Which pulse sequence is optimal for myo-inositol detection at 3T?

Neurometabolite mapping highlights elevated myo-inositol profiles within the developing brain in down syndrome

The neurodevelopmental phenotype in Down Syndrome (DS), or Trisomy 21, is variable including a wide spectrum of cognitive impairment and a high risk of early-onset Alzheimer's disease (AD). A key metabolite of interest within the brain in DS is Myo-inositol (mIns). The NA+/mIns co-transporter is located on human chromosome 21 and is overexpressed in DS. In adults with DS, elevated brain mIns was previously associated with cognitive impairment and proposed as a risk marker for progression to AD. However, it is unknown if brain mIns is increased earlier in development. The aim of this study was to estimate mIns concentration levels and key brain metabolites [N-acetylaspartate (NAA), Choline (Cho) and Creatine (Cr)] in the developing brain in DS and aged-matched controls. We used in vivo magnetic resonance spectroscopy (MRS) in neonates with DS (n = 12) and age-matched controls (n = 26) scanned just after birth (36-45 weeks postmenstrual age). Moreover, we used Mass Spectrometry in early (10-20 weeks post conception) ex vivo fetal brain tissue samples from DS (n = 14) and control (n = 30) cases. Relative to [Cho] and [Cr], we report elevated ratios of [mIns] in vivo in the basal ganglia/thalamus, in neonates with DS, when compared to age-matched typically developing controls. Glycine concentration ratios [Gly]/[Cr] and [Cho]/[Cr] also appear elevated. We observed elevated [mIns] in the ex vivo fetal cortical brain tissue in DS compared with controls. In conclusion, a higher level of brain mIns was evident as early as 10 weeks post conception and was measurable in vivo from 36 weeks post-menstrual age. Future work will determine if this early difference in metabolites is linked to cognitive outcomes in childhood or has utility as a potential treatment biomarker for early intervention.

White Matter Neurometabolic Signatures Support the Deficit and Nondeficit Distinction in Antipsychotic-Naïve First-Episode Psychosis Patients

The deficit syndrome is thought to be a more homogenous clinical subgroup within the syndrome of schizophrenia that is characterized by enduring negative symptoms. It is hypothesized that distinct pathophysiological processes underlie the subtypes, where the deficit syndrome reflects an early onset nonprogressive developmental process, and the nondeficit form of the illness is characterized by attenuated neuroplasticity secondary to elevated glutamate levels. We used single-voxel magnetic resonance spectroscopy (PRESS; TE: 30 ms) to measure left frontal white matter neurometabolite levels in 61 antipsychotic-naïve first-episode psychosis patients (39 who did not display deficit features, 22 who did display deficit features, assessed with the Schedule for the Deficit Syndrome) and 59 healthy controls. Metabolite levels were quantified with the LCModel. We used a MANCOVA to determine neurometabolite differences between healthy controls, deficit syndrome patients, and nondeficit patients. We report a significant group difference when all metabolites were considered jointly (F[10,208] = 2.16; P = .02). Post hoc analyses showed that patients presenting without deficit features had higher glutamate levels than patients with deficit features and controls. Patients presenting without deficit features also had significantly higher myoinositol levels than controls; myoinositol levels were trend-level higher in patients presenting with deficit features compared to controls. Our data support the idea that the pathophysiology of patients presenting without deficit features may differ from those presenting with deficit features.

Reproducibility of whole-brain temperature mapping and metabolite quantification using proton magnetic resonance spectroscopy

Assessing brain temperature can provide important information about disease processes (e.g., stroke, trauma) and therapeutic effects (e.g., cerebral hypothermia treatment). Whole-brain magnetic resonance spectroscopic imaging (WB-MRSI) is increasingly used to quantify brain metabolites across the entire brain. However, its feasibility and reliability for estimating brain temperature needs further validation. Therefore, the present study evaluates the reproducibility of WB-MRSI for temperature mapping as well as metabolite quantification across the whole brain in healthy volunteers. Ten healthy adults were scanned on three occasions 1 week apart. Brain temperature, along with four commonly assessed brain metabolites-total N-acetyl-aspartate (tNAA), total creatine (tCr), total choline (tCho) and myo-inositol (mI)-were measured from WB-MRSI data. Reproducibility was evaluated using the coefficient of variation (CV). The measured mean (range) of the intra-subject CVs was 0.9% (0.6%-1.6%) for brain temperature mapping, and 4.7% (2.5%-15.7%), 6.4% (2.4%-18.9%) and 14.2% (4.4%-52.6%) for tNAA, tCho and mI, respectively, with reference to tCr. Consistently larger variability was found when using H₂ O as the reference for metabolite quantifications: 7.8% (3.3%-17.8%), 7.8% (3.1%-18.0%), 9.8% (3.7%-31.0%) and 17.0% (5.9%-54.0%) for tNAA, tCr, tCho and mI, respectively. Further, the larger the brain region (indicated by a greater number of voxels within that region), the better the reproducibility for both temperature and metabolite estimates. Our results demonstrate good reproducibility of whole-brain temperature and metabolite measurements using the WB-MRSI technique.

Validation of in vivo MRS measures of metabolite concentrations in the human brain

PURPOSE

In vivo magnetic resonance spectroscopy (MRS) is the only technique capable of non-invasively assessing metabolite concentrations in the brain. The lack of alternative methods makes validation of MRS measures challenging. The aim of this study is to assess the validity of MRS measures of human brain metabolite concentrations by comparing multiple MRS measures acquired using different MRS acquisition sequences.

METHODS

Single-voxel SPECIAL and MEGA-PRESS MR spectra were acquired from both the dorsolateral prefrontal cortex and primary motor cortices in 15 healthy subjects. The SPECIAL spectrum, as well as both the edit-off and difference spectra of MEGA-PRESS were each analyzed in LCModel to obtain estimates of the absolute concentrations of total choline (TCh; glycerophosphocholine + phosphocholine), total creatine (TCr; creatine + phosphocreatine), N-acetylaspartate (NAA), N-acetylaspartylglutamate (NAAG), NAA + NAAG, glutamate (Glu), glutamine (Gln), Glu + Gln, scyllo-inositol (Scyllo), myo-inositol (Ins), glutathione (GSH), γ-aminobutyric acid (GABA), lactate (Lac) and aspartate (Asp). Then, having obtained up to three independent measures of each metabolite per brain region per subject, correlations between the different measures were assessed.

RESULTS

The degree of correlation between measures varied greatly across both the metabolites and sequences tested. As expected, metabolites with the most prominent spectral peaks (TCh, TCr, NAA + NAAG, Ins and Glu) had the most well-correlated measures between methods, while metabolites with less prominent spectral peaks (Lac, Gln, GABA, Asp, and NAAG) tended to have poorly-correlated measures between methods. Some metabolites with relatively less prominent spectral peaks (GSH, Scyllo) had fairly well-correlated measures between some methods. Combining metabolites improved the agreement between methods for measures of NAA + NAAG, but not for Glu + Gln.

CONCLUSIONS

Given that the ground truth for in vivo MRS measures is never known, the method proposed here provides a promising means to assess the validity of in vivo MRS measures, which has not yet been explored widely.

Characterization of Metabolic, Diffusion, and Perfusion Properties in GBM: Contrast-Enhancing versus Non-Enhancing Tumor

BACKGROUND

Although the contrast-enhancing (CE) lesion on T₁-weighted MR images is widely used as a surrogate for glioblastoma (GBM), there are also non-enhancing regions of infiltrative tumor within the T₂-weighted lesion, which elude radiologic detection. Because non-enhancing GBM (Enh-) challenges clinical patient management as latent disease, this study sought to characterize ex vivo metabolic profiles from Enh- and CE GBM (Enh+) samples, alongside histological and in vivo MR parameters, to assist in defining criteria for estimating total tumor burden.

METHODS

Fifty-six patients with newly diagnosed GBM received a multi-parametric pre-surgical MR examination. Targets for obtaining image-guided tissue samples were defined based on in vivo parameters that were suspicious for tumor. The actual location from where tissue samples were obtained was recorded, and half of each sample was analyzed for histopathology while the other half was scanned using HR-MAS spectroscopy.

RESULTS

The Enh+ and Enh- tumor samples demonstrated comparable mitotic activity, but also significant heterogeneity in microvascular morphology. Ex vivo spectroscopic parameters indicated similar levels of total choline and N-acetylaspartate between these contrast-based radiographic subtypes of GBM, and characteristic differences in the levels of myo-inositol, creatine/phosphocreatine, and phosphoethanolamine. Analysis of in vivo parameters at the sample locations were consistent with histological and ex vivo metabolic data.

CONCLUSIONS

The similarity between ex vivo levels of choline and NAA, and between in vivo levels of choline, NAA and nADC in Enh+ and Enh- tumor, indicate that these parameters can be used in defining non-invasive metrics of total tumor burden for patients with GBM.

Unedited in vivo detection and quantification of γ-aminobutyric acid in the occipital cortex using short-TE MRS at 3 T

Short-TE MRS has been proposed recently as a method for the in vivo detection and quantification of γ-aminobutyric acid (GABA) in the human brain at 3 T. In this study, we investigated the accuracy and reproducibility of short-TE MRS measurements of GABA at 3 T using both simulations and experiments. LCModel analysis was performed on a large number of simulated spectra with known metabolite input concentrations. Simulated spectra were generated using a range of spectral linewidths and signal-to-noise ratios to investigate the effect of varying experimental conditions, and analyses were performed using two different baseline models to investigate the effect of an inaccurate baseline model on GABA quantification. The results of these analyses indicated that, under experimental conditions corresponding to those typically observed in the occipital cortex, GABA concentration estimates are reproducible (mean reproducibility error, <20%), even when an incorrect baseline model is used. However, simulations indicate that the accuracy of GABA concentration estimates depends strongly on the experimental conditions (linewidth and signal-to-noise ratio). In addition to simulations, in vivo GABA measurements were performed using both spectral editing and short-TE MRS in the occipital cortex of 14 healthy volunteers. Short-TE MRS measurements of GABA exhibited a significant positive correlation with edited GABA measurements (R = 0.58, p < 0.05), suggesting that short-TE measurements of GABA correspond well with measurements made using spectral editing techniques. Finally, within-session reproducibility was assessed in the same 14 subjects using four consecutive short-TE GABA measurements in the occipital cortex. Across all subjects, the average coefficient of variation of these four GABA measurements was 8.7 ± 4.9%. This study demonstrates that, under some experimental conditions, short-TE MRS can be employed for the reproducible detection of GABA at 3 T, but that the technique should be used with caution, as the results are dependent on the experimental conditions.

The case of the missing glutamine

A theoretical study was performed to determine the accuracy and repeatability of multiple one-dimensional pulse sequences in the quantification of glutamine concentration at 3 T. Variable repeatability (12% to > 50%) and significant absolute error (-50% to +70%) were noted for the eight pulse sequences considered. Data acquired in vivo using three of the pulse sequences used for simulation matched the predicted repeatability well; among the pulse sequences considered, point-resolved spectroscopy (TE = 80 ms) offered minimal error and acceptable repeatability (12%) for brain glutamine measurements. Following correction for the expected bias of each pulse sequence, consistent glutamine measurements, in the 1-mM range, were reported with the three sequences. An explanation for the mismatch between in vivo (1)H MRS and in vitro (13)C/(1)H MRS at high field was attempted.

Improved myo-inositol detection through Carr-Purcell PRESS: a tool for more sensitive mild cognitive impairment diagnosis

A 3-T study is presented, comparing the ability of two (1) H spectroscopy pulse sequences, Carr-Purcell point resolved spectroscopy (CPRESS; TE = 45 msec), and conventional PRESS (TE = 35 msec), to separate between groups of 20 normal control (NC) and 20 mild cognitive impairment (MCI) subjects. Both sequences showed higher myo-inositol (mI) and mI/N-acetyl-aspartate (NAA) levels in the posterior cingulate gyrus of the MCI subjects. The increased intrasubject repeatability of mI and mI/NAA CPRESS measurements (∼ 6% vs. 9% for PRESS) translated into decreased intraclass variability. A 22% intraclass mI PRESS variability was reduced to 16% for CPRESS, and an 18% intraclass mI/NAA PRESS variability was reduced to 12% for CPRESS for the group of NC subjects. Similar results were observed for the MCI subjects. Decreased intraclass variability led to improved separation between NC and MCI subjects (P = 0.017 for PRESS and P < 0.0001 for CPRESS mI/NAA, the best NC/MCI discriminant for each method). Seventy-five percent sensitivity at eighty percent specificity was demonstrated by mI/NAA CPRESS measurements in separating NC from MCI subjects. High correlations were also observed between subject performance on a number of neuropsychological tests (probing verbal memory, visuoconstruction performance, and visual motor integration) and the mI/NAA ratio; higher correlation coefficients (with stronger statistical significance) were consistently evident for CPRESS than for PRESS data.

Noninvasive evaluation of cerebral glioma grade by using multivoxel 3D proton MR spectroscopy

OBJECTIVE

To determine whether metabolite ratios in multivoxel 3D proton MR spectroscopy (¹H MRS) is different between low-grade and high-grade gliomas and may be useful for glioma grading.

MATERIALS AND METHODS

Thirty-nine patients (23 male and 16 female; 22-75 years old; mean age, 44.92±12.65 years) suspected of having gliomas underwent 3D ¹H MRS examinations. Metabolite ratios [choline (Cho)/creatine (Cr), N-acetylaspartate (NAA)/Cr and Cho/NAA] were measured. Tumor grade was determined by using the histopathologic grading. Receiver operating characteristic analysis of metabolite ratios was performed, and optimum thresholds for tumor grading were determined. The resulting sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for identifying high-grade gliomas were calculated.

RESULTS

Diagnostic-quality 3D ¹H MRS with readily quantifiable Cho, Cr and NAA peaks was obtained in 94.87% of the cases. The Cho/Cr and Cho/NAA ratios were significantly higher in high-grade than in low-grade glioma (P<.001), whereas the NAA/Cr ratios were significantly lower in high-grade than in low-grade glioma (P<.001). Receiver operating characteristic analysis demonstrated a threshold value of 2.04 for Cho/Cr ratio to provide sensitivity, specificity, PPV and NPV of 84.00%, 83.33%, 91.30% and 71.43%, respectively. Threshold value of 2.20 for Cho/NAA ratio resulted in sensitivity, specificity, PPV and NPV of 88.00%, 66.67%, 84.62% and 72.73%, respectively. Overall diagnostic accuracy was not statistically significantly different between Cho/Cr and Cho/NAA ratios (χ²=0.093, P=.76).

CONCLUSION

Metabolite ratios of low-grade gliomas were significantly different from high-grade gliomas. Cho/Cr and Cho/NAA ratios could have the superior diagnostic performance in predicting the glioma grade.

Optimized glutamate detection at 3T

PURPOSE

To identify the pulse sequence and acquisition parameters that result in the most accurate and repeatable measurements of glutamate (Glu) concentration in the brain at 3T.

MATERIALS AND METHODS

Simulations were performed to compare the accuracy and repeatability of 11 pulse sequences and acquisition parameters, within four general classes (PRESS, STEAM, Carr-Purcell PRESS [CPRESS] and TE averaged PRESS [JPRESS]), the majority of which were previously suggested as optimal for Glu detection. Three of the simulated acquisitions were implemented in a clinical scanner and measures of repeatability in vivo were compared to their simulated values.

RESULTS

Good agreement was demonstrated between simulated and experimentally determined measures of repeatability. Among the acquisitions considered, a CPRESS sequence with minimal echo time, together with, possibly, a short TE PRESS sequence, result in the most repeatable within session Glu measurements, while slightly overestimating the Glu concentration. Excellent accuracy is demonstrated by the simulations for a JPRESS sequence, at the expense of lower repeatability than optimal PRESS or CPRESS sequences.

CONCLUSION

Further proof of concept is presented toward validation of a simulation approach to understand pulse sequence performance in measuring the concentration of a given metabolite. Improved within session Glu measurement repeatability is predicted for CPRESS and demonstrated in vivo.