Jean-Christophe Ginefri

High-temperature superconducting surface coil for in vivo microimaging of the human skin

A small, high‐temperature superconducting (HTS) surface coil was used to improve the signal‐to‐noise ratio (SNR) for in vivo human skin microscopy at 1.5 T. The internal noise of the conventional copper coil limits the SNR for this application. Inductive measurements of the HTS coil parameters indicated that at 77 K its internal noise contributed about 4% of the total noise, and the predicted SNR gain was about 3.2‐fold over that of a room‐temperature copper coil. In vivo images of the human skin produced with the HTS coil showed highly resolved details and a 3.7‐fold improvement in SNR over that obtained with the room‐temperature copper coil. Magn Reson Med 45:376–382, 2001.

In vivo single cell detection of tumor-infiltrating lymphocytes with a clinical 1.5 Tesla MRI system

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.

Quick measurement of nuclear magnetic resonance coil sensitivity with a single-loop probe

The coil sensitivity for nuclear magnetic resonance (NMR) signal detection can be defined as B1/P, which represents the magnetic field that the coil induces per unit supplied power. An inductive probe was previously proposed to quickly evaluate the sensitivity of a tuned NMR coil. As this probe uses two loops, decoupled by means of a slight overlapping, it is not applicable to small NMR coils since the decoupling efficiency is limited by the loop dimensions. We present a new method, derived from the former, which uses a single-loop probe and allows accurate sensitivity mapping even for very small NMR coils. This new method proves to be in good agreement with both the theoretical formula and a reference method for assessing a simple coil’s sensitivity. We conclude that it is both reliable and particularly convenient for development and optimization of small coils for surface MR imaging.

Performance of a miniature high‐temperature superconducting (HTS) surface coil for in vivo microimaging of the mouse in a standard 1.5T clinical whole‐body scanner

The performance of a 12‐mm high‐temperature superconducting (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 subcutaneous implanted tumors, respectively, over a series of mice. Excellent in vivo image quality was demonstrated in 16 min with native voxels down to (59 μm)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 mm3 in living mice, which until now were only accessible to specialized high‐field MR microscopes. Magn Reson Med 60:917–927, 2008.

Novel inductive decoupling technique for flexible transceiver arrays of monolithic transmission line resonators.

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.

Comparison of radio-frequency and microwave superconducting properties of YBaCuO dedicated to magnetic resonance imaging

Properties of YBaCuO thin films are evaluated in two distinct frequency ranges using different patterns made during the same process on LaAlO/sub 3/ substrate. Microwave superconducting properties in the range 1-45 GHz are determined by S-parameters measurement of a superconducting coplanar waveguide in the range 53-95 K. We obtain a surface resistance of 0.4 m/spl Omega/ at 10.8 GHz and 77 K. Radio-frequency properties are obtained by measuring the Q-factor of a superconducting resonator (YBCO multiturn transmission lines separated by a sapphire sheet) dedicated to surface magnetic resonance imaging. At 52 MHz and 77 K we measure a Q-factor of 33180. The extraction of the radio-frequency surface resistance from Q-factor measurements in the 64-95 K range takes into account external loss mechanisms and nonuniform normal current distribution and leads to a 0.0093-/spl mu//spl Omega/ surface resistance at 52 MHz and 77 K, in good agreement with the value extrapolated from microwave measurements assuming an /spl omega//sup 2/ frequency dependence. The evaluation of /spl lambda//sub 0/ is carried out by using several models for X/sub L/(t). Least squares fits to data in the microwave and radio-frequency domain are performed using the Gorter-Casimir expression for X/sub L/(t) and give the same /spl lambda//sub 0/ value for both devices.

Implanted, inductively-coupled, radiofrequency coils fabricated on flexible polymeric material: application to in vivo rat brain MRI at 7 T.

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.

Preliminary ex vivo 3D microscopy of coronary arteries using a standard 1.5 T MRI scanner and a superconducting RF coil

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 μm3 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 mm3 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.

Method for nonlinear characterization of radio frequency coils made of high temperature superconducting material in view of magnetic resonance imaging applications

A contactless method based on reflectometry to accurately characterize an inductive radio frequency (rf) 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 (MR) applications with superconducting probe heads or analysis of rf properties in nonlinear materials. The method is demonstrated here by characterizing a high temperature superconducting (HTS) 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 density) by including any pertinent model.

Multi-turn multi-gap transmission line resonators – Concept, design and first implementation at 4.7 T and 7 T

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 B1 distribution of MTMG-TLRs strongly depends on the number and distribution of turns. A comparison to conventional loop coils revealed that the B1 performance of MTMG-TLRs is comparable to a loop coil with the same mean diameter; however, lower 10g SAR values were found for MTMG-TLRs. The MTMG-TLR design is expected to bring most benefits at high static field, where it allows for independent size and frequency selection, which cannot be achieved with standard TLR design. However, it also enables more accurate geometric optimization at low static field. Thereby, the MTMG-TLR design preserves the intrinsic advantages of TLRs, i.e. mechanical flexibility, high SAR efficiency, mass production, and coil miniaturization.