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

Abstract

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 ωQ) can reduce Mxy 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 B0). Increased flip-rates imply an associated signal loss and are to be expected when ωQ ~ ω1. The second experiment shows that a prescribed flip-angle decrease from 90° to 20°, with concomitant decrease in TE from 0.25 ms to 0.10 ms and no T1 weighting, results in a 14%-26% saline calibration phantom normalized signal (SN) increase in the white matter regions. The third experiment shows that this (SN) increase is primarily due to a residual quadrupole effect, with a small contribution from T2 weighting. There is an observed deviation from the spin 3/2 biexponential curve, also suggesting ωQ dephasing. Using simulation to explain the results of all three experiments, a model of brain tissue is hypothesized. It includes one pool (50%) with ωQ = 0, and another (50%) in which ωQ 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 ω1 will provide more accurate measures of sodium concentration than 'standard' methods using 90° pulses. Alternatively, further study of sodium ωQmay provide a means to explore tissue structure and organization.

Double quantum filtered 23Na MRI with magic angle excitation of human skeletal muscle in the presence of B0 and B1 inhomogeneities

“…Also, the assumption of sufficiently strong time‐averaged quadrupolar interaction might not be fulfilled in human muscle tissue. To further investigate DQF‐MA signal intensity dependences, a numerical simulation—for example as presented by Stobbe and Beaulieu—might be used in addition to the analytical signal equations.…”

“…In contrast, agarose gel represents an isotropic environment. The quadrupolar interactions between sodium ions and the surrounding molecules are time‐averaged to zero , as shown by Stobbe and Beaulieu . Therefore, no even rank DQCs can be generated in agarose gel.…”

X‐nuclei imaging: Current state, technical challenges, and future directions

“…Furthermore, the ratio of 23 Na DQ to SQ signal in muscle was proposed to be a measure for Na + retention . The presence of magic angle DQ signal in the human brain was shown to influence TSC quantification …”

Radiofrequency excitation–related 23Na MRI signal loss in skeletal muscle, cartilage, and skin

“…Another approach is to reconsider the flip angle used for excitation. We have previously demonstrated in the context of brain analysis that smaller flip angles yield less spin‐3/2 weighting than commonly used 90° flip angles . This is shown with simulation in Figure for cartilage and skin.…”

“…In 1987, Joseph and Summers suggested that this flip‐angle effect provides a mechanism to detect residual quadrupole splitting and its associated signal loss, although no flip‐angle effect was detected in samples of porcine skeletal muscle or feline brain . However, more recently a flip‐angle effect was detected in vivo in human brain white matter, particularly in the posterior limb of the internal capsule . Although residual quadrupole splitting is known to exist in cartilage, quantification of this effect is limited to the study of excised bovine nasal cartilage.…”