Exploring and enhancing relaxation-based sodium MRI contrast

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

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.

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 (23)Na T 2), as well as residual quadrupole interaction dependence.

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