Category Archives: RF Circuits

Microcoils for Broadband Multinuclei Detection

Anders, Jens, and Aldrik H. Velders. “Microcoils for Broadband Multinuclei Detection.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 265–96. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

NMR techniques are among the most influential analytical tools developed in the past century and widely used in various disciplines from oil well drilling to medicine. To date, two major hurdles inhibit an even more widespread use of NMR spectroscopy in science and society: first, NMR’s relatively low sensitivity severely constrains applications of mass- and volume-limited samples including lab-on-chip integration, in-cell analysis, and bioanalyte detection. Typical NMR samples contain micromole quantities of material in a relatively large sample volume of about 0.5ml; this large sample volume in turn imposes stringent requirements on the magnetic field – both for the generation but also on the susceptibility of the materials utilized in the probe head – which has to be homogenous in the whole sample volume with ppb resolution. Second, NMR equipment is very complex and costly. A major contribution to the high price of NMR equipment is constituted by the (cryogenic) superconducting magnets used to generate the static magnetic field.This problem will hopefully be tackled by the introduction of new magnet-manufacturing techniques and materials, for example, high-temperature superconductors, and the development of miniaturized spectrometers. Another complex and costly aspect concerns the heart of spectrometers consisting of intricate multifrequency probes, with coils integrated in sophisticated tuning–matching circuits connected to complex RF transceiver circuits. In viewof these limitations of currentNMRsystems, to make NMR more versatile and affordable, a key challenge is improving sensitivity and, at the same time, reducing cost and complexity of NMR probes and electronics.

Inductance Calculation in Magnetic Resonance Solenoid Coils with Strip and Wire Conductors #Instrumentation #NMR

If you’ve ever built your own RF circuit for NMR you know that calculating the inductance for a solenoid can be challenging. This article gives some insight where the discrepancies are coming from and how to make the prediction more accurately. However, in the end you still need to get the soldering iron out and adjust capacitors to make it work.

Giovannetti, Giulio, and Francesca Frijia. “Inductance Calculation in Magnetic Resonance Solenoid Coils with Strip and Wire Conductors.” Applied Magnetic Resonance 51, no. 8 (August 2020): 703–10.

Solenoids are employed in Magnetic Resonance (MR) as radiofrequency (RF) coils due to their high sensitivity. In particular, their cylindrical symmetry is optimal for circular cross-sectional samples. Solenoid inductance estimation is a constraint for a correct design and tuning of the resonant circuit constituting the RF coil, suitable to be used for transmitting and receiving the RF signal of the given X-nucleus with the available MR scanner. However, the different literature formulation for solenoid inductance estimation is not optimized for a wide variety of coil geometries and doesn’t take into account conductor geometry. This paper proposes an analytical method for the solenoid inductance calculation in dependence on the conductor cross-sectional geometry (flat strip and circular wire). Simulations accuracy was evaluated with workbench experimental measurement performed on a home-built strip solenoid and by comparisons with literature data referred to wire solenoids.

Design of a local quasi-distributed tuning and matching circuit for dissolution DNP cross polarization #DNPNMR

Vinther, Joachim M.O., Vitaliy Zhurbenko, Mohammed M. Albannay, and Jan Henrik Ardenkjær-Larsen. “Design of a Local Quasi-Distributed Tuning and Matching Circuit for Dissolution DNP Cross Polarization.” Solid State Nuclear Magnetic Resonance 102 (October 2019): 12–20.

Dynamic nuclear polarization (DNP) build-up times at low temperature for low-gamma nuclei can be unfavorably long and can be accelerated by transfer of polarization from protons. The efficiency of the cross polarization (CP) depends on the B1-field strengths, the pulse sequence chosen for cross polarization and the sample composition. CP experiments rely on high B1-fields, which typically lead to electrical discharge and breakdown in the circuit. This problem is particularly severe in the low pressure helium atmosphere due to easily ionized helium atoms. The purpose of this study is to identify strategies to minimize voltages across components in a tuning and matching circuit of the coil to avoid electrical discharge during CP experiments. Design equations for three tuning and matching network configurations are derived. The results of the study are then used in the design of a single coil double resonance DNP probe operating at 71.8 MHz (13C frequency) and 285.5 MHz (1H frequency). In the current setup we achieve 28% polarization on 13C in urea with a build-up time of 11.6 min with CP compared to 14% and 53 min by direct polarization using TEMPOL as the radical. Different cross polarization sequences are compared.

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