Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

Parallel radiofrequency transmission has garnered much attention for its wide range of benefits in magnetic resonance imaging (MRI), including reduced power deposition and radiofrequency excitation with improved spatial uniformity. However, few pTx systems are commercially available and most are expensive. This manuscript introduces another alternative parallel transmit architecture at 3 T based on field-programmable gate array technology, and explores the utility of a low cost, integrated circuit approach to signal modulation that is easily scaled to high channel counts. The technical and engineering specifications of a complete 4-channel signal modulation module are presented in detail, including radiofrequency characterization and MRI results. The experimental results are additionally compared to a commercially available 4-channel modulation system. The findings indicate that the proposed device is easy to use, provides fine control of phase and amplitude on existing MRI systems, and can be fabricated for approximately 30 USD per channel. Initial estimates suggest that the complete 4-channel modulation system (including the required software licenses and multi-function reconfigurable input/output devices) can be implemented for approximately 10 000 USD.

High signal-to-noise ratio (SNR) of the NMR signal has always been a key target that drives massive research effort in many fields. Among several parameters, a high filling factor of the MR coil has proven to boost the SNR. In case of small-volume samples, a high filling factor and thus a high SNR can be achieved through miniaturizing the MR coil. However, under certain circumstances, this can be impractical. In this paper, we present an extensive theoretical and experimental investigation of the inductively coupled LC resonator and the magnetic Lenz lens as two candidate approaches that can enhance the SNR in such circumstances. The results demonstrate that the narrow-band LC resonator is superior in terms of SNR, while the non-tuned nature of the Lenz lens makes it preferable in broadband applications.

An accurate coil design is a fundamental task to maximize signal-to-noise ratio in magnetic resonance applications. Coil design techniques take advantage of computer simulations especially when coil size is comparable to the radiofrequency (RF) wavelength. In particular, the estimation of the losses within the conductors as well as the radiative losses, both as a function of frequency, is instrumental to a complete coil performance characterization. However, the cross-sectional shape of the conductors strongly affects the radiofrequency coil's performance, especially at those frequencies where conductor losses represent the dominant power dissipation mechanism. Indeed, at radiofrequencies, the current flowing in the conductor is distributed in the proximity of its surface instead of being uniformly distributed over the cross section; it follows that an accurate conductor losses estimation can be performed only in the case of wire conductors by using analytical formulations. For strip conductors, although different theoretical approaches have been proposed in literature by taking into account the losses, no closed-form expression for conductors resistance is available which takes into account both classical and lateral skin effects. In this work, finite element method (FEM) simulations have been performed for estimating conductor and radiative losses in planar surface loops made of strips and circular wires; the results have been compared against analytical formulations and literature data. Workbench tests performed on two circular coil prototypes, the first one constituted by a strip and the second one by circular wire conductors, tuned at 63.9 MHz and 127.8 MHz, showed a good agreement with FEM simulations.

A continuous wave, homodyne, low frequency electron paramagnetic resonance spectrometer is described which can accommodate 15 cm diameter objects. The spectrometer can utilize small volume and surface coil probes operating between 100 and 500 MHz. The magnetic field can be scanned between 0 and 35 mT and is thus suitable for g < 2 spins and wide absorption lines. The spectrometer can record conventional field swept, field cycled, and spatially resolved spectra. Details of the instrument design and representative spectra from six different samples are presented. This design has applications to study objects with cultural heritage significance.

A simple scheme for dynamically switching the quality factor, Q, of a loop-gap resonator (LGR); working at 250 MHz is presented. The addition of this Q-modulator resulted in 30% improvement in electron paramagnetic resonance imager signal-to-noise ratio. During pulse excitation, this scheme lowered the Q, while higher Q was obtained during signal detection. These conditions favored the image acquisition. The Q-modulator is passive; the transition between different states was actuated by the radio frequency power itself.

In this study, a simple air thermostatic system for a temperature range from -10 to +90?C combined with a NMR probe, which can be easily fabricated in a laboratory is proposed. The thermostatic system is suitable for NMR spectrometers for relaxation and diffusion measurement using portable Halbach magnets.