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Quantitative sodium imaging with a flexible twisted projection pulse sequence

The quantification of sodium MR images from an arbitrary intensity scale into a bioscale fosters image interpretation in terms of the spatially resolved biochemical process of sodium ion homeostasis. A methodology for quantifying tissue sodium concentration using a flexible twisted projection imaging sequence is proposed that allows for optimization of tradeoffs between readout time, signal‐to‐noise ratio efficiency, and sensitivity to static field susceptibility artifacts. The gradient amplitude supported by the slew rate at each k‐space radius regularizes the readout gradient waveform design to avoid slew rate violation. Static field inhomogeneity artifacts are corrected using a frequency‐segmented conjugate phase reconstruction approach, with field maps obtained quickly from coregistered proton imaging. High‐quality quantitative sodium images have been achieved in phantom and volunteer studies with real isotropic spatial resolution of 7.5 × 7.5 × 7.5 mm3 for the slow T2 component in ∼8 min on a clinical 3‐T scanner. After correcting for coil sensitivity inhomogeneity and water fraction, the tissue sodium concentration in gray matter and white matter was measured to be 36.6 ± 0.6 μmol/g wet weight and 27.6 ± 1.2 μmol/g wet weight, respectively. Magn Reson Med 63:1583–1593, 2010. © 2010 Wiley‐Liss, Inc.