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Les antennes radiofréquence (RF) sont un élément essentiel de l'IRM, compte tenu de leur impact direct sur la qualité de l’image. Ce chapitre examine les principes fondamentaux d'une antenne RF du point de vue de l’ingénieur, du physicien et du clinicien. Il résume les avancées et les développements actuels de la technologie.

International audience; Magnetic resonance elastography aims to non-invasively and remotely characterize the mechanical properties of living tissues. To quantitatively and regionally map the shear viscoelastic moduli in vivo, the technique must achieve proper mechanical excitation throughout the targeted tissues. Although it is straightforward, ante manibus, in close organs such as the liver or the breast, which practitioners clinically palpate already, it is somewhat fortunately highly challenging to trick the natural protective barriers of remote organs such as the brain. So far, mechanical waves have been induced in the latter by shaking the surrounding cranial bones. Here, the skull was circumvented by guiding pressure waves inside the subject's buccal cavity so mechanical waves could propagate from within through the brainstem up to the brain. Repeatable, reproducible and robust displacement fields were recorded in phantoms and in vivo by magnetic resonance elastography with guided pressure waves such that quantitative mechanical outcomes were extracted in the human brain.

International audience; Radiofrequency (RF) coils fashioned from high-temperature superconductor (HTS) have the potential to increase the sensitivity of the magnetic resonance imaging (MRI) experiment by more than a dozen times compared to conventional copper coils. Progress, however, has been slow due to a series of technological hurdles. In this article, we present the developments that recently led to new perspectives for HTS coil in MRI, and challenges that still need to be solved. First, we recall the motivations for the implementations of HTS coils in MRI by presenting the limits of cooled copper coil technology, such as the anomalous skin effect limiting the decrease of the electric resistance of normal conductors at low temperature. Then, we address the progress made in the development of MRI compatible cryostats. New commercially available low-noise pulsed-tube cryocoolers and new materials removed the need for liquid nitrogen-based systems, allowing the design of cryogen-free and more user-friendly cryostats. Another recent advance was the understanding of how to mitigate the imaging artifacts induced by HTS diamagnetism through field cooling or temperature control of the HTS coil. Furthermore, artifacts can also originate from the RF field coupling between the transmission coil and the HTS reception coil. Here, we present the results of an experiment implementing a decoupling strategy exploiting nonlinearities in the electric response of HTS materials. Finally, we discuss the potential applications of HTS coils in bio-imaging and its prospects for further improvements. These include making the technology more user-friendly, implementing the HTS coils as coil arrays, and proposing solutions for the ongoing issue of decoupling. HTS coil still faces several challenges ahead, but the significant increase in sensitivity it offers lends it the prospect of being ultimately disruptive.

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.

Magnetic resonance imaging (MRI) is a major imaging modality, giving access to anatomical and functional information with high diagnostic value. To achieve high-quality images, optimization of the radio-frequency coil that detects the MR signal is of utmost importance. A widely applied strategy is to use arrays of small coils in parallel on MR scanners equipped with multiple receive channels that achieve high local detection sensitivity over an extended lateral coverage while allowing for accelerated acquisition and SNR optimization by proper signal weighting of the channels. However, the development of high-density coil arrays gives rise to several challenges due to the increased complexity with respect to mutual decoupling as well as electronic circuitry required for coil interfacing. In this work, we investigate a novel single-element coil design composed of small loops in series, referred to as “multi-loop coil (MLC)”. The MLC concept exploits the high sensitivity of small coils while reducing sample induced noise together with an extended field of view, similar to arrays. The expected sensitivity improvement using the MLC principle is first roughly estimated using analytical formulae. The proof of concept is then established through fullwave 3D electromagnetic simulations and validated by B1 mapping in MR experiments on phantom. Investigations were performed using two MLCs, each composed of 19 loops, targeting MRI at high (3 T) and at ultra-high field strength (7 T). The 3 T and 7 T MLCs have an overall diameter of 12 cm and 6 cm, respectively. For all investigated MLCs, we demonstrate a sensitivity improvement as compared to single loop coils. For small distances inside the sample, i.e. close to the coil, a sensitivity gain by a factor between 2 and 4 was obtained experimentally depending on the set-up. Further away inside the sample, the performance of MLCs is comparable to single loop coils. The MLC principle brings additional degrees of freedom for coil design and sensitivity optimization and appears advantageous for the development of single coils but also individual elements of arrays, especially for applications with a larger area and shallow target depth, such as skin imaging or high-resolution MRI of brain slices.

The Multi-turn Transmission Line Resonator (MTLR) design allows for developing very small and highly sensitive radiofrequency (RF) coils for magnetic resonance imaging. However the monolithic feature and the small size of the developed coils, imposes to employ dedicated contact-free tuning techniques. In this work, we investigated two contact-free tuning principles for small monolithic RF coils: a dielectric tuning principle based on the interception of the electric field lines by a dielectric layer, and an inductive tuning principle based on the interception of magnetic field lines by a closed conducting ring. For both tuning principles, we used experimental measurements co-supported by electromagnetic simulation and analytical modelling to determine the accessible tuning range as a function of both the tuning element's characteristics and its distance to the RF coil. A maximal tuning range about 9% and 24% was achieved using the dielectric tuning principle and the inductive tuning principle, respectively. Contrarily to the dielectric tuning principle, the inductive tuning principle was found to strongly limit the quality factor of the coil. To implement the two investigated tuning principles, we have developed an automated system based on piezoelectric actuators that achieves microscopic displacements of the tuning elements. The automated system was also used to perform inductive impedance matching of the coil. For purposes of both tuning and matching, when starting from a strongly un-tuned or un-matched configuration, the automated system reaches convergence within a few tens of seconds. First MRI experiments were performed to evaluate the MRI compatibility of the automated system.

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.

PURPOSE To investigate the feasibility of detecting a living cell monolayer labeled with gadoterate (Gd-DOTA) in a microfluidic environment, by micromagnetic resonance imaging (MRI) in a 2.35T small-animal system. The development of new targeted contrast agents (CAs) requires proof-of-concept studies in order to establish the detectability of the CA and to predict the role of biodistribution in its uptake mechanisms. A promising approach is to carefully mimic the in vivo pharmacokinetic context with reduced experimental complexity compared to in vivo situations. MATERIALS AND METHODS A dedicated experimental system was built by combining a microfluidic slide and a radiofrequency probe based on a 6 mm diameter multiturn transmission-line resonator. Adherent KB cells were incubated with different concentrations of Gd. MRI data were acquired at 2.35T with a 3D gradient echo and a resolution of 12.4 μm perpendicular to the cell layer. The longitudinal relaxation rate, R1 , was measured as a function of the amount of Gd internalized by the cells. RESULTS R1 measurements for different Gd concentrations per cell were performed using data with an signal-to-noise ratio (SNR) of 100. Relaxation-rate variations ΔR1 of 0.035 s(-1) were measured. A quenching effect was observed at Gd concentrations above 20 fmol/cell. CONCLUSION Our results suggest that this dedicated experimental system is suitable for specifically assessing new high-relaxivity targeted CAs under real-time uptake conditions.

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Purpose This article presents a novel inductive decoupling technique for form-fitting coil arrays of monolithic transmission line resonators, which target biomedical applications requiring high signal-to-noise ratio over a large field of view to image anatomical structures varying in size and shape from patient to patient. Methods Individual transmission line resonator elements are mutually decoupled using magnetic flux sharing by overlapping annexes. This decoupling technique was evaluated by electromagnetic simulations and bench measurements for two- and four-element arrays, comparing single- and double-gap transmission line resonator designs, combined either with a basic capacitive matching scheme or inductive pickup loop matching. The best performing array was used in 7T MRI experiments demonstrating its form-fitting ability and parallel imaging potential. Results The inductively matched double-gap transmission line resonator array provided the best decoupling efficiency in simulations and bench measurements (

To improve the detection sensitivity of Micro Magnetic Resonance Imaging (μ-MRI) in humans, small animals or biological sample, it is necessary to reach higher spatial resolution. Radio Frequency (RF) YBaCuO film coils, in superconducting state, are used for that purpose, instead of copper coils. In working condition, these miniature RF coils are located in clinical MRI scanners (few Tesla) and very closed to the biological sample under investigation. A non magnetic cryostat has been designed and constructed to operate these RF coils in this particular medical environment. This cryostat is principally made of polymer materials to reduce the electromagnetic perturbations to the MRI magnet and the cryogenic scheme has been chosen to be cryogen free. In this paper, we describe the cryostat and present the experimental thermal characterizations and the first images obtained in a clinical MRI scanner.

Purpose To demonstrate the feasibility of a highly sensitive superconducting surface coil for microscopic MRI of the human skin in vivo in a clinical 1.5 Tesla (T) scanner. Materials and Methods A 12.4-mm high-temperature superconducting coil was used at 1.5T for phantom and in vivo skin imaging. Images were inspected to identify fine anatomical skin structures. Signal-to-noise ratio (SNR) improvement by the high-temperature superconducting (HTS) coil, as compared to a commercial MR microscopy coil was quantified from phantom imaging; the gain over a geometrically identical coil made from copper (cooled or not) was theoretically deduced. Noise sources were identified to evaluate the potential of HTS coils for future studies. Results In vivo skin images with isotropic 80 μm resolution were demonstrated revealing fine anatomical structures. The HTS coil improved SNR by a factor 32 over the reference coil in a nonloading phantom. For calf imaging, SNR gains of 380% and 30% can be expected over an identical copper coil at room temperature and 77 K, respectively. Conclusion The high sensitivity of HTS coils allows for microscopic imaging of the skin at 1.5T and could serve as a tool for dermatology in a clinical setting. J. Magn. Reson. Imaging 2015;41:496–504.© 2013 Wiley Periodicals, Inc.

International audience; Quantification strategies have been a long standing challenge since the early days of MR molecular and cellular imaging. Quantitative imaging is desirable in the framework of comparative or follow-up studies in order to establish objective evidences of a specific target expression pertaining to a specific disease. Numerous strategies of increasing complexity have been proposed to address this need. In this chapter, we briefly review common strategies related to quantitative molecular imaging for MRI and we discuss their respective outcomes. We focus on relaxation Contrast Agents (CAs) that generate MR contrast by affecting the proton relaxation times, and which are the most common kind of CAs used in MRI. Particular attention is paid on cutting edge applications enabling local CA quantification as well as distinction of the bound/activated sub-population of the CA.

Molecular magnetic resonance imaging targeted to an endothelial integrin involved in neoangiogenesis was compared to DCE-US and immunochemistry to assess the early response of three different therapeutic agents in renal cell carcinoma. Human A498 renal cells carcinoma was subcutaneously inoculated into 24 nude mice. Mice received either phosphate-buffered saline solution, sunitinib, everolimus, or bevacizumab during 4 days. DCE-US and molecular MRI targetingαvβ3 were performed at baseline and 4 days after treatment initiation. PI, AUC, relaxation rate variationsΔR2⁎, and percentage of vessels area quantified on CD31-stained microvessels were compared. Significant decreases were observed for PI and AUC parameters measured by DCE-US for bevacizumab group as early as 4 days, whereas molecularαvβ3-targeted MRI was able to detect significant changes in both bevacizumab and everolimus groups. Percentage of CD31-stained microvessels was significantly correlated with DCE-US parameters, PI (R=0.87,p=0.0003) and AUC (R=0.81,p=0.0013). The percentage of vessel tissue area was significantly reduced (p<0.01) in both sunitinib and bevacizumab groups. We report an early detection of neoangiogenesis modification after induction of targeted therapies, using DCE-US orαvβ3-targeted MRI. We consider these outcomes should encourage clinical trial developments to further evaluate the potential of this molecular MRI technique.

A novel design scheme for monolithic transmission line resonators (TLRs) is presented - the multi-turn multi-gap TLR (MTMG-TLR) design. The MTMG-TLR design enables the construction of TLRs with multiple turns and multiple gaps. This presents an additional degree of freedom in tuning self-resonant TLRs, as their resonance frequency is fully determined by the coil geometry (e.g. diameter, number of turns, conductor width, etc.). The novel design is evaluated at 4.7T and 7T by simulations and experiments, where it is demonstrated that MTMG-TLRs can be used for MRI, and that the B

Combined with high-field MRI scanners, small implanted coils allow for high resolution imaging with locally improved SNR, as compared to external coils. Small flexible implantable coils dedicated to in vivo MRI of the rat brain at 7 T were developed. Based on the Multi-turn Transmission Line Resonator design, they were fabricated with a Teflon substrate using copper micromolding process and a specific metal–polymer adhesion treatment. The implanted coils were made biocompatible by PolyDimethylSiloxane (PDMS) encapsulation. The use of low loss tangent material achieves low dielectric losses within the substrate and the use of the PDMS layer reduces the parasitic coupling with the surrounding media. An implanted coil was implemented in a 7 T MRI system using inductive coupling and a dedicated external pick-up coil for signal transmission. In vivo images of the rat brain acquired with in plane resolution of (150 μm)2 thanks to the implanted coil revealed high SNR near the coil, allowing for the visualization of fine cerebral structures.

International audience; Aims: Submicron membrane fragments termed microparticles (MPs), which are released by apoptotic or activated cells, are newly considered as vectors of biological information and actors of pathology development. We propose the tagging of MPs with magnetic nanoparticles as a new approach allowing imaging, manipulation and targeting of cell-derived MPs. Materials & methods: MPs generated in vitro from human endothelial cells or isolated from atherosclerotic plaques were labeled using citrate-coated 8 nm iron-oxide nanoparticles. MPs were tagged with magnetic nanoparticles on their surface and detected as Annexin-V positive by flow cytometry. Results: Labeled MPs could be mobilized, isolated and manipulated at a distance in a magnetic field gradient. Magnetic mobility of labeled MPs was quantified by micromagnetophoresis. Interactions of labeled MPs with endothelial cells could be triggered and modulated by magnetic guidance. Nanoparticles served as tracers at different scales: at the subcellular level by electron microscopy, at the cellular level by histology and at the macroscopic level by MRI. Conclusion: Magnetic labeling of biogenic MPs opens new prospects for noninvasive monitoring and distal manipulations of these biological effectors.

We demonstrate the feasibility of detecting individual tumor-infiltrating cells in vivo, by means of cellular magnetic labeling and a 1.5 Tesla clinical MRI device equipped with a high-resolution surface coil. Using a recently developed high-temperature superconducting (HTS) surface coil, single cells were detected in vitro in voxels of (60 μm)3 at magnetic loads as low as 0.2 pg of iron per cell. The same imaging protocol was used in vivo to monitor infiltration of ovalbumin-expressing tumors by transferred OVA antigen-specific cytotoxic lymphocytes with low iron load. Magn Reson Med 60:1292–1297, 2008. © 2008 Wiley-Liss, Inc.

In magnetic resonance imaging (MRI), the electrical performance of the RF coil is critical to achieve sufficient signal to noise ratio (SNR), especially when microscopic structures [about (100 μm)3] have to be observed. In this field of application, we have developed a device (microcoil) based on the original concept of monolithic resonator, dedicated to superficial region imaging (human skin) or small animal imaging. This paper presents the developed process based on micromoulding. Flexible thin films of polymer have been used as dielectric substrate so that the microcoil could be form-fitted to non-plane surfaces. First, electrical characterizations of the RF coils have been performed. The results were compared to the expected values. A flexible RF coil of 15 mm diameter was then used to perform proton MRI of a saline phantom. When the coil was form-fitted to the phantom surface, a maximum SNR gain of 2 was achieved with respect to identical but plane RF coil. Finally, the flexible coil was used to perform MRI in vivo with high spatial resolution on a mouse using a small animal dedicated scanner operating at 2.35 T.

Magnetic resonance imaging (MRI) is well suited for small animal model investigations to study various human pathologies. However, the assessment of microscopic information requires a high-spatial resolution (HSR) leading to a critical problem of signal-to-noise ratio limitations in standard whole-body imager. As contrast mechanisms are field dependent, working at high field do not allow to derive MRI criteria that may apply to clinical settings done in standard whole-body systems. In this work, a contrast-enhanced dynamic MRI protocol with improved spatial and time resolution was used to perform in vivo tumor model imaging on the mouse at 1.5 T. The needed sensitivity is provided by the use of a 12-mm superconducting surface coil operating at 77 K. High quality in vivo images were obtained and revealed well-defined internal structures of the tumor. A 3-D HSR sequence with voxels of 59×59×300 μm 3 encoded within 6.9 min and a 2-D sequence with subsecond acquisition time and isotropic in-plane resolution of 234 μm were used to analyze the contrast enhancement kinetics in tumoral structures at long and short time scales. This work is a first step to better characterize and differentiate the dynamic behavior of tumoral heterogeneities.

To investigate the feasibility of detecting a living cell monolayer labeled with gadoterate (Gd-DOTA) in a microfluidic environment, by micromagnetic resonance imaging (MRI) in a 2.35T small-animal system. The development of new targeted contrast agents (CAs) requires proof-of-concept studies in order to establish the detectability of the CA and to predict the role of biodistribution in its uptake mechanisms. A promising approach is to carefully mimic the in vivo pharmacokinetic context with reduced experimental complexity compared to in vivo situations.A dedicated experimental system was built by combining a microfluidic slide and a radiofrequency probe based on a 6 mm diameter multiturn transmission-line resonator. Adherent KB cells were incubated with different concentrations of Gd. MRI data were acquired at 2.35T with a 3D gradient echo and a resolution of 12.4 μm perpendicular to the cell layer. The longitudinal relaxation rate, R1 , was measured as a function of the amount of Gd internalized by the cells.R1 measurements for different Gd concentrations per cell were performed using data with an signal-to-noise ratio (SNR) of 100. Relaxation-rate variations ΔR1 of 0.035 s(-1) were measured. A quenching effect was observed at Gd concentrations above 20 fmol/cell.Our results suggest that this dedicated experimental system is suitable for specifically assessing new high-relaxivity targeted CAs under real-time uptake conditions.

International audience; In this chapter we describe computer‐based tomographic methods using magnetic resonance imaging (MRI) and X‐ray absorption as well as X‐ray phase‐contrast imaging techniques. The latter are based on refraction and near‐field diffraction of X‐rays and are gaining increased attention due to the increase in sensitivity they offer over conventional, attenuation‐based X‐ray techniques. The increased sensitivity is of particular interest in marine biology imaging, especially for soft tissue visualization and analysis. The basic principles of the imaging techniques are described, starting with an analysis of the image formation process, followed by a review of several tomography methods, such as MRI or X‐ray imaging, and practical guidelines for parameter selection. Finally, two relevant examples of imaging are presented: imaging in marine biology (segmentation and visualization) and small animal (sea horse) imaging. These examples illustrate imaging under different conditions and using different reconstruction options.

This article presents a novel inductive decoupling technique for form-fitting coil arrays of monolithic transmission line resonators, which target biomedical applications requiring high signal-to-noise ratio over a large field of view to image anatomical structures varying in size and shape from patient to patient.Individual transmission line resonator elements are mutually decoupled using magnetic flux sharing by overlapping annexes. This decoupling technique was evaluated by electromagnetic simulations and bench measurements for two- and four-element arrays, comparing single- and double-gap transmission line resonator designs, combined either with a basic capacitive matching scheme or inductive pickup loop matching. The best performing array was used in 7T MRI experiments demonstrating its form-fitting ability and parallel imaging potential.The inductively matched double-gap transmission line resonator array provided the best decoupling efficiency in simulations and bench measurements (-15 dB). The decoupling and parallel imaging performance proved robust against mechanical deformation of the array.The presented decoupling technique combines the robustness of conventional overlap decoupling regarding coil loading and operating frequency with the extended field of view of nonoverlapped coils. While demonstrated on four-element arrays, it can be easily expanded to fabricate readily decoupled form-fitting 2D arrays with an arbitrary number of elements in a single etching process.

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In this paper, we detail how microantennae dedicated to Magnetic Resonance Imaging (MRI) can benefit from the advantages offered by polymer substrates, especially flexibility and dielectric properties. We present a monolithic and wireless design based on the transmission lines between conductor windings on both sides of a dielectric substrate and its fabrication process. This last one requires specific plasma treatments to improve polymer/metal adhesion. We have led a comparative study on the effects of the ageing time on the wettability and the metal adhesion to Kapton and Teflon surfaces. Correlation between wettability (water contact angle) and adhesion (tensile strength) has been established. Then, the use of PolyDiMethylSiloxane (PDMS) as biocompatible packaging material and the optimization of its thickness allows us to conserve suitable f0 and Q values in a conducting environment such as the biological tissues. These studies allow us to perform 7 Tesla in vivo MRI of the rat brain with a high spatial resolution of 100 x 100 x 200 µm3 and a Signal to Noise Ratio of 80.

International audience; Methods and Materials Yeast suspensions with increasing [RR e ]: Yeast suspensions at 30% wet wt/vol in minimal medium were bubbled with 95% O 2 /5% CO 2 (n =2) or 95% N 2 /5% CO 2 (n=1) for 1.5 hours before the first 1 H 2 O T 1 IR measurement. In one suspension bubbled with 95% O 2 /5% CO 2 , 10 mole of the uncoupler of oxidative phosphorylation, 2,4-dinitrophenol, (DNP) was added after 1.5 hour of O 2 bubbling, just prior to the first addition of RR e. Serial addition of RR to the yeast suspensions resulted in the following [RR e ] (mM) values in the medium: 0, 0.3, 0.6, 0.9, 1.2, 1.8, 2.4, 2.9, 3.5, 4.7, 7.0, 9.3, 11.6, 16.1, and 20.5. The total volume increased by 18% by the end of the titration, which was accounted for in calculating [RR e ]. The IR data acquired at each [RR e ] were analyzed by 2SX fitting and relaxivity fitting as described below. MR Measurements and Data Analyses: The 64 t I delay increments between the 180 o (composite) pulse and 90 o RF pulses of the IR pulse sequence are as follows (s): 0. Relaxograms: A 1D Inverse Laplace Transform (ILT) written in Matlab (TwoDLaplaceInverse, Magritek Limited, Wellington New Zealand) of [(M Z (∞)-M Z (t I))/2M Z (∞)] produced the longitudinal relaxogram, the apparent relaxation time constant (T 1 ) distribution. A two peak relaxogram yielded H 2 O i T 1 ′ (T′ 1i) and H 2 O e T 1 ′ (T′ 1e) values, where the T 1 ′ was taken as the peak position. The apparent relative water mole fractions were taken as the relative peak area values. Peak positions and areas were determined using Matlab routines (MathWorks Inc, Natick, MA). Extracting exchange parameters: As noted in the main manuscript, two different approaches for extracting exchange parameters from relaxographic data were employed. The first method, "2SX fitting," substitutes Eqs. (4-6) into the right hand side of Eq. (7) [(M Z (∞)-M Z (t I))/2M Z (∞)] = a L exp(-t I R 1L) + a s exp(-t I R 1s) (7) (main manuscript) The values of τ i , p i , and r 1e with fixed values for R 1i and R 1e0 are adjusted to match the IR time-course, [(M Z (∞)-M Z (t I))/2M Z (∞)], observed at each [RR e ] value. The second method, "relaxivity fitting," adjusts Eq. (4) and/or (5) to match the [RR e ]-dependence of the R 1L and/or R 1S obtained from the relaxograms (1, 2). Matlab was used for the 2SX and relaxivity fittings.

International audience; Tissue-engineered scaffolds are made of biocompatible polymers with various structures, allowing cell seeding, growth, and differentiation. Noninvasive imaging methods are needed to study tissue-engineered constructs before and after implantation. Here, we show that high-resolution magnetic resonance imaging (MRI) performed on a clinical 1.5-T device is a reliable technique to assess three-dimensional structures of porous scaffolds and to validate cell-seeding procedures. A high-temperature superconducting detection coil was used to achieve a resolution of 30Â30Â30 mm 3 when imaging the scaffolds. Three types of structures with tuneable architectures were prepared from naturally derived polysaccharides and evaluated as scaffolds for mesenchymal stem cell (MSC) culture. To monitor cell seeding, MSCs were magnetically labeled using simple incubation with anionic citrate-coated iron-oxide nanoparticles for 30 min. Iron uptake was quantified using single-cell magnetophoresis, and cell proliferation was checked for 7 days after labeling. Three-dimensional (3D) microstructures of scaffolds were assessed using MRI, revealing lamellar or globular porous organization according to the scaffold preparation process. MSCs with different iron load (5, 12 and 31 pg of iron per cell) were seeded on scaffolds at low density (132 cells=mm 3) and detected on 3D gradient-echo MR images according to phase distortions and areas of intensely low signal, whose size increased with cell iron load and echo time. Overall signal loss in the scaffold correlated with the number of seeded cells and their iron load. Different organizations of cells were observed depending on the scaffold architecture. After subcutaneous implantation in mice, scaffolds seeded with labeled cells could be distinguished in vivo from scaffold with nonlabeled cells by observation of signal and phase heterogeneities and by measuring the global signal loss. High-resolution 1.5-T MRI combined with efficient intracellular contrast agents shows promise for noninvasive 3D visualization of tissue-engineered constructs before and after in vivo implantation.

We demonstrate the feasibility of detecting individual tumor-infiltrating cells in vivo, by means of cellular magnetic labeling and a 1.5 Tesla clinical MRI device equipped with a high-resolution surface coil. Using a recently developed high-temperature superconducting (HTS) surface coil, single cells were detected in vitro in voxels of (60 microm)(3) at magnetic loads as low as 0.2 pg of iron per cell. The same imaging protocol was used in vivo to monitor infiltration of ovalbumin-expressing tumors by transferred OVA antigen-specific cytotoxic lymphocytes with low iron load.

International audience; The performance of a 12-mm high-temperature superconduct-ing (HTS) surface coil for in vivo microimaging of mice in a standard 1.5T clinical whole-body scanner was investigated. Systematic evaluation of MR image quality was conducted on saline phantoms with various conductivities to derive the sensitivity improvement brought by the HTS coil compared with a similar room-temperature copper coil. The observed signal-to-noise ratio (SNR) was correlated to the loaded quality factor of the radio frequency (RF) coils and is theoretically validated with respect to the noise contribution of the MR acquisition channel. The expected in vivo SNR gain was then extrapolated for different anatomical sites by monitoring the quality factor in situ during animal imaging experiments. Typical SNR gains of 9.8, 9.8, 5.4, and 11.6 were found for brain, knee, back, and subcu-taneous implanted tumors, respectively, over a series of mice. Excellent in vivo image quality was demonstrated in 16 min with native voxels down to (59 m) 3 with an SNR of 20. The HTS coil technology opens the way, for the first time at the current field strength of clinical MR scanners, to spatial resolutions below 10-3 mm 3 in living mice, which until now were only accessible to specialized high-field MR microscopes. Magn Reson Med 60: 917-927, 2008.

Teaching imaging of magnetic resonance (MR) today is still as challenging as it has always been, because it requires admitting that we cannot express fundamental questions of quantum mechanics with straightforward language or without using extensive theory. Here we allow students to face a real MR setup based on the Earth's magnetic field. We address the applied side of teaching MR using a device that is affordable and that proves to be sufficiently robust, at universities in Orsay, France, and San Sebastian, Spain, in experimental practicals at undergraduate and graduate levels. We specifically present some of the advantages of low field for measuring R2 relaxation rates, reaching a power of separation of 1.5 μmol on Mn(II) ions between two water bottles each of half a liter. Finally we propose key approaches for the lecturers to adopt when they are asked to pass from theoretical knowledge to teachable knowhow. The outcomes are fast calibration and the MR acquisition protocols, demonstrating the reproducibility of energy transfer during the saturation pulses, and the quantitative nature of MR, with water protons and a helium-3 sample.

Signal-to-noise ratio improvement is of major importance to achieve microscopic spatial resolution in magnetic resonance experiments. Magnetic resonance imaging of small animals is particularly concerned since it typically requires voxels of less than (100 μm) 3 to observe the small anatomical structures having size reduction by a factor of more than 10 as compared to human being. The signal-to-noise ratio can be increased by working at high static magnetic field strengths, but the biomedical interest of such high-field systems may be limited due to field-dependant contrast mechanisms and severe technological difficulties. An alternative approach that allows working in clinical imaging system is to improve the sensitivity of the radio-frequency receiver coil. This can be done using small cryogenically operated coils made either of copper or high-temperature superconducting material. We report the technological development of cryo-cooled superconducting coils for high-resolution imaging in a whole-body magnetic resonance scanner operating at 1.5 T. The technological background supporting this development is first addressed, including HTS coil design, simulation tools, cryogenic mean description and electrical characterization procedure. To illustrate the performances of superconducting coils for magnetic resonance imaging at intermediate field strength, in-vivo mouse images of various anatomic sites acquired with a 12 mm diameter cryo-cooled superconducting coil are presented.

International audience; This paper presents the feasibility of three-dimensional (3D) magnetic resonance (MR) histology of atheromatous coronary lesions in the entire human heart ex vivo using a standard 1.5-T scanner and a 12-mm high-temperature superconducting (HTS) surface coil. The HTS coil was a five-turn transmission-line resonator operated at 77 K, affording a signal-to-noise ratio (SNR) gain of about ninefold as compared to a similar, room-temperature copper coil. Local microscopy at the surface of an explanted, entire heart was achieved by a 3D spoiled gradient echo sequence and assessed by comparison with conventional histology. One hundred and twenty four adjacent cross sections of the coronary artery, with voxels of 59 × 59 × 100 µm 3 and an SNR of about 20, were obtained in 25 min. Consecutive data sets were combined to reconstruct extended views along the artery. Compared to histology, MR microscopy allowed precise nondestructive 3D depiction of the architecture of the atheromatous plaques. This is the first report of microscopic details (less than 10 −3 mm 3 voxels) of diseased arteries obtained in an entire human heart preserving the arterial integrity and the spatial geometry of atheroma. This noninvasive microscopy approach using a HTS surface coil might be applied in vivo to study the architecture and components of superficial human structures, using routine MR scanners.

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 (T(HTS)). 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 T(HTS), except for the orthogonal orientation above 1 T. The improvement of Q by lowering T(HTS) 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.

International audience; A contactless method based on reflectometry to accurately characterize an inductive radio frequency ͑rf͒ 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 ͑MR͒ applications with superconducting probe heads or analysis of rf properties in nonlinear materials. The method is demonstrated here by characterizing a high temperature superconducting ͑HTS͒ 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 density͒ by including any pertinent model.