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 x 59 x 100 microm3 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.

Perspectives with cryogenic RF probes in biomedical MRI.

Since discovery of high-temperature superconductive (HTS) ceramics by Bednorz and Muller in 1986, there has been an accelerated development of cold technologies in industry, including the domain of NMR detection. The purpose of this paper is to fix ideas about the stage that cryogenic radio frequency (RF) probe techniques have reached in biomedical magnetic resonance imaging (MRI). Readers confronted to the literature about this emerging topic have to understand a large range of motivations with somewhat unclearly defined technical limitations and actual outlets. An overview of sensitivity issues in the general context of biomedical MRI is provided here and the contribution of RF coil techniques to recent advances is identified. The domains where cooled coil materials such as copper, low- or high-temperature superconductors, could actually increase the RF coil sensitivity are delimited by a quantitative analysis of noise mechanisms. Technical keys, cryogenic means and cold RF coil technologies are considered, and first achievements in different fields of biomedical MRI are reviewed. This survey provides a basis for discussing about the future impact of cryogenic probes for MRI investigations.

CPMG measurements and ultrafast imaging in human lungs with hyperpolarized helium-3 at low field (0.1 T).

This work reports the use of single-shot spin echo sequences to achieve in vivo diffusion gas measurements and ultrafast imaging of human lungs, in vivo, with hyperpolarized (3)He at 0.1 T. The observed transverse relaxation time of (3)He lasted up to 10 s, which made it possible to use long Carr-Purcell-Meiboom-Gill echo trains. Preliminary NMR studies showed that the resolution of lung images acquired with hyperpolarized (3)He and single-shot sequences is limited to about 6 mm because of the diffusion of the gas in applied field gradients. Ultrafast images of human lungs in normal subjects, achieved in less than 0.4 s with the equivalent of only 130 micromol of fully polarized (3)He, are presented. Comparison with other studies shows that there is no SNR penalty by using low fields in the hyperpolarized case. Advantage was taken of the self diffusion-weighting of the rapid acquisition with relaxation enhancement (RARE) sequence to acquire apparent diffusion coefficient (ADC) images of the lungs. Time scales of seconds could be explored for the first time because there is no hindrance from T(*)(2) as with the usual approaches. At 0.1 T, 180 degrees RF pulses can be repeated every 10 ms without exceeding specific absorption rate limits, which would not be the case for higher fields. Moreover, at low field, susceptibility-induced phenomena are expected to be milder. This supports the idea that low-field imagers can be used for hyperpolarized noble gas MRI of lungs and may be preferred for ADC measurements.

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.

Magnetization transfer characteristics in atherosclerotic plaque components assessed by adapted binomial preparation pulses.

Increasing the contrast between atheromatous plaque components is a major issue in cardiovascular MRI research. It would allow one to identify unstable plaque by differentiating the lipid core associated with vulnerability, from the fibrous cap, considered as a factor of stability. T2 and diffusion-weighted imaging have already provided satisfying results. Magnetization transfer (MT) between restricted protons Hr and free-water protons Hf could achieve a different contrast related to collagen and lipoprotein macromolecules present in the fibrous cap and lipid core, respectively. The purpose of this work was to evaluate in vitro the MT effect produced by adapted T2-selective 1-3-3-1 binomial pulses on isolated samples of atheromatous arteries at 3 T. A method based on simulation was used in order to improve the MT specificity: it is shown that 50% 1-3-3-1 pulses (the percentage indicating the level of Hr saturation) allow an estimation of T2r, the Hr T2. Using this technique, magnetization transfer was observed for the first time in atherosclerotic plaque components, an effect more pronounced for the fibrous cap and media than for the lipid core and adventitia. The T2r estimation gave values ranging from 20 to 25 micros for the four samples. This preliminary study provides a basis for establishing an MT imaging sequence of atheromatous arteries, by using 50% 1-3-3-1 pulses calibrated for saturating protons with a 20 micros T2. This MT protocol should be further compared to T2 and diffusion-weighted imaging.

Multi-turn split-conductor transmission-line resonators.

Multi-turn split-conductor transmission-line resonators (MSTR) can be designed to operate over a wide range of NMR operating frequencies without lumped tuning capacitors. This architecture constitutes an alternative solution to recent designs proposed for high-Q, thin-film, high-temperature superconducting NMR probes. An advantage is that the resonant frequency can be calculated in a relatively simple manner in terms of coil turns or total coil length, coil width, substrate thickness, and dielectric constant. Analytical calculation of the resonant frequency is provided. Also, a series of MSTRs was constructed on a double copper-clad substrate, and their resonant frequencies are noted. The results obtained were in good agreement with the predicted values.

Optimization of T2-selective binomial pulses for magnetization transfer.

Accurate rules have been established to build binomial pulses up to fifth order, for selectively saturating protons at any given T2 with minimum power deposition. Pulse performance and sensitivity to experimental defects have been evaluated; the third order is generally found to be best suited. It is shown, by combining theory and experiment performed at 0.1 T, that matching the saturation pulse to T2 of the motionally restricted pool is essential to reveal exchange with free water protons. It is emphasized that, to date, lack of magnetization transfer contrast with binomial pulses is due mainly to insufficient RF level available with most MR imaging systems, especially at high magnetic field.

Implementation and optimization by the simplex method of a 3D double echo sequence in steady-state free precession.

Under steady-state conditions, the resulting echoes have very complex T1 and T2 relationships. Many authors exploited these echoes in different sequences to produce either T1- or T2-weighted images. The simultaneous acquisition of two echoes in a single sequence provides two images of clearly different contrasts. We implemented such a sequence, in a 3D-acquisition mode, combining the advantages of thin and contiguous slices with those of a multi-echo sequence. The contrast of the images was correlated with theoretical results, derived from Bloch equations. In order to estimate the acquisition parameters (alpha, TE, TR) to obtain an optimal T1- or T2-contrast between two tissues, a computer simulation of these equations was used in conjunction with the simplex method. The results show that this sequence improves the clinical efficiency of MRI, particularly in neurological and articular disease.

Optimization of NMR receiver bandwidth by inductive coupling.

We show, by theory and experiment, that inductive coupling can be used in overcoupled mode to widen the bandwidth of a high-Q NMR coil with only a negligible degradation of signal-to-noise ratio over the bandwidth of interest. The receiver bandwidth depends on the coupling coefficient between the NMR coil and the coupling coil rather than on the quality factor of the NMR coil alone. The overall bandwidth can be optimized by a judicious choice of the coupling coefficient. Moreover, this method permits wireless reception without the need for retuning and rematching despite changes of NMR coils or samples. This technique has been incorporated in a 0.1-T imager for clinical routine. It achieves a typical bandwidth five times greater than that using a classic 50-omega matching method.

In vivo high-resolution MR imaging of the skin in a whole-body system at 1.5 T.

High-resolution magnetic resonance (MR) images of the skin were acquired with a whole-body MR system at 1.5 T by adding a specific imaging module: A saddle-shaped surface gradient coil was connected in place of one of the gradient coils of the system, and a surface radio-frequency coil with a 1.5-cm radius was placed at the center of the gradient coil. The images, acquired in 3 minutes 25 seconds, represent a field of view of 18 x 50 mm2, which corresponds to a pixel size of 70 x 390 micro2; the section thickness was 1.2 mm. With this spatial resolution, the different layers of the skin are clearly delineated: Epidermis appears as a high-signal-intensity layer, while dermis appears hypointense due to its very short T2. Pilosebaceous units appear as inclusions of epidermis inside dermis. The high quality of the images obtained enables in vivo MR characterization of skin.

The slotted cylinder: an efficient probe for NMR imaging.

The slotted cylinder, an inductive structure with low self-inductance, low electric field, has been studied as a probe for NMR imaging applications. A theoretical calculation allows us to map the magnetic field and to evaluate electrical parameters of the structure. Several implementations, including new designs, have been experimentally tested over a wide range of frequencies (4-40 MHz), and compared to a classical coil probe. This study demonstrates the efficiency of the slotted cylinder for NMR imaging. It is optimized for large conductive samples when imaging at high frequencies (for human head, above 20 MHz).