Non-invasive assessment of tumor proliferation using triple quantum filtered 23/Na MRI: technical challenges and solutions

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.

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

Quantitative sodium imaging and gliomas: a feasibility study

PURPOSE

Recent advances in sodium brain MRI have allowed for increased signal-to-noise ratio, faster imaging, and the ability of differentiating intracellular from extracellular sodium concentration, opening a new window of opportunity for clinical application. In gliomas, there are significant alterations in sodium metabolism, including increase in the total sodium concentration and extracellular volume fraction. The purpose of this study is to assess the feasibility of using sodium MRI quantitative measurements to evaluate gliomas.

METHODS

Eight patients with treatment-naïve gliomas were scanned at 3 T with a homemade ¹H/²³Na head coil, generating maps of pseudo-intracellular sodium concentration (C₁), pseudo-extracellular volume fraction (α₂), apparent intracellular sodium concentration (aISC), and apparent total sodium concentration (aTSC). Measurements were made within the contralateral normal-appearing putamen, contralateral normal-appearing white matter (NAWM), and solid tumor regions (area of T2-FLAIR abnormality, excluding highly likely areas of edema, cysts, or necrosis). Paired samples t test were performed comparing NAWM and putamen and between NAWM and solid tumor.

RESULTS

The normal-appearing putamen demonstrated significantly higher values for aTSC, aISC, C₁ (p < 0.001), and α₂ (p = 0.002) when compared to those of NAWM. The mean average of all solid tumors, when compared to that of NAWM, demonstrated significantly higher values of aTSC and α₂ (p < 0.001), and significantly lower values of aISC (p = 0.02) for each patient. There was no significant difference between the values of C₁ (p = 0.19).

CONCLUSION

Quantitative sodium measurements can be done in glioma patients and also has provided further evidence that total sodium and extracellular volume fraction are increased in gliomas.

Neuroimaging of Peptide-based Vaccine Therapy in Pediatric Brain Tumors: Initial Experience

The potential benefits of peptide-based immunotherapy for pediatric brain tumors are under investigation. Treatment-related heterogeneity has resulted in radiographic challenges, including pseudoprogression. Conventional MR imaging has limitations in assessment of different forms of treatment-related heterogeneity, particularly regarding distinguishing true tumor progression from efficacious treatment responses. Advanced neuroimaging techniques, including diffusion magnetic resonance (MR), perfusion MR, and MR spectroscopy, may add value in the assessment of treatment-related heterogeneity. Observations suggest that recent delineation of specific response criteria for immunotherapy of adult brain tumors is likely relevant to the pediatric population and further validation in multicenter pediatric brain tumor peptide-based vaccine studies is warranted.

In vivo magnetic resonance imaging of sodium and diffusion in rat glioma at 21.1 T

Sodium and diffusion magnetic resonance imaging (MRI) in intracranial rat 9L gliomas were evaluated over 6-8 days using the advanced sensitivity of sodium MRI at 21.1 T. Glioma doubling time was 2.4-2.6 days. Glioma sodium signal was detected using the ultra-short echo time of 0.15 ms. The high resolution 3D sodium MRI with pixels of 0.125 μL allowed for minimizing a partial volume effect often relevant to the MRI of low intensity signals. Tumor sodium and diffusion MRI were evaluated for two separate subclones of 9L cells with different resistance to 1,3-bis(2-chloroethyl)-1-nitrosurea detected by pre-surgery assays. In vivo, after implantation, resistant 9L cells created tumors with significantly reduced sodium concentrations (57 ± 3 mM) compared with nonresistant 9L cells (78 ± 3 mM). The corresponding differences in diffusion were less, but also statistically significant. During tumor progression, an increase of glioma sodium concentration was observed in both cell types with a rate of 2.4-5.8 %/day relative to normal brain. Tumor diffusion was not significantly changed at this time, indicative of no alterations in glioma cellularity. Thus, changes in sodium during tumor progression reflect increasing intracellular sodium concentration and mounting metabolic stress. These experiments also demonstrate an enhanced sensitivity of sodium MRI to reflect tumor cell resistance.