Determination of the antisymmetric part of the chemical shift anisotropy tensor via spin relaxation in nuclear magnetic resonance

Relaxation processes induced by the antisymmetric part of the chemical shift anisotropy tensor (henceforth called anti-CSA) are usually neglected in NMR relaxation studies. It is shown here that anti-CSA components contribute to longitudinal relaxation rates of the indole N-15 nucleus in tryptophan in solution at different magnetic fields and temperatures. To determine the parameters of several models for rotational diffusion and internal dynamics, we measured the longitudinal relaxation rates R-1=1/T-1 of N-15, the N-15-H-1 dipole-dipole (DD) cross-relaxation rates (Overhauser effects), and the cross-correlated CSA/DD relaxation rates involving the second-rank symmetric part of the CSA tensor of N-15 at four magnetic fields B-0=9.4, 14.1, 18.8, and 22.3 T (400, 600, 800, and 950 MHz for protons) over a temperature range of 270

Method for nmr spectrometry or mri measurement using dissolution dynamic nuclear polarization (dnp) with scavenging of free radicals

Geoffrey Bodenhausen, Sami Jannin, Pascal Miéville

PROBLEM TO BE SOLVED: To provide a method for extending longitudinal relaxation time and transverse relaxation time in nuclear magnetic resonance (NMR), while removing free radicals from hyperpolarized solution. SOLUTION: A method for preparing samples for magnetic resonance measurement by using dissolution dynamic nuclear polarization-based hyperpolarization, includes the step (1.1) to prepare frozen beads of a first kind containing paramagnetic substances in addition to a solute under examination; the step (2.1) to insert the frozen beads into a polarization magnet; the step (3.1) to create a higher polarization state of the nuclei in a magnetic field; the step (4.1) to heat the sample up to room temperature, the step (5.1) to transfer samples into MR magnet; and the step (6.1) to perform MR measurement. Further, additional steps are included, the step (1.2) to prepare frozen beads of a second kind containing a reduction agent, and the step (2.2) to insert the frozen beads of a second kind together with the frozen beads of the first kind into the polarization magnet. ;COPYRIGHT: (C)2011,JPO&INPIT

2011

Measurement of cross relaxation between two selected nuclei by synchronous nutation of magnetization in nuclear magnetic resonance

Geoffrey Bodenhausen, Robert Konrat

A novel technique is described that allows one to measure cross-relaxation rates (Overhauser effects) between 2 selected nuclei in high-resoln. NMR. The 2 chosen sites are irradiated simultaneously with the sidebands of an amplitude-modulated radiofrequency field, so that their magnetization vectors are forced to undergo a simultaneous motion, which is referred to as synchronous nutation. From the time-dependence obsd. for different initial conditions, one may derive cross-relaxation rates, and hence det. internuclear distances. The scalar interactions between the selected spins and other spins belonging to the same coupling network are effectively decoupled. Also, cross relaxation to other spins in the environment does not affect the transient response of the selected spins, which are therefore in effect isolated from their environment in terms of dipolar interactions. The method is particularly suitable to study cases where normal Overhauser effects are perturbed by spin-diffusion effects due to the presence of further spins. The technique is applied to the protein bovine pancreatic trypsin inhibitor. [on SciFinder (R)]

1993

Engineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla

Jérémie Daniel Clément

Magnetic resonance imaging is widely used in medical diagnosis to obtain anatomical details of the human body in a non-invasive way. Clinical MR scanners typically operate at a static magnetic field strength (B0) of 1.5T or 3T. However, going to higher field is of great interest since the signal-to-noise ratio is proportional to B0. Therefore, higher image resolution and better contrast between the human tissues could be achieved. Nevertheless, new challenges arise when increasing B0. The wavelength associated with the radio-frequency field B1+ has smaller dimensions - approx. 12 cm for human brain tissues - than the human brain itself (20 cm in length), the organ of interest in this thesis. The main consequence is that the transmit field distribution pattern (B1+) is altered and the final MR images present bright and dark signal spots. These effects prevent the ultra-high field MR scanners (>= 7T) to be used for routine clinical diagnosis. Parallel-transmit is one approach to address these new challenges. Instead of using an RF coil connected to a single power input as it is commonly done at lower magnetic fields, multiple RF coils are used with independent power inputs. The subsequent distinct RF signals can be manipulated separately, which provides an additional degree of freedom to generate homogeneous B1+-field distributions over large or specific regions in the human body. A transmit/receive RF coil array optimized for whole-brain MR imaging was developed and is described in this thesis. Dipoles antennas were used since they could provide a large longitudinal (vertical axis-head to neck) coverage and high transmit field efficiency. Results demonstrated a complete coverage of the human brain, and particularly high homogeneity over the cerebellum. However, since the receive sensitivity over large field-of-views is related to the number of channels available to detect the NMR signal, the next work was to add a 32-channel receive loop coil array to the transmit coil array. The complete coverage of the human brain was assessed with a substantial increase in signal-to-noise compared to the transmit/receive dipole coil array alone. Moreover, acquisition time was shortened since higher acceleration factors could be used. To optimize the individual RF fields and generate an homogeneous B1+-field, a method was developed making use of the particle-swarm algorithm. A user-friendly graphical interface was implemented. Good homogeneity could be achieved over the whole-brain after optimization with the coil array built in this study. Moreover, the optimization was shown to be robust across multiple subjects. The last project was focused on the single transmit system. Local volume coils (single transmit) present pronounced transmit field inhomogeneities in specific regions of the human brain such as the temporal lobes. A widely used approach to address locally these challenges is to add dielectric pads inside the volume coils to enhance the local transmit field efficiency. It was shown in this thesis that constructing dedicated surface coils is a valuable alternative to the dielectric pads in terms of transmit field efficiency and MR spectroscopy results. Two RF coil setups were developed for the temporal and frontal lobes of the human brain, respectively. This thesis provides extensive insight on MR engineering of RF coils at ultra-high field and the potential of parallel-transmit to address the future needs in clinical applications.

EPFL2019

Engineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla

Jérémie Daniel Clément

EPFL2019