Implementing High Q-Factor HTS Resonators to Enhance Probe Sensitivity in 13C NMR Spectroscopy.

Nuclear magnetic resonance spectroscopy (NMR) probes using thin-film high temperature superconducting (HTS) resonators provide exceptional mass sensitivity in small-sample NMR experiments for natural products chemistry and metabolomics. We report improvements in sensitivity to our 1.5 mm 13C-optimized NMR probe based on HTS resonators. The probe has a sample volume of 35 microliters and operates in a 14.1 T magnet. The probe also features HTS resonators for 1H transmission and detection and the 2H lock. The probe utilizes a 13C resonator design that provides greater efficiency than our previous design. The quality factor of the new resonator in the 14.1 T background field was measured to be 4,300, which is over 3x the value of the previous design. To effectively implement the improved quality factor, we demonstrate the effect of adding a shorted transmission line stub to increase the bandwidth and reduce the rise/fall time of 13C irradiation pulses. Initial NMR measurements verify 13C NMR sensitivity is significantly improved while preserving detection bandwidth. The probe will be used for applications in metabolomics.

13C-Optimized HTS NMR RF Coil Design at 21.1 T.

We present the design of a novel high-temperature superconductor double-sided racetrack resonator for a 13C optimized nuclear magnetic resonance (NMR) transmitter/receiver coil. The coils operate in a 21.1 T magnet and accommodate a 3 mm × 6.2 mm cross-section rectangular sample tube. The design includes the incorporation of revised finger lengths to improve the homogeneity of current density across the fingers, a new laser trimming approach for adjusting the resonance frequency, and improved ability to shift higher-order modes for suitability in 1H/13C NMR probes. Resonator design methodology, simulations and experimental results are presented.

Modeling the Resonance Shifts Due to Coupling Between HTS Coils in NMR Probes.

Nuclear magnetic resonance (NMR) probes using thin-film HTS coils offer high sensitivity and are particularly suitable for small-sample applications. Typically, HTS probes are optimized for the detection of multiple nuclei and require several coils to be located within a small volume near the sample. Coupling between the coils shifts coil resonances and complicates coil trimming when tuning HTS probes. We have modeled the magnetic coupling between the coils of a 1.5-mm all-HTS NMR probe with 13C, 1H, and 2H channels. By measuring the magnetic coupling coefficients between individual coils, we solve the general coupling matrix given by KVL for six coupled resonators. Our results indicate that required trims can be accurately predicted by applying single coil trimming simulations to this magnetic coupling model. Use of the magnetic coupling model significantly improves the efficiency of tuning HTS probes.

Ultra-wide bore 900 MHz high-resolution NMR at the National High Magnetic Field Laboratory.

Access to an ultra-wide bore (105 mm) 21.1 T magnet makes possible numerous advances in NMR spectroscopy and MR imaging, as well as novel applications. This magnet was developed, designed, manufactured and tested at the National High Magnetic Field Laboratory and on July 21, 2004 it was energized to 21.1 T. Commercial and unique homebuilt probes, along with a standard commercial NMR console have been installed and tested with many science applications to develop this spectrometer as a user facility. Solution NMR of membrane proteins with enhanced resolution, new pulse sequences for solid state NMR taking advantage of narrowed proton linewidths, and enhanced spatial resolution and contrast leading to improved animal imaging have been documented. In addition, it is demonstrated that spectroscopy of single site (17)O labeled macromolecules in a hydrated lipid bilayer environment can be recorded in a remarkably short period of time. (17)O spectra of aligned samples show the potential for using this data for orientational restraints and for characterizing unique details of cation binding properties to ion channels. The success of this NHMFL magnet illustrates the potential for using a similar magnet design as an outsert for high temperature superconducting insert coils to achieve an NMR magnet with a field >25 T.

Correction for intensity falloff in surface coil magnetic resonance imaging.

A method has been developed to compensate for sensitivity variation in surface coil images. An algorithm to derive the surface coil profile by acquiring a crude body coil image of the region under study is presented and tested using a homogeneous phantom. Practical application is demonstrated on images of a tomato and rabbit acquired with a 2 T 33-cm bore magnetic resonance imager/spectrometer.

Reduction of pulsed gradient settling time in the superconducting magnet of a magnetic resonance instrument.

The implementation of electronic compensation for eddy currents to reduce the gradient settling times on a superconducting magnet is described. Field plots inside the magnet indicate the importance of assessing the field changes at several positions in the magnet. The effect of an asymmetry between the gradient coil system and the cryostat is demonstrated. A modified compensation circuit is described to overcome this mechanical asymmetry.

Fat tissue and fat suppression.

Fat tissues consist of fat cells, capillaries, and collagen fibers. In order to completely suppress the signals from fat tissues in clinical magnetic resonance imaging, the signal from capillaries and collagen fibers as well as from fat cells should all be suppressed. We have previously reported that fat signal can be uniformly suppressed by applying an optimized presaturation pulse. The inhomogeneously broadened fat peak of tissue spectrum is excited by the optimized pulse and dephased by a subsequent field gradient. The broadened water peak is not affected. In this paper we discuss a technique that suppresses signals from fat tissues completely as well as uniformly. This technique is based on the cancellation of fat and water signals in the same image voxel by combining the optimized selective excitation with the opposite phase imaging technique. Experimental and clinical images demonstrate that the new technique improves the delineation and depiction of anatomy in clinical fat suppression imaging.

Compensation for surface coil sensitivity variation in magnetic resonance imaging.

A procedure which incorporates the aspects of both correction matrix and digital filtering to compensate for sensitivity fall-off of the surface coil has been applied to clinical imaging. In this method the surface coil profile is determined using the surface coil image and a crude body coil image. Our results indicate that the corrected surface coil image exhibits the homogeneity of the body coil image while essentially preserving the sensitivity of the surface coil image.

A field-gradient coil using concentric return paths.

An approach to transverse field-gradient coil design is presented which locates all current elements in planes perpendicular to the main magnetic field. The linear volume of the resulting coil structure can be made to extend nearly to both ends of the coil. This design is based on forming a concentric return path in the same plane as the field-producing arcs. The coil structure consists of stacked planar units which give a desired field-gradient configuration. The size of the linear-field region is optimized by varying the current in each plane, by varying the location of a plane relative to the others in the stack, or by varying both current and location. Coils with linear regions having a wide range of length-to-diameter ratios can be designed, as illustrated by two examples. The construction of a prototype coil is presented and its performance in imaging confirms the analysis. This type of design is suited to magnetic resonance of the human head without obstruction from the shoulders.

In vivo proton spin-lattice relaxation times of normal and dystrophic muscles.

In vivo spin-lattice relaxation times (T1) of water and lipid protons were measured in normal and dystrophic chicken pectoralis muscles at different ages. Values were obtained with a surface coil used as both a receiver and a transmitter. A 2 theta-T1-theta-Acquisition sequence was used for these measurements. Accuracy was verified with an inversion-recovery method using a slotted tube resonator as the transmitter and a surface coil as the receiver. It was observed that the T1 values of water protons in normal muscles decrease with age, the T1 values of water protons do not change with age in dystrophic muscles, and the T1 values of lipid protons increase with age in normal and dystrophic muscles. These results indicate a failure of the normal maturation of dystrophic muscles.

A high-temperature superconducting Helmholtz probe for microscopy at 9.4 T.

The design and operation of a high-temperature superconducting (HTS) probe for magnetic resonance microscopy (MRM) at 400 MHz are presented. The design of the probe includes a Helmholtz coil configuration and a stable open-cycle cooling mechanism. Characterization of coil operating parameters is presented to demonstrate the suitability of cryo-cooled coils for MRM. Specifically, the performance of the probe is evaluated by comparison of signal-to-noise (SNR) performance with that of a copper Helmholtz pair, analysis of B1 field homogeneity, and quantification of thermal stability. Images are presented to demonstrate the SNR advantage of the probe for typical MRM applications.