Electrophysiological stimulation of excised rat muscle elicits a measurable change in tissue sodium concentration using 23 Na-MRI Frank Riemer 1 , Joshua Kaggie 1 , Cormac O'Neill 2 , Mary McLean 3 , James Grist 1 , Myfanwy Hill 3 , Joe Guy 3 , Rolf Schulte 4 , Martin Graves 1,2 , James Fraser 5 , and Ferdia Gallagher 1 1 Department of Radiology, University of Cambridge, Cambridge, United Kingdom, 2 Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom, 3 Cancer Research UK Cambridge Institute, University of Cambridge, Cambridge, United Kingdom, 4 GE Healthcare, Munich, Germany, 5 Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, United Kingdom Synopsis Changes in the tissue sodium gradient play an important role in cell signalling such as at the neuromuscular junction and as part of neuronal action potentials. 23 Na-MRI has the ability to measure the macroscopic sodium distribution. In this study we investigated the changes in tissue sodium in an electrically stimulated and freshly excised rat leg muscle. Introduction Changes in the tissue sodium gradient play an important role in cell signalling such as at the neuromuscular junction and as part of neuronal action potentials. 23 Na-MRI has the ability to measure the macroscopic sodium distribution. In this study we investigated the changes in tissue sodium in an electrically stimulated and freshly excised rat leg muscle. Methods The rat (Lister, age: 14 weeks) was sacrificed according to local ethical rules. Rat hind leg muscles were obtained within 1 hour of imaging. Muscles were placed in a container filled to 1 mm depth with buffer solution 1 (fig.1). The container was placed on top of a custom 23 Na T/R coil of 15 cm diameter in a clinical 3T MRI (GE MR750, GE Healthcare, Waukesha, WI). A 3D-Cones sequence 2 was used for 23 Na-imaging (resolution = 3.8x3.8x3.8 mm 3 , FOV = 20 cm, TE/TR = 0.5/50 ms, flip angle = 90°, readout length = 30 ms, 197 readouts, duration of one scan = 10 seconds). A waveform generator was placed outside of the room, and the cable connected to an MRI pass panel to stimulate the muscle with prongs attached to the muscle for field excitation (20 ms pulses, 1 second between pulses). A trap circuit at 33 MHz (>30 dB attenuation) was placed on the cables to remove noise at the imaging frequency for sodium. Images were corrected for B 1 sensitivity using the double angle method (FA = 30° and 60°) 3 . 20 and 85 mM NaCl in 4% agar phantoms were placed in the FOV for quantification 4 . Imaging was performed in 1 minute blocks (6 volumes per block), 4 blocks in total (2 blocks of stimulation interleaved with 2 rest blocks). SPM12 (UCL, London, UK) was used to find voxels undergoing significant sodium signal changes in the FOV. No