Yannick Crémillieux

Helium-3 MRI diffusion coefficient: correlation to morphometry in a model of mild emphysema

Hyperpolarised gases have been most recently used in magnetic resonance imaging to demonstrate new image-derived pulmonary function parameters. One of these parameters is the apparent diffusion coefficient, which reflects the sizes of the structures that compartmentalise gas within the lung (i.e. alveolar space). In the present study, noninvasive parameters were compared to microscopic measurements (mean linear intercept and mean alveolar internal area). Nonselective helium‐3 gas density coronal ex vivo images and apparent diffusion maps were acquired in control and elastase-induced panacinar emphysema rats. Total lung capacity was considered the reference for both imaging experiments and lung fixation. A mild degree of emphysema was found based on mean linear intercept (134±25 µm) versus control (85±14 µm). The apparent diffusion coefficients were significantly different between the two groups (0.18±0.02 and 0.15±0.01 cm2·s−1 for elastase and control, respectively). A significant correlation between the apparent diffusion coefficient and corresponding morphometric parameters in mild emphysema was demonstrated for the first time. This study opens the possibility of estimating absolute airspace size using noninvasive techniques.

Correlation between magnetic resonance imaging disturbances and the magnetic susceptibility of dental materials

OBJECTIVES The purpose of this study was to correlate the size and the orientation of image disturbances observed on specific samples of dental materials with their magnetic susceptibility. The measurement of the magnetic susceptibility was performed in the 10(-5) or 10(-6) range using MRI to establish a classification of dental materials. METHODS Cylindrical dental alloy samples incorporating gold, silver, and palladium were placed in a Pyrex beaker filled with distilled water. Images were performed at 0.13 Tesla using two-dimensional Fourier transformation and projection reconstruction at 360 degrees imaging methods. The magnetic susceptibilities were obtained by measuring distances between spots having the highest intensity on the image. RESULTS A very discriminating classification may be established on MRI criteria. This method permits one to determine the dia- or para-magnetic character of the dental materials tested. Only palladium-based alloys have been detected to be paramagnetic with kappa > 0. One of the silver-based alloys did not induce detectable distortion because its susceptibility was very close to that of distilled water. Based on this MRI data, the use of this material may be recommended for applications that may be subjected to MRI evaluation. SIGNIFICANCE With the increasing use of MRI as a diagnostic tool, it is useful to establish a classification of prosthetic biomaterials compatible with MRI.

Dynamic MR-imaging with radial scanning, a post-acquisition keyhole approach

A new method for 2D/3D dynamic MR-Imaging with radial scanning is proposed. It exploits the inherent strong oversampling in the centre of-space, which holds crucial temporal information of the contrast evolution. It is based on (1) a rearrangement of (novel 3D) isotropic distributions of trajectories during the scan according to the desired time resolution and (2) a post-acquisition keyhole approach. The 2D/3D dynamic images are reconstructed using 2D/3D-gridding and 2D/3D-IFFT. The scan time is not increased with respect to a conventional 2D/3D radial scan of the same image resolution, in addition one benefits from the dynamic information. An application to in vivo ventilation of rat lungs using hyperpolarized helium is demonstrated.

Hyperpolarized Helium3 Encapsulated in Microbubbles: A New Class of Blood Pool MRI Contrast Agent

A novel blood pool agent, based on hyperpolarized helium3 (HP 3He), was investigated for tissue perfusion using MRI. The results are reported in this paper. Thanks to the considerable polarization value obtained by optical pumping process, HP 3He has already demonstrated a great potential for anatomical and functional lung ventilation studies (1,2). In order to benefit from this large polarization and NMR signal advantages, we have studied the feasibility of this new contrast agent for perfusion imaging. To overcome the problem of the low helium solubility in blood, methods based on helium transportation using carrier agents have been investigated (3). In our study, a phospholipid-based substrate has been developed for helium encapsulation. Injections of 3m-diameter microbubble solutions have been used for in vivo intravascular imaging. In this work, the lung parenchyma perfusion imaging (4) as well as imaging of the coronary arteries (5) and myocardium perfusion are investigated.

Standardized MR protocol for the evaluation of MRA sequences and/or contrast agents effects in high-degree arterial stenosis analysis

PurposeTo investigate the relative role of high resolution (spatial or temporal) magnetic resonance angiography (MRA) sequence and of contrast agent properties in the evaluation of high-degree arterial stenosis.MethodsWe qualitatively and quantitatively studied both 50 and 95% (300 μm diameter) stenosis of a 6 mm arterial phantom with two contrast agents (CA), Gd-DOTA (r1 =2.9 mM−1 s−1) versus P760 (r1 =25 mM−1 s−1) at several CA concentrations, including arterial peak concentration after injection of either a single or double dose of CA, using either a high temporal (booster) or high spatial (HR) resolution 3D MRA sequences. Experimental data were then compared to theoretical data.ResultsWith the 3D HR sequence, both visual and quantitative analysis were significantly better compared to the 3D booster sequence, at each phantom diameter. Quantitative analysis was significantly improved by injection of a double versus a single dose of each CA (Gd-DOTA or P760), primarily in high degree stenosis.ConclusionCombined MRA spatial resolution and high CA efficiency are mandatory to correctly evaluate high degree stenosis.

A simple switching unit for gradient reorientation in fast projection-reconstruction NMR imaging

This note describes a simple unit dedicated to monitor the three gradient values during a projection-reconstruction NMR imaging sequence. Typically, the unit can be used to acquire two- or three-dimensional fast gradient-echo images.