Quantitative T1rho NMR spectroscopy of rat cerebral metabolites in vivo: effects of global ischemia.

The NMR relaxation times (T(1rho), T(2), and T(1)) of water, N-acetylaspartate (NAA), creatine (Cr), choline-containing compounds (Cho), and lactate (Lac) were quantified in rat brain at 4.7 T. In control animals, the cerebral T(1rho) figures, as determined with a spin-lock field of 1.0 G, were 575 +/- 30 ms, 380 +/- 19 ms, 705 +/- 53 ms, and 90 +/- 1 ms for NAA, Cr, Cho, and water, respectively. The T(1rho) figures were 62-103% longer than their respective T(2) values determined by a multiecho method. In global (ischemic) ischemia, T(1rho) of NAA declined by 34%, that of Cr and Cho did not change, and that of water increased by 10%. The T(1rho) of lactate in ischemic brain was 367 +/- 44 ms. Similar patterns of changes were observed in the multiecho T(2) of these cerebral metabolites. The T(1) of water and NAA changed in a fashion similar to that of T(1rho) and T(2). These results show differential responses in metabolite and water T(1rho) relaxation times following ischemia, and indicate that metabolite T(1rho) and T(2) relaxation times behave similarly in the ischemic brain. The contributions of dipolar and nondipolar effects on T(1rho) relaxation in vivo are discussed in this work.     

Changes in the proton T2 relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 T.

Proton T(2) relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy ((1)H-MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T(2) values of water and N-acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (approximately 10%) during ischemia, and this T(2) reduction is reversed by reperfusion. The T(2) reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level-dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T(2) changes for tCr can probably be explained by the fact that the BOLD-related T(2) decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T(2) than PCr, during ischemia. The changes in T(2) should be taken into account for the quantification of metabolite concentrations during ischemia.     

Molecular self-diffusion of intracellular metabolites in rat brain in vivo Investigated by Localized Proton NMR Diffusion Spectroscopy

Molecular self-diffusion coefficients of water (0.75 ± 0.05), Nacetylaspartate (0.27 ± 0.04), creatines (0.27 ± 0.04), and cholines (0.28 ± 0.08) × 10^?5 cm² s^?1 were obtained from localized proton NMR spectra of rat brain in vivo using diffusion-weighted stimulated-echo (STEAM) sequences with a diffusion time of (Δ ? δ/3) = 17 ms.     

Proton T1 relaxation times of metabolites in human occipital white and gray matter at 7 T

Proton T₁ relaxation times of metabolites in the human brain have not previously been published at 7 T. In this study, T₁ values of CH₃ and CH₂ group of N‐acetylaspartate and total creatine as well as nine other brain metabolites were measured in occipital white matter and gray matter at 7 T using an inversion‐recovery technique combined with a newly implemented semi‐adiabatic spin‐echo full‐intensity acquired localized spectroscopy sequence (echo time = 12 ms). The mean T₁ values of metabolites in occipital white matter and gray matter ranged from 0.9 to 2.2 s. Among them, the T₁ of glutathione, scyllo‐inositol, taurine, phosphorylethanolamine, and N‐acetylaspartylglutamate were determined for the first time in the human brain. Significant differences in T₁ between white matter and gray matter were found for water (?28%), total choline (?14%), N‐acetylaspartylglutamate (?29%), N‐acetylaspartate (+4%), and glutamate (+8%). An increasing trend in T₁ was observed when compared with previously reported values of N‐acetylaspartate (CH₃), total creatine (CH₃), and total choline at 3 T. However, for N‐acetylaspartate (CH₃), total creatine, and total choline, no substantial differences compared to previously reported values at 9.4 T were discernible. The T₁ values reported here will be useful for the quantification of metabolites and signal‐to‐noise optimization in human brain at 7 T. Magn Reson Med 69:931–936, 2013. © 2012 Wiley Periodicals, Inc.     

Fast mapping of the T2 relaxation time of cerebral metabolites using proton echo-planar spectroscopic imaging (PEPSI).

Metabolite T2 is necessary for accurate quantification of the absolute concentration of metabolites using long-echo-time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T2. In this study we used proton echo-planar spectroscopic imaging (PEPSI) at 3T to obtain fast T2 maps of three major cerebral metabolites: N-acetyl-aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T2 values obtained using single-voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 x 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal-to-noise ratio (SNR) for T2 estimation was validated by high Pearson's correlation coefficients between logarithmic MR signals and TEs (R2 = 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T2 values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T2 estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T2 were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite T2.     

Proton transfer ratio,lactate, and intracellular pH in acute cerebral ischemia

The amide proton transfer ratio (APTR) from the asymmetry of the Z‐spectrum was determined in rat brain tissue during and after unilateral middle cerebral artery occlusion (MCAo). Cerebral lactate (Lac) as determined by ¹H NMR spectroscopy, water diffusion, and T_1ρ were quantified as well. Lac concentrations were used to estimate intracellular pH (pH_i) in the brain during the MCA occlusion. A decrease in APTR during occlusion indicated acidification from 7.1 to 6.79 ± 0.19 (a drop by 0.3 ± 0.2 pH units), whereas pH_i computed from Lac concentration was 6.3 ± 0.2 (a drop by 0.8 ± 0.2 pH units). Despite the disagreement between the two methods in terms of the size of the change in the absolute pH_i during ischemia, ΔAPTR and pH_i (and Lac concentration) displayed a strong correlation during the MCAo. Diffusion and T_1ρ indicated cytotoxic edema following MCA occlusion; however, APTR returned slowly toward the values determined in the contralateral hemisphere post‐ischemia. These data argue that the APTR during ischemia is affected not only by pH_i but by other physicochemical factors as well, and indicates different aspects of pathology in the post‐ischemic brain compared to those that influence water diffusion and T_1ρ. Magn Reson Med 57:647–653, 2007. © 2007 Wiley‐Liss, Inc.     

Human brain-structure resolved T(2) relaxation times of proton metabolites at 3 Tesla.

The transverse relaxation times, T(2), of N-acetylaspartate (NAA), total choline (Cho), and creatine (Cr) obtained at 3T in several human brain regions of eight healthy volunteers are reported. They were obtained simultaneously in 320 voxels with three-dimensional (3D) proton MR spectroscopy ((1)H-MRS) at 1 cm(3) spatial resolution. A two-point protocol, optimized for the least error per given time by adjusting both the echo delay (TE(i)) and number of averages, N(i), at each point, was used. Eight healthy subjects (four males and four females, age = 26 +/- 2 years) underwent the hour-long procedure of four 15-min, 3D acquisitions (TE(1) = 35 ms, N(1) = 1; and TE(2) = 285 ms, N(2) = 3). The results reveal that across all subjects the NAA and Cr T(2)s in gray matter (GM) structures (226 +/- 17 and 137 +/- 12 ms, respectively) were 13-17% shorter than the corresponding T(2)s in white matter (WM; 264 +/- 10 and 155 +/- 7 ms, respectively). The T(2)s of Cho did not differ between GM and WM (207 +/- 17 and 202 +/- 8, respectively). For the purpose of metabolic quantification, these values justify to within +/-10% the previous use of one T(2) per metabolite for 1) the entire brain and 2) all subjects. These T(2) values (which to our knowledge were obtained for the first time at this field, spatial resolution, coverage, and precision) are essential for reliable absolute metabolic quantification.     

Cerebral T1rho relaxation time increases immediately upon global ischemia in the rat independently of blood glucose and anoxic depolarization.

Time-dependent changes of T1 in the rotating frame (T1rho), diffusion, T2, and magnetization transfer contrast on cardiac arrest-induced global ischemia in rat were investigated. T1rho, as acquired with spin lock amplitudes >0.6 G, started to increase 10-20 sec after cardiac arrest followed by an increase within 3-4 min to a level that was 6-8% greater than in normal brain. The ischemic T1rho response coincided with the drop of water diffusion coefficient in normoglycemic animals. However, unlike the rate of diffusion, the kinetics of T1rho were not affected by either preischemic hypoglycemia or hyperglycemia. Similar to diffusion, the kinetics of anoxic depolarization were dependent on preischemic blood glucose levels. Ischemia caused a reduction in the Hahn spin echo T2 as a result of blood oxygenation level-dependent (BOLD) effect; maximal negative BOLD seen by 40 sec. In the animals injected with an ironoxide particle contrast agent, AMI-227, prior to the insult, both T1rho and T2 immediately increased in concert on induction of ischemia. In contrast to the T1rho and diffusion changes, a much slower change in magnetization transfer contrast was evident over the first 20 min of ischemia. These data demonstrate that T1rho immediately increases following ischemia and that the pathophysiological mechanisms affecting this relaxation time may not directly involve magnetization transfer. The mechanisms prolonging T1rho differ from those affecting water diffusion with respect to their sensitivities to glucose and are apparently independent of membrane depolarization.     

T(2) + measurement during acute cerebral ischemia by Carr-Purcell MRI at 4T.

Metabolic and structural changes occur in brain tissue within minutes of ischemia. The adiabatic multi-echo (Carr-Purcell) localization pulse sequence LASER has shown promise in detecting tissue contrast changes within the first hour of ischemia. The purpose of this initial study was to combine the LASER localization sequence with fast 3D echo-planar imaging (EPI) to quantify the regional apparent transverse relaxation (T(2) (dagger)) in a rabbit model of acute embolic ischemia at 4 Tesla. Carr-Purcell T(2) (dagger)-weighted images were acquired at 7 different echo-times and used to estimate T(2) (dagger) in both cortex and striatum. In ischemic tissue identified by 2,3,5-triphenyltetrazolium chloride (TTC) staining, the T(2) (dagger) increased by approximately 31% after 1 hour of ischemia and remained elevated until study completion at 4 h of ischemia. Lesion volume, defined as the number of pixels with T(2) (dagger) greater than 90 ms, increased by 40% between 1 and 4 h after induction of ischemia. Carr-Purcell LASER-EPI T(2) (dagger)-weighted images show promise in detecting early tissue changes in focal cerebral ischemia.     

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo.

Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.     

Diffusion of metabolites in normal and ischemic rat brain measured by localized 1H MRS

The apparent diffusion coefficient (ADC) of choline-containing compounds (Cho), creatine and phosphocreatine (Cre), N-acetyl-aspartate (NAA), lactate, and water was measured in normal rat brain, and in the ischemic and contralateral region of rat brain approximately 3 and 24 h after induction of focal cerebral ischemia. After 3 h of ischemia, the ADC of Cre and NAA in the ischemic region had significantly decreased by 29% and 19%, respectively (P < 0.05). Lactate ADC was also obtained in the ischemic region. After 24 h of focal ischemia, no ADC values could be measured for NAA, Cre and Cho in the ischemic region because their concentrations had become too low. The ADCs of lactate and water in the ischemic volume were virtually identical at 3 and 24 h after occlusion. The experiments suggest that the ADC decrease of water after induction of ischemia is partly caused by changes in the diffusion characteristics of the intracellular compartment.     

Detection of cerebral metabolites by single-voxel-based PRESS and COSY techniques at 3T

PURPOSE: To compare point-resolved spectroscopy (PRESS) and localized two-dimensional (2D) correlated spectroscopy (L-COSY) in the detection of cerebral metabolites in humans on a clinical scanner at 3T and to estimate their respective inter- and intrasubject variances. MATERIALS AND METHODS: Measurements were made on nine healthy subjects to assess intersubject variance, and daily on a single subject over a period of seven days to assess intrasubject variance. All L-COSY measurements were performed with a voxel size of 27 mL (3 x 3 x 3 cm(3)) and a measurement time of approximately 34 minutes in the occipitoparietal lobe of the brain. Relative metabolite concentrations were estimated with respect to N-methyl creatine. RESULTS: While the sensitivity of PRESS is twice that of L-COSY, the greater spectral resolution offered by L-COSY resulted in greater consistency in estimates of the concentrations of several cerebral metabolites, as indicated by a superior intraclass correlation and a significantly lower standard deviation (SD) in a matched pair intrasubject analysis. CONCLUSION: Our pilot results demonstrate that L-COSY is an effective approach for resolving cerebral metabolites, and demonstrates a lower coefficient of variance (CV) than the conventional 1D localized spectroscopic approach using LC Model for quantification.     

Effects of severe global ischemia on N-acetylaspartate and other metabolites in the rat brain

N-acetylaspartate (NAA) is found exclusively in neurons and their processes in the adult brain. Since the regional distribution of NAA may be imaged using magnetic resonance spectroscopic imaging (¹H-MRSI), a regional measure of neuronal density may be noninvasively obtained. The technique may be particularly useful in the diagnosis of diseases where neurons are selectively injured, since these diseases do not result in definitive changes on conventional imaging studies. The goal of this study was to determine whether ¹H-MRSI measurement of NAA detects neuronal loss following global ischemia. ¹H-MRSI was performed in rats 24 h after global ischemia was induced by bilateral carotid occlusion plus hypotension. ¹H-MRSI showed that NAA was decreased by 29–74% in vulnerable regions, including the cortex, striatum, hippocampus, and, to a lesser extent, the thalamus. No change was observed in the brain stem or cerebellum. Regions where ¹H-MRSI observed NAA was decreased also had histological evidence of selective neuronal necrosis and showed marked increase of lactate and alanine. These results show that ¹H-MRSI detected loss of NAA in brain regions with selective neuronal loss, suggesting that ¹H-MRSI measurements of NAA could detect neuronal loss in a variety of disease states where there is selective neuronal necrosis.     

Temporal and regional changes during focal ischemia in rat brain studied by proton spectroscopic imaging and quantitative diffusion NMR imaging

The early development of focal ischemia after permanent occlusion of the right middle cerebral artery (MCA) was studied in six rats using interleaved measurements by diffusion-weighted NMR imaging (DWI) of water and two variants of proton spectroscopic imaging (SI), multiecho SI (TE: 136, 272, 408 ms) and short TE SI (TE: 20 ms). Measurements on a 4.7-T NMR imaging system were performed between the control phase and approximately 6 h postocclusion. In the center of the ischemic lesion of all rats, the apparent diffusion coefficient (ADC) decreased rapidly to 84.4 ± 4.2% (mean ± SD) of the control values approximately 2 min postocclusion. Approximately 6 h postocclusion, the ADC was reduced to 67.1 ± 5.9%. In contrast, large differences between the animals were observed for the temporal increase of lactate (Lac) in the ipsilateral hemisphere. The maximum Lac signal was reached in four rats after 0.5-1.5 h, and in two rats was not reached even after 6 h postocclusion. Six h postocclusion, SI spectra measured at a TE of 136 ms revealed a decrease in the CH³ signal of N-acetylaspartate (NAA) to 67 ± 13% of the control values. Differences were observed between the spatial regions of decreased NAA and increased Lac. In the lesions, a T2 relaxation time of Lac of 292 ± 40 ms, considering a J-cou-pling constant of 6.9 Hz, was measured. Furthermore, a prolongation of the T₂ of the CH₃ signal of creatine/phosphocre-atine (Cr/PCr) was observed in the lesion, from 163 ± 22 ms during control to 211 ± 41 ms approximately 6 h postocclusion. The experiments proved that DWI and proton SI are valuable tools to provide complementary information on processes associated with brain infarcts.     

Proton T2 relaxation study of water, N-acetylaspartate, and creatine in human brain using Hahn and Carr-Purcell spin echoes at 4T and 7T.

Carr-Purcell and Hahn spin-echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T2) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T2 values of proton resonances (water, N-acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T2 decay can be substantially reduced using a Carr-Purcell-type SE sequence.