Abstract
The use of high temperature superconducting (HTS) radio frequency (RF) coils in Magnetic Resonance Imaging (MRI) greatly improves the signal-to-noise ratio (SNR) in many biomedical applications and particularly in micro-MRI. However, a detailed understanding of the electrical behavior of HTS coils is important in order to optimize their performance through MR experiments. This paper presents a simple and versatile cryogen-free cryostat designed to characterize the RF properties of HTS coils prior to their use in MRI. The cryostat can be used at temperatures from 50 K to 300 K, with a control precision of approximately 3 mK at 70 K, and can measure the RF electrical power transmitted to an HTS coil over a range from 1 μW to 10 W. The quality factor and resonance frequency of the tested HTS coil are determined as a function of the temperature and the power it dissipates. This cryostat also permits the dynamic adjustment of the coil resonance frequency via temperature control. Finally, this study demonstrates that the HTS coil takes less than 12 μs to transit from the superconducting to the dissipative state, which is compatible with MRI requirements.
Highlights
Magnetic resonance imaging (MRI) is a safe and non-invasive imaging technique that provides anatomical, structural and functional information
This paper describes the development and validation of a custom-made low-cost cryostat for the characterization of the nonlinear response of high temperature superconducting (HTS) coils as a function of incident RF power, at working temperatures ranging between 60 and 300 K
We have demonstrated that the resistance Rs of the HTS coil changes as a function of the incident power level, and the transition time is achieved in less than 12 μs
Summary
Magnetic resonance imaging (MRI) is a safe and non-invasive imaging technique that provides anatomical, structural and functional information. The detection of the spin response is achieved by placing a surface receive coil close to the region of interest; surface coils have a higher local sensitivity than volume coils. The receive coil must be inactive when the transmit coil is active (i.e., during spin excitation) in order to prevent RF coupling between the two coils. This is usually obtained by using an active or passive decoupling circuit that is soldered on the coil, attenuating or shifting the resonance of the coil [1].
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