Premium
Residual quadrupole interaction in brain and its effect on quantitative sodium imaging
Author(s) -
Stobbe Robert W.,
Beaulieu Christian
Publication year - 2016
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.3376
Subject(s) - flip angle , dephasing , quadrupole , chemistry , residual , imaging phantom , excitation , nuclear magnetic resonance , white matter , weighting , signal (programming language) , physics , magnetic resonance imaging , atomic physics , optics , condensed matter physics , mathematics , algorithm , medicine , quantum mechanics , computer science , acoustics , radiology , programming language
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 frequencyω ¯ 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 whenω ¯ Q ~ ω 1 . The second experiment shows that a prescribed flip‐angle decrease from 90° to 20°, with concomitant decrease in T E from 0.25 ms to 0.10 ms 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 suggestingω ¯ Q dephasing. Using simulation to explain the results of all three experiments, a model of brain tissue is hypothesized. It includes one pool (50%) withω ¯ Q = 0, and another (50%) in whichω ¯ Q has a Gaussian distribution with a standard deviation of 625 Hz. Given the result of the second experiment, it is suggested that the use of reduced flip‐angles with large ω 1 will provide more accurate measures of sodium concentration than ‘standard’ methods using 90°pulses. Alternatively, further study of sodiumω ¯ Q may provide a means to explore tissue structure and organization. Copyright © 2015 John Wiley & Sons, Ltd.