Residual quadrupole interaction in brain and its effect on quantitative sodium imaging

Sodium MRI is particularly interesting given the key role that sodium ions play in cellular metabolism. To measure concentration, images must be free from contrast unrelated to sodium density. However, spin 3/2 NMR is complicated by more than rapid biexponential signal decay. Residual quadrupole interactions (described by frequency Q) can reduce M-xy development during RF excitation. Three experiments, each performed on the same four healthy volunteers, demonstrate that residual quadrupole interactions are of concern in quantitative sodium imaging of the brain. The first experiment shows a reliable increase in the rate of excitation flipping' (1%-6%), particularly in white matter with tracts running superior-inferior (i.e. parallel to B-0). Increased flip-rates imply an associated signal loss and are to be expected when Q(-1). The second experiment shows that a prescribed flip-angle decrease from 90 degrees to 20 degrees, with concomitant decrease in T-E from 0.25ms to 0.10ms and no T-1 weighting, results in a 14%-26% saline calibration phantom normalized signal (S-N) increase in the white matter regions. The third experiment shows that this (S-N) increase is primarily due to a residual quadrupole effect, with a small contribution from T-2 weighting. There is an observed deviation from the spin 3/2 biexponential curve, also suggesting Q dephasing. Using simulation to explain the results of all three experiments, a model of brain tissue is hypothesized. It includes one pool (50%) with Q = 0, and another (50%) in which Q has a Gaussian distribution with a standard deviation of 625Hz. Given the result of the second experiment, it is suggested that the use of reduced flip-angles with large (1) will provide more accurate measures of sodium concentration than standard' methods using 90 degrees pulses. Alternatively, further study of sodium Qmay provide a means to explore tissue structure and organization. Copyright (c) 2015 John Wiley & Sons, Ltd.