Luc Darrasse

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 3He at 0.1 T. The observed transverse relaxation time of 3He 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 3He 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 μmol of fully polarized 3He, 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° 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. Magn Reson Med 47:75–81, 2002.

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.

Quick measurement of NMR‐coil sensitivity with a dual‐loop probe

The radio‐frequency coil sensitivity for NMR signal detection can be defined as B1/√P, which is the magnetic field that the coil induces at a given point per unit supplied power. We propose a new method, based on the reciprocity theorem, to quickly evaluate the sensitivity of a tuned coil. The only requirement of the method is a voltage gain measurement with a small dual‐loop probe, without mobilization of the NMR unit. The probe can be used to gauge and map the sensitivity of the coil with good accuracy, while monitoring its tuning frequency ω0 and its quality factor Q. Our method is particularly convenient for development and maintenance of coils for MR imaging, and more generally could be applied to other fields involving radio‐frequency characterization of inductive circuits.

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.

Drying of a porous rock monitored by NMR imaging

The drying of a limestone block was studied by analysing one-dimensional projections (profiles) and three-dimensional (3D) NMR images as well as free induction decay (FID), as a function of the amount of water remaining in the sample at various stages of the drying process. From these results, the water repartition in the sample stays continuous and fairly uniform down to a water content W of the order of 2% in weight of water per weight of rock. However, at the end of the drying, for W<2%, water is no longer uniformly distributed. The remaining water is concentrated in the centre of the block where the NMR signal is intense, whereas in the surrounding region the NMR signal is drastically weaker. This is the first report on drying of a porous stone monitored by NMR imaging through to its late stage. These first results show spatial compartmentation and modification of the physicochemical state of the protons during the drying, which would be difficult to observe with other techniques. The authors suggest a two-compartment model which is consistent with images and profiles.

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.

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.

Phase-contrast velocimetry with hyperpolarized 3He for in vitro and in vivo characterization of airflow

This paper describes a technique that combines radial MRI and phase contrast (PC) to map the velocities of hyperpolarized gases (3He) in respiratory airways. The method was evaluated on well known geometries (straight and U‐shaped pipes) before it was applied in vivo. Dynamic 2D maps of the three velocity components were obtained from a 10‐mm slice with an in‐plane spatial resolution of 1.6 mm within 1 s. Integration of the in vitro through‐plane velocity over the slice matched the input flow within a relative precision of 6.4%. As expected for the given Reynolds number, a parabolic velocity profile was obtained in the straight pipe. In the U‐shaped pipe the three velocity components were measured and compared to a fluid‐dynamics simulation so the precision was evaluated as fine as 0.025 m s−1. The technique also demonstrated its ability to visualize vortices and localize characteristic points, such as the maximum velocity and vortex‐center positions. Finally, in vivo feasibility was demonstrated in the human trachea during inhalation. Magn Reson Med, 2006.

Low-field 3He nuclear magnetic resonance in human lungs

We report the first in vivo NMR experiments performed with hyperpolarised 3He gas at 0.1 T. Hyperpolarisation is achieved by laser optical pumping of 3He atoms in the metastable triplet state. Longitudinal relaxation times of nuclear magnetisation of the order of 35 s are measured in human lungs. Very long transverse relaxation times of a few seconds are also observed. This work establishes the possibility of magnetic resonance imaging of human lung airways at low field.

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.