme enables comprehensive information on lung morphometry and function to be obtained within a single breath‐hold. In vivo experimental results demonstrate that DWCSSR has great potential for the evaluation and diagnosis of pulmonary diseases.     

Simultaneous detection of valine and lactate using MEGA‐PRESS editing in pyogenic brain abscess

Valine and lactate have been recognized as important metabolic markers to diagnose brain abscess by means of MRS. However, in vivo unambiguous detection and quantification is hampered by macromolecular contamination. In this work, MEGA‐PRESS difference editing of valine and lactate is proposed. The method is validated in vitro and applied for quantitative in vivo experiments in one healthy subject and two brain abscess patients. It is demonstrated that with this technique the overlapping lipid signal can be reduced by more than an order of magnitude and thus the robustness of valine and lactate detection in vivo can be enhanced. Quantification of the two abscess MEGA‐PRESS spectra yielded valine/lactate concentration ratios of 0.10 and 0.27. These ratios agreed with the concentration ratios determined from concomitantly acquired short‐T_E PRESS data and were in line with literature values. The quantification accuracy of lactate (as measured with Cramér‐Rao lower bounds in LCModel processing) was better for MEGA‐PRESS than for short‐T_E PRESS in all acquired in vivo datasets. The Cramér‐Rao lower bounds of valine were only better for MEGA‐PRESS in one of the two abscess cases, while in the other case coediting of isoleucine confounded the quantification in the MEGA‐PRESS analysis. MEGA‐PRESS and short‐T_E PRESS should be combined for unambiguous quantification of amino acids in abscess measurements. Simultaneous valine/lactate MEGA‐PRESS editing might benefit the distinction of brain abscesses from tumors, and further categorization of bacteria with reasonable sensitivity and specificity.     

Lithium compartmentation in brain by 7Li MRS: effect of total lithium concentration

In previous work at 4.7 T, the individual components of biexponential ⁷Li transverse (T₂) spin relaxation in rat brain in vivo were tentatively identified with intra‐ and extracellular Li. The goal in this work was to estimate Li's compartmental distribution as a function of total Li concentration in brain from the biexponential decays. Here a localized, biexponential ⁷Li T₂ MR spin‐relaxation study with isotopically enriched ⁷LiCl is reported in rat brain in vivo at 7 T. Additionally, a simple linear interpolation using the biexponential T₂ values to estimate intracellular Li from individual monoexponential T₂ decays was assessed. Intracellular T₂ was 14.8 ± 4.3 ms and extracellular T₂ was 295 ± 61 ms. The fraction of intracellular brain Li ranged from 37.3 to 64.8% (mean 54.5 ± 6.7%) and did not correlate with total Li concentration. The estimated intracellular Li concentration ranged from 47 to 80% (mean 68.3 ± 8.5%) of the total brain Li concentration and was highly correlated with it. The monoexponential estimates of the intracellular‐Li fractions and derived concentrations averaged about 15% higher than the corresponding biexponential estimates. This work supports the previous conclusion that a large fraction of Li in the brain is within the intracellular compartment. Copyright © 2013 John Wiley & Sons, Ltd.     

In‐vivo assessment of tissue metabolite levels using 1H MRS and the Electric REference To access In vivo Concentrations (ERETIC) method

Quantitative values of metabolite concentrations in ¹H magnetic resonance spectroscopy have been obtained using the Electric REference To access In vivo Concentrations (ERETIC) method, whereby a synthetic reference signal is injected during the acquisition of spectra. The method has been improved to enable quantification of metabolite concentrations in vivo. Optical signal transmission was used to eliminate random fluctuations in ERETIC signal coupling to the receiver coil due to changes in position of cables and highly dielectric human tissue. Stability and reliability of the signal were tested in vitro, achieving stability with a mean error of 2.83%. Scaling of the signal in variable loading conditions was demonstrated and in‐vivo measurements of brain were acquired on a 3T Philips system using a transmit/receive coil. The quantitative brain water and metabolite concentration values are in good agreement with those in the literature. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of an optimally sensitive and specific chemical exchange saturation transfer MRI quantification metric in relevant biological phantoms

The purpose of this study was to develop realistic phantom models of the intracellular environment of metastatic breast tumour and naïve brain, and using these models determine an analysis metric for quantification of CEST MRI data that is sensitive to only labile proton exchange rate and concentration. The ability of the optimal metric to quantify pH differences in the phantoms was also evaluated. Novel phantom models were produced, by adding perchloric acid extracts of either metastatic mouse breast carcinoma cells or healthy mouse brain to bovine serum albumin. The phantom model was validated using ¹H NMR spectroscopy, then utilized to determine the sensitivity of CEST MRI to changes in pH, labile proton concentration, T₁ time and T₂ time; six different CEST MRI analysis metrics (MTR_asym, APT*, MTR_Rex, AREX and CESTR* with and without T₁/T₂ compensation) were compared. The new phantom models were highly representative of the in vivo intracellular environment of both tumour and brain tissue. Of the analysis methods compared, CESTR* with T₁ and T₂ time compensation was optimally specific to changes in the CEST effect (i.e. minimal contamination from T₁ or T₂ variation). In phantoms with identical protein concentrations, pH differences between phantoms could be quantified with a mean accuracy of 0.6 pH units. We propose that CESTR* with T₁ and T₂ time compensation is the optimal analysis method for these phantoms. Analysis of CEST MRI data with T₁/T₂ time compensated CESTR* is reproducible between phantoms, and its application in vivo may resolve the intracellular alkalosis associated with breast cancer brain metastases without the need for exogenous contrast agents.     

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

In this study, in vivo ¹³C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1‐¹³C]ethanol. After [1‐¹³C]ethanol had been administered systemically to rats, ¹³C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. ¹³C‐labeled bicarbonate HCO (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 [1‐¹³C]ethanol and [¹³C₆]‐D ‐glucose. In contrast with labeling by [¹³C₆]‐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 [1‐¹³C]ethanol. As ¹³C labels originating from ethanol enter the brain after being converted into [1‐¹³C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered ¹³C‐labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo ¹³C MRS of rat brain following intragastric administration of ¹³C‐labeled ethanol is demonstrated. Published in 2011 by John Wiley & Sons, Ltd.     

In vivo mouse myocardial 31P MRS using three‐dimensional image‐selected in vivo spectroscopy (3D ISIS): technical considerations and biochemical validations

³¹P MRS provides a unique non‐invasive window into myocardial energy homeostasis. Mouse models of cardiac disease are widely used in preclinical studies, but the application of ³¹P MRS in the in vivo mouse heart has been limited. The small‐sized, fast‐beating mouse heart imposes challenges regarding localized signal acquisition devoid of contamination with signal originating from surrounding tissues. Here, we report the implementation and validation of three‐dimensional image‐selected in vivo spectroscopy (3D ISIS) for localized ³¹P MRS of the in vivo mouse heart at 9.4 T. Cardiac ³¹P MR spectra were acquired in vivo in healthy mice (n = 9) and in transverse aortic constricted (TAC) mice (n = 8) using respiratory‐gated, cardiac‐triggered 3D ISIS. Localization and potential signal contamination were assessed with ³¹P MRS experiments in the anterior myocardial wall, liver, skeletal muscle and blood. For healthy hearts, results were validated against ex vivo biochemical assays. Effects of isoflurane anesthesia were assessed by measuring in vivo hemodynamics and blood gases. The myocardial energy status, assessed via the phosphocreatine (PCr) to adenosine 5′‐triphosphate (ATP) ratio, was approximately 25% lower in TAC mice compared with controls (0.76 ± 0.13 versus 1.00 ± 0.15; P < 0.01). Localization with one‐dimensional (1D) ISIS resulted in two‐fold higher PCr/ATP ratios than measured with 3D ISIS, because of the high PCr levels of chest skeletal muscle that contaminate the 1D ISIS measurements. Ex vivo determinations of the myocardial PCr/ATP ratio (0.94 ± 0.24; n = 8) confirmed the in vivo observations in control mice. Heart rate (497 ± 76 beats/min), mean arterial pressure (90 ± 3.3 mmHg) and blood oxygen saturation (96.2 ± 0.6%) during the experimental conditions of in vivo ³¹P MRS were within the normal physiological range. Our results show that respiratory‐gated, cardiac‐triggered 3D ISIS allows for non‐invasive assessments of in vivo mouse myocardial energy homeostasis with ³¹P MRS under physiological conditions. Copyright © 2015 John Wiley & Sons, Ltd.