Initial reports of 23Na-magnetic resonance imaging (MRI) date back to the 1970s¹. However, methodological challenges of the technique - low in-vivo sodium concentrations and low available field strengths - hampered its widespread adoption for many years. Recent technical developments such as clinical 3 Tesla MRI, improved coils, and faster gradients have overcome some of these limitations and have enhanced clinical scenarios for 23Na-MRI. Several recent publications have demonstrated the feasibility of 23Na-MRI for in-vivo imaging, including cerebral (stroke, brain tumors, multiple sclerosis)², abdominal (kidney)³, musculoskeletal (knee, spine)⁴ and heart imaging⁵. Still, sodium imaging has not found its way into clinical routine. This may be due high costs, availability but also technical obstacles. A lot of studies use 23Na coils additionally to 1H-coils for imaging. This usually increases acquisition times and decreases patient comfort. So far, no study has addressed the question whether morphologic/functional sequences from a double-tuned sodium coil can be considered as diagnostic and thus a coil switch to a 1H coil can be considered as dispensable. The purpose of this study was therefore to prospectively compare image quality of a double-tuned (1H/23Na) vs. a clinically used 1H head coil in healthy subjects.

In this IRB-approved, intra-individual comparison study 10 healthy volunteers (6f, 4m; mean age 25 years) underwent two consecutive cerebral MRI exams with both a double-tuned (1H/23Na) birdcage radiofrequency head coil (Rapid Biomedical, Rimpar, Germany) vs. a clinically used 12-channel 1H coil (Siemens Healthcare, Erlangen, Germany) at 3.0T (TimTrio, Siemens Healthcare, Erlangen, Germany). The following MR sequences were included: localizer, axial T2 TSE, T1 flash, T2 TIRM, T1 MP-Rage and a DW-EPI. No contrast media was applied. Subjective image quality was evaluated in consensus by two radiologists on a 5-point-Likert scale (1= poor image quality; 5= excellent image quality). Objective image quality was evaluated by circular region of interest (ROI) placement in the white matter in all sequences, respectively. JMP 10.0 (SAS Institute Inc., Cary, North Carolina, USA) was used for statistical analysis. Descriptive statistics (median, mean, 95% confidence interval and standard deviation (SD)) were used for all data. Signal-to-noise ratio (SNR) was compared using a two-sided Mann-Whitney-U-Test.Acquisition time was 20 minutes for the 12-channel 1H coil vs. 30 minutes (including the 23Na scan) with the double tuned (1H/23Na) birdcage head coil. Overall image data was rated as diagnostic for both coils. Subjective image quality was excellent for both coils (1H: 5; 25-75% quartile 5-5 vs. 1H/23Na: 5; 25-75% quartile 4-5) (Fig. 1). SNR did not significantly differ between images acquired by the (1H/23Na) head coil and the clinically used head coil (all sequences p>0.05). 23Na-MRI can offer additional information regarding tissue function and viability compared to 1H-MRI. Nonetheless, there are several obstacles, which have impeded its clinical implementation, including hardware costs and technical challenges. One step towards a more widespread clinical use of 23Na-MRI would be the use of double-tuned (1H/23Na) coils. This would complement 1H-MRI without moving the patients, which would enable co-registration of images and also save time, e.g. in stroke patients where acquisition time can be critical. A prerequisite would be that morphologic and functional images acquired by a double-tuned coil can confidently be rated as diagnostic, which we could show in our study. Image data of a double-tuned (1H/23Na) coil vs. a clinically used 1H coil show comparable, diagnostic image quality. The sole use of a double-tuned coil could reduce imaging time significantly and increase patient comfort. This may narrow the gap between pre-clinical/human research studies and clinical implementation of sodium MRI.