A comparison of imaging sequences for sodium MR imaging on a 9.4T whole body machine

Introduction MR Imaging of sodium, and generally of X-nuclei, given their low concentration in tissues and their fast relaxation properties will greatly benefit from the increase of signal at Ultra High Fields (≥ 7T). Imaging of these nuclei typically requires the use of dedicated sequences and a high degree of optimisation. Several different approaches have been used in the past [1-7] with characteristic advantages and disadvantages. In this initial study, six sequences used for sodium imaging were compared on a 9.4T whole-body machine. Methods Six sequences for 23Na imaging were implemented on a Siemens (Erlangen, Germany) 9.4T whole-body scanner. The sequences were: Gradient Echo (GRE), 3D Cones (3DC), Twisted Projection Imaging (TPI), SPRITE, Cylindrical stack of radial spokes (CSRS), and 3D spokes (3DS). A novel, home-built RF coil comprising a birdcage transmitter and an 8-channel phased-array receiver was used during this study. A phantom with 6 compartments filled with gels at three sodium concentrations (30, 100, 150 mmol/l) and two agarose concentrations (2%, 6% w/w) was used in order to mimic the conditions found in the human brain. An automatic shimming procedure which made use of the sodium signal corrected the static field. The same set of shim values were used for all measurements. Sequence parameters are summarised in Table 1. In order to enable a fair compare, the sequences parameters were optimised such that the acquisition was under 10 minutes and at 95% of the imposed (first level) SAR limits. Results Figure 1 shows the images acquired with the different methods. All images have sufficient SNR in order to distinguish the compartments with the lowest sodium concentration. Different levels of T2* weighting are also visible. SNR values calculated for the compartment (outlined in yellow) of the lowest [23Na] (30 mmol/l) and highest [Agarose] (6% w/w) are summarised in Table 1. Discussion Each sequence showed advantages and disadvantages that can be summarised as following: 3DGRE was the easiest sequence to implement. Its shows good SNR, good image quality (PSF) but it suffers strongly from T2* weighting due to rather long minimum achievable TE (> 1 ms). 3D