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 3 7.5 3 7.5 mm 3 for the slow T 2 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 6 0.6 mmol/g wet weight and 27.6 6 1.2 mmol/g wet weight, respectively. Magn Reson Med 63:1583-1593, 2010. V C 2010 Wiley-Liss, Inc. Key words: sodium imaging; twisted projection imaging; quantitative imaging; tissue sodium concentration; ultra-short TE imagingRegulation of sodium homeostasis through counterbalancing low intracellular and high extracellular sodium ion concentrations with potassium ions is of vital importance for cellular function (1,2). These ion gradients across the cell membrane provide the potential energy for many important cellular transport processes. Action potentials, intracellular pH regulation, and many membrane transport processes are all directly dependent on the sodium ion gradient across the cell membrane. Damage to brain cell integrity and disruption of cell packing produce local increases in tissue sodium concentration (TSC) (3,4). TSC, determined by quantitative MRI, has been shown to have a potential role in monitoring tissue viability in humans with diseases such as stroke and in monitoring treatment of brain tumors (3-6).Despite these potential medical applications described more than two decades ago (7), quantitative sodium imaging has been slow to evolve. The sodium MR signal has a detection sensitivity of four orders of magnitude lower than that of the proton signal. It exhibits biexponential relaxation behavior with fast and slow transverse relaxation characteristics (T 2fast $1-3 ms and T 2slow $12-25 ms, respectively) in biologic tissues (8). Therefore, sodium imaging requires an imaging sequence with a short excitation radiofrequency (RF) pulse and a short echo time (TE) to reduce signal loss from the rapid decay of the transverse magnetization.Twisted projection imaging (TPI) is a three-dimensional (3D) projection reconstruction sequence-based approach that can achieve short TE values and high a...

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Purpose: To assess whether exposure to a 9.4T static magnetic field during sodium imaging at 105.92 MHz affects human vital signs and cognitive function. Materials and Methods:Measurements of human vital signs and cognitive ability made before and after exposure to a 9.4T MR scanner and a mock scanner with no magnetic field are compared using a protocol approved by the U.S. Food and Drug Administration (FDA).Results: Exposure to a 9.4T static magnetic field during sodium imaging did not result in a statistically significant change in the vital signs or cognitive ability of healthy normal volunteers. Conclusion:Vital sign and cognitive ability measurements made before and after sodium imaging at 9.4T suggest that performing human MRI at 105.92 MHz in a 9.4T static magnetic field does not pose a health risk.