Diffusion-weighted MR spectroscopy: Consensus, recommendations, and resources from acquisition to modeling.

Brain cell structure and function reflect neurodevelopment, plasticity, and aging; and changes can help flag pathological processes such as neurodegeneration and neuroinflammation. Accurate and quantitative methods to noninvasively disentangle cellular structural features are needed and are a substantial focus of brain research. Diffusion-weighted MRS (dMRS) gives access to diffusion properties of endogenous intracellular brain metabolites that are preferentially located inside specific brain cell populations. Despite its great potential, dMRS remains a challenging technique on all levels: from the data acquisition to the analysis, quantification, modeling, and interpretation of results. These challenges were the motivation behind the organization of the Lorentz Center workshop on "Best Practices & Tools for Diffusion MR Spectroscopy" held in Leiden, the Netherlands, in September 2021. During the workshop, the dMRS community established a set of recommendations to execute robust dMRS studies. This paper provides a description of the steps needed for acquiring, processing, fitting, and modeling dMRS data, and provides links to useful resources.

Probing diffusion of water and metabolites to assess white matter microstructure in Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a progressive X-linked neuromuscular disorder caused by the absence of functional dystrophin protein. In addition to muscle, dystrophin is expressed in the brain in both neurons and glial cells. Previous studies have shown altered white matter microstructure in patients with DMD using diffusion tensor imaging (DTI). However, DTI measures the diffusion properties of water, a ubiquitous molecule, making it difficult to unravel the underlying pathology. Diffusion-weighted spectroscopy (DWS) is a complementary technique which measures diffusion properties of cell-specific intracellular metabolites. Here we performed both DWS and DTI measurements to disentangle intra- and extracellular contributions to white matter changes in patients with DMD. Scans were conducted in patients with DMD (15.5 ± 4.6 y/o) and age- and sex-matched healthy controls (16.3 ± 3.3 y/o). DWS measurements were obtained in a volume of interest (VOI) positioned in the left parietal white matter. Apparent diffusion coefficients (ADCs) were calculated for total N-acetylaspartate (tNAA), choline compounds (tCho), and total creatine (tCr). The tNAA/tCr and tCho/tCr ratios were calculated from the non-diffusion-weighted spectrum. Mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD), and fractional anisotropy of water within the VOI were extracted from DTI measurements. DWS and DTI data from patients with DMD (respectively n = 20 and n = 18) and n = 10 healthy controls were included. No differences in metabolite ADC or in concentration ratios were found between patients with DMD and controls. In contrast, water diffusion (MD, t = -2.727, p = 0.011; RD, t = -2.720, p = 0.011; AD, t = -2.715, p = 0.012) within the VOI was significantly higher in patients compared with healthy controls. Taken together, our study illustrates the potential of combining DTI and DWS to gain a better understanding of microstructural changes and their association with disease mechanisms in a clinical setting.

Longitudinal Monitoring of Microstructural Alterations in Cerebral Ischemia with in Vivo Diffusion-weighted MR Spectroscopy.

Background The time course of cellular damage after acute ischemic stroke (IS) is currently not well known, and specific noninvasive markers of microstructural alterations linked to inflammation are lacking, which hinders the monitoring of anti-inflammatory treatment. Purpose To evaluate the temporal pattern of neuronal and glial microstructural changes after stroke using in vivo single-voxel diffusion-weighted MR spectroscopy. Materials and Methods In this prospective longitudinal study, participants with IS and healthy volunteers (HVs) underwent MRI at 3.0 T. In participants with IS, apparent diffusion coefficients (ADCs) and concentrations of total N-acetyl-aspartate (tNAA), total creatine (tCr), and total choline (tCho) were measured in volumes of interest (VOIs), including the lesion VOI (VOIles) and the contralateral VOI (VOIcl) at 2 weeks, 1 month, and 3 months after IS. HVs were examined once, with VOIs located in the same brain regions as participants with IS. Within- and between-group differences and longitudinal changes were examined using linear mixed-effects models. Results Twenty participants with IS (mean age, 61 years ± 13 [SD]; 12 women) and 20 HVs (mean age, 59 years ± 13; 12 women) were evaluated. No differences in ADCs or concentrations were observed in VOIcl between HVs and participants with IS. In participants with IS, the ADC of tCr was higher in VOIles than in VOIcl at 1 month (+14.4%, P = .004) and 3 months after IS (+19.0%, P < .001), while the ADC of tCho was higher only at 1 month (+16.7%, P = .001). No difference in the ADC of tNAA was observed between the two VOIs at any time point. tNAA and tCr concentrations were lower in VOIles than in VOIcl and were stable over time (approximately -50% and -30%, respectively; P < .001). Conclusion High diffusivity of choline-containing compounds and total creatine (tCr) in the ischemic lesion 1 month after ischemic stroke (IS) indicates glial morphologic changes, suggesting that active inflammation is still ongoing at this time point. High tCr diffusivity up to 3 months after IS likely reflects the presence of astrogliosis at the chronic stage of cerebral ischemia. Clinical trial registration no. NCT02833961 © RSNA, 2022 Online supplemental material is available for this article.

Neuropsychiatric systemic lupus erythematosus is associated with a distinct type and shape of cerebral white matter hyperintensities.

OBJECTIVES: Advanced white matter hyperintensity (WMH) markers on brain MRI may help reveal underlying mechanisms and aid in the diagnosis of different phenotypes of SLE patients experiencing neuropsychiatric (NP) manifestations. METHODS: In this prospective cohort study, we included a clinically well-defined cohort of 155 patients consisting of 38 patients with NPSLE (26 inflammatory and 12 ischaemic phenotype) and 117 non-NPSLE patients. Differences in 3 T MRI WMH markers (volume, type and shape) were compared between patients with NPSLE and non-NPSLE and between patients with inflammatory and ischaemic NPSLE by linear and logistic regression analyses corrected for age, sex and intracranial volume. RESULTS: Compared with non-NPSLE [92% female; mean age 42 (13) years], patients with NPSLE [87% female; mean age 40 (14) years] showed a higher total WMH volume [B (95%-CI)]: 0.46 (0.0 7 ↔ 0.86); P = 0.021], a higher periventricular/confluent WMH volume [0.46 (0.0 6 ↔ 0.86); P = 0.024], a higher occurrence of periventricular with deep WMH type [0.32 (0.1 3 ↔ 0.77); P = 0.011], a higher number of deep WMH lesions [3.06 (1.2 1 ↔ 4.90); P = 0.001] and a more complex WMH shape [convexity: ‒0.07 (‒0.12 ↔ ‒0.02); P = 0.011, concavity index: 0.05 (0.0 1 ↔ 0.08); P = 0.007]. WMH shape was more complex in inflammatory NPSLE patients [89% female; mean age 39 (15) years] compared with patients with the ischaemic phenotype [83% female; mean age 41 (11) years] [concavity index: 0.08 (0.0 1 ↔ 0.15); P = 0.034]. CONCLUSION: We demonstrated that patients with NPSLE showed a higher periventricular/confluent WMH volume and more complex shape of WMH compared with non-NPSLE patients. This finding was particularly significant in inflammatory NPLSE patients, suggesting different or more severe underlying pathophysiological abnormalities.

Improved detection limits of J-coupled neurometabolites in the human brain at 7 T with a J-refocused sLASER sequence.

In a standard spin echo, the time evolution due to homonuclear couplings is not reversed, leading to echo time (TE)-dependent modulation of the signal amplitude and signal loss in the case of overlapping multiplet resonances. This has an adverse effect on quantification of several important metabolites such as glutamate and glutamine. Here, we propose a J-refocused variant of the sLASER sequence (J-sLASER) to improve quantification of J-coupled metabolites at ultrahigh field (UHF). The use of the sLASER sequence is particularly advantageous at UHF as it minimizes chemical shift displacement error and results in relatively homogenous refocusing. We simulated the MRS signal from brain metabolites over a broad range of TE values with sLASER and J-sLASER, and showed that the signal of J-coupled metabolites was increased with J-sLASER with TE values up to ~80 ms. We further simulated "brain-like" spectra with both sequences at the shortest TE available on our scanner. We showed that, despite the slightly longer TE, the J-sLASER sequence results in significantly lower Cramer-Rao lower bounds (CRLBs) for J-coupled metabolites compared with those obtained with sLASER. Following phantom validation, we acquired spectra from two brain regions in 10 healthy volunteers (age 38 ± 15 years) using both sequences. We showed that using J-sLASER results in a decrease of CRLBs for J-coupled metabolites. In particular, we measured a robust ~38% decrease in the mean CRLB (glutamine) in parietal white matter and posterior cingulate cortex (PCC). We further showed, in 10 additional healthy volunteers (age 34 ± 15 years), that metabolite quantification following two separate acquisitions with J-sLASER in the PCC was repeatable. The improvement in quantification of glutamine may in turn improve the independent quantification of glutamate, the main excitatory neurotransmitter in the brain, and will simultaneously help to track possible modulations of glutamine, which is a key player in the glutamatergic cycle in astrocytes.

Diffusion-weighted MR spectroscopy (DW-MRS) is sensitive to LPS-induced changes in human glial morphometry: A preliminary study.

BACKGROUND: Low-dose lipopolysaccharide (LPS) is a well-established experimental method for inducing systemic inflammation and shown by microscopy to activate microglia in rodents. Currently, techniques for in-vivo imaging of glia in humans are limited to TSPO (Translocator protein) PET, which is expensive, methodologically challenging, and has poor cellular specificity. Diffusion-weighted magnetic resonance spectroscopy (DW-MRS) sensitizes MR spectra to diffusion of intracellular metabolites, potentially providing cell-specific information about cellular morphology. In this preliminary study, we applied DW-MRS to measure changes in the apparent diffusion coefficients (ADC) of glial and neuronal metabolites to healthy participants who underwent an LPS administration protocol. We hypothesized that the ADC of glial metabolites will be selectively modulated by LPS-induced glial activation. METHODS: Seven healthy male volunteers, (mean 25.3 ± 5.9 years) were each tested in two separate sessions once after LPS (1 ng/Kg intravenously) and once after placebo (saline). Physiological responses were monitored during each session and serial blood samples and Profile of Mood States (POMS) completed to quantify white blood cell (WBC), cytokine and mood responses. DW-MRS data were acquired 5-5½ hours after injection from two brain regions: grey matter in the left thalamus, and frontal white matter. RESULTS: Body temperature, heart rate, WBC and inflammatory cytokines were significantly higher in the LPS compared to the placebo condition (p < 0.001). The ADC of the glial metabolite choline (tCho) was also significantly increased after LPS administration compared to placebo (p = 0.008) in the thalamus which scaled with LPS-induced changes in POMS total and negative mood (Adj R2 = 0.83; p = 0.004). CONCLUSIONS: DW-MRS may be a powerful new tool sensitive to glial cytomorphological changes in grey matter induced by systemic inflammation.

Compartmental diffusion and microstructural properties of human brain gray and white matter studied with double diffusion encoding magnetic resonance spectroscopy of metabolites and water.

Double diffusion encoding (DDE) of the water signal offers a unique ability to separate the effect of microscopic anisotropic diffusion in structural units of tissue from the overall macroscopic orientational distribution of cells. However, the specificity in detected microscopic anisotropy is limited as the signal is averaged over different cell types and across tissue compartments. Performing side-by-side water and metabolite DDE spectroscopic (DDES) experiments provides complementary measures from which intracellular and extracellular microscopic fractional anisotropies (µFA) and diffusivities can be estimated. Metabolites are largely confined to the intracellular space and therefore provide a benchmark for intracellular µFA and diffusivities of specific cell types. By contrast, water DDES measurements allow examination of the separate contributions to water µFA and diffusivity from the intra- and extracellular spaces, by using a wide range of b values to gradually eliminate the extracellular contribution. Here, we aimed to estimate tissue and compartment specific human brain microstructure by combining water and metabolites DDES experiments. We performed our DDES measurements in two brain regions that contain widely different amounts of white matter (WM) and gray matter (GM): parietal white matter (PWM) and occipital gray matter (OGM) in a total of 20 healthy volunteers at 7 Tesla. Metabolite DDES measurements were performed at b = 7199 s/mm2, while water DDES measurements were performed with a range of b values from 918 to 7199 s/mm2. The experimental framework we employed here resulted in a set of insights pertaining to the morphology of the intracellular and extracellular spaces in both gray and white matter. Results of the metabolite DDES experiments in both PWM and OGM suggest a highly anisotropic intracellular space within neurons and glia, with the possible exception of gray matter glia. The water µFA obtained from the DDES results at high b values in both regions converged with that of the metabolite DDES, suggesting that the signal from the extracellular space is indeed effectively suppressed at the highest b value. The µFA measured in the OGM significantly decreased at lower b values, suggesting a considerably lower anisotropy of the extracellular space in GM compared to WM. In PWM, the water µFA remained high even at the lowest b value, indicating a high degree of organization in the interstitial space in WM. Tortuosity values in the cytoplasm for water and tNAA, obtained with correlation analysis of microscopic parallel diffusivity with respect to GM/WM tissue fraction in the volume of interest, are remarkably similar for both molecules, while exhibiting a clear difference between gray and white matter, suggesting a more crowded cytoplasm and more complex cytomorphology of neuronal cell bodies and dendrites in GM than those found in long-range axons in WM.

Longitudinal changes in cerebral white matter microstructure in newly diagnosed systemic lupus erythematosus patients.

OBJECTIVES: To evaluate longitudinal variations in diffusion tensor imaging (DTI) metrics of different white matter (WM) tracts of newly diagnosed SLE patients, and to assess whether DTI changes relate to changes in clinical characteristics over time. METHODS: A total of 17 newly diagnosed SLE patients (19-55 years) were assessed within 24 months from diagnosis with brain MRI (1.5 T Philips Achieva) at baseline, and after at least 12 months. Fractional anisotropy, mean diffusivity (MD), radial diffusivity (RD) and axial diffusivity values were calculated in several normal-appearing WM tracts. Longitudinal variations in DTI metrics were analysed by repeated measures analysis of variance. DTI changes were separately assessed for 21 WM tracts. Associations between longitudinal alterations of DTI metrics and clinical variables (SLEDAI-2K, complement levels, glucocorticoid dosage) were evaluated using adjusted Spearman correlation analysis. RESULTS: Mean MD and RD values from the normal-appearing WM significantly increased over time (P = 0.019 and P = 0.021, respectively). A significant increase in RD (P = 0.005) and MD (P = 0.012) was found in the left posterior limb of the internal capsule; RD significantly increased in the left retro-lenticular part of the internal capsule (P = 0.013), and fractional anisotropy significantly decreased in the left corticospinal tract (P = 0.029). No significant correlation was found between the longitudinal change in DTI metrics and the change in clinical measures. CONCLUSION: Increase in diffusivity, reflecting a compromised WM tissue microstructure, starts in initial phases of the SLE disease course, even in the absence of overt neuropsychiatric (NP) symptoms. These results indicate the importance of monitoring NP involvement in SLE, even shortly after diagnosis.

Cytosolic diffusivity and microscopic anisotropy of N-acetyl aspartate in human white matter with diffusion-weighted MRS at 7 T.

Metabolite diffusion measurable in humans in vivo with diffusion-weighted spectroscopy (DW-MRS) provides a window into the intracellular morphology and state of specific cell types. Anisotropic diffusion in white matter is governed by the microscopic properties of the individual cell types and their structural units (axons, soma, dendrites). However, anisotropy is also markedly affected by the macroscopic orientational distribution over the imaging voxel, particularly in DW-MRS, where the dimensions of the volume of interest (VOI) are much larger than those typically used in diffusion-weighted imaging. One way to address the confound of macroscopic structural features is to average the measurements acquired with uniformly distributed gradient directions to mimic a situation where fibers present in the VOI are orientationally uniformly distributed. This situation allows the extraction of relevant microstructural features such as transverse and longitudinal diffusivities within axons and the related microscopic fractional anisotropy. We present human DW-MRS data acquired at 7 T in two different white matter regions, processed and analyzed as described above, and find that intra-axonal diffusion of the neuronal metabolite N-acetyl aspartate is in good correspondence to simple model interpretations, such as multi-Gaussian diffusion from disperse fibers where the transverse diffusivity can be neglected. We also discuss the implications of our approach for current and future applications of DW-MRS for cell-specific measurements.

In vivo diffusion-weighted MRS using semi-LASER in the human brain at 3 T: Methodological aspects and clinical feasibility.

Diffusion-weighted (DW-) MRS investigates non-invasively microstructural properties of tissue by probing metabolite diffusion in vivo. Despite the growing interest in DW-MRS for clinical applications, little has been published on the reproducibility of this technique. In this study, we explored the optimization of a single-voxel DW-semi-LASER sequence for clinical applications at 3 T, and evaluated the reproducibility of the method under different experimental conditions. DW-MRS measurements were carried out in 10 healthy participants and repeated across three sessions. Metabolite apparent diffusion coefficients (ADCs) were calculated from mono-exponential fits (ADCexp ) up to b = 3300 s/mm2 , and from the diffusional kurtosis approach (ADCK ) up to b = 7300 s/mm2 . The inter-subject variabilities of ADCs of N-acetylaspartate + N-acetylaspartylglutamate (tNAA), creatine + phosphocreatine, choline containing compounds, and myo-inositol were calculated in the posterior cingulate cortex (PCC) and in the corona radiata (CR). We explored the effect of physiological motion on the DW-MRS signal and the importance of cardiac gating and peak thresholding to account for signal amplitude fluctuations. Additionally, we investigated the dependence of the intra-subject variability on the acquisition scheme using a bootstrapping resampling method. Coefficients of variation were lower in PCC than CR, likely due to the different sensitivities to motion artifacts of the two regions. Finally, we computed coefficients of repeatability for ADCexp and performed power calculations needed for designing clinical studies. The power calculation for ADCexp of tNAA showed that in the PCC seven subjects per group are sufficient to detect a difference of 5% between two groups with an acquisition time of 4 min, suggesting that ADCexp of tNAA is a suitable marker for disease-related intracellular alteration even in small case-control studies. In the CR, further work is needed to evaluate the voxel size and location that minimize the motion artifacts and variability of the ADC measurements.

Different phenotypes of neuropsychiatric systemic lupus erythematosus are related to a distinct pattern of structural changes on brain MRI.

OBJECTIVES: The underlying structural brain correlates of neuropsychiatric involvement in systemic lupus erythematosus (NPSLE) remain unclear, thus hindering correct diagnosis. We compared brain tissue volumes between a clinically well-defined cohort of patients with NPSLE and SLE patients with neuropsychiatric syndromes not attributed to SLE (non-NPSLE). Within the NPSLE patients, we also examined differences between patients with two distinct disease phenotypes: ischemic and inflammatory. METHODS: In this prospective (May 2007 to April 2015) cohort study, we included 38 NPSLE patients (26 inflammatory and 12 ischemic) and 117 non-NPSLE patients. All patients underwent a 3-T brain MRI scan that was used to automatically determine white matter, grey matter, white matter hyperintensities (WMH) and total brain volumes. Group differences in brain tissue volumes were studied with linear regression analyses corrected for age, gender, and total intracranial volume and expressed as B values and 95% confidence intervals. RESULTS: NPSLE patients showed higher WMH volume compared to non-NPSLE patients (p = 0.004). NPSLE inflammatory patients showed lower total brain (p = 0.014) and white matter volumes (p = 0.020), and higher WMH volume (p = 0.002) compared to non-NPSLE patients. Additionally, NPSLE inflammatory patients showed lower white matter (p = 0.020) and total brain volumes (p = 0.038) compared to NPSLE ischemic patients. CONCLUSION: We showed that different phenotypes of NPSLE were related to distinct patterns of underlying structural brain MRI changes. Especially the inflammatory phenotype of NPSLE was associated with the most pronounced brain volume changes, which might facilitate the diagnostic process in SLE patients with neuropsychiatric symptoms. KEY POINTS: • Neuropsychiatric systemic lupus erythematosus (NPSLE) patients showed a higher WMH volume compared to SLE patients with neuropsychiatric syndromes not attributed to SLE (non-NPSLE). • NPSLE patients with inflammatory phenotype showed a lower total brain and white matter volume, and a higher volume of white matter hyperintensities, compared to non-NPSLE patients. • NPSLE patients with inflammatory phenotype showed lower white matter and total brain volumes compared to NPSLE patients with ischemic phenotype.

Quantitative MRI and laser ablation-inductively coupled plasma-mass spectrometry imaging of iron in the frontal cortex of healthy controls and Alzheimer's disease patients.

Accumulation of iron within the cortex of Alzheimer's disease (AD) patients has been reported by numerous MRI studies using iron-sensitive methods. Validation of iron-sensitive MRI is important for the interpretation of in vivo findings. In this study, the relation between the spatial iron distribution and T2∗-weighted MRI in the human brain was investigated using a direct comparison of spatial maps of iron as detected by T2∗-weighted MRI, iron histochemistry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), in postmortem brain tissue of the medial frontal gyrus of three control subjects and six AD patients. In addition, iron levels measured by LA-ICP-MS and three quantitative MRI methods, namely R2∗ (=1/T2∗), image phase and quantitative susceptibility mapping (QSM), were compared between 19 AD and 11 controls. Histochemistry results we obtained with the modified Meguro staining were highly correlated with iron levels as detected by LA-ICP-MS (r2 â€‹= â€‹0.82, P â€‹< â€‹0.0001). Significant positive correlations were also found between LA-ICP-MS and the three quantitative MRI measurements: R2∗ (r2 â€‹= â€‹0.63), image phase (r2 â€‹= â€‹0.70) and QSM (r2 â€‹= â€‹0.74 (all p â€‹< â€‹0.0001)). R2∗ and QSM showed the strongest correlation with iron content; the correlation of phase with iron clearly showed increased variation, probably due to its high orientation dependence. Despite the obvious differences in iron distribution patterns within the cortex between AD patients and controls, no overall significant differences were found in iron as measured by LA-ICP-MS, nor in R2∗, phase or susceptibility. In conclusion, our results show that histochemistry as well as quantitative MRI methods such as R2∗ mapping and QSM provide reliable measures of iron distribution in the cortex. These results support the use of MRI studies focusing on iron distribution in both the healthy and the diseased brain.

Insights into brain microstructure from in vivo DW-MRS.

Many developmental processes, such as plasticity and aging, or pathological processes such as neurological diseases are characterized by modulations of specific cellular types and their microstructures. Diffusion-weighted Magnetic Resonance Imaging (DW-MRI) is a powerful technique for probing microstructure, yet its information arises from the ubiquitous, non-specific water signal. By contrast, diffusion-weighted Magnetic Resonance Spectroscopy (DW-MRS) allows specific characterizations of tissues such as brain and muscle in vivo by quantifying the diffusion properties of MR-observable metabolites. Many brain metabolites are predominantly intracellular, and some of them are preferentially localized in specific brain cell populations, e.g., neurons and glia. Given the microstructural sensitivity of diffusion-encoding filters, investigation of metabolite diffusion properties using DW-MRS can thus provide exclusive cell and compartment-specific information. Furthermore, since many models and assumptions are used for quantification of water diffusion, metabolite diffusion may serve to generate a-priori information for model selection in DW-MRI. However, DW-MRS measurements are extremely challenging, from the acquisition to the accurate and correct analysis and quantification stages. In this review, we survey the state-of-the-art methods that have been developed for the robust acquisition, quantification and analysis of DW-MRS data and discuss the potential relevance of DW-MRS for elucidating brain microstructure in vivo. The review highlights that when accurate data on the diffusion of multiple metabolites is combined with accurate computational and geometrical modeling, DW-MRS can provide unique cell-specific information on the intracellular structure of brain tissue, in health and disease, which could serve as incentives for further application in vivo in human research and clinical MRI.

Dysregulation of energy metabolism in multiple sclerosis measured in vivo with diffusion-weighted spectroscopy.

OBJECTIVE: We employed diffusion-weighted magnetic resonance spectroscopy (DW-MRS), which allows to measure in vivo the diffusion properties of metabolites, to explore the functional neuro-axonal damage and the ongoing energetic dysregulation in multiple sclerosis (MS). METHODS: Twenty-five patients with MS and 18 healthy controls (HC) underwent conventional magnetic resonance imaging (MRI) and DW-MRS. The apparent diffusion coefficient (ADC) of total N-acetyl-aspartate (tNAA) and creatine-phosphocreatine (tCr) were measured in the parietal normal-appearing white matter (NAWM) and in the thalamic grey matter (TGM). Multiple regressions were used to compare metabolite ADCs between groups and to explore clinical correlations. RESULTS: In patients compared with HCs, we found a reduction in ADC(tNAA) in the TGM, reflecting functional and structural neuro-axonal damage, and in ADC(tCr) in both NAWM and TGM, possibly reflecting a reduction in energy supply in neurons and glial cells. Metabolite ADCs did not correlate with tissue atrophy, lesional volume or metabolite concentrations, while in TGM metabolite ADCs correlated with clinical scores. CONCLUSION: DW-MRS showed a reduction in tCr diffusivity in the normal-appearing brain of patients with MS, which might reflect a state of ongoing energy dysregulation affecting neurons and/or glial cells. Reversing this energy dysregulation before neuro-axonal degeneration arises may become a key objective in future neuroprotective strategies.

Cortical glutamate in migraine.

Cortical hyperexcitability due to enhanced glutamatergic activity has been implicated in migraine pathophysiology but direct evidence is lacking. Here we assessed glutamate levels and intracellular mobility of glutamate in the visual cortex of migraineurs in-between attacks. We included 50 migraineurs (23 with aura and 27 without aura) and 24 age- and gender-matched non-headache controls. We used proton magnetic resonance spectroscopy (1H-MRS) and diffusion weighted spectroscopy at 7 T with a single volume of interest (2 × 2 × 3 cm) located in the primary and secondary visual cortex. For 1H-MRS we used a semi-LASER sequence with water referencing for absolute quantification. For diffusion weighted spectroscopy we used an adapted PRESS sequence with gradients applied in three directions and two different gradient amplitudes. Between-group differences were evaluated using analysis of covariance with the grey matter fraction in the volume of interest as covariate and post hoc comparisons with Bonferroni correction. Glutamate concentrations differed between groups (P = 0.047) and were higher in migraineurs without aura (mean ± standard deviation: 7.02 ± 0.50 mM) compared to controls (mean ± standard deviation: 6.40 ± 0.78 mM, P = 0.042). The apparent diffusion coefficient of glutamate was similar among groups (P = 0.129) suggesting similar inter- and intracellular mobility of glutamate in all three study groups. No differences were observed for concentrations and diffusion constants of other metabolites. The present study suggests that interictal glutamate levels are increased in the visual cortex of migraineurs without aura, supporting the hypothesis of cortical hyperexcitability in migraine.

Glial and axonal changes in systemic lupus erythematosus measured with diffusion of intracellular metabolites.

Systemic lupus erythematosus is an inflammatory autoimmune disease with multi-organ involvement. Central nervous system involvement in systemic lupus erythematosus is common and results in several neurological and psychiatric symptoms that are poorly linked to standard magnetic resonance imaging outcome. Magnetic resonance imaging methods sensitive to tissue microstructural changes, such as diffusion tensor imaging and magnetization transfer imaging, show some correlation with neuropsychiatric systemic lupus erythematosus (NPSLE) symptoms. Histological examination of NPSLE brains reveals presence of cerebral oedema, loss of neurons and myelinated axons, microglial proliferation and reactive astrocytosis, microinfacrts and diffuse ischaemic changes, all of which can affect both diffusion tensor imaging and magnetization transfer imaging in a non-specific manner. Here we investigated the underlying cell-type specific microstructural alterations in the brain of patients with systemic lupus erythematosus with and without a history of central nervous system involvement. We did so combining diffusion tensor imaging with diffusion-weighted magnetic resonance spectroscopy, a powerful tool capable of characterizing cell-specific cytomorphological changes based on diffusion of intracellular metabolites. We used a 7 T magnetic resonance imaging scanner to acquire T1-weighted images, diffusion tensor imaging datasets, and single volume diffusion-weighted magnetic resonance spectroscopy data from the anterior body of the corpus callosum of 13 patients with systemic lupus erythematosus with past NPSLE, 16 patients with systemic lupus erythematosus without past NPSLE, and 19 healthy control subjects. Group comparisons were made between patients with systemic lupus erythematosus with/without past NPSLE and healthy controls on diffusion tensor imaging metrics and on diffusion coefficients of three brain metabolites: the exclusively neuronal/axonal N-acetylaspartate, and the predominantly glial creatine + phosphocreatine and choline compounds. In patients with systemic lupus erythematosus with past NPSLE, significantly higher diffusion tensor imaging mean and radial diffusivities were accompanied by a significantly higher intracellular diffusion of total creatine (0.202 ± 0.032 µm(2)/ms, P = 0.018) and total choline (0.142 ± 0.031 µm(2)/ms, P = 0.044) compared to healthy controls (0.171 ± 0.024 µm(2)/ms, 0.124 ± 0.018 µm(2)/ms, respectively). Total N-acetylaspartate, total creatine and total choline diffusion values from all patients with systemic lupus erythematosus correlated positively with systemic lupus erythematosus disease activity index score (P = 0.033, P = 0.040, P = 0.008, respectively). Our results indicate that intracellular alterations, and in particular changes in glia, as evidenced by increase in the average diffusivities of total choline and total creatine, correlate with systemic lupus erythematosus activity. The higher diffusivity of total creatine and total choline in patients with NPSLE, as well as the positive correlation of these diffusivities with the systemic lupus erythematosus disease activity index are in line with cytomorphological changes in reactive glia, suggesting that the diffusivities of choline compounds and of total creatine are potentially unique markers for glial reactivity in response to inflammation.

Diffusion-weighted chemical shift imaging of human brain metabolites at 7T.

PURPOSE: Diffusion-weighted chemical shift imaging (DW-CSI) of brain metabolites poses significant challenges associated with the acquisition of spectroscopic data in the presence of strong diffusion weighting gradients. We present a reproducible DW-CSI acquisition and processing scheme that addresses most of the potential sources of instability and provides reproducible and anatomically meaningful diffusion-weighted and apparent diffusion coefficient (ADC) metabolite maps. METHODS: A real-time navigator-based acquisition scheme was used, allowing instantaneous reacquisition of corrupted k-space data and postprocessing correction of gradient-induced phase fluctuations. Eddy current correction based on residual water resonance was implemented and improved the quality of the data significantly. RESULTS: Highly reproducible diffusion-weighted metabolite maps of three highest concentration brain metabolites are shown. The navigator-based accept/reject strategy and the postacquisition corrections improved the stability of the DW-CSI signal and the reproducibility of the resulting DW-CSI maps significantly. The metabolite ADC values could be related to the underlying tissue cellular composition. CONCLUSION: Robust investigation of DW-CSI of brain metabolites is feasible and may provide information complementary to that obtained from more sensitive but less specific methods such as diffusion tensor imaging.

Reproducibility and optimization of in vivo human diffusion-weighted MRS of the corpus callosum at 3 T and 7 T.

Diffusion-weighted MRS (DWS) of brain metabolites enables the study of cell-specific alterations in tissue microstructure by probing the diffusion of intracellular metabolites. In particular, the diffusion properties of neuronal N-acetylaspartate (NAA), typically co-measured with N-acetylaspartyl glutamate (NAAG) (NAA + NAAG = tNAA), have been shown to be sensitive to intraneuronal/axonal damage in pathologies such as stroke and multiple sclerosis. Lacking, so far, are empirical assessments of the reproducibility of DWS measures across time and subjects, as well as a systematic investigation of the optimal acquisition parameters for DWS experiments, both of which are sorely needed for clinical applications of the method. In this study, we acquired comprehensive single-volume DWS datasets of the human corpus callosum at 3 T and 7 T. We investigated the inter- and intra-subject variability of empirical and modeled diffusion properties of tNAA [D(avg) (tNAA) and D(model) (tNAA), respectively]. Subsequently, we used a jackknife-like resampling approach to explore the variance of these properties in partial data subsets reflecting different total scan durations. The coefficients of variation (C(V)) and repeatability coefficients (C(R)) for D(avg) (tNAA) and D(model) (tNAA) were calculated for both 3 T and 7 T, with overall lower variability in the 7 T results. Although this work is limited to the estimation of the diffusion properties in the corpus callosum, we show that a careful choice of diffusion-weighting conditions at both field strengths allows the accurate measurement of tNAA diffusion properties in clinically relevant experimental time. Based on the resampling results, we suggest optimized acquisition schemes of 13-min duration at 3T and 10-min duration at 7 T, whilst retaining low variability (C(V) ≈ 8%) for the tNAA diffusion measures. Power calculations for the estimation of D(model )(tNAA) and D(avg) (tNAA) based on the suggested schemes show that less than 21 subjects per group are sufficient for the detection of a 10% effect between two groups in case-control studies.

The interaction between apparent diffusion coefficients and transverse relaxation rates of human brain metabolites and water studied by diffusion-weighted spectroscopy at 7 T.

The dependence of apparent diffusion coefficients (ADCs) of molecules in biological tissues on an acquisition-specific timescale is a powerful mechanism for studying tissue microstructure. Unlike water, metabolites are confined mainly to intracellular compartments, thus providing higher specificity to tissue microstructure. Compartment-specific structural and chemical properties may also affect molecule transverse relaxation times (T2). Here, we investigated the correlation between diffusion and relaxation for N-acetylaspartate, creatine and choline compounds in human brain white matter in vivo at 7 T, and compared them with those of water under the same experimental conditions. Data were acquired in a volume of interest in parietal white matter at two different diffusion times, Δ = 44 and 246 ms, using a matrix of three echo times (T(E)) and five diffusion weighting values (up to 4575 s/mm²). Significant differences in the dependence of the ADCs on T(E) were found between water and metabolites, as well as among the different metabolites. A significant decrease in water ADC as a function of TE was observed only at the longest diffusion time (p < 0.001), supporting the hypothesis that at least part of the restricted water pool can be associated with longer T2, as suggested by previous studies in vitro. Metabolite data showed an increase of creatine (p < 0.05) and N-acetylaspartate (p < 0.05) ADCs with TE at Δ = 44 ms, and a decrease of creatine (p < 0.05) and N-acetylaspartate (p = 0.1) ADCs with TE at Δ = 246 ms. No dependence of choline ADC on TE was observed. The metabolite results suggest that diffusion and relaxation properties are dictated not only by metabolite distribution in different cell types, but also by other mechanisms, such as interactions with membranes, exchange between "free" and "bound" states or interactions with microsusceptibility gradients.

Histological validation of DW-MRI tractography in human postmortem tissue.

Despite several previous attempts, histological validation of diffusion-weighted magnetic resonance imaging (DW-MRI)-based tractography as true axonal fiber pathways remains difficult. In the present study, we establish a method to compare histological and tractography data precisely enough for statements on the level of single tractography pathways. To this end, we used carbocyanine dyes to trace connections in human postmortem tissue and aligned them to high-resolution DW-MRI of the same tissue processed within the diffusion tensor imaging (DTI) formalism. We provide robust definitions of sensitivity (true positives) and specificity (true negatives) for DTI tractography and characterize tractography paths in terms of receiver operating characteristics. With sensitivity and specificity rates of approximately 80%, we could show a clear correspondence between histological and inferred tracts. Furthermore, we investigated the effect of fractional anisotropy (FA) thresholds for the tractography and identified FA values between 0.02 and 0.08 as optimal in our study. Last, we validated the course of entire tractography curves to move beyond correctness determination based on pairs of single points on a tract. Thus, histological techniques, in conjunction with alignment and processing tools, may serve as an important validation method of DW-MRI on the level of inferred tractography projections between brain areas.