Using ²³Na Magnetic Resonance Imaging to Measure Sodium in Muscle Tissues

All procedures involving human participants have been performed in compliance with the institutional, national, and international guidelines for human welfare and have been reviewed by the local institutional review board.

1. Preparation of the phantom holder

1. Four standard sodium solutions, including aqueous NaCl in the physiologic range of peripheral tissue sodium content 10-40 mmol/L, are embedded in a platform in the coil.

2. MRI examination

The quantification of Na⁺ content at the mid-calf area is measured using an MRI exam on a 3.0 T MR scanner (Philips Healthcare, Cleveland, OH, USA) and a single-tuned receive-only quadrature sodium coil (Rapid Biomedical GmbH, Rimpar, Germany).

1. Place the participant in a supine position on the MRI patient table.
2. For imaging of the calf at the mid gastrocnemius muscle, place the phantom holder inside the RF coil and rest the left lower leg on top of the phantom holder before closing the top half of the RF coil.
3. Use the three-dimensional gradient-echo sequence as follows.
4. Field of view = 192 × 192 mm², acquired matrix size = 64 × 64, and slice thickness = 30 mm. A repetition time (TR) and echo time (TE) of 130/0.99 milliseconds is preferred to reduce error from residual T1 signal effects.
5. Acquire fat and water images using the mDixon protocol, with matching field of view (FOV) and matrix size to the three-dimensional gradient-echo sequence.
6. The sodium image acquisition time is approximately 15 minutes.
7. The total multinuclear MRI exam of the lower extremity is approximately 30 minutes.

3. Image Analysis

1. Measure the mean signal intensity in the standard sodium solutions.
2. Calibrate the magnitude of sodium signal intensity to the standard sodium concentrations on a per-voxel basis.
3. Make a quantitative sodium map and match the matrix size of the fat/water mDixon images.
4. Segment each region of interest using the water image from mDixon, including the outer and inner borders of the skin, total muscle, bone, blood vessels, and sub-adipose tissue (SAT).
5. Segment the skin region further into the posterior semiperimeter of the skin.
6. Remove the voxels in the bone and blood vessels.
7. Quantified metrics include Skin sodium (mM/L), SAT sodium (mM/L), Muscle sodium (mM/L), Circumference (cm), SAT volume (mm²), Muscle volume (mm²), Fat/Water ratio, and Fat Signal Fraction.