epuis une quinzaine d’annees, de nombreux developpements methodologiques et instrumentaux en IRM sont realises afin d’evaluer leur impact sur la securite du patient. Les consequences de ces etudes sont diverses mais principalement fondees sur le respect des normes de securite en vigueur. C’est ainsi que pour repondre a des contraintes liees a la securite radiofrequence, un examen par resonance magnetique est actuellement restreint a l’utilisation soit de capteurs radiofrequence (RF) externes soit de capteurs endocavitaires fortement isoles et exclusivement en utilisation perineale (MEDRAD – Endorectal probe with planar movable MRI coil – Patent number: WO9412102, US5365928). Au cours de notre travail, nous avons etudie l’echauffement RF de conducteurs metalliques qui seraient places au contact ou a l’interieur d’un patient au cours d’un examen IRM. La complexite des phenomenes sousjacents et la difficulte d’interpretation des resultats expliquent en partie la discordance des resultats (1, 2) selon les auteurs. Cette difficulte provient essentiellement de la nature du champ electromagnetique radiofrequence qui est non stationnaire spatialement, et qui varie tres rapidement avec la distance. Nous avons tout d’abord considere un cas simple, celui de fils metalliques en presence d’un champ electromagnetique RF cree par une antenne d’emission corps entier d’un imageur clinique (MAGNETOM Symphony, Siemens Medical Solutions, Erlangen, Allemagne) installe sur un site hospitalo-universitaire. Nous nous sommes mis en situation extreme afin de mieux caracteriser le phenomene et de determiner le danger potentiel pour le patient mais aussi pour le radiologue. L’antenne corps entier a ete chargee avec deux fantomes standard de 2 litres chacun, remplis avec une solution de 1,25 g de NiSO 4 × 6H 2 O et 5 g de NaCl par litre. Le poids introduit dans le fichier patient a ete de 60 kg. Les experiences ont ete realisees sur des fils en cuivre et en Nitinol. Le diametre des fils utilises etaient de 0,14 mm et les longueurs ont variees entre λ /16 et λ, avec la longueur d’onde λ d’une valeur de 4,7 m dans l’air. L’elevation de temperature a ete mesuree par un systeme de mesure par fibre optique (Luxtron model 3204 ; Luxtron Corp., Northwestern Parkway, CA). Ainsi, nous avons montre que l’echauffement RF induit par l’antenne d’emission corps entier, se manifestait aux extremites du fil (effet de pointe) mais aussi le long du fil. Cet echauffement le long du fil apparait pour des distances qui dependent d’une part de la longueur electrique du fil et d’autre part de parametres lies a l’examen qui sont la position du fil dans le tunnel et les parametres de la sequence d’imagerie utilises.

This experimental investigation is focused on a radiation induced red emission in Ge doped silica materials, elaborated with different methods and processes. The differently irradiated samples as well as the pristine ones were analyzed with various spectroscopic techniques, such as confocal microscopy luminescence (CML), time resolved luminescence (TRL), photoluminescence excitation (PLE) and electron paramagnetic resonance (EPR). Our data prove that irradiation induces a red luminescence related to the presence of the Ge atoms. Such emission features a photoexcitation spectrum in the UV-blue spectral range and, TRL measurements show that its decrease differs from a single exponential law with a lifetime of tens of nanoseconds. CML measurements under laser at 633 nm evidenced the lack of correlation of the emission here reported with that of the Ge- or Si- non bridging oxygen hole centers. Moreover, our EPR experiments highlighted the lack of correlation between the red emitting defect with other radiation induced paramagnetic centers such as the E′Ge and Ge(2). The relation of the investigated emission with the H(II) defects, previously considered as responsible for a red emission, can not be totally excluded.

With the development of interventional MRI, heating of biological tissues along the metallic wires in the MRI scanner has become an important issue. To assess thermal response to RF exposure during MRI, we studied the temperature elevation near nonmagnetic metallic wires. All tests were performed on a 1.5 T clinical scanner. Four experiments were conducted to investigate the effects of the wire diameter, the excitation flip angle, the temperature distribution along the wire, and the wire length. Electromagnetic simulations of the experimental setup were made with the use of commercial method of moments (MoM) software and numerical simulations of Hallén's equations. Comparisons between measured and calculated values of the electric field are presented. This study demonstrates that 1) temperature decreases with the diameter of the wire,2)temperature increases quadratically with the excitation flip angle, 3) heating occurs not only at the tip but also along the wire, and 4) the heating peaks are not obtained for the classical resonant length multiple of λ/4 (where λ is the RF field wavelength). In addition, significant and rapid heating increases were observed in the close vicinity of the wire. Magn Reson Med 52:1200–1206, 2004. © 2004 Wiley‐Liss, Inc.

Synopsis Heating of biological tissues along metallic wires placed in MRI scanner has become an important question with the development of interventional MRI. This paper presents results about temperature variation due to RF heating along a wire with two different lengths during a MRI examination. Measured values are compared with simulation results. Study shows that the temperature elevation may be important and does not appear only at wire tips.

Feasibility of gastrointestinal walls imaging using endoluminal coils is described. A single-loop coil was built to be inserted into a gastric tube. Endoluminal coil performance was evaluated on a 1.5 T clinical scanner and in vivo high-resolution Magnetic Resonance Imaging (MRI) of rabbit gastrointestinal walls was performed. Images allow visualization and identification of rabbit gastrointestinal walls. Tuning/matching circuit and decoupling strategies are described. Tracking capabilities and safety issues are discussed. Dedicated endoluminal RF coil provided a dramatic increase in signal-to-noise ratio (SNR) at the region of interest as compared with clinical multi-elements array coil. Very high resolution images of in vivo rabbit colon walls was achieved providing detailed information about the gastrointestinal wall layers. This technique could be considered on human for accurate tumoral and inflammatory bowel diseases diagnosis.