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.

During magnetic resonance imaging (MRI) examinations, the average specific absorption rate (SAR) of the whole body is calculated as an index of global energy deposition in biological tissue without taking into account the presence of metallic implants or conductive materials. However, this global SAR calculation is not sufficient to ensure patient safety and a local SAR measurement should be carried out. Several measurement techniques have already been used to evaluate the local SAR, in particular electric field (E-field) probes, but the accuracy of the measurements and the resolutions (spatial and temporal) depend strongly on the measurement method/probe. This work presents an MR-compatible, subcentimeter probe based on an electro-optic (EO) principle enabling a real-time measurement of the local E-field during MRI scans. The experiments using these probes were performed on two different MR systems (preclinical and clinical) having different static magnetic field strengths and with different volume coil geometries. The E-field was measured with unloaded (in air) and loaded volume coils in order to assess the sensing characteristics of the optical probe. The results show an excellent linearity between the measured E-field and the radiofrequency (RF) magnetic field in both experimental conditions. Moreover, the distribution of the E-field throughout the volume coil was experimentally determined and was in good agreement with numerical simulations. Finally, we demonstrate through our measurements that the E-field depends strongly on the dielectric properties of the medium.