Guide to Simulating Complex NMR Probe Circuits

Integration of multi-coil shim and RF microstrip coils for high-resolution microfluidic nuclear magnetic resonance detection

The integration of nuclear magnetic resonance (NMR) and microfluidic technology provides an excellent detection method for detecting nanoscale micro-samples and analysing intermediates during in situ reaction processes. However, the non-cylindrical symmetric structure of microfluidic chips and micro-coils, along with magnetic susceptibility mismatches, results in a complex distorted magnetic field and reduces spectral resolution. Traditional spherical harmonic shimming methods and coils are global in nature for the target area, forming field distributions with a symmetrical form about the origin. This has significant limitations for shimming local distortions in non-origin-symmetric fields. In this paper, we propose a novel integrated probe suitable for high-resolution detection of microfluidic NMR, as well as a shimming method for local distortions. Specifically: (1) two pairs of double-layer multi-coil (MC) shimming structures suitable for local distortions and global inhomogeneity of the static magnetic field in the detection area of the microfluidic chip are proposed. (2) To reduce interference between the shimming multiple coils (MCs) and the microstrip RF coil, an RF field confinement structure and the corresponding RF tuning matching circuit are designed. (3) A double-layer MC shimming method based on local field distortions is proposed. The integrated probe incorporates two pairs of double-layer MC shimming plates and a pair of double-layer microstrip RF coils on both sides of the microfluidic chip, has a ground layer between the shim coils and the microstrip RF coils to shield interfering signals, and uses Bluetooth communication to transmit shimming data with the host. The proposed shimming method establishes an asymmetric distortion field model based on different microfluidic chip structures and samples, and then controls the inner and outer MCs to compensate for local distortions and global inhomogeneity of the magnetic field. Compared with traditional SH shimming, the proposed MC shimming method and system can flexibly achieve three-dimensional shimming of different target magnetic fields for local field distortion fields in planar microfluidic structures, and can use the small current in a single-turn coil to meet the shimming strength requirements. NMR experiments demonstrated that the proposed integrated probe and shimming method could significantly improve local magnetic field inhomogeneity caused by the magnetic susceptibility effects, enhance static magnetic field uniformity, and effectively improve the NMR signal resolution and spectral line shape. The integrated structural design provides a promising method for achieving high-performance on-chip detection and advancing device development for micro-sample NMR detection.

A theoretical and experimental investigation on a volume coil with slotted end-rings for rat MRI at 7 T

OBJECTIVE

A volume coil with squared slots-end ring was developed to attain improved sensitivity for imaging of rat's brain at 7 T.

MATERIAL AND METHODS

The principles of the high cavity resonator for the low-pass case and the law of Biot-Savart were used to derive a theoretical expression of [Formula: see text]. The slotted-end ring resonator showed a theoretical 2.22-fold sensitivity improvement over the standard birdcage coil with similar dimensions. Numerical studies were carried out for the electromagnetic fields and specific absorption rates for our coil and a birdcage coil loaded with a saline-filled spherical phantom and a digital brain of a rat.

RESULTS

An improvement of the signal-to-noise ratio (SNR) can be observed for the slotted volume coil over the birdcage regardless of the load used in the electromagnetic simulations. The specific absorption rate simulations show a decrement for the digital brain and quite similar values with the saline solution phantom. Phantom and rat's brain images were acquired at 7 T to prove the viability of the coil design. The experimental noise figure of our coil design was four times less than the standard birdcage with similar dimensions, which showed a 44.5% increase in experimental SNR.

DISCUSSION

There is remarkable agreement among the theoretical, numerical and experimental sensitivity values, which all demonstrate that the coil performance for MR imaging of small rodents can be improved using slotted end-rings.

Reimagining magnetic resonance instrumentation using open maker tools and hardware as protocol

Over the course of its history, the field of nuclear magnetic resonance spectroscopy has been characterized by alternating periods of intensive instrumentation development and rapid expansion into new chemical application areas. NMR is now both a mainstay of routine analysis for laboratories at all levels of education and research. On the other hand, new instrumentation and methodological advances promise expanded functionality in the future. At the core of this success is a community fundamentally dedicated to sharing ideas and collaborative advancements, as exemplified by the extensive remixing and repurposing of pulse sequences. Recent progress in modularity, automation, and 3D printing have reignited the tinkering spirit and demonstrate great promise to mature into a maker space that will enable similarly facile sharing of new applications and broader access to magnetic resonance.