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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.

International audience; 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.

International audience; The dependence of the nuclear magnetic resonance relaxation rate on the magnetic field has been widely studied, in particular, in biomedical areas with the objectives to better understand the underlying microscopic mechanisms in tissues and provide biomarkers of diseases. By combining fast-field cycling (FFC) and magnetic resonance imaging (MRI), it is possible to provide localized relaxation dispersion measurements in heterogeneous systems with recent demonstrations in solutions, biological samples, human beings, and small animals. We report here the developments and performances of a device designed for small animal FFC-MRI comprising a resistive insert technology operating inside a 1.5 T MRI system. Specific measurement methods were developed to characterize the system efficiency, response time, homogeneity, stability, and compensation. By adding a non-linear element in the system and using a dual amplifier strategy, it is shown that large field offsets can be produced during relaxation periods while maintaining precise field control during detection periods. The measurement of longitudinal nuclear magnetic relaxation dispersion (NMRD) profiles in the range of 1.08 T-1.92 T is reported, essentially displaying a linear variation in this range for common MRI contrast agents. The slopes of both the longitudinal and transverse relaxation dispersion profiles at 1.5 T are measured and validated, extending the capabilities of previous approaches. The performances of a longitudinal relaxation dispersion mapping method are finally reported, opening the way to quantitative preclinical dispersion imaging studies at a high FFC-MRI field.

This paper addresses the scientific and technological challenges related to the development of wireless radio frequency (RF) coils for magnetic resonance imaging (MRI) based on published literature together with the authors' interpretation and further considerations. Key requirements and possible strategies for the wireless implementation of three important subsystems, namely the MR receive signal chain, control signaling, and on-coil power supply, are presented and discussed. For RF signals of modern MRI setups (e.g., 3 T, 64 RF receive channels), with on-coil digitization and advanced methods for dynamic range (DR ≥ 16-bit) and data rate compression, still data rates > 500 Mbps will be required. For wireless high-speed MR data transmission, 60 GHz WiGig and optical wireless communication appear to be suitable strategies; however, on-coil functionality during MRI scans remains to be verified. Besides RF signals, control signals for on-coil components, e.g., active detuning, synchronization to the MR system, and B0 shimming, have to be managed. Wireless power supply becomes an important issue, especially with a large amount of additional on-coil components. Wireless power transfer systems (>10 W) seem to be an attractive solution compared to bulky MR-compatible batteries and energy harvesting with low power output. In our opinion, completely wireless RF coils will ultimately become feasible in the future by combining efficient available strategies from recent scientific advances and novel research. Besides ongoing improvement of all three subsystems, innovations are specifically required regarding wireless technologies, MR compatibility, and wireless power supply.

Purpose Aerosol toxicology and drug delivery through the lungs, which depend on various parameters, require methods to quantify particle deposition. Intrapulmonary-administered MRI contrast agent combined with lung-specific imaging sequences has been proposed as a high performance technique for aerosol research. Here, aerosol deposition is assessed using ultra-short echo (UTE) sequences. Methods Before and after administration of Gd-DOTA-based aerosol delivered nose-only in free-breathing healthy rats, a T1-weighted 3D UTE sequence was applied in a clinical 1.5 Tesla scanner. Administration lasted 14 min, and the experiment was performed on six rats. A contrast-enhanced quantitative analysis was done. Results Fifty percent signal enhancement was obtained in the lung parenchyma. Lung clearance of the contrast agent was evaluated to be 14% per h (corresponding to a characteristic clearance time of 3.6 h) and aerosol deposition was shown to be homogeneous throughout the lung in healthy rats. The total deposited dose was estimated to be 1.05 µmol/kg body weight, and the concentration precision was 0.02 mM. Conclusion The UTE protocol with nebulized Gd-DOTA is replicable to significantly enhance the lung parenchyma and to map aerosol deposition. This functional strategy, applied in a clinical system with a clinical nebulization setup and a low inhaled dose, suggests a feasible translation to human. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.