Results Minimum radial variation of signal intensity was found for an angle between 50 and 70°, depending on the radius. The quality factor Q of the “50° coil” embedded in salty water mimicking loading conditions was 124. SNR measured at the close vicinity of the coil (about 5 mm from the center) compared with SNR measured with the regular four-element phased array body coil used for abdominal clinical exams was 30 times higher. The sensitivity pattern falls off rapidly with distance from the coil. The sensitivity of the double loop coil was rather uniform to a fixed radius and improved compared to a signal loop coil. Moreover, it provides a better SNR than the four-element phased-array body coil up to about a radius of 30 mm from the coil centre. The anal sphincter complex was visualized with detail (Fig. 3). The high spatial resolution and excellent tissue contrast in images allowed excellent visualization of anal mucosa/submucosa complex, internal sphincter, and the external sphincter. Conclusion An endoluminal loop RF coil has been developed for the anal apparatus. The internal coil provides a dramatic increase in SNR local to the coil, compared to usual external coils. High-resolution magnetic resonance imaging acquired in the swine provides excellent visualizations of the normal anal sphincter complex anatomy. The results are promising and suggest useful applications in the management of anorectal diseases with minimally invasive procedure.

2 internal and 18 x 20 mm 2 external dimensions). One conductor of this loop is common for the two elements. The S-parameters and the quality factor were measured with an ENA300 network analyzer (Agilent Technologies Inc., Santa Clara, CA, USA). The decoupling between the two channels was achieved using a fixed capacitor inserted in the common conductor. The value determined by simulation was experimentally adjusted to minimize the S21 transfer parameter between the two channels. Both channels were tuned at 300.3 MHz corresponding to the proton's resonance frequency at 7 T and matched to 50 impedance line using non-magnetic case A series 100 and 710 ATC capacitors (American Technical Ceramics, New York, USA). The tuning/matching and active decoupling circuit was designed to be interfaced with the system decoupling box. The MRI experiments were performed on a 7T Biospec (Bruker, Ettlingen, Germany) equipped with 4-proton receiver channels. The designed phased array coil was compared to a Bruker 15 mm diameter surface coil. Experimental characterization (signal uniformity and SNR) was performed on cylindrical phantoms filled with salty water (NaCl 0.45%) mimicking load conditions. For in vivo experiments, the ethical guidelines for experimental investigations with animals were followed, and the experimental protocol was approved by the Animal Ethics Committee of our institution. Gaseous anesthesia was performed on adult rats placed in supine position. The 15 mm diameter surface coil was placed in contact with the medial side of knee joint and the designed array coil was placed on top of patella to encompass the whole knee joint. The HR-MRI of the rat knee joint was performed using a 3D Gradient-Echo Fast Imaging (GEFI) sequence with the following parameters: 25° flip angle, 50 ms TR, 3.4 ms TE, 42 kHz rbw. A total of 64 partitions (312 µ m thick) were acquired with a FOV of 30 x 30 mm 2 and an acquisition matrix size of 512 x 384. Acquisition volume was reconstructed to a 512 x 512 x 128 matrix leading to a 156 µ m partition thickness and an in-plane pixel of 59 x 59 µ m 2 . The scan time for the GEFI sequence was 45 min. The femoral and tibial plateaus (medial and lateral) articular cartilage volumes were extracted using an interactive touch-sensitive screen with a 1280 × 1024 pixel matrix. The user segmented the knee cartilage compartments directly on this screen using the supplied pen. The articular cartilage was segmented on each MRI slice. Each segmented area was then assigned to its corresponding cartilage compartment using gray-scale code labels leading to the three articular cartilage volumes. Additionally, a proof of concept was performed using two independent phased array coils (one for each knee joint). The phased array coils were decoupled using a 20 x 25 mm 2 copper sheet, placed at equal distance between the two array coils. Additional supply voltage sources were used for tuning and matching of the second array coil. Both legs were acquired within the same scan using a similar acquisition as for a single knee joint with the same pixel size but with a larger FOV in a coronal plane. Results The measured quality factor of the unloaded coil was about 130 for every single channel. The quality factor of the loaded coil decreased to 110. The decoupling capacitor value mounted on the circuit was about 56 pF corresponding to a simulated mutual inductance of 9.4 nH. The isolation between the two channels was 27 dB. The decoupling between the two array coils separated by a thin copper layer for multiple knee imaging was 28 dB. The SNR gain in the ROI for the two-channel array coil was up to 2.2 compared to the SNR obtained with the 15 mm diameter surface coil. The signal intensity was more uniform with a SNR standard deviation of 5 compared to 27 measured on images acquired with the surface coil. The 2.2 gain in SNR was used in vivo to decrease the voxel size from 59 x 59 x 156 µ m 3

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.

Objective: To develop a quantitative non-invasive in vivo three-dimensional (3D) high resolution (HR) micro-magnetic resonance imaging (mu MRI) protocol to measure the medial tibial cartilage thickness (MT.ThC) in the normal rabbit and in the anterior cruciate ligament transection (ACLT) rabbit model of osteoarthritis and quantify the progression of MT.ThC.Methods: The left knee of 10 control and 40 operated rabbits was imaged in vivo with a 7 T mu MRI system at 3 and 5 months after ACLT. A 3D fast low angle short (FLASH) fat-suppressed MRI protocol was implemented leading to 44 x 176 mu m(3) spatial resolution and to 44 mu m(3) isotropic voxel after cubic interpolation. Semi-automatic MT.ThC measurements were made in 3D, in four different locations, in vivo and longitudinally in both groups. At 5 months, gross macroscopy, visual analogical evaluation of the cartilage and histology were compared to the MR-based MT.ThC.Results: At 3 and 5 months, the MT.ThC measured in the minimum interbone distance area was the thinnest MR-based MT.ThC. It was significantly lower in the operated group and among the four evaluated MT.ThC, it was the most discriminative between the normal and the operated groups (P < 0.05). The MT.ThC measured in the minimum interbone distance area was also the most sensitive to change in the operated group (66.4% MT.ThC loss, P = 0.003) while no significant changes were observed in the control group.Conclusion: Quantitative 3D HR mu MRI allowed for non-invasive longitudinal MT.ThC measurements in four different locations in both the normal and the operated rabbits. We concluded the MT.ThC measured in the minimum interbone distance area reflected the severity of the disease and was the most effective to measure the progression of the medial tibial cartilage destruction. (c) 2007 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.

PURPOSE:To assess in vivo distal colon wall magnetic resonance imaging (MRI) feasibility on rabbits using an endoluminal radio frequency (RF) coil on a 1.5-T clinical scanner.MATERIALS AND METHODS:The endoluminal coil signal-to-noise ratio (SNR) was compared to a clinical four-element phased-array body coil. High-resolution (HR) MRI of rabbit colon walls was performed on six rabbits. The imaging protocol combined T1-weighted fast low-angle-shot (FLASH) sequences with and without fat saturation (FS), T2-weighted True-Fast imaging with steady state precession (Fisp), turbo spin-echo (TSE), and T1-weighted FLASH FS after contrast media injection. Images were compared to histological sections. Catheter tracking using an endoluminal coil in addition to external coils was also evaluated on two rabbits.RESULTS:HR images allow visualization and identification of rabbit colon wall layers. Real-time tracking allows a clear visualization and a good positioning of the endoluminal coil within the rabbit.CONCLUSION:Compared to a clinical multielement array coil, a dedicated endoluminal RF coil provides an important SNR increase at the region of interest (ROI). Very HR images of in vivo rabbit colon walls were achieved providing detailed information on the different wall layers. This technique could be considered on humans for accurate tumoral and inflammatory bowel disease diagnosis.

The present growing field of molecular imaging, including multimodality microimaging techniques and spectroscopic approaches, is mainly based on small animal studies. Monitoring such models requires an efficient treatment and use of electrophysiological signals which may be spoiled by environmental effects especially when working with nuclear magnetic resonance (NMR) since radiofrequency (RF) pulses and magnetic field gradient commutations may create spurious supplementary signals. In this work, a method is given for EEG and EMG denoising of signals acquired during phosphorous magnetic resonance (MR) brain spectroscopy data acquisition on a rat model developed for sleeplawake studies. The proposed approach is based on wavelet decomposition and the key method is to turn into profit the shape variations of EMG during the time course of sleep/awake cycles. Statistical properties of the noise are studied using EMG recorded during paradoxical sleep as noise model. A specific estimation of noise level using EMC recorded during slow sleep leads to an optimal wavelet coefficients thresholding. This approach is well suited to improve signal to noise ratio of EEG and EMG and to preserve small amplitude electrophysiological signals.

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.

Rationale and Objectives. Despite improvements, spatial resolution and image quality with routine surface coils are too limited when detailed information about the gastrointestinal layers is requested. The objective of our feasibility study was to evaluate the potential of a dedicated endoluminal coil to depict different layers of the colonic wall in an in vitro small animal model.Materials and Methods. A single-loop coil (40 mm length, 5 mm width) was built using IC (printed circuit) technology. The coil was tuned to a frequency of 63.7 MHz and matched at 50 Omega for this frequency. The coil was housed in a biocompatible tube with an outer diameter of 18 F (6 mm). Ten segments of rat colon, surgically excised 5 hours earlier, were completely immersed in an isotonic solution. The coil was introduced through the lumen of colonic specimens. MRI experiments were performed on a 1.5 T MR Symphony system (Siemens, Erlangen, Germany) using imaging protocol combining high-resolution 2D Flash, fast imaging employing steady-state acquisition (TrueFISP), turbo spin echo (TSE), and 3D FastLow-Angle Shot (FLASH) sequences. After a 24-hour period of fixation in 10% formalin, colonic specimens were excised along the longitudinal axis for histologic analysis.Results. The endoluminal coil provided high SNR allowing for the visualization of different layers of rat colonic walls. All the performed sequences made it possible to identify at least two different layers. On T1-weighted gradient-echo sequences, the mucosa was of high signal intensity, whereas the muscle layers had an intermediate to low signal intensity. The signal intensity of different wall layers was similar in different sequences. Histologic analysis identified three main layers.Conclusion. These results are well correlated with histologic findings and suggest that endoluminal MR imaging may have potential for accurate staging of colonic tumor or inflammatory process.

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.

The extraction of nuclear magnetic resonance (NMR) spectra of samples having smaller and smaller volumes is a real challenge. Reductions of volume are dictated by the difficulties of production of sufficiently large samples or by necessities of miniaturization of the analyzing system. In both cases a careful design of the radiofrequency (RF) coil, ensuring an optimum reception of the NMR signal, is mandatory. We evaluated the usefulness of an electromagnetic simulation software for the design and optimization of these radio-frequency coils, which are more and more used in biology and health research projects. The contribution of different effects (dc, skin and proximity) at the total resistance of the coils were assessed as well as the expected SNR per sample volume unit and the quality factor. Designed for a biological application, these coils have to be as less invasive as possible and they must allow small quantities analysis of metabolites inside capillary tubs or surrounding the microcoil. In order to evaluate detection efficiency and spectral resolution, preliminary experiments have been performed at 85.13 MHz under a static magnetic field of 2 T.

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.

The extraction of the Nuclear Magnetic Resonance (NMR) spectra of samples having smaller and smaller volumes is a real challenge. Either these reductions of volume are dictated by the difficulties of production of sufficiently large samples or by necessities of miniaturisation of the analysing system, in both cases a careful design of the radiofrequency coil, ensuring an optimum reception of the NMR signal, is required. We have also evaluated the usefulness of electromagnetic simulation software for the design and optimisation of these radio-frequency coils, which are more and more used in biology and health research projects. (C) 2002 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

This communication reports the design, the fabrication and preliminary tests of planar microcoils used as nuclear magnetic resonance (NMR) radio frequency (rf) detection coils. The magnetic coupling between a rf coil and a sample is maximized by physically designing the coil as small as possible as to just accommodate the sample. Maximum coupling corresponds directly to an optimal sensitivity for the signal reception. Designed for a biological application, these coils have to be as less invasive as possible and they must allow studying small quantities of metabolites surrounding the microcoil. NMR microspectroscopy experiments have been performed in water and butter samples using a 500μm×1000μm planar microcoil tuned and matched at 85.13MHz (proton’s frequency at 2T).

The present growing field of molecular imaging, including multimodality microimaging techniques and spectroscopic approaches, is mainly based on small animal studies. Monitoring such models requires an efficient treatment and use of electrophysiological signals which may be spoiled by environmental effects especially when working with nuclear magnetic resonance (NMR) since radiofrequency (RF) pulses and magnetic field gradient commutations may create spurious supplementary signals. In this work, a method is given for EEG and EMG denoising of signals acquired during phosphorous magnetic resonance (MR) brain spectroscopy data acquisition on a rat model developed for sleep/awake studies. The proposed approach is based on wavelet decomposition and the key method is to turn into profit the shape variations of EMG during the time course of sleep/awake cycles. Statistical properties of the noise are studied using EMG recorded during paradoxical sleep as noise model. A specific estimation of noise level using EMG recorded during slow sleep leads to an optimal wavelet coefficients thresholding. This approach is well suited to improve signal to noise ratio of EEG and EMG and to preserve small amplitude electrophysiological signals.