Exploring and enhancing relaxation-based sodium MRI contrast

Object

Sodium MRI is typically concerned with measuring tissue sodium concentration. This requires the minimization of relaxation weighting. However, ²³Na relaxation may itself be interesting to explore, given an underlying mechanism (i.e. the electric-quadrupole-moment–electric-field-gradient interaction) that differs from ¹H. A new sodium sequence was developed to enhance ²³Na relaxation contrast without decreasing signal-to-noise ratio.

Materials and Methods

The new sequence, labeled Projection Acquisition in the steady-state with Coherent MAgNetization (PACMAN), uses gradient refocusing of transverse magnetization following readout, a short repetition time, and a long radiofrequency excitation pulse. It was developed using simulation, verified in model environments (saline and agar), and evaluated in the brain of three healthy adult volunteers.

Results

Projection Acquisition in the steady-state with Coherent MAgNetization generates a large positive contrast-to-noise ratio (CNR) between saline and agar, matching simulation-based design. In addition to enhanced CNR between cerebral spinal fluid and brain tissue in vivo, PACMAN develops substantial contrast between gray and white matter. Further simulation shows that PACMAN has a ln(T _2f/T ₁) contrast dependence (where T _2f is the fast component of ²³Na T ₂), as well as residual quadrupole interaction dependence.

Conclusion

The relaxation dependence of PACMAN sodium MRI may provide contrast related to macromolecular tissue structure.