Xavier Maître

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.

Magnetic susceptibility matching at the air–tissue interface in rat lung by using a superparamagnetic intravascular contrast agent: Influence on transverse relaxation time of hyperpolarized helium-3†

Transverse relaxation of hyperpolarized helium‐3 magnetization in respiratory airways highly depends on local magnetic field gradients induced by the magnetic susceptibility difference between gas and pulmonary tissue. Fast transverse relaxation is known to be an important feature that yields information about lung microstructure and function, but it is also an essential limitation in designing efficient strategies for lung imaging. Using intravascular injections of a superparamagnetic contrast agent in rats, it was possible to increase the overall susceptibility of the perfused lung tissues and hence to match it with the gas susceptibility. The transverse decay time constant of inhaled hyperpolarized helium‐3 was measured in multiple‐spin‐echo experiments at 1.5 T as a function of the superparamagnetic contrast agent concentration in the animal blood. The time constant was increased by a factor of 3 when an optimal concentration was reached as predicted for susceptibility matching by combining intrinsic susceptibilities of tissue, blood, and gas. Magn Reson Med 54:28–33, 2005. Published 2005 Wiley‐Liss, Inc.

High nuclear polarization of 3He at low and high pressure by metastability exchange optical pumping at 1.5?tesla

Metastability exchange optical pumping of helium-3 is performed in a strong magnetic field of 1.5 T. The achieved nuclear polarizations, between 80% at 1.33 mbar and 25% at 67 mbar, show a substantial improvement at high pressures with respect to standard low-field optical pumping. The specific mechanisms of metastability exchange optical pumping at high field are investigated, advantages and intrinsic limitations are discussed. From a practical point of view, these results open the way to alternative technological solutions for polarized helium-3 applications and in particular for magnetic-resonance imaging of human lungs.

Velocity-selective RF pulses in MRI.

A family of velocity‐selective pulses consisting of a series of RF hard pulses followed by bipolar gradients was designed. The succession of required pulses was deduced using a k‐space approach within a small tip‐angle approximation. Fourier transform of the desired velocity excitation determined the flip‐angle series, and the corresponding position in the generalized k‐space identified the bipolar‐gradient first moments. Spins from any velocity class can be selected. To illustrate this approach we designed and experimentally tested a velocity‐slice selection that is analogous to standard spatial‐slice selection but involves excitation of spins moving at a chosen velocity (velocity‐slice center) and within a given interval (velocity‐slice thickness). The assumed approximation does not limit the design to small angles, because velocity selection still holds for angles up to 90°. Velocity slices were experimentally selected, centered on velocities ranging from −1 m s−1 to 1 m s−1 with a velocity‐slice thickness of 0.4 m s−1. The experimental velocity‐slice profile was assessed and the flow was quantified. Magn Reson Med, 2006.

Phase-contrast helium-3 MRI of aerosol deposition in human airways.

One of the key challenges in the study of health‐related aerosols is predicting and monitoring sites of particle deposition in the respiratory tract. The potential health risks of ambient exposure to environmental or workplace aerosols and the beneficial effects of medical aerosols are strongly influenced by the site of aerosol deposition along the respiratory tract. Nuclear medicine is the only current modality that combines quantification and regional localization of aerosol deposition, and this technique remains limited by its spatial and temporal resolutions and by patient exposure to radiation. Recent work in MRI has shed light on techniques to quantify micro‐sized magnetic particles in living bodies by the measurement of associated static magnetic field variations. With regard to lung MRI, hyperpolarized helium‐3 may be used as a tracer gas to compensate for the lack of MR signal in the airways, so as to allow assessment of pulmonary function and morphology. The extrathoracic region of the human respiratory system plays a critical role in determining aerosol deposition patterns, as it acts as a filter upstream from the lungs. In the present work, aerosol deposition in a mouth–throat phantom was measured using helium‐3 MRI and compared with single‐photon emission computed tomography. By providing high sensitivity with high spatial and temporal resolutions, phase‐contrast helium‐3 MRI offers new insights for the study of particle transport and deposition. Copyright

Space and phase normalisations in motion correction for magnetic resonance elastography

Magnetic resonance elastography (MRE) aims at characterising the properties of tissues by following a mechanical wave induced in a target tissue along the three spatial directions. Hence, the shear viscoelastic moduli may be inferred from the inversion of the wave equations (Sinkus et al. 2005). Long acquisition times may lead to a motion-induced mismatch of the displacement field components as a result of physiological, cardiac, respiratory, physical and patient motions. In this work, an original motion-correction scheme, including space and phase normalisations, is presented. It was implemented and tested onto an easily-motion-controlled whole-brain MRE dataset, in which motion was added. This approach improved the processing of MRE data for tissue rotation of already less than a degree.