Differences in apparent diffusion coefficients of brain metabolites between grey and white matter in the human brain measured at 7 T

Diffusion weighted spectroscopy can provide microstructural information that is specific to compartmental geometry. So far, in human brain, apparent diffusion coefficients (ADCs) of only the metabolites N-acetyl aspartate, creatine (tCr) and choline (tCho) have been assessed. High field MR at 7 T allows the collection and analysis of diffusion weighted spectroscopy data of additional metabolites of interest such as glutamate (Glu), N-acetyl aspartyl glutamate, and glutamine (Gln), which are of interest due to their different compartmentalization and role in brain physiology. In this study, we performed (1)H diffusion weighted spectroscopy at 7 T using a diffusion-weighted PRESS sequence in parietal white matter (n = 6) and occipital grey matter (n = 7). Data were analyzed using the LCmodel. ADCs could reliably be obtained of N-acetyl aspartate, tCr, tCho, Glu, Gln in grey and white matter, and N-acetyl aspartyl glutamate in white matter. Significant differences in ADC values were observed between grey and white matter for all metabolites. ADCs in grey matter were consistently lower than in white matter. These differences can probably be attributed to different compartmentalization as well as to the differential impact of diffusion time on ADC of different molecules under conditions of restricted diffusion.     

Measurement of brain glutamate using TE-averaged PRESS at 3T.

A method is introduced that provides improved in vivo spectroscopic measurements of glutamate (Glu), glutamine (Gln), choline (Cho), creatine (Cre), N-acetyl compounds (NAtot, NAA + NAAG), and the inositols (mI and sI). It was found that at 3T, TE averaging, the f1 = 0 slice of a 2D J-resolved spectrum, yielded unobstructed signals for Glu, Glu + Gln (Glx), mI, NA(tot), Cre, and Cho. The C4 protons of Glu at 2.35 ppm, and the C2 protons of Glx at 3.75 ppm were well resolved and yielded reliable measures of Glu/Gln stasis. Apparent T1/T2 values were obtained from the raw data, and metabolite tissue levels were determined relative to a readily available standard. A repeatibility error of <5%, and a coefficient of variation (CV) of <10% were observed for brain Glu levels in a study of six normal volunteers.     

Prostate spectroscopy at 3 Tesla using two‐dimensional S‐PRESS

Two‐dimensional (2D) strong‐coupling point‐resolved spectroscopy (S‐PRESS) is introduced as a novel approach to ¹H MR spectroscopy (MRS) in the prostate. The technique provides full spectral information and allows for an accurate characterization of the citrate (Cit) signal. The method is based on acquiring a series of PRESS spectra with constant total echo time (TE). The indirect dimension is encoded by varying the relative lengths of the first and second TEs (TE₁ + TE₂ = TE). In the resulting 2D spectra, only the signal of strongly coupled spin systems is spread into the second dimension, which leads to more clearly arranged spectra. Furthermore, the spectral parameters of Cit (coupling constant J and chemical shift difference δ of the AB spin system) can be determined with high accuracy in vivo. The sequence is analytically optimized for maximal “strong coupling peaks” of Cit at 3T. 2D S‐PRESS spectra are compared with JPRESS spectra in vitro as well as in vivo. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.     

Very short echo time improves the precision of glutamate detection at 3T in 1H magnetic resonance spectroscopy

Purpose:

To examine the precision of glutamate detection using a very short echo time (TE) phase rotation STEAM (PR‐STEAM) sequence.

Materials and Methods:

Spectrosopic data were acquired from the anterior cingulate gyrus in nine healthy adults using 6.5‐msec TE PR‐STEAM, 40‐msec TE PRESS, 72‐msec TE STEAM, and TE‐Averaging with an effective TE of 105 msec on a clinical 3T magnetic resonance imaging (MRI) system. All data were quantified using LCModel and reported as ratios relative to total creatine.

Results:

Glutamate Cramer‐Rao lower bounds were less than 8% for all sequences. The 6.5‐msec TE PR‐STEAM identified glutamate with the greatest precision (coefficient of variation [CV] of 7.1%), followed by TE‐Averaging (CV of 8.9%), 40‐msec TE PRESS (CV of 11.9%), and 72‐msec TE STEAM (CV of 13.8%).

Conclusion:

In the absence of spectral editing, glutamate is best detected in the human brain at 3T using very short TEs. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Measuring glucose concentrations in the rat brain using echo‐time‐averaged point resolved spectroscopy at 7 tesla

Glucose has multiple functions in the brain, and there is interest in estimating in vivo concentrations rather than merely the uptake determined by nuclear medicine. Glucose can be estimated using magnetic resonance spectroscopy, but measurement is difficult due to its multiple J‐coupled proton signals overlapping with other metabolite signals. To minimize the effect of interfering signals, echo time (TE) values between 60 and 95 ms were averaged, and the loss in signal due to the T₂ effect was corrected in both the estimation of glucose concentration and in creation of the basis files for fitting. The effectiveness of the TE‐averaging method was evaluated by measuring the glucose concentration in fasted rats before and after feeding. The brain glucose in all rats increased after feeding with fasted and fed glucose‐to‐creatine ratios of 0.15 ± 0.03 and 0.24 ± 0.04, respectively. Data at a short TE of 13 ms measured ratios of 0.30 ± 0.16 and 0.36 ± 0.24 for the fasted and fed rats, respectively, demonstrating the difficulty in obtaining reliable glucose measurements at short TE. Overall, TE averaging minimizes the influence of macromolecular signals and nearby peaks to give precise, consistent estimates of glucose. Magn Reson Med 70:301–308, 2013. © 2012 Wiley Periodicals, Inc.     

Short echo‐time 3D radial gradient‐echo MRI using concurrent dephasing and excitation

Ultrashort echo‐time imaging and sweep imaging with Fourier transformation are powerful techniques developed for imaging ultrashort T₂ species. However, it can be challenging to implement them on standard clinical MRI systems due to demanding hardware requirements. In this article, the limits of what is possible in terms of the minimum echo‐time and repetition time with 3D radial gradient‐echo sequences, which can be readily implemented on a standard clinical scanner, are investigated. Additionally, a new 3D radial gradient‐echo sequence is introduced, called COncurrent Dephasing and Excitation (CODE). The unique feature of CODE is that the initial dephasing of the readout gradient is performed during RF excitation, which allows CODE to effectively achieve echo‐times on the order of ～0.2 ms and larger in a clinical setting. The minimum echo‐time achievable with CODE is analytically described and compared with a standard 3D radial gradient‐echo sequence. CODE was implemented on a clinical 3 T scanner (Siemens 3 T MAGNETOM Trio), and both phantom and in vivo human knee images are shown for demonstration. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Late stimulation of the sphenopalatine‐ganglion in ischemic rats: Improvement in N‐acetyl‐aspartate levels and diffusion weighted imaging characteristics as seen by MR

Purpose:

To assess, by MR spectroscopy (MRS) and diffusion weighted imaging (DWI), the ability of electrical stimulation of the sphenopalatine ganglion (SPG) to augment stroke recovery in transient middle cerebral artery occluded (t‐MCAO) rats, when treatment is started 18 ± 2 h post‐occlusion.

Materials and Methods:

¹H‐MRS imaging (¹H‐MRSI) and DWI were used to evaluate ischemic brain tissue after SPG stimulation in rats subjected to 2 h of t‐MCAO. Rats were examined by ¹H‐MRSI, DWI, and behavioral tests at 16 ± 2 h, 8 days, and 28 days post‐MCAO.

Results:

N‐Acetyl‐aspartate (NAA) levels of the stimulated and control rats were the same 16 ± 2 h post‐MCAO (0.52 ± 0.03, 0.54 ± 0.03). At 28 days post‐occlusion, NAA levels were significantly higher in the treated group (0.60 ± 0.04) compared with those of the untreated animals (0.50 ± 0.04; P < 0.05). This effect was more pronounced for regions with low NAA values (0.16 ± 0.03) that changed to 0.32 ± 0.03 (P = 0.04) for the treated group and to 0.10 ± 0.03 (P = 0.20) for the controls. DWI data showed better ischemic tissue condition for the treated rats, but the measured parameters showed only a trend of improvement. The MR results were corroborated by behavioral examinations.

Conclusion:

Our findings suggest that SPG stimulation may ameliorate MR tissue characteristics following t‐MCAO even if treatment is started 18 h post‐occlusion. J. Magn. Reson. Imaging 2010;31:1355–1363. © 2010 Wiley‐Liss, Inc.     

Measurement of glycine in the human brain in vivo by 1H‐MRS at 3 T: application in brain tumors

Glycine is a key metabolic intermediate required for the synthesis of proteins, nucleic acids, and other molecules, and its detection in cancer could, therefore, provide biologically relevant information about the growth of the tumor. Here, we report measurement of glycine in human brain and gliomas by an optimized point‐resolved spectroscopy sequence at 3 T. Echo time dependence of the major obstacle, myo‐inositol (mI) multiplet, was investigated with numerical simulations, incorporating the 3D volume localization. The simulations indicated that a subecho pair (TE₁, TE₂) = (60, 100) ms permits detection of both glycine and mI with optimum selectivity. In vivo validation of the optimized point‐resolved spectroscopy was conducted on the right parietal cortex of five healthy volunteers. Metabolite signals estimated from LCModel were normalized with respect to the brain water signal, and the concentrations were evaluated assuming the total creatine concentration at 8 mM. The glycine concentration was estimated as 0.6 ± 0.1 mM (mean ± SD, n = 5), with a mean Cramér‐Rao lower bound of 9 ± 1%. The point‐resolved spectroscopy sequence was applied to measure the glycine levels in patients with glioblastoma multiforme. Metabolite concentrations were obtained using the water signal from the tumor mass. The study revealed that a subset of human gliomas contains glycine levels elevated 1.5–8 fold relative to normal. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Detection of glutamate in the human brain at 3 T using optimized constant time point resolved spectroscopy.

A CT-PRESS sequence was implemented on a 3-T MR scanner and optimized for the detection of the C4 resonance of glutamate. By simulating the sequence using the full density matrix it was found that 121 chemical shift encoding steps in t1 with an increment delta t1 = 1.6 ms were sufficient to separate the glutamate C4 resonance. The simulations also showed that the highest signal-to-noise ratio was achieved at an average echo time of 131 ms. When using an eightfold undersampling scheme in f1 in order to reduce the minimum total measurement time, the average echo time was 139 ms with 17 encoding steps (delta t1 = 12.8 ms). The sequence was tested on phantoms containing solutions of various brain metabolites and on healthy human volunteers. Besides resolving glutamate, other resonances detected in vivo comprised N-acetyl aspartate, total creatine, choline containing compounds, and myo-inositol. However, glutamine resonances could not be resolved due to severe signal overlap from glutamate and N-acetyl aspartate.