Repeatability and reproducibility of cerebral 23Na imaging in healthy subjects

Hardware and Software Setup for Quantitative 23Na Magnetic Resonance Imaging at 3T: A Phantom Study

Magnetic resonance (MR) with sodium (23Na) is a noninvasive tool providing quantitative biochemical information regarding physiology, cellular metabolism, and viability, with the potential to extend MR beyond anatomical proton imaging. However, when using clinical scanners, the low detectable 23Na signal and the low 23Na gyromagnetic ratio require the design of dedicated radiofrequency (RF) coils tuned to the 23Na Larmor frequency and sequences, as well as the development of dedicated phantoms for testing the image quality, and an MR scanner with multinuclear spectroscopy (MNS) capabilities. In this work, we propose a hardware and software setup for evaluating the potential of 23Na magnetic resonance imaging (MRI) with a clinical scanner. In particular, the reliability of the proposed setup and the reproducibility of the measurements were verified by multiple acquisitions from a 3T MR scanner using a homebuilt RF volume coil and a dedicated sequence for the imaging of a phantom specifically designed for evaluating the accuracy of the technique. The final goal of this study is to propose a setup for standardizing clinical and research 23Na MRI protocols.

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).

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, I2Total=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.

Quantitative Sodium (23Na) MRI in Pediatric Gliomas: Initial Experience

Background: 23Na MRI correlates with tumor proliferation, and studies in pediatric patients are lacking. The purpose of the study: (1) to compare total sodium concentration (TSC) between pediatric glioma and non-neoplastic brain tissue using 23Na MRI; (2) compare tissue conspicuity of bound sodium concentration (BSC) using 23Na MRI dual echo relative to TSC imaging. Methods: TSC was measured in: (1) non-neoplastic brain tissues and (2) three types of manually segmented gliomas (diffuse intrinsic brainstem glioma (DIPG), recurrent supratentorial low-grade glioma (LGG), and high-grade glioma (HGG)). In a subset of patients, serial changes in both TSC and BSC (dual echo 23Na MRI) were assessed. Results: Twenty-six pediatric patients with gliomas (median age of 12.0 years, range 4.9−23.3 years) were scanned with 23Na MRI. DIPG treated with RT demonstrated higher TSC values than the uninvolved infratentorial tissues (p < 0.001). Recurrent supratentorial LGG and HGG exhibited higher TSC values than the uninvolved white matter (WM) and gray matter (GM) (p < 0.002 for LGG, and p < 0.02 for HGG). The dual echo 23Na MRI suppressed the sodium signal within both CSF and necrotic foci. Conclusion: Quantitative 23Na MRI of pediatric gliomas demonstrates a range of values that are higher than non-neoplastic tissues. Dual echo 23Na MRI of BCS improves tissue conspicuity relative to TSC imaging.

Evaluation of Sodium (23Na) MR-imaging as a Biomarker and Predictor for Neurodegenerative Changes in Patients With Alzheimer's Disease

BACKGROUND/AIM

Sodium (²³Na) MR imaging is a noninvasive MRI technique that has been shown to be sensitive to visualize biochemical information about tissue viability, their cell integrity, and cell function in various studies. The aim of this study was to evaluate differences in regional brain ²³Na signal intensity between Alzheimer's disease (AD) and healthy controls to preliminarily evaluate the capability of ²³Na imaging as a biomarker for AD.

PATIENTS AND METHODS

A total of 14 patients diagnosed with AD were included: 12 in the state of dementia and 2 with mild cognitive impairment (MCI), and 12 healthy controls (HC); they were all scanned on a 3T clinical scanner with a double tuned ¹H/²³Na birdcage head coil. After normalizing the signal intensity with that of the vitreous humor, relative tissue sodium concentration (rTSC) was measured after automated segmentation in the hippocampus, amygdala, basal ganglia, white matter (WM) and grey matter (GM) in both cerebral hemispheres.

RESULTS

Patients with AD showed a significant increase in rTSC in comparison to healthy controls in the following brain regions: WM 13.6%; p=0.007, hippocampus 12.9%; p=0.003, amygdala 18.9%; p=0.0007.

CONCLUSION

²³Na-MRI has the potential to be developed as a useful biomarker for the diagnosis of AD.

High-altitude cerebral edema: its own entity or end-stage acute mountain sickness?

High-altitude cerebral edema (HACE) and acute mountain sickness (AMS) are neuropathologies associated with rapid exposure to hypoxia. However, speculation remains regarding the exact etiology of both HACE and AMS and whether they share a common mechanistic pathology. This review outlines the basic principles of HACE development, highlighting how edema could develop from 1) a progression from cytotoxic swelling to ionic edema or 2) permeation of the blood brain barrier (BBB) with or without ionic edema. Thereafter, discussion turns to the available neuroimaging literature in the context of cytotoxic, ionic, or vasogenic edema in both HACE and AMS. Although HACE is clearly caused by an increase in brain water of ionic and/or vasogenic origin, there is very little evidence that this type of edema is present when AMS develops. However, cerebral vasodilation, increased intracranial blood volume, and concomitant intracranial fluid shifts from the extracellular to the intracellular space, as interpreted from changes in diffusion indices within white matter, are observed consistently in persons acutely exposed to hypoxia and with AMS. Therefore, herein we explore the idea that intracellular swelling occurs alongside AMS, and is a critical precursor to extracellular ionic edema formation. We propose that this process produces a subtle modulation of the BBB, which either together with or independent of vasogenic edema provides a transvascular segue from the end-stage of AMS to HACE. Ultimately, this review seeks to shed light on the possible processes underlying HACE pathophysiology, and thus highlights potential avenues for future prevention and treatment.

Frontiers of Sodium MRI Revisited: From Cartilage to Brain Imaging

Sodium magnetic resonance imaging (²³ Na-MRI) is a highly promising imaging modality that offers the possibility to noninvasively quantify sodium content in the tissue, one of the most relevant parameters for biochemical investigations. Despite its great potential, due to the intrinsically low signal-to-noise ratio (SNR) of sodium imaging generated by low in vivo sodium concentrations, low gyromagnetic ratio, and substantially shorter relaxation times than for proton (¹ H) imaging, ²³ Na-MRI is extremely challenging. In this article, we aim to provide a comprehensive overview of the literature that has been published in the last 10-15 years and which has demonstrated different technical designs for a range of ²³ Na-MRI methods applicable for disease diagnoses and treatment efficacy evaluations. Currently, a wider use of 3.0T and 7.0T systems provide imaging with the expected increase in SNR and, consequently, an increased image resolution and a reduced scanning time. A great interest in translational research has enlarged the field of sodium MRI applications to almost all parts of the body: articular cartilage tendons, spine, heart, breast, muscle, kidney, and brain, etc., and several pathological conditions, such as tumors, neurological and degenerative diseases, and others. The quantitative parameter, tissue sodium concentration, which reflects changes in intracellular sodium concentration, extracellular sodium concentration, and intra-/extracellular volume fractions is becoming acknowledged as a reliable biomarker. Although the great potential of this technique is evident, there must be steady technical development for ²³ Na-MRI to become a standard imaging tool. The future role of sodium imaging is not to be considered as an alternative to ¹ H MRI, but to provide early, diagnostically valuable information about altered metabolism or tissue function associated with disease genesis and progression. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

Assessing the variability of 23 Na MRI in skeletal muscle tissue: Reproducibility and repeatability of tissue sodium concentration measurements in the lower leg at 3 T

The goal of this study was to evaluate the reproducibility and repeatability of tissue sodium concentration (TSC) measurements using ²³ Na MRI in skeletal muscle tissue. ²³ Na MRI was performed at 3 T on the right lower leg of eight healthy volunteers (aged 28 ± 4 years). The examinations were repeated at the same site after ~ 22 weeks to assess the variability over a medium-term period. Additionally, they were scanned at a second site shortly before or shortly after the first visit (within 3 weeks) to evaluate the inter-site reproducibility. Moreover, we analysed the effect of B₀ correction on the variability. Coefficients of variations (CVs) from mean TSC values as well as Bland-Altman plots were used to assess intra-site repeatability and inter-site reproducibility. In phantom measurements, the B₀ correction improved the quantitative accuracy. We observed differences of up to 4.9 mmol/L between the first and second visit and a difference of up to 3.7 mmol/L between the two different sites. The CV for the medium-term repeatability was 15% and the reproducibility CV was 9%. The Bland-Altman plots indicated high agreement between the visits in all muscle regions. The systematic bias of -0.68 mmol/L between site X and Y (P = 0.03) was slightly reduced to -0.64 mmol/L after B₀ correction (P = 0.04). This work shows that TSC measurements in healthy skeletal muscle tissue can be performed with good repeatability and reproducibility, which is of importance for future longitudinal or multicentre studies.

23 Na-MRI as a Noninvasive Biomarker for Cancer Diagnosis and Prognosis

The influx of sodium (Na⁺) ions into a resting cell is regulated by Na⁺ channels and by Na⁺/H⁺ and Na⁺/Ca²⁺ exchangers, whereas Na⁺ ion efflux is mediated by the activity of Na⁺/K⁺‐ATPase to maintain a high transmembrane Na⁺ ion gradient. Dysfunction of this system leads to changes in the intracellular sodium concentration that promotes cancer metastasis by mediating invasion and migration. In addition, the accumulation of extracellular Na⁺ ions in cancer due to inflammation contributes to tumor immunogenicity. Thus, alterations in the Na⁺ ion concentration may potentially be used as a biomarker for malignant tumor diagnosis and prognosis. However, current limitations in detection technology and a complex tumor microenvironment present significant challenges for the in vivo assessment of Na⁺ concentration in tumor. ²³Na‐magnetic resonance imaging (²³Na‐MRI) offers a unique opportunity to study the effects of Na⁺ ion concentration changes in cancer. Although challenged by a low signal‐to‐noise ratio, the development of ultrahigh magnetic field scanners and specialized sodium acquisition sequences has significantly advanced ²³Na‐MRI. ²³Na‐MRI provides biochemical information that reflects cell viability, structural integrity, and energy metabolism, and has been shown to reveal rapid treatment response at the molecular level before morphological changes occur. Here we review the basis of ²³Na‐MRI technology and discuss its potential as a direct noninvasive in vivo diagnostic and prognostic biomarker for cancer therapy, particularly in cancer immunotherapy. We propose that ²³Na‐MRI is a promising method with a wide range of applications in the tumor immuno‐microenvironment research field and in cancer immunotherapy monitoring.

Level of Evidence

2

Technical Efficacy

Stage 2