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...

Sodium ion homeostasis is a fundamental property of viable tissue, allowing the tissue sodium concentration to be modeled as the tissue cell volume fraction. The modern neuropathology literature using ex vivo tissue from selected brain regions indicates that human brain cell density remains constant during normal aging and attributes the volume loss that occurs with advancing age to changes in neuronal size and dendritic arborization. Quantitative sodium MRI performed with the enhanced sensitivity of ultrahigh‐field 9.4 T has been used to investigate tissue cell volume fraction during normal aging. This cross‐sectional study (n = 49; 21–80 years) finds that the in vivo tissue cell volume fraction remains constant in all regions of the brain with advancing age in individuals who remain cognitively normal, extending the ex vivo literature reporting constant neuronal cell density across the normal adult age range. Cell volume fraction, as measured by quantitative sodium MRI, is decreased in diseases of cell loss, such as stroke, on a time scale of minutes to hours, and in response to treatment of brain tumors on a time scale of days to weeks. Neurodegenerative diseases often have prodromal periods of decades in which regional neuronal cell loss occurs prior to clinical presentation. If tissue cell volume fraction can detect such early pathology, this quantitative parameter may permit the objective measurement of preclinical disease progression. This current study in cognitively normal aging individuals provides the basis for the pursuance of investigations directed towards such neurodegenerative diseases. Copyright © 2015 John Wiley & Sons, Ltd.

Purpose: To demonstrate the feasibility of performing 39-potassium MR imaging of a human brain. Methods: 39-Potassium magnetic resonance imaging of a human brain was performed at 9.4 T using a flexible twisted projection imaging acquisition with a nominal isotropic spatial resolution of 10 mm in 40 min using a single-tuned birdcage radiofrequency coil. Co-registered sodium imaging with a nominal isotropic spatial resolution of 3.5 mm was performed on the same subject in 10 min. Results: The 39-potassium flexible twisted projection imaging imaging had a signal-to-noise ratio of 5.2 in brain paranchyma. This qualitative imaging showed the expected features when compared to co-registered high-and low-resolution sodium imaging of the same subject. Conclusion: Potassium MR images may offer complementary information to that of sodium MR images by sampling the intracellular rather that interstitial environment. Quantification will require additional improvement in signal-to-noise ratio to produce clinically useful bioscales as are developing for sodium MR imaging.