A temperature-controlled cryogen free cryostat integrated with transceiver-mode superconducting coil for high-resolution magnetic resonance imaging

Abstract

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.

Recent Advances and Challenges in the Development of Radiofrequency HTS Coil for MRI

“…Pulsed tube refrigerators without moving parts close to the cold finger have been used at close proximity to the MRI magnet, i.e. on the patient bed [42], or even inside the magnet within the RF coil cryostat [37]. The latter solution was designed to allow a variable orientation of the RF coil with a very compact design; however, it actually needs to be operated in a downward orientation (i.e.…”

“…The use of a refrigerant was eliminated in the cryogen-free system proposed by [40], later described by [42], and displayed in Figure 2. The cold source, provided by a commercial single-stage pulse tube cryocooler connected to a remote motor, is located directly on the non-magnetic cryostat vessel.…”

“…This solution allows the transfer of sufficient cooling power to the RF coil; offers great adaptability to the RF coil bedding geometry; minimizes static and RF magnetic field perturbations induced by the system; and minimizes the transfer of mechanical vibrations to the HTS coil. Moreover, a non-magnetic heat exchanger tip anchored to the coil bed and a fully automated control system enable the control of the coil temperature to within 0.1 K, [41,42]. Thus, the coil frequency can be tuned through temperature as a parameter.…”

“…(E) Cryostat on the patient bed which can be inserted in the bore of the 1.5 T MRI scanner. This system was fully described in [42]. characteristic dimensions.…”

“…Setup 2-Then, the HTS coil is cooled inside the MR compatible cryostat [42] integrated into a 1.5 T clinical MRI (Achieva, Philips) shown in Figure 2. Fine frequency tuning of the HTS coil at ω 0 is performed using a tuning coil [3], while impedance matching is obtained with a coupling coil (Figure 2D).…”