Brain intra- and extracellular sodium concentration in multiple sclerosis: a 7 T MRI study

Advanced Brain Imaging in Central Nervous System Demyelinating Diseases

In recent decades, advances in neuroimaging have profoundly transformed our comprehension of central nervous system demyelinating diseases. Remarkable technological progress has enabled the integration of cutting-edge acquisition and postprocessing techniques, proving instrumental in characterizing subtle focal changes, diffuse microstructural alterations, and macroscopic pathologic processes. This review delves into state-of-the-art modalities applied to multiple sclerosis, neuromyelitis optica spectrum disorders, and myelin oligodendrocyte glycoprotein antibody-associated disease. Furthermore, it explores how this dynamic landscape holds significant promise for the development of effective and personalized clinical management strategies, encompassing support for differential diagnosis, prognosis, monitoring treatment response, and patient stratification.

Functionalized DNA Origami-Enabled Detection of Biomarkers

Biomarkers are crucial physiological and pathological indicators in the host. Over the years, numerous detection methods have been developed for biomarkers, given their significant potential in various biological and biomedical applications. Among these, the detection system based on functionalized DNA origami has emerged as a promising approach due to its precise control over sensing modules, enabling sensitive, specific, and programmable biomarker detection. We summarize the advancements in biomarker detection using functionalized DNA origami, focusing on strategies for DNA origami functionalization, mechanisms of biomarker recognition, and applications in disease diagnosis and monitoring. These applications are organized into sections based on the type of biomarkers - nucleic acids, proteins, small molecules, and ions - and concludes with a discussion on the advantages and challenges associated with using functionalized DNA origami systems for biomarker detection.

Mice and minipigs with compromised expression of the Alzheimer's disease gene SORL1 show cerebral metabolic disturbances on hyperpolarized [1-13C]pyruvate and sodium MRI

The sortilin-related receptor 1 (SORL1) gene, encoding the cellular endosomal sorting-related receptor with A-type repeats (SORLA), is now established as a causal gene for Alzheimer's disease. As the latest addition to the list of causal genes, the pathophysiological effects and biomarker potential of SORL1 variants remain relatively undiscovered. Metabolic dysfunction is, however, well described in patients with Alzheimer's disease and is used as an imaging biomarker in clinical diagnosis settings. To understand the metabolic consequences of loss-of-function SORL1 mutations, we applied two metabolic MRI technologies, sodium (23Na) MRI and MRI with hyperpolarized [1-13C]pyruvate, in minipigs and mice with compromised expression of SORL1. At the age analysed here, both animal models display no conventional imaging evidence of neurodegeneration but show biochemical signs of elevated amyloid production, thus representing the early preclinical disease. With hyperpolarized MRI, the exchange from [1-13C]pyruvate to [1-13C]lactate and 13C-bicarbonate was decreased by 32 and 23%, respectively, in the cerebrum of SORL1-haploinsufficient minipigs. A robust 11% decrease in the sodium content was observed with 23Na-MRI in the same minipigs. Comparably, the brain sodium concentration gradually decreased from control to SORL1 haploinsufficient (-11%) to SORL1 knockout mice (-23%), suggesting a gene dose dependence in the metabolic dysfunction. The present study highlights that metabolic MRI technologies are sensitive to the functional, metabolic consequences of Alzheimer's disease and Alzheimer's disease-linked genotypes. Further, the study suggests a potential avenue of research into the mechanisms of metabolic alterations by SORL1 mutations and their potential role in neurodegeneration.

Recent technical developments and clinical research applications of sodium (23Na) MRI

Sodium is an essential ion that plays a central role in many physiological processes including the transmembrane electrochemical gradient and the maintenance of the body's homeostasis. Due to the crucial role of sodium in the human body, the sodium nucleus is a promising candidate for non-invasively assessing (patho-)physiological changes. Almost 10 years ago, Madelin et al. provided a comprehensive review of methods and applications of sodium (23Na) MRI (Madelin et al., 2014) [1]. More recent review articles have focused mainly on specific applications of 23Na MRI. For example, several articles covered 23Na MRI applications for diseases such as osteoarthritis (Zbyn et al., 2016, Zaric et al., 2020) [2,3], multiple sclerosis (Petracca et al., 2016, Huhn et al., 2019) [4,5] and brain tumors (Schepkin, 2016) [6], or for imaging certain organs such as the kidneys (Zollner et al., 2016) [7], the brain (Shah et al., 2016, Thulborn et al., 2018) [8,9], and the heart (Bottomley, 2016) [10]. Other articles have reviewed technical developments such as radiofrequency (RF) coils for 23Na MRI (Wiggins et al., 2016, Bangerter et al., 2016) [11,12], pulse sequences (Konstandin et al., 2014) [13], image reconstruction methods (Chen et al., 2021) [14], and interleaved/simultaneous imaging techniques (Lopez Kolkovsky et al., 2022) [15]. In addition, 23Na MRI topics have been covered in review articles with broader topics such as multinuclear MRI or ultra-high-field MRI (Niesporek et al., 2019, Hu et al., 2019, Ladd et al., 2018) [16-18]. During the past decade, various research groups have continued working on technical improvements to sodium MRI and have investigated its potential to serve as a diagnostic and prognostic tool. Clinical research applications of 23Na MRI have covered a broad spectrum of diseases, mainly focusing on the brain, cartilage, and skeletal muscle (see Fig. 1). In this article, we aim to provide a comprehensive summary of methodological and hardware developments, as well as a review of various clinical research applications of sodium (23Na) MRI in the last decade (i.e., published from the beginning of 2013 to the end of 2022).

Photoacoustic Chemical Imaging Sodium Nano-Sensor Utilizing a Solvatochromic Dye Transducer for In Vivo Application

Sodium has many vital and diverse roles in the human body, including maintaining the cellular pH, generating action potential, and regulating osmotic pressure. In cancer, sodium dysregulation has been correlated with tumor growth, metastasis, and immune cell inhibition. However, most in vivo sodium measurements are performed via Na23 NMR, which is handicapped by slow acquisition times, a low spatial resolution (in mm), and low signal-to-noise ratios. We present here a plasticizer-free, ionophore-based sodium-sensing nanoparticle that utilizes a solvatochromic dye transducer to circumvent the pH cross-sensitivity of most previously reported sodium nano-sensors. We demonstrate that this nano-sensor is non-toxic, boasts a 200 μM detection limit, and is over 1000 times more selective for sodium than potassium. Further, the in vitro photoacoustic calibration curve presented demonstrates the potential of this nano-sensor for performing the in vivo chemical imaging of sodium over the entire physiologically relevant concentration range.

A combined X-ray fluorescence and infrared microspectroscopy study for new insights into elemental-biomolecular obesity-induced changes in rat brain structures

The objective of our research was to determine the brain changes at the molecular and elemental levels typical of early-stage obesity. Therefore a combined approach using Fourier transform infrared micro-spectroscopy (FTIR-MS) and synchrotron radiation induced X-ray fluorescence (SRXRF) was introduced to evaluate some brain macromolecular and elemental parameters in high-calorie diet (HCD)- induced obese rats (OB, n = 6) and in their lean counterparts (L, n = 6). A HCD was found to alter the lipid- and protein- related structure and elemental composition of the certain brain areas important for energy homeostasis. The increased lipid unsaturation in the frontal cortex and ventral tegmental area, the increased fatty acyl chain length in the lateral hypothalamus and substantia nigra as well as the decreased both protein α helix to protein β- sheet ratio and the percentage fraction of β-turns and β-sheets in the nucleus accumbens were revealed in the OB group reflecting obesity-related brain biomolecular aberrations. In addition, the certain brain elements including P, K and Ca were found to differentiate the lean and obese groups at the best extent. We can conclude that HCD-induced obesity triggers lipid- and protein- related structural changes as well as elemental redistribution within various brain structures important for energy homeostasis. In addition, an approach applying combined X-ray and infrared spectroscopy was shown to be a reliable tool for identifying elemental-biomolecular rat brain changes for better understanding the interplay between the chemical and structural processes involved in appetite control.

Patterns of inflammation, microstructural alterations, and sodium accumulation define multiple sclerosis subtypes after 15 years from onset

Introduction

Conventional MRI is routinely used for the characterization of pathological changes in multiple sclerosis (MS), but due to its lack of specificity is unable to provide accurate prognoses, explain disease heterogeneity and reconcile the gap between observed clinical symptoms and radiological evidence. Quantitative MRI provides measures of physiological abnormalities, otherwise invisible to conventional MRI, that correlate with MS severity. Analyzing quantitative MRI measures through machine learning techniques has been shown to improve the understanding of the underlying disease by better delineating its alteration patterns.

Methods

In this retrospective study, a cohort of healthy controls (HC) and MS patients with different subtypes, followed up 15 years from clinically isolated syndrome (CIS), was analyzed to produce a multi-modal set of quantitative MRI features encompassing relaxometry, microstructure, sodium ion concentration, and tissue volumetry. Random forest classifiers were used to train a model able to discriminate between HC, CIS, relapsing remitting (RR) and secondary progressive (SP) MS patients based on these features and, for each classification task, to identify the relative contribution of each MRI-derived tissue property to the classification task itself.

Results and discussion

Average classification accuracy scores of 99 and 95% were obtained when discriminating HC and CIS vs. SP, respectively; 82 and 83% for HC and CIS vs. RR; 76% for RR vs. SP, and 79% for HC vs. CIS. Different patterns of alterations were observed for each classification task, offering key insights in the understanding of MS phenotypes pathophysiology: atrophy and relaxometry emerged particularly in the classification of HC and CIS vs. MS, relaxometry within lesions in RR vs. SP, sodium ion concentration in HC vs. CIS, and microstructural alterations were involved across all tasks.

Variability by region and method in human brain sodium concentrations estimated by 23Na magnetic resonance imaging: a meta-analysis

Sodium imaging (²³Na-MRI) is of interest in neurological conditions given potential sensitivity to the physiological and metabolic status of tissues. Benchmarks have so far been restricted to parenchyma or grey/white matter (GM/WM). We investigate (1) the availability of evidence, (2) regional pooled estimates and (3) variability attributable to region/methodology. MEDLINE literature search for tissue sodium concentration (TSC) measured in specified 'healthy' brain regions returned 127 reports, plus 278 retrieved from bibliographies. 28 studies met inclusion criteria, including 400 individuals. Reporting variability led to nested data structure, so we used multilevel meta-analysis and a random effects model to pool effect sizes. The pooled mean from 141 TSC estimates was 40.51 mM (95% CI 37.59-43.44; p < 0.001, I²_Total=99.4%). Tissue as a moderator was significant (F²₁₄ = 65.34, p-val < .01). Six sub-regional pooled means with requisite statistical power were derived. We were unable to consider most methodological and demographic factors sought because of non-reporting, but each factor included beyond tissue improved model fit. Significant residual heterogeneity remained. The current estimates provide an empirical point of departure for better understanding in ²³Na-MRI. Improving on current estimates supports: (1) larger, more representative data collection/sharing, including (2) regional data, and (3) agreement on full reporting standards.

Women in the field of multiple sclerosis: How they contributed to paradigm shifts

History is full of women who made enormous contributions to science. While there is little to no imbalance at the early career stage, a decreasing proportion of women is found as seniority increases. In the multiple sclerosis (MS) field, 44% of first authors and only 35% of senior authors were female. So, in this review, we highlight ground-breaking research done by women in the field of MS, focusing mostly on their work as principal investigators. MS is an autoimmune disorder of the central nervous system (CNS), with evident paradigm shifts in the understating of its pathophysiology. It is known that the immune system becomes overactivated and attacks myelin sheath surrounding axons. The resulting demyelination disrupts the communication signals to and from the CNS, which causes unpredictable symptoms, depending on the neurons that are affected. Classically, MS was reported to cause mostly physical and motor disabilities. However, it is now recognized that cognitive impairment affects more than 50% of the MS patients. Another shifting paradigm was the involvement of gray matter in MS pathology, formerly considered to be a white matter disease. Additionally, the identification of different T cell immune subsets and the mechanisms underlying the involvement of B cells and peripheral macrophages provided a better understanding of the immunopathophysiological processes present in MS. Relevantly, the gut-brain axis, recognized as a bi-directional communication system between the CNS and the gut, was found to be crucial in MS. Indeed, gut microbiota influences not only different susceptibilities to MS pathology, but it can also be modulated in order to positively act in MS course. Also, after the identification of the first microRNA in 1993, the role of microRNAs has been investigated in MS, either as potential biomarkers or therapeutic agents. Finally, concerning MS therapeutical approaches, remyelination-based studies have arisen on the spotlight aiming to repair myelin loss/neuronal connectivity. Altogether, here we emphasize the new insights of remarkable women that have voiced the impact of cognitive impairment, white and gray matter pathology, immune response, and that of the CNS-peripheral interplay on MS diagnosis, progression, and/or therapy efficacy, leading to huge breakthroughs in the MS field.

MR-Nucleomics: The study of pathological cellular processes with multinuclear magnetic resonance spectroscopy and imaging in vivo

Clinical medicine has experienced a rapid development in recent decades, during which therapies targeting specific cellular signaling pathways, or specific cell surface receptors, have been increasingly adopted. While these developments in clinical medicine call for improved precision in diagnosis and treatment monitoring, modern medical imaging methods are restricted mainly to anatomical imaging, lagging behind the requirements of precision medicine. Although positron emission tomography and single photon emission computed tomography have been used clinically for studies of metabolism, their applications have been limited by the exposure risk to ionizing radiation, the subsequent limitation in repeated and longitudinal studies, and the incapability in assessing downstream metabolism. Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) are, in theory, capable of assessing molecular activities in vivo, although they are often limited by sensitivity. Here, we review some recent developments in MRS and MRSI of multiple nuclei that have potential as molecular imaging tools in the clinic.

Quantitative 7T sodium magnetic resonance imaging of the human brain using a 32-channel phased-array head coil: Application to patients with secondary progressive multiple sclerosis

Apparent tissue sodium concentrations (aTSCs) determined by ²³ Na brain magnetic resonance imaging (MRI) have the potential to serve as a biomarker in pathologies such as multiple sclerosis (MS). However, the quantification is hindered by the intrinsically low signal-to-noise ratio of ²³ Na MRI. The purpose of this study was to improve the accuracy and reliability of quantitative ²³ Na brain MRI by implementing a dedicated postprocessing pipeline and to evaluate the applicability of the developed approach for the examination of MS patients. ²³ Na brain MRI measurements of 13 healthy volunteers and 17 patients with secondary progressive multiple sclerosis (SPMS) were performed at 7 T using a dual-tuned ²³ Na/¹ H birdcage coil with a receive-only 32-channel phased array. The aTSC values were determined for normal appearing white matter (NAWM) and normal appearing gray matter (NAGM) in healthy subjects and SPMS patients. Signal intensities were normalized using the mean cerebrospinal fluid (CSF) sodium concentration determined in 37 separate patients receiving a spinal tap for routine diagnostic purposes. Five volunteers underwent MRI examinations three times in a row to assess repeatability. Coefficients of variation (CoVs) were used to quantify the repeatability of the proposed method. aTSC values were compared regarding brain regions and subject cohort using the paired-samples Wilcoxon rank-sum test. Laboratory CSF sodium concentration did not differ significantly between patients without and with MS (p = 0.42). The proposed quantification workflow for ²³ Na MRI was highly repeatable with CoVs averaged over all five volunteers of 1.9% ± 0.9% for NAWM and 2.2% ± 1.6% for NAGM. Average NAWM aTSC was significantly higher in patients with SPMS compared with the control group (p = 0.009). Average NAGM aTSC did not differ significantly between healthy volunteers and MS patients (p = 0.98). The proposed postprocessing pipeline shows high repeatability and the results can serve as a baseline for further studies establishing ²³ Na brain MRI as a biomarker in diseases such as MS.

Grey-matter sodium concentration as an individual marker of multiple sclerosis severity

OBJECTIVE

Quantification of brain injury in patients with variable disability despite similar disease duration may be relevant to identify the mechanisms underlying disability in multiple sclerosis (MS). We aimed to compare grey-matter sodium abnormalities (GMSAs), a parameter reflecting neuronal and astrocyte dysfunction, in MS patients with benign multiple sclerosis (BMS) and non-benign multiple sclerosis (NBMS).

METHODS

We identified never-treated BMS patients in our local MS database of 1352 patients. A group with NBMS was identified with same disease duration. All participants underwent ²³Na magnetic resonance imaging (MRI). The existence of GMSA was detected by statistical analysis.

RESULTS

In total, 102 individuals were included (21 BMS, 25 NBMS and 56 controls). GMSA was detected in 10 BMS and 19 NBMS (11/16 relapsing-remitting multiple sclerosis (RRMS) and 8/9 secondary progressive multiple sclerosis (SPMS) patients) (p = 0.05). On logistic regression including the presence or absence of GMSA, thalamic volume, cortical grey-matter volume and T2-weighted lesion load, thalamic volume was independently associated with BMS status (odds ratio (OR) = 0.64 for each unit). Nonetheless, the absence of GMSA was independently associated when excluding patients with significant cognitive alteration (n = 7) from the BMS group (OR = 4.6).

CONCLUSION

Detection of GMSA in individuals and thalamic volume are promising to differentiate BMS from NBMS as compared with cortical or whole grey-matter atrophy and T2-weighted lesions.

A Narrative Review on Axonal Neuroprotection in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) resulting in demyelination and neurodegeneration. The therapeutic strategy is now largely based on reducing inflammation with immunosuppressive drugs. Unfortunately, when disease progression is observed, no drug offers neuroprotection apart from its anti-inflammatory effect. In this review, we explore current knowledge on the assessment of neurodegeneration in MS and look at putative targets that might prove useful in protecting the axon from degeneration. Among them, Bruton's tyrosine kinase inhibitors, anti-apoptotic and antioxidant agents, sex hormones, statins, channel blockers, growth factors, and molecules preventing glutamate excitotoxicity have already been studied. Some of them have reached phase III clinical trials and carry a great message of hope for our patients with MS.

Opportunities for Molecular Imaging in Multiple Sclerosis Management: Linking Probe to Treatment

Imaging has been a critical component of multiple sclerosis (MS) management for nearly 40 years. The visual information derived from structural MRI, that is, signs of blood-brain barrier disruption, inflammation and demyelination, and brain and spinal cord atrophy, are the primary metrics used to evaluate therapeutic efficacy in MS. The development of targeted imaging probes has expanded our ability to evaluate and monitor MS and its therapies at the molecular level. Most molecular imaging probes evaluated for MS applications are small molecules initially developed for PET, nearly half of which are derived from U.S. Food and Drug Administration-approved drugs and those currently undergoing clinical trials. Superparamagnetic and fluorinated particles have been used for tracking circulating immune cells (in situ labeling) and immunosuppressive or remyelinating therapeutic stem cells (ex vivo labeling) clinically using proton (hydrogen 1 [¹H]) and preclinically using fluorine 19 MRI. Translocator protein PET and ¹H MR spectroscopy have been demonstrated to complement imaging metrics from structural (gadolinium-enhanced) MRI in nine and six trials for MS disease-modifying therapies, respectively. Still, despite multiple demonstrations of the utility of molecular imaging probes to evaluate the target location and to elucidate the mechanisms of disease-modifying therapies for MS applications, their use has been sparse in both preclinical and clinical settings.

Increased brain tissue sodium concentration in Friedreich ataxia: A multimodal MR imaging study

In patients with Friedreich ataxia, structural MRI is typically used to detect abnormalities primarily in the brainstem, cerebellum, and spinal cord. The aim of the present study was to additionally investigate possible metabolic changes in Friedreich ataxia using in vivo sodium MRI that may precede macroanatomical alterations, and to explore potential associations with clinical parameters of disease progression. Tissue sodium concentration across the whole brain was estimated from sodium MRI maps acquired at 3 T and compared between 24 patients with Friedreich ataxia (21-57 years old, 13 females) and 23 controls (21-60 years old, 12 females). Tensor-based morphometry was used to assess volumetric changes. Total sodium concentrations and volumetric data in brainstem and cerebellum were correlated with clinical parameters, such as severity of ataxia, activity of daily living and disability stage, age, age at onset, and disease duration. Compared to controls, patients showed reduced brain volume in the right cerebellar lobules I-V (difference in means: -0.039% of total intracranial volume [TICV]; Cohen's d = 0.83), cerebellar white matter (WM) (-0.105%TICV; d = 1.16), and brainstem (-0.167%TICV; d = 1.22), including pons (-0.102%TICV; d = 1.00), medulla (-0.036%TICV; d = 1.72), and midbrain (-0.028%TICV; d = 1.05). Increased sodium concentration was additionally detected in the total cerebellum (difference in means: 2.865 mmol; d = 0.68), and in several subregions with highest effect sizes in left (5.284 mmol; d = 1.01) and right cerebellar lobules I-V (5.456 mmol; d = 1.00), followed by increases in the vermis (4.261 mmol; d = 0.72), and in left (2.988 mmol; d = 0.67) and right lobules VI-VII (2.816 mmol; d = 0.68). In addition, sodium increases were also detected in all brainstem areas (3.807 mmol; d = 0.71 to 5.42 mmol; d = 1.19). After controlling for age, elevated total sodium concentrations in right cerebellar lobules IV were associated with younger age at onset (r = -0.43) and accordingly with longer disease duration in patients (r = 0.43). Our findings support the potential of in vivo sodium MRI to detect metabolic changes of increased total sodium concentration in the cerebellum and brainstem, the key regions in Friedreich ataxia. In addition to structural changes, sodium changes were present in cerebellar hemispheres and vermis without concomitant significant atrophy. Given the association with age at disease onset or disease duration, metabolic changes should be further investigated longitudinally and in larger cohorts of early disease stages to determine the usefulness of sodium MRI as a biomarker for early neuropathological changes in Friedreich ataxia and efficacy measure for future clinical trials.

Effects of Cations on HPTS Fluorescence and Quantification of Free Gadolinium Ions in Solution; Assessment of Intracellular Release of Gd3+ from Gd-Based MRI Contrast Agents

8-Hydroxypyrene-1,3,6-trisulfonate (HPTS) is a small, hydrophilic fluorescent molecule. Since the pKa of the hydroxyl group is close to neutrality and quickly responds to pH changes, it is widely used as a pH-reporter in cell biology for measurements of intracellular pH. HPTS fluorescence (both excitation and emission spectra) at variable pH was measured in pure water in the presence of NaCl solution or in the presence of different buffers (PBS or hepes in the presence or not of NaCl) and in a solution containing BSA. pKa values have been obtained from the sigmoidal curves. Herein, we investigated the effect of mono-, di-, and trivalent cations (Na+, Ca2+, La3+, Gd3+) on fluorescence changes and proposed its use for the quantification of trivalent cations (e.g., gadolinium ions) present in solution as acqua-ions. Starting from the linear regression, the LoD value of 6.32 µM for the Gd3+ detection was calculated. The effects on the emission were also analyzed in the presence of a combination of Gd3+ at two different concentrations and the previously indicated mono and di-valent ions. The study demonstrated the feasibility of a qualitative method to investigate the intracellular Gd3+ release upon the administration of Gd-based contrast agents in murine macrophages.

Does Excess Tissue Sodium Storage Regulate Blood Pressure?

Purpose of Review

The regulation of blood pressure is conventionally conceptualised into the product of “circulating blood volume” and “vasoconstriction components”. Over the last few years, however, demonstration of tissue sodium storage challenged this dichotomous view.

Recent Findings

We review the available evidence pertaining to this phenomenon and the early association made with blood pressure; we discuss open questions regarding its originally proposed hypertonic nature, recently challenged by the suggestion of a systemic, isotonic, water paralleled accumulation that mirrors absolute or relative extracellular volume expansion; we present the established and speculate on the putative implications of this extravascular sodium excess, in either volume-associated or -independent form, on blood pressure regulation; finally, we highlight the prevalence of high tissue sodium in cardiovascular, metabolic and inflammatory conditions other than hypertension.

Summary

We conclude on approaches to reduce sodium excess and on the potential of emerging imaging technologies in hypertension and other conditions.

The Role of Molecular Imaging as a Marker of Remyelination and Repair in Multiple Sclerosis

The appearance of new disease-modifying therapies in multiple sclerosis (MS) has revolutionized our ability to fight inflammatory relapses and has immensely improved patients’ quality of life. Although remarkable, this achievement has not carried over into reducing long-term disability. In MS, clinical disability progression can continue relentlessly irrespective of acute inflammation. This “silent” disease progression is the main contributor to long-term clinical disability in MS and results from chronic inflammation, neurodegeneration, and repair failure. Investigating silent disease progression and its underlying mechanisms is a challenge. Standard MRI excels in depicting acute inflammation but lacks the pathophysiological lens required for a more targeted exploration of molecular-based processes. Novel modalities that utilize nuclear magnetic resonance’s ability to display in vivo information on imaging look to bridge this gap. Displaying the CNS through a molecular prism is becoming an undeniable reality. This review will focus on “molecular imaging biomarkers” of disease progression, modalities that can harmoniously depict anatomy and pathophysiology, making them attractive candidates to become the first valid biomarkers of neuroprotection and remyelination.

Compressed Sensing in Sodium Magnetic Resonance Imaging: Techniques, Applications, and Future Prospects

Sodium (²³ Na) yields the second strongest nuclear magnetic resonance (NMR) signal in biological tissues and plays a vital role in cell physiology. Sodium magnetic resonance imaging (MRI) can provide insights into cell integrity and tissue viability relative to pathologies without significant anatomical alternations, and thus it is considered to be a potential surrogate biomarker that provides complementary information for standard hydrogen (¹ H) MRI in a noninvasive and quantitative manner. However, sodium MRI suffers from a relatively low signal-to-noise ratio and long acquisition times due to its relatively low NMR sensitivity. Compressed sensing-based (CS-based) methods have been shown to accelerate sodium imaging and/or improve sodium image quality significantly. In this manuscript, the basic concepts of CS and how CS might be applied to improve sodium MRI are described, and the historical milestones of CS-based sodium MRI are briefly presented. Representative advanced techniques and evaluation methods are discussed in detail, followed by an expose of clinical applications in multiple anatomical regions and diseases as well as thoughts and suggestions on potential future research prospects of CS in sodium MRI. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

Development of an Add-on 23Na-MRI Radiofrequency Platform for a 1H-MRI System Using a Crossband Repeater: Proof-of-concept

²³Na-MRI provides information on Na⁺ content, and its application in the medical field has been highly anticipated. However, for existing clinical ¹H-MRI systems, its implementation requires an additional broadband RF transmitter, dedicated transceivers, and RF coils for Na⁺ imaging. However, a standard medical MRI system cannot often be modified to perform ²³Na imaging. We have developed an add-on crossband RF repeater system that enables ²³Na-MRI simply by inserting it into the magnet bore of an existing ¹H MRI. The three axis gradient fields controlled by the ¹H-MRI system were directly used for ²³Na imaging without any deformation. A crossband repeater is a common technique used for amateur radio. This concept was proven by a saline solution phantom and in vivo mouse experiments. This add-on RF platform is applicable to medical ¹H MRI systems and can enhance the application of ²³Na-MRI in clinical usage.

What can 7T sodium MRI tell us about cellular energy depletion and neurotransmission in Alzheimer's disease?

The pathophysiological processes underlying the development and progression of Alzheimer's disease (AD) on the neuronal level are still unclear. Previous research has hinted at metabolic energy deficits and altered sodium homeostasis with impaired neuronal function as a potential metabolic marker relevant for neurotransmission in AD. Using sodium (²³ Na) magnetic resonance (MR) imaging on an ultra-high-field 7 Tesla MR scanner, we found increased cerebral tissue sodium concentration (TSC) in 17 biomarker-defined AD patients compared to 22 age-matched control subjects in vivo. TSC was highly discriminative between controls and early AD stages and was predictive for cognitive state, and associated with regional tau load assessed with flortaucipir-positron emission tomography as a possible mediator of TSC-associated neurodegeneration. TSC could therefore serve as a non-invasive, stage-dependent, metabolic imaging marker. Setting a focus on cellular metabolism and potentially disturbed interneuronal communication due to energy-dependent altered cell homeostasis could hamper progressive cognitive decline by targeting these processes in future interventions.

New Prospects for Ultra-High-Field Magnetic Resonance Imaging in Multiple Sclerosis

ABSTRACT

There is growing interest in imaging multiple sclerosis (MS) through the ultra-high-field (UHF) lens, which currently means a static magnetic field strength of 7 T or higher. Because of higher signal-to-noise ratio and enhanced susceptibility effects, UHF magnetic resonance imaging improves conspicuity of MS pathological hallmarks, among them cortical demyelination and the central vein sign. This could, in turn, improve confidence in MS diagnosis and might also facilitate therapeutic monitoring of MS patients. Furthermore, UHF imaging offers unique insight into iron-related pathology, leptomeningeal inflammation, and spinal cord pathologies in neuroinflammation. Yet, limitations such as the longer scanning times to achieve improved resolution and incipient safety data on implanted medical devices need to be considered. In this review, we discuss applications of UHF imaging in MS, its advantages and limitations, and practical aspects of UHF in the clinical setting.

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

Magnetic resonance imaging determination of tissue sodium in patients with chronic kidney disease

Excess sodium is a major modifiable contributor to hypertension and cardiovascular risk. Knowledge of sodium storage and metabolism has derived mainly from indirect measurements of dietary sodium intake and urinary sodium excretion, however both attempt to measure body sodium and fluid in a two-compartment model of intracellular and extracellular spaces. Our understanding of total body sodium has recently included a storage pool in tissues. In the last two decades, sodium-23 magnetic resonance imaging (²³ Na MRI) has allowed dynamic quantification of tissue sodium in vivo. Tissue sodium is independently associated with cardiovascular dysfunction and inflammation. This review explores (i) The revolution of our understanding of sodium physiology, (ii) The development and potential clinical adoption of ²³ Na MRI to provide improved measurement of total body sodium in CKD and (iii) How we can better understand mechanistic and clinical implications of tissue sodium in hypertension, cardiovascular disease and immune dysregulation, especially in the CKD population.

Sodium Intensity Changes Differ Between Relaxation- and Density-Weighted MRI in Multiple Sclerosis

Introduction: The source of Tissue Sodium Concentration (TSC) increase in Multiple Sclerosis (MS) remains unclear, and could be attributed to altered intracellular sodium concentration or tissue microstructure. This paper investigates sodium in MS using three new MRI sequences. Methods: Three sodium scans were acquired at 4.7 T from 30 patients (11 relapsing-remitting, 10 secondary-progressive, 9 primary-progressive) and 9 healthy controls including: Density-Weighted (NaDW), with very short 30° excitation for more accurate TSC measurement; Projection Acquisition with Coherent MAgNetization (NaPACMAN), designed for enhanced relaxation-based contrast; and Soft Inversion Recovery FLuid Attenuation (NaSIRFLA), developed to reduce fluid space contribution. Signal was measured in both lesions (n = 397) and normal appearing white matter (NAWM) relative to controls in the splenium of corpus callosum and the anterior and posterior limbs of internal capsule. Correlations with clinical and cognitive evaluations were tested over all MS patients. Results: Sodium intensity in MS lesions was elevated over control WM by a greater amount for NaPACMAN (75%) than NaDW (35%), the latter representing TSC. In contrast, NaSIRFLA exhibited lower intensity, but only for region specific analysis in the SCC (-7%). Sodium intensity in average MS NAWM was not significantly different than control WM for either of the three scans. NaSIRFLA in the average NAWM and specifically the posterior limb of internal capsules positively correlated with the Paced Auditory Serial Addition Test (PASAT). Discussion: Lower NaSIRFLA signal in lesions and ~2× greater NaPACMAN signal elevation over control WM than NaDW can be explained with a demyelination model that also includes edema. A NAWM demyelination model that includes tissue atrophy suggests no signal change for NaSIRFLA, and only slightly greater NAWM signal than control WM for both NaDW and NaPACMAN, reflecting experimental results. Models were derived from previous total and myelin water fraction study in MS with T2-relaxometry, and for the first time include sodium within the myelin water space. Reduced auditory processing association with lower signal on NaSIRFLA cannot be explained by greater demyelination and its modeled impact on the three sodium MRI sequences. Alternative explanations include intra- or extracellular sodium concentration change. Relaxation-weighted sodium MRI in combination with sodium-density MRI may help elucidate microstructural and metabolic changes in MS.

Longitudinal Sodium MRI of Multiple Sclerosis Lesions: Is there Added Value of Sodium Inversion Recovery MRI

BACKGROUND

Sodium enhancement has been demonstrated in multiple sclerosis (MS) lesions.

PURPOSE

To investigate sodium MRI with and without an inversion recovery pulse in acute MS lesions in an MS relapse and during recovery.

STUDY TYPE

Prospective.

SUBJECTS

Twenty-nine relapsing-remitting MS patients with an acute relapse were included.

FIELD STRENGTH/SEQUENCE

A 3D density-adapted radial sodium sequence at 3 T using a dual-tuned (²³ Na/¹ H) head coil.

ASSESSMENT

Full-brain images of the tissue sodium concentration (TSC1, n = 29) and a sodium inversion recovery sequence (SIR1, n = 20) at the beginning of the anti-inflammatory therapy and on medium-term follow-up visits (days 27-99, n = 12 [TSC], n = 5 [SIR]) were measured. Regions of interest (RoIs) with contrast enhancement (T₁ CE+) and without change in T1-weighted imaging (FL + T1n) were normalized (nTSC and nSIR). To gain insight on the origin of the TSC enhancement at time point 1, it is investigated whether the nTSC enhancement of the lesions is accompanied by a change of the respective nSIR. Potential prognostic value of nSIR1 is examined referring to the nTSC progression. STATISTICAL TESTS: nTSC and nSIR were compared regarding the type of lesion and the time point using a one-way ANOVA. Pearson's correlation coefficient was calculated for nTSC over nSIR and for nTSC1-nTSC2 over nSIR1. A P-value <0.05 was considered statistically significant.

RESULTS

At the first measurement, all lesion types showed increased nTSC, while nSIR was decreased in the FL + T₁ n and the T₁ CE+ lesions in comparison to the normal-appearing white matter. For acute lesions, the difference between nTSC at baseline and nTSC at time point 2 showed a significant correlation with the baseline nSIR.

DATA CONCLUSION

At time point 1, nTSC is increased, while nSIR is unchanged or decreased in the lesions. The mean sodium IR signal at baseline correlates with recovery or progression of an acute lesion.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 4.

Modeling of sustained spontaneous network oscillations of a sexually dimorphic brainstem nucleus: the role of potassium equilibrium potential

Intrinsic oscillators in the central nervous system play a preeminent role in the neural control of rhythmic behaviors, yet little is known about how the ionic milieu regulates their output patterns. A powerful system to address this question is the pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus. A neural network comprised of an average of 87 pacemaker cells and 20 relay cells produces tonic oscillations, with higher frequencies in males compared to females. Previous empirical studies have suggested that this sexual dimorphism develops and is maintained through modulation of buffering of extracellular K⁺ by a massive meshwork of astrocytes enveloping the pacemaker and relay cells. Here, we constructed a model of this neural network that can generate sustained spontaneous oscillations. Sensitivity analysis revealed the potassium equilibrium potential, E_K (as a proxy of extracellular K⁺ concentration), and corresponding somatic channel conductances as critical determinants of oscillation frequency and amplitude. In models of both the pacemaker nucleus network and isolated pacemaker and relay cells, the frequency increased almost linearly with E_K, whereas the amplitude decreased nonlinearly with increasing E_K. Our simulations predict that this frequency increase is largely caused by a shift in the minimum K⁺ conductance over one oscillation period. This minimum is close to zero at more negative E_K, converging to the corresponding maximum at less negative E_K. This brings the resting membrane potential closer to the threshold potential at which voltage-gated Na⁺ channels become active, increasing the excitability, and thus the frequency, of pacemaker and relay cells.

Brain microstructural and metabolic alterations detected in vivo at onset of the first demyelinating event

In early multiple sclerosis, a clearer understanding of normal-brain tissue microstructural and metabolic abnormalities will provide valuable insights into its pathophysiology. We used multi-parametric quantitative MRI to detect alterations in brain tissues of patients with their first demyelinating episode. We acquired neurite orientation dispersion and density imaging [to investigate morphology of neurites (dendrites and axons)] and ²³Na MRI (to estimate total sodium concentration, a reflection of underlying changes in metabolic function). In this cross-sectional study, we enrolled 42 patients diagnosed with clinically isolated syndrome or multiple sclerosis within 3 months of their first demyelinating event and 16 healthy controls. Physical and cognitive scales were assessed. At 3 T, we acquired brain and spinal cord structural scans, and neurite orientation dispersion and density imaging. Thirty-two patients and 13 healthy controls also underwent brain ²³Na MRI. We measured neurite density and orientation dispersion indices and total sodium concentration in brain normal-appearing white matter, white matter lesions, and grey matter. We used linear regression models (adjusting for brain parenchymal fraction and lesion load) and Spearman correlation tests (significance level P ≤ 0.01). Patients showed higher orientation dispersion index in normal-appearing white matter, including the corpus callosum, where they also showed lower neurite density index and higher total sodium concentration, compared with healthy controls. In grey matter, compared with healthy controls, patients demonstrated: lower orientation dispersion index in frontal, parietal and temporal cortices; lower neurite density index in parietal, temporal and occipital cortices; and higher total sodium concentration in limbic and frontal cortices. Brain volumes did not differ between patients and controls. In patients, higher orientation dispersion index in corpus callosum was associated with worse performance on timed walk test (P = 0.009, B = 0.01, 99% confidence interval = 0.0001 to 0.02), independent of brain and lesion volumes. Higher total sodium concentration in left frontal middle gyrus was associated with higher disability on Expanded Disability Status Scale (r_s = 0.5, P = 0.005). Increased axonal dispersion was found in normal-appearing white matter, particularly corpus callosum, where there was also axonal degeneration and total sodium accumulation. The association between increased axonal dispersion in the corpus callosum and worse walking performance implies that morphological and metabolic alterations in this structure could mechanistically contribute to disability in multiple sclerosis. As brain volumes were neither altered nor related to disability in patients, our findings suggest that these two advanced MRI techniques are more sensitive at detecting clinically relevant pathology in early multiple sclerosis.

Global decrease in brain sodium concentration after mild traumatic brain injury

The pathological cascade of tissue damage in mild traumatic brain injury is set forth by a perturbation in ionic homeostasis. However, whether this class of injury can be detected in vivo and serve as a surrogate marker of clinical outcome is unknown. We employ sodium MRI to test the hypotheses that regional and global total sodium concentrations: (i) are higher in patients than in controls and (ii) correlate with clinical presentation and neuropsychological function. Given the novelty of sodium imaging in traumatic brain injury, effect sizes from (i), and correlation types and strength from (ii), were compared to those obtained using standard diffusion imaging metrics. Twenty-seven patients (20 female, age 35.9 ± 12.2 years) within 2 months after injury and 19 controls were scanned with proton and sodium MRI at 3 Tesla. Total sodium concentration, fractional anisotropy and apparent diffusion coefficient were obtained with voxel averaging across 12 grey and white matter regions. Linear regression was used to obtain global grey and white matter total sodium concentrations. Patient outcome was assessed with global functioning, symptom profiles and neuropsychological function assessments. In the regional analysis, there were no statistically significant differences between patients and controls in apparent diffusion coefficient, while differences in sodium concentration and fractional anisotropy were found only in single regions. However, for each of the 12 regions, sodium concentration effect sizes were uni-directional, due to lower mean sodium concentration in patients compared to controls. Consequently, linear regression analysis found statistically significant lower global grey and white matter sodium concentrations in patients compared to controls. The strongest correlation with outcome was between global grey matter sodium concentration and the composite z-score from the neuropsychological testing. In conclusion, both sodium concentration and diffusion showed poor utility in differentiating patients from controls, and weak correlations with clinical presentation, when using a region-based approach. In contrast, sodium linear regression, capitalizing on partial volume correction and high sensitivity to global changes, revealed high effect sizes and associations with patient outcome. This suggests that well-recognized sodium imbalances in traumatic brain injury are (i) detectable non-invasively; (ii) non-focal; (iii) occur even when the antecedent injury is clinically mild. Finally, in contrast to our principle hypothesis, patients' sodium concentrations were lower than controls, indicating that the biological effect of traumatic brain injury on the sodium homeostasis may differ from that in other neurological disorders. Note: This figure has been annotated.

Tissue Sodium Concentration within White Matter Correlates with the Extent of Small Vessel Disease

INTRODUCTION

Sodium MRI (23Na MRI) derived biomarkers such as tissue sodium concentration (TSC) provide valuable information on cell function and brain tissue viability and has become a reliable tool for the assessment of brain tumors and ischemic stroke beyond pathoanatomical morphology. Patients with major stroke often suffer from different degrees of underlying white matter lesions (WMLs) attributed to chronic small vessel disease. This study aimed to evaluate the WM TSC in patients with an acute ischemic stroke and to correlate the TSC with the extent of small vessel disease. Furthermore, the reliability of relative TSC (rTSC) compared to absolute TSC in these patients was analyzed.

METHODOLOGY

We prospectively examined 62 patients with acute ischemic stroke (73 ± 13 years) between November 2016 and August 2019 from which 18 patients were excluded and thus 44 patients were evaluated. A 3D 23Na MRI was acquired in addition to a T2-TIRM and a diffusion-weighted image. Coregistration and segmentation were performed with SPM 12 based on the T2-TIRM image. The extension of WM T2 hyperintense lesions in each patient was classified using the Fazekas scale of WMLs. The absolute TSC in the WM region was correlated to the Fazekas grades. The stroke region was manually segmented on the coregistered absolute diffusion coefficient image and absolute, and rTSC was calculated in the stroke region and compared to nonischemic WM region. Statistical significance was evaluated using the Student t-test.

RESULTS

For patients with Fazekas grade I (n = 25, age: 68.5 ± 15.1 years), mean TSC in WM was 55.57 ± 7.43 mM, and it was not statistically significant different from patients with Fazekas grade II (n = 7, age: 77.9 ± 6.4 years) with a mean TSC in WM of 53.9 ± 6.4 mM, p = 0.58. For patients with Fazekas grade III (n = 9, age: 81.4 ± 7.9 years), mean TSC in WM was 68.7 ± 10.5 mM, which is statistically significantly higher than the TSC in patients with Fazekas grade I and II (p < 0.001 and p = 0.05, respectively). There was a positive correlation between the TSC in WM and the Fazekas grade with r = 0.48 p < 0.001. The rTSC in the stroke region was statistically significant difference between low (0 and I) and high (2 and 3) Fazekas grades (p = 0.0353) whereas there was no statistically significant difference in absolute TSC in the stroke region between low (0 and I) and high (2 and 3) Fazekas grades.

CONCLUSION

The significant difference in absolute TSC in WM in patients with severe small vessel disease; Fazekas grade 3 can lead to inaccuracies using rTSC quantification for evaluation of acute ischemic stroke using 23 Na MRI. The study, therefore, emphasizes the importance of absolute tissue sodium quantification.

Neuroimaging Correlates of Cognitive Dysfunction in Adults with Multiple Sclerosis

Cognitive impairment is a frequent and meaningful symptom in multiple sclerosis (MS), caused by the accrual of brain structural damage only partially counteracted by effective functional reorganization. As both these aspects can be successfully investigated through the application of advanced neuroimaging, here, we offer an up-to-date overview of the latest findings on structural, functional and metabolic correlates of cognitive impairment in adults with MS, focusing on the mechanisms sustaining damage accrual and on the identification of useful imaging markers of cognitive decline.

Voxel-wise partial volume correction method for accurate estimation of tissue sodium concentration in 23 Na-MRI at 7 T

Sodium is crucial for the maintenance of cell physiology, and its regulation of the sodium-potassium pump has implications for various neurological conditions. The distribution of sodium concentrations in tissue can be quantitatively evaluated by means of sodium MRI (²³ Na-MRI). Despite its usefulness in diagnosing particular disease conditions, tissue sodium concentration (TSC) estimated from ²³ Na-MRI can be strongly biased by partial volume effects (PVEs) that are induced by broad point spread functions (PSFs) as well as tissue fraction effects. In this work, we aimed to propose a robust voxel-wise partial volume correction (PVC) method for ²³ Na-MRI. The method is based on a linear regression (LR) approach to correct for tissue fraction effects, but it utilizes a 3D kernel combined with a modified least trimmed square (3D-mLTS) method in order to minimize regression-induced inherent smoothing effects. We acquired ²³ Na-MRI data with conventional Cartesian sampling at 7 T, and spill-over effects due to the PSF were considered prior to correcting for tissue fraction effects using 3D-mLTS. In the simulation, we found that the TSCs of gray matter (GM) and white matter (WM) were underestimated by 20% and 11% respectively without correcting tissue fraction effects, but the differences between ground truth and PVE-corrected data after the PVC using the 3D-mLTS method were only approximately 0.6% and 0.4% for GM and WM, respectively. The capability of the 3D-mLTS method was further demonstrated with in vivo ²³ Na-MRI data, showing significantly lower regression errors (ie root mean squared error) as compared with conventional LR methods (p < 0.001). The results of simulation and in vivo experiments revealed that 3D-mLTS is superior for determining under- or overestimated TSCs while preserving anatomical details. This suggests that the 3D-mLTS method is well suited for the accurate determination of TSC, especially in small focal lesions associated with pathological conditions.

Cerebral Vasoreactivity as an Indirect MRI Marker of White Matter Tracts Alterations in Multiple Sclerosis

Patients with multiple sclerosis (MS) show a diffuse cerebral perfusion decrease, presumably related to multiple metabolism and vascular alterations. It is assumed that white matter fiber alterations cause a localized cerebral vasoreactivity (CVR) disruption through astrocytes metabolism alteration, leading to hypoperfusion. We proposed to (1) evaluate the CVR disruptions in MS, (2) in relation to white matter lesions and (3) compare CVR disruptions maps with standard imaging biomarkers. Thirty-five MS patients (10 progressive, 25 relapsing-remitting) and 22 controls underwent MRI with hypercapnic challenge, DTI imaging and neuropsychological assessment. Areas with disrupted CVR were assessed using a general linear model. Resulting maps were associated with clinical scores, compared between groups, and related to DTI metrics and white matter lesions. MS patients showed stronger disrupted CVR within supratentorial white matter, linking the left anterior insula to both the precentral gyrus and the right middle and superior frontal gyrus through the corpus callosum (P < 0.05, FWE corrected). Patient's verbal intellectual quotient was negatively associated with a pathway linking both hippocampi to the ispilateral prefrontal cortex (P < 0.05, FWE corrected). Disrupted CVR maps unrelated to DTI metrics and white matter lesions. We have demonstrated for the first time that white matter alterations can be indirectly identified through surrounding vessel alterations, and are related to clinical signs of MS. This offers a new, likely independent marker to monitor MS and supports a mediator role of the astrocytes in the fibers/vessels relationship.

Ultra-high-field 7-T MRI in multiple sclerosis and other demyelinating diseases: from pathology to clinical practice

Magnetic resonance imaging (MRI) is essential for the early diagnosis of multiple sclerosis (MS), for investigating the disease pathophysiology, and for discriminating MS from other neurological diseases. Ultra-high-field strength (7-T) MRI provides a new tool for studying MS and other demyelinating diseases both in research and in clinical settings. We present an overview of 7-T MRI application in MS focusing on increased sensitivity and specificity for lesion detection and characterisation in the brain and spinal cord, central vein sign identification, and leptomeningeal enhancement detection. We also discuss the role of 7-T MRI in improving our understanding of MS pathophysiology with the aid of metabolic imaging. In addition, we present 7-T MRI applications in other demyelinating diseases. 7-T MRI allows better detection of the anatomical, pathological, and functional features of MS, thus improving our understanding of MS pathology in vivo. 7-T MRI also represents a potential tool for earlier and more accurate diagnosis.

Sodium in the Relapsing-Remitting Multiple Sclerosis Spinal Cord: Increased Concentrations and Associations With Microstructural Tissue Anisotropy

BACKGROUND

Associations between brain total sodium concentration, disability, and disease progression have recently been reported in multiple sclerosis. However, such measures in spinal cord have not been reported.

PURPOSE

To measure total sodium concentration (TSC) alterations in the cervical spinal cord of people with relapsing-remitting multiple sclerosis (RRMS) and a control cohort using sodium MR spectroscopy (MRS).

STUDY TYPE

Retrospective cohort.

SUBJECTS

Nineteen people with RRMS and 21 healthy controls.

FIELD STRENGTH/SEQUENCE

3 T sodium MRS, diffusion tensor imaging, and 3D gradient echo.

ASSESSMENT

Quantification of total sodium concentration in the cervical cord using a reference phantom. Measures of spinal cord cross-sectional area, fractional anisotropy, mean diffusivity, radial diffusivity, and axial diffusivity from ¹ H MRI. Clinical assessments of 9-Hole Peg Test, 25-Foot Timed walk test, Paced Auditory Serial Addition Test with 3-second intervals, grip strength, vibration sensitivity, and posturography were performed on the RRMS cohort as well as reporting lesions in the C2/3 area.

STATISTICAL TESTS

Multiple linear regression models were run between sodium and clinical scores, cross-sectional area, and diffusion metrics to establish any correlations.

RESULTS

A significant increase in spinal cord total sodium concentration was found in people with RRMS relative to healthy controls (57.6 ± 18 mmol and 38.0 ± 8.6 mmol, respectively, P < 0.001). Increased TSC correlated with reduced fractional anisotropy (P = 0.034) and clinically with decreased mediolateral stability assessed with posturography (P = 0.045).

DATA CONCLUSION

Total sodium concentration in the cervical spinal cord is elevated in RRMS. This alteration is associated with reduced fractional anisotropy, which may be due to changes in tissue microstructure and, hence, in the integrity of spinal cord tissue.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 2.

Microstructural correlates of 23Na relaxation in human brain at 7 Tesla

²³Na provides the second strongest MR-observable signal in biological tissue and exhibits bi-exponential T₂^∗ relaxation in micro-environments such as the brain. There is significant interest in developing ²³Na biomarkers for neurological diseases that are associated with sodium channel dysfunction such as multiple sclerosis and epilepsy. We have previously reported methods for acquisition of multi-echo sodium MRI and continuous distribution modelling of sodium relaxation properties as surrogate markers of brain microstructure. This study aimed to compare ²³Na T₂^∗ relaxation times to more established measures of tissue microstructure derived from advanced diffusion MRI at 7 ​T. Six healthy volunteers were scanned using a 3D multi-echo radial ultra-short TE sequence using a dual-tuned ¹H/²³Na birdcage coil, and a high-resolution multi-shell, high angular resolution diffusion imaging sequence using a 32-channel ¹H receive coil. ²³Na T₂^∗ relaxation parameters [mean T₂^∗ (T₂^∗_mean) and fast relaxation fraction (T₂^∗_ff)] were calculated from a voxel-wise continuous gamma distribution signal model. White matter (restricted anisotropic diffusion) and grey matter (restricted isotropic diffusion) density were calculated from multi-shell multi-tissue constrained spherical deconvolution. Sodium parameters were compared with white and grey matter diffusion properties. Sodium T₂^∗_mean and T₂^∗_ff showed little variation across a range of white matter axonal fibre and grey matter densities. We conclude that sodium T₂^∗ relaxation parameters are not greatly influenced by relative differences in intra- and extracellular partial volumes. We suggest that care be taken when interpreting sodium relaxation changes in terms of tissue microstructure in healthy tissue.

The role of glial-neuronal metabolic cooperation in modulating progression of multiple sclerosis and neuropathic pain

While the etiology of multiple sclerosis (MS) remains unclear, research from the clinic and preclinical models identified the essential role of inflammation and demyelination in the pathogenesis of MS. Current treatments focused on anti-inflammatory processes are effective against acute episodes and relapsing-remitting MS, but patients still move on to develop secondary progressive MS. MS progression is associated with activation of microglia and astrocytes, and importantly, metabolic dysfunction leading to neuronal death. Neuronal death also contributes to chronic neuropathic pain. Metabolic support of neurons by glia may play central roles in preventing progression of MS and chronic neuropathic pain. Here, we review mechanisms of metabolic cooperation between glia and neurons and outline future perspectives exploring metabolic support of neurons by glia.

Detecting neurodegenerative pathology in multiple sclerosis before irreversible brain tissue loss sets in

Background

Multiple sclerosis (MS) is a complex chronic inflammatory and degenerative disorder of the central nervous system. Accelerated brain volume loss, or also termed atrophy, is currently emerging as a popular imaging marker of neurodegeneration in affected patients, but, unfortunately, can only be reliably interpreted at the time when irreversible tissue damage likely has already occurred. Timing of treatment decisions based on brain atrophy may therefore be viewed as suboptimal.

Main body

This Narrative Review focuses on alternative techniques with the potential of detecting neurodegenerative events in the brain of subjects with MS prior to the atrophic stage. First, metabolic and molecular imaging provide the opportunity to identify early subcellular changes associated with energy dysfunction, which is an assumed core mechanism of axonal degeneration in MS. Second, cerebral hypoperfusion has been observed throughout the entire clinical spectrum of the disorder but it remains an open question whether this serves as an alternative marker of reduced metabolic activity, or exists as an independent contributing process, mediated by endothelin-1 hyperexpression. Third, both metabolic and perfusion alterations may lead to repercussions at the level of network performance and structural connectivity, respectively assessable by functional and diffusion tensor imaging. Fourth and finally, elevated body fluid levels of neurofilaments are gaining interest as a biochemical mirror of axonal damage in a wide range of neurological conditions, with early rises in patients with MS appearing to be predictive of future brain atrophy.

Conclusions

Recent findings from the fields of advanced neuroradiology and neurochemistry provide the promising prospect of demonstrating degenerative brain pathology in patients with MS before atrophy has installed. Although the overall level of evidence on the presented topic is still preliminary, this Review may pave the way for further longitudinal and multimodal studies exploring the relationships between the abovementioned measures, possibly leading to novel insights in early disease mechanisms and therapeutic intervention strategies.

CEST MRI with distribution-based analysis for assessment of early stage disease activity in a mouse model of multiple sclerosis: An initial study

Imaging biomarkers that can detect pathological changes at an early stage of multiple sclerosis (MS) may allow earlier therapeutic intervention with an improved outcome. Using a mouse model of MS, termed as experimental autoimmune encephalomyelitis (EAE), we performed chemical exchange saturation transfer (CEST) MRI at a very early stage before symptom onset (6 days post-induction) for assessment of changes in tissues that appear "normal" with conventional MRI. The collected CEST Z-spectra signals (S_sat /S₀ ) were analyzed using a histogram-guided method to determine the contributions from various offset frequencies. Histogram analysis showed that EAE mice exhibit a more heterogeneous distribution with lower peak heights in the hindbrain compared with naïve mice at saturation offsets of 1 and 2 ppm. At these two offsets, both the mean S_sat /S₀ and the mean MTR_asym values in the cerebellum and brain stem are significantly different between EAE and naïve mice (P < 0.05). Immunofluorescent staining validated the presence of neuroinflammation, with IBA1-positive cells detected throughout the hindbrain including the cerebellum and brain stem. Follow-up MRI at the symptom onset (score = 1.5-2.5, 13 days post-induction) confirmed gadolinium-enhanced periventricular lesions. CEST Z-spectra signals also changed by this time. The proposed three-level histogram-oriented analysis is simple to execute and robust for detecting subtle changes in Z-spectra signals, which does not require a priori knowledge of damage locations or contributing offset components. CEST MRI signals at 1 and 2 ppm were sensitive to the subtle pathological changes at an early stage in EAE mice, and have potential as novel imaging biomarkers complementary to functional and physiological MRI measures.

Cortical grey matter sodium accumulation is associated with disability and secondary progressive disease course in relapse-onset multiple sclerosis

OBJECTIVE

Sodium (²³Na)-MRI is an emerging imaging technique to investigate in vivo changes in tissue viability, reflecting neuroaxonal integrity and metabolism. Using an optimised ²³Na-MRI protocol with smaller voxel sizes and improved tissue contrast, we wanted to investigate whether brain total sodium concentration (TSC) is a biomarker for long-term disease outcomes in a cohort of patients with relapse-onset multiple sclerosis (MS), followed from disease onset.

METHODS

We performed a cross-sectional study in 96 patients followed up ~ 15 years after a clinically isolated syndrome (CIS) and 34 healthy controls. Disease course was classified as CIS, relapsing-remitting MS or secondary progressive MS (SPMS). We acquired ¹H-MRI and ²³Na-MRI and calculated the TSC in cortical grey matter (CGM), deep grey matter, normal-appearing white matter (WM) and WM lesions. Multivariable linear regression was used to identify independent associations of tissue-specific TSC with physical disability and cognition, with adjustment for tissue volumes.

RESULTS

TSC in all tissues was higher in patients with MS compared with healthy controls and patients who remained CIS, with differences driven by patients with SPMS. Higher CGM TSC was independently associated with Expanded Disability Status Scale (R²=0.26), timed 25-foot walk test (R²=0.23), 9-hole peg test (R²=0.23), Paced Auditory Serial Addition Test (R²=0.29), Symbol Digit Modalities Test (R²=0.31) and executive function (R²=0.36) test scores, independent of grey matter atrophy.

CONCLUSIONS

Sodium accumulation in CGM reflects underlying neuroaxonal metabolic abnormalities relevant to disease course heterogeneity and disability in relapse-onset MS. TSC and should be considered as an outcome measure in future neuroprotection trials.

Advances in brain imaging in multiple sclerosis

Brain imaging is increasingly used to support clinicians in diagnosing multiple sclerosis (MS) and monitoring its progression. However, the role of magnetic resonance imaging (MRI) in MS goes far beyond its clinical application. Indeed, advanced imaging techniques have helped to detect different components of MS pathogenesis in vivo, which is now considered a heterogeneous process characterized by widespread damage of the central nervous system, rather than multifocal demyelination of white matter. Recently, MRI biomarkers more sensitive to disease activity than clinical disability outcome measures, have been used to monitor response to anti-inflammatory agents in patients with relapsing-remitting MS. Similarly, MRI markers of neurodegeneration exhibit the potential as primary and secondary outcomes in clinical trials for progressive phenotypes. This review will summarize recent advances in brain neuroimaging in MS from the research setting to clinical applications.

Lifestyle-based modifiable risk factors in multiple sclerosis: review of experimental and clinical findings

Multiple sclerosis (MS) is a lifelong inflammatory and neurodegenerative disease influenced by multiple lifestyle-based factors. We provide a narrative review of the effects of modifiable risk factors that are identified as being associated with risk to develop MS and/or influencing the future clinical disease outcomes. The emerging data regarding the beneficial effects of diet modifications and exercise are further reviewed. In contrast, obesity and comorbid cardiovascular diseases are associated with increased MS susceptibility and worse disease progression. In addition, the potential influence of smoking, coffee and alcohol consumption on MS onset and disability development are discussed. Successful management of the modifiable risk factors may lead to better long-term outcomes and improve patients' quality of life. MS specialists should participate in educating and facilitating lifestyle-based modifications as part of their neurological consults.

Advances in spinal cord imaging in multiple sclerosis

The spinal cord is frequently affected in multiple sclerosis (MS), causing motor, sensory and autonomic dysfunction. A number of pathological abnormalities, including demyelination and neuroaxonal loss, occur in the MS spinal cord and are studied in vivo with magnetic resonance imaging (MRI). The aim of this review is to summarise and discuss recent advances in spinal cord MRI. Advances in conventional spinal cord MRI include improved identification of MS lesions, recommended spinal cord MRI protocols, enhanced recognition of MRI lesion characteristics that allow MS to be distinguished from other myelopathies, evidence for the role of spinal cord lesions in predicting prognosis and monitoring disease course, and novel post-processing methods to obtain lesion probability maps. The rate of spinal cord atrophy is greater than that of brain atrophy (-1.78% versus -0.5% per year), and reflects neuroaxonal loss in an eloquent site of the central nervous system, suggesting that it can become an important outcome measure in clinical trials, especially in progressive MS. Recent developments allow the calculation of spinal cord atrophy from brain volumetric scans and evaluation of its progression over time with registration-based techniques. Fully automated analysis methods, including segmentation of grey matter and intramedullary lesions, will facilitate the use of spinal cord atrophy in trial designs and observational studies. Advances in quantitative imaging techniques to evaluate neuroaxonal integrity, myelin content, metabolic changes, and functional connectivity, have provided new insights into the mechanisms of damage in MS. Future directions of research and the possible impact of 7T scanners on spinal cord imaging will be discussed.

MRI in multiple sclerosis: clinical and research update

PURPOSE OF REVIEW

Clinical MRI is of paramount importance for multiple sclerosis diagnosis but lacks the specificity to investigate the pathogenic mechanisms underlying disease onset and progression. The application of advanced MR sequences allows the characterization of diverse and complex pathological mechanisms, granting insights into multiple sclerosis natural history and response to treatment.

RECENT FINDINGS

This review provides an update on the most recent international guidelines for optimal standard imaging of multiple sclerosis and discusses advantages and limitations of advanced imaging approaches for investigating inflammation, demyelination and neurodegeneration. An overview is provided for methods devoted to imaging leptomeningeal enhancement, microglial activation, demyelination, neuronal metabolic damage and neuronal loss.

SUMMARY

The application of magnetic resonance (MR) guidelines to standard-of-care MR protocols, although still limited, would substantially contribute to the optimization of multiple sclerosis management. From an academic perspective, different mechanism-specific imaging techniques are available and offer a powerful tool to elucidate multiple sclerosis pathogenesis, monitor disease progression and guide therapeutic choices.

MRI in multiple sclerosis: what is changing?

PURPOSE OF REVIEW

To summarize recent findings from the application of MRI in the diagnostic work-up of patients with suspected multiple sclerosis (MS), and to review the insights into disease pathophysiology and the utility of MRI for monitoring treatment response.

RECENT FINDINGS

New evidence from the application of MRI in patients with clinically isolated syndromes has guided the 2017 revision of the McDonald criteria for MS diagnosis, which has simplified their clinical use while preserving accuracy. Other MRI measures (e.g., cortical lesions and central vein signs) may improve diagnostic specificity, but their assessment still needs to be standardized, and their reliability confirmed. Novel MRI techniques are providing fundamental insights into the pathological substrates of the disease and are helping to give a better understanding of its clinical manifestations. Combined clinical-MRI measures of disease activity and progression, together with the use of clinically relevant MRI measures (e.g., brain atrophy) might improve treatment monitoring, but these are still not ready for the clinical setting.

SUMMARY

Advances in MRI technology are improving the diagnostic work-up and monitoring of MS, even in the earliest phases of the disease, and are providing MRI measures that are more specific and sensitive to disease pathological substrates.

Potential of Sodium MRI as a Biomarker for Neurodegeneration and Neuroinflammation in Multiple Sclerosis

In multiple sclerosis (MS), experimental and ex vivo studies indicate that pathologic intra- and extracellular sodium accumulation may play a pivotal role in inflammatory as well as neurodegenerative processes. Yet, in vivo assessment of sodium in the microenvironment is hard to achieve. Here, sodium magnetic resonance imaging (²³NaMRI) with its non-invasive properties offers a unique opportunity to further elucidate the effects of sodium disequilibrium in MS pathology in vivo in addition to regular proton based MRI. However, unfavorable physical properties and low in vivo concentrations of sodium ions resulting in low signal-to-noise-ratio (SNR) as well as low spatial resolution resulting in partial volume effects limited the application of ²³NaMRI. With the recent advent of high-field MRI scanners and more sophisticated sodium MRI acquisition techniques enabling better resolution and higher SNR, ²³NaMRI revived. These studies revealed pathologic total sodium concentrations in MS brains now even allowing for the (partial) differentiation of intra- and extracellular sodium accumulation. Within this review we (1) demonstrate the physical basis and imaging techniques of ²³NaMRI and (2) analyze the present and future clinical application of ²³NaMRI focusing on the field of MS thus highlighting its potential as biomarker for neuroinflammation and -degeneration.

Much Ado about N…atrium: modelling tissue sodium as a highly sensitive marker of subclinical and localized oedema

Hypertonic Na⁺ accumulation in peripheral tissues is a recently described phenomenon: it has been associated with ageing, hypertension, diabetes, chronic kidney disease and heart failure, but its clinical meaning has yet to be determined. This concept conflicts with the classic physiological paradigm of constant balance between salt intake and excretion, and its water-independent nature is still a matter of debate.

We developed a theoretical model explaining changes in the chemical composition of tissues as a function of extracellular volume fraction and excess extracellular fluid, i.e. oedema. The model suggests that the proportional increase in absolute Na⁺ content and concentration due to different degrees of oedema is higher than the parallel increase in water content, thus making Na⁺ a more sensitive index to detect this oedema.

Our model would explain some of the recent findings of high tissue Na⁺ content in pathological conditions. More importantly, it prompts the reappraisal of tissue Na⁺ analysis from being a topic of niche interest to a potential diagnostic tool with broad applicability in the investigation of subclinical systemic and localized oedema.