• Sherif A Mohamed, Anne Adlung, Arne M Ruder, Michaela A U Hoesl, Lothar Schad, Christoph Groden, Frank A Giordano, and Eva Neumaier-Probst.
• Department of Neuroradiology, University Medical Center Mannheim, Heidelberg University, Mannheim, Germany.
• J Neuroimaging. 2021 Mar 1; 31 (2): 297-305.

Background And PurposeTo date, treatment response to stereotactic radiosurgery (SRS) in brain metastases (BM) can only be determined by MRI evaluation of contrast-enhancing lesions in a long-time follow-up. Sodium MRI has been a subject of immense interest in imaging research as the measure of tissue sodium concentration (TSC) can give valuable quantitative information on cell viability. We aimed to analyze the longitudinal changes of TSC in BM measured with 23 Na MRI before and after SRS for assessment of early local tumor effects.MethodsSeven patients with a total of 12 previously untreated BM underwent SRS with 22 Gy. In addition to a standard MRI protocol including dynamic susceptibility-weighted contrast-enhanced perfusion, a 23 Na MRI was performed at three time points: (I) 2 days before, (II) 5 days, and (III) 40 days after SRS. Nine BMs were evaluated. The absolute TSC in the BM, the respective peritumoral edemas, and the normal-appearing corresponding contralateral brain area were assessed and the relative TSC were correlated to the changes in BM longest axial diameters.ResultsTSC was elevated in nine BM at baseline before SRS with a mean of 73.4 ± 12.3 mM. A further increase in TSC was observed 5 days after SRS in all the nine BM with a mean of 86.9 ± 13 mM. Eight of nine BM showed a mean 60.6 ± 13.3% decrease in the longest axial diameter 40 days after SRS; at this time point, the TSC also had decreased to a mean 65.1 ± 7.9 mM. In contrast, one of the nine BM had a 13.4% increase in the largest axial diameter at time point III. The TSC of this BM showed a further TSC increase of 80.1 mM 40 days after SRS.ConclusionChanges in TSC using 23 Na MRI shows the possible capability to detect radiobiological changes in BM after SRS.© 2020 The Authors. Journal of Neuroimaging published by Wiley Periodicals LLC on behalf of American Society of Neuroimaging.

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