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High-Temperature Superconducting (HTS) radio frequency coils can greatly improve the sensitivity of Magnetic Resonance Imaging (MRI), with signal to noise ratios increased by up to 16 times. However, as homogeneity of the static field B0 is a major condition for MR acquisitions, their propensity to warp the magnetic field in their vicinity as a result of magnetic flux expulsion could cause severe image degradation. In this Letter, we report on the observation of artifacts generated by magnetic flux expulsion from a HTS coil. To that end, we acquired MR images of a water sample in the vicinity of a nonresonant HTS coil for different cooling conditions. These are zero field cooling and field cooling of the coil inside a clinical MRI bore magnet of 1.5 T, for a range of working temperatures of the HTS coil. After zero-field cooling, we observed significant artifacts in the MR images, increasing with lower temperature; images acquired after field cooling did not display these alterations. We propose a phenomenological model for the magnetic field disturbance induced by the HTS coil. This model reproduces, in good agreement with the experiments, the phase distortion observed in the MR images and allows one to extract the critical current density of the HTS coil, an important technological parameter of superconductors. Finally, we discuss the necessity of field cooling as a way to avoid these B0 inhomogeneity artifacts. [ABSTRACT FROM AUTHOR]

Small-sized High Temperature Superconducting (HTS) radiofrequency coils are used in a number of micro-magnetic resonance imaging applications and demonstrate a high detection sensitivity that improves the signal-to-noise ratio. However, the use of HTS coils could be limited by the rarity of cryostats that are suitable for the MR environment. This study presents a magnetic resonance (MR)-compatible and easily operated cryogen-free cryostat based on the pulse tube cryocooler technology for the cooling and monitoring of HTS coils below the temperature of liquid nitrogen. This cryostat features a real-time temperature control function that allows the precise frequency adjustment of the HTS coil. The influence of the temperature on the electrical properties, resonance frequency (f0), and quality factor (Q) of the HTS coil was investigated. Temperature control is obtained with an accuracy of over 0.55 K from 60 K to 86 K, and the sensitivity of the system, extracted from the frequency measurement from 60 K to 75 K, is of about 2 kHz/K, allowing a fine retuning (within few Hz, compared to 10 kHz bandwidth) in good agreement with experimental requirements. We demonstrated that the cryostat, which is mainly composed of non-magnetic materials, does not perturb the electromagnetic field in any way. MR images of a 10 × 10 × 15 mm3 liquid phantom were acquired using the HTS coil as a transceiver with a spatial resolution of 100 × 100 × 300 µm3 in less than 20 min under experimental conditions at 1.5 T. [ABSTRACT FROM AUTHOR]

The present work investigates the joined effects of temperature and static magnetic field on the electrical properties of a 64 MHz planar high-temperature superconducting (HTS) coil, in order to enhance the signal-to-noise ratio (SNR) in nuclear magnetic resonance (NMR) applications with a moderate decrease of the HTS coil temperature (THTS). Temperature control is provided with accuracy better than 0.1 K from 80 to 66 K by regulating the pressure of the liquid nitrogen bath of a dedicated cryostat. The actual temperature of the HTS coil is obtained using a straightforward wireless method that eliminates the risks of coupling electromagnetic interference to the HTS coil and of disturbing the static magnetic field by DC currents near the region of interest. The resonance frequency ( f0) and the quality factor (Q) of the HTS coil are measured as a function of temperature in the 0-4.7 T field range with parallel and orthogonal orientations relative to the coil plane. The intrinsic HTS coil sensitivity and the detuning effect are then analyzed from the Q and f0 data. In the presence of the static magnetic field, the initial value of f0 in Earth's field could be entirely recovered by decreasing THTS, except for the orthogonal orientation above 1 T. The improvement of Q by lowering THTS was substantial. From 80 to 66 K, Q was multiplied by a factor of 6 at 1.5 T in orthogonal orientation. In parallel orientation, the maximum measured improvement of Q from 80 K to 66 K was a factor of 2. From 80 to 66 K, the improvement of the RF sensitivity relative to the initial value at the Earth's field and ambient pressure was up to 4.4 dB in parallel orientation. It was even more important in orthogonal orientation and continued to increase, up to 8.4 dB, at the maximum explored field of 1.5 T. Assuming that the noise contributions from the RF receiver are negligible, the SNR improvement using enhanced HTS coil cooling in NMR experiments was extracted from Q measurements either with or without the presence of the sample. Notably, the additional cooling in the presence of conductive samples appears more beneficial at higher field strengths and with an orthogonal incidence than with parallel. The temperature range accessible here, involving a relatively straightforward cryogenic design, brings a gain in RF sensitivity that is of great significance to cutting-edge applications with very weakly conducting samples, small biological specimens, or small animals in vivo. This work also demonstrates a better tolerance to thin-film orientation misalignments relative to the magnetic field, and this could eventually play a role in designing effective non-planar HTS coils or coil arrays which include elements of various orientations. Finally, the data provided in this work may help understand some critical aspects in the design of HTS coils for NMR and MRI applications and accounts for the presence of the static magnetic field, particularly regarding the SNR loss due to a decreased quality factor and detuning issues. [ABSTRACT FROM AUTHOR]

A contactless method based on reflectometry to accurately characterize an inductive radio frequency (rf) resonator even in the occurrence of a strong electrical nonlinearity is presented. Nonlinear extraction of the unloaded quality factor and resonance frequency is possible by combining an initial low-level swept-frequency calibration with high-level single-frequency measurements. The extraction protocol relies on a simple intrinsic R, L, C model and does not involve a fitting procedure according to a particular nonlinearity model. It includes a correction for strong coupling conditions between the probe and the rf coil, which allows extending the analysis over a wide range of transmitted power. Electrical modeling based on the extracted intrinsic data allows predicting the coil behavior when loaded by any kind of matching network. The method will have implications in different domains such as Magnetic Resonance (MR) applications with superconducting probe heads or analysis of rf properties in nonlinear materials. The method is demonstrated here by characterizing a high temperature superconducting (HTS) coil dedicated to MR imaging at 64 MHz. The coil consists in a multiturn spiral design that is self-resonant close to the MR frequency of interest. The Q factor and the resonance frequency are determined as a function of the actual power dissipated in the HTS coil accounting for losses occurring in the measurement system. Further characteristics of the HTS coil are considered in the present paper. The relation between the transmitted power and the magnetic field generated by the coil, which is the most relevant characteristics for MR applications, is directly accessible. The equivalent impedance of the coil under test is also expressed as a function of the total current flowing in the windings. The method could be extended to assess the fundamental properties of the nonlinear material (e.g., the London penetration depth or the critical current density) by including any pertinent model. [ABSTRACT FROM AUTHOR]

Describes a Helmholtz coil made from two bracelet resonators instead of the conventional untuned coils spaced axially half a diameter apart. Design procedure; Features of the resonator; Application for nuclear magnetic resonance experiments.

The radio-frequency coil sensitivity for NMR signal detection can be defined as B[sub 1]/√P, which is the magnetic field that the coil induces at a given point per unit supplied power. We propose a new method, based on the reciprocity theorem, to quickly evaluate the sensitivity of a tuned coil. The only requirement of the method is a voltage gain measurement with a small dual-loop probe, without mobilization of the NMR unit. The probe can be used to gauge and map the sensitivity of the coil with good accuracy, while monitoring its tuning frequency ω[sub 0] and its quality factor Q. Our method is particularly convenient for development and maintenance of coils for MR imaging, and more generally could be applied to other fields involving radio-frequency characterization of inductive circuits. [ABSTRACT FROM AUTHOR]

Examines a simple device for highly accurate and real time measurement of magnetic field gradient pulses. Use of analog integration of the differential voltage induced in a pair of sensor coils; Confirmation of insensitivity of the method to static magnetic field inhomogeneities; Suggestion of using two different solutions for analog interrogation.