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

Abstract

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.