Myelin water mapping by spatially regularized longitudinal relaxographic imaging at high magnetic fields.

PURPOSE: Magnetic resonance T1 -weighted images are routinely used for human brain segmentation, brain parcellation, and clinical diagnosis of demyelinating diseases. Myelin is thought to influence the longitudinal relaxation commonly described by a mono-exponential recovery, although reports of bi-exponential longitudinal relaxation have been published. The purpose of this work was to investigate if a myelin water T1 contribution could be separated in geometrically sampled Look-Locker trains of low flip angle gradient echoes. METHODS: T1 relaxograms from normal human brain were computed by a spatially regularized inverse Laplace transform after estimating the apparent inversion efficiency. RESULTS: With sufficiently long inversion-time sampling (ca. 5 × T1 of cerebrospinal fluid), the T1 relaxogram revealed a short-T1 peak (106-225 ms). The apparent fraction of this water component increased in human brain white matter from 8.3% at 3 T, to 11.3% at 4 T and 15.0% at 7 T. The T2 * of the short-T1 peak at 3 T was shorter, 27.9 ± 13.0 ms, than that of the long-T1 peak, 51.3 ± 5.6 ms. CONCLUSION: The short-T1 fraction is interpreted as the water resident in myelin. Its detection is facilitated by longer T1 of axoplasmic water at higher magnetic field.

Center-out echo-planar spectroscopic imaging with correction of gradient-echo phase and time shifts.

A procedure to prevent the formation of image and spectral Nyquist ghosts in echo-planar spectroscopic imaging is introduced. It is based on a novel Cartesian center-out echo-planar spectroscopic imaging trajectory, referred to as EPSICO, and combined with a correction of the gradient-echo phase and time shifts. Processing of homogenous sets of forward and reflected echoes is no longer necessary, resulting in an optimized spectral width. The proposed center-out trajectory passively prevents the formation of Nyquist ghosts by privileging the acquisition of the center k-space line with forward echoes at the beginning of an echo-planar spectroscopic imaging dwell time and by ensuring that all k-space lines and their respective complex conjugates are acquired at equal time intervals. With the proposed procedure, concentrations of N-acetyl aspartate, creatine, choline, glutamate, and myo-inositol were reliably determined in human white matter at 3 T.

Transition dipole moments between the low-lying Ω(g,u)(+∕-) states of the Rb2 and Cs2 molecules.

For the Rb(2) and Cs(2) molecules, the adiabatic potential-energy curves and the transition dipole moments of the 43 Ω((+∕-)) (g,u) low-lying states dissociating adiabatically to the limits up to ns+(n-1)d (n = 5,6 for Rb(2) and Cs(2), respectively), have been computed as a function of the internuclear distance R for a large and dense grid. Each molecule was treated as a two-electron system. We used an ab initio approach involving a relativistic non-empirical pseudo-potential for Rb and Cs cores, core-valence polarization potentials, and full valence configuration interaction calculations for the two valence electrons. Spin-orbit effects were taken into account through semi-empirical spin-orbit pseudopotentials. Equilibrium distances, transition energies, rotational constants, and harmonic frequencies as well as depths of wells and heights of barriers are reported for all the molecular states investigated in Hund's cases (a) and (c). Extensive tables of energy values and transition dipole moments are given in an auxiliary (EPAPS) files as a database for future studies on Rb(2) and Cs(2).

Rapid, accurate and simple model to predict NMR chemical shifts for biological molecules.

We present a new model to predict chemical shifts for biological molecules. It is simple, fast, and involves a limited number of parameters. It is particularly adapted to be used in molecular dynamics studies with a molecular mechanic potential. We test the model for polyamines, which are rather small molecules, as well as for proteins for which a lot of NMR chemical shifts are available. The tests show that our simple and fast model is competitive, by its accuracy, with sophisticated models specifically developed for proteins. It was also seen to be successful for polyamines.