Geneviève Guillot

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.

Evaluation of cartilage repair tissue after biomaterial implantation in rat patella by using T2 mapping

To evaluate the ability of MR T2 mapping (8.5 T) to characterize ex vivo longitudinally, morphologically and quantitatively, alginate-based tissue engineering in a rat model of patellar cartilage chondral focal defect. Calibrated rat patellar cartilage defects (1.3 mm) were created at day 0 (D0) and alginate sponge with (Sp/C+) or without (Sp/C−) autologous chondrocytes were implanted. Animals were sacrificed sequentially at D20, D40 and D60 after surgery and dissected patellae underwent MRI exploration (8.5 T). T2 values were calculated from eight SE images by using nonlinear least-squares curve fitting on a pixel-by-pixel basis (constant repetition time of 1.5 s, eight different echo times: 5.5, 7.5, 10.5, 12.5, 15.0, 20.0, 25.0 and 30.0 ms). On the T2 map, acquired in a transversal plane through the repair zone, global T2 values and zonal variation of T2 values of repair tissue were evaluated versus control group and compared with macroscopic score and histological studies (toluidine blue, sirius red and hematoxylin-eosin). “Partial”, “total” and “hypertrophic” repair patterns were identified. At D40 and D60, Sp/C+ group was characterized by a higher proportion of “total” repair in comparison to Sp/C− group. At D60, the proportion of “hypertrophic” repair was two fold in Sp/C− group versus Sp/C+ group. As confirmed morphologically and histologically, the T2 map also permitted the distinction of three types of repair tissue: “total”, “partial” and “hypertrophic”. “Total” repair tissue was characterized by high T2 values versus normal cartilage (p<0.05). Zonal variation, reflecting the collagen network organization, appeared only at D60 for Sp/C+ group (p<0.05). “Hypertrophic” tissue, mainly observed at D60, presented high T2 global values without zonal variation with cartilage depth. These results confirm the potency of the MR T2 map (8.5 T) to characterize macroscopically and microscopically the patterns of the scaffold guided-tissue repair of a focal chondral lesion in the rat patella (“total”, “partial” and “hypertrophic”). On T2 map, three parameters (i.e. MRI macroscopic pattern, T2 global values and zonal variation of T2 values) permit to characterize chondral repair tissue, as a virtual biopsy.

Characteristics of submicron pores obtained by chemical etching of nuclear tracks in polycarbonate films

Calibrated pores in the range 102–2×103 A have been obtained by chemical etching of polycarbonate thin films irradiated with high energy krypton ions (500 MeV, Kr25+). Both the amorphous and the crystalline forms of polycarbonate (Makrofol, Bayer), further designated by their respective trade names N and KG, have been investigated up to thicknesses of 60 μm, close to the theoretical ion range of 77 μm. From conductivity studies, three different domains have been separated around the ion track: A highly damaged core of radius ?50 A with a fast etching rate vT ≊104 A/min, an intermediate zone of radius ?500 A with an etching rate vI = 0.9 A/min, an outer region with an etching rate equal to that of the undamaged material, vG = 0.47 A/min. These observations are compatible with the delta ray model for track formation in plastic materials. Scanning electron microscope investigations have revealed that the pores formed in the N material are straight cylinders with a narrow distribution of the pore entrance dia...

Monitoring of tracer dispersion in porous media by NMR imaging

The dispersion of a paramagnetic tracer has been measured by NMR imaging in a non-consolidated porous medium made of 2 and 6 mm diameter glass beads. The technique allows one to obtain 3D images of the tracer repartition as well as instantaneous 1D concentration profiles along the flow axis. Flow reversibility was checked by examining concentration profile variations in both forward and backward flows. Dispersion coefficients were obtained by fitting 1D profiles with the solutions of the convection diffusion equation. They are completely consistent with usual results on bead packings for experiments performed with tracer injection stable with respect to gravity. Moreover, a total irreversibility of the dispersion is established after displacement lengths of a few bead diameters. The tracer distributions as observed from 3D images differ markedly in stable and unstable experiments. For gravitationally unstable injections, on both profiles and images, an additional, partly reversible, convective spreading was observed-it is probably due to permeability inhomogeneities of the bead packing.

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.

3D analysis from micro-MRI during in situ compression on cancellous bone

A mini-compression jig was built to perform in situ tests on bovine trabecular bone monitored by micro-MRI. The MRI antenna provided an isotropic resolution of 78 microm that allows for a volume correlation method to be used. Three-dimensional displacement fields are then evaluated within the bone sample during the compression test. The performances of the correlation method are evaluated and discussed to validate the technique on trabecular bone. By considering correlation residuals and estimates of acquisition noise, the measured results are shown to be trustworthy. By analyzing average strain levels for different interrogation volumes along the loading direction, it is shown that the sample size is less than that of a representative volume element. This study shows the feasibility of the 3D-displacement and strain field analyses from micro-MRI images. Other biological tissues could be considered in future work.

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.

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.

Freezing or supercooling: how does an aquatic subterranean crustacean survive exposures at subzero temperatures?

SUMMARY Crystallization temperature (Tc), resistance to inoculative freezing (IF), ice contents, bound water, protein and glycogen body contents were measured in the aquatic subterranean crustacean Niphargus rhenorhodanensis and in the morphologically close surface-dwelling aquatic crustacean Gammarus fossarum, both acclimated at 12°C, 3°C and -2°C. Cold acclimation induced an increase in the Tc values in both species but no survival was observed after thawing. However, after inoculation at high sub-zero temperatures, cold-acclimated N. rhenorhodanensis survived whereas all others, including the 3°C and -2°C acclimated G. fossarum died. In its aquatic environment, N. rhenorhodanensis is likely to encounter inoculative freezing before reaching the Tc and IF tolerance appears as a highly adaptive trait in this species. Bound water and glycogen were found to increase in the 3°C and -2°C acclimated N. rhenorhodanensis, whereas no variation was observed in G. fossarum. Considering the hydrophilic properties of glycogen, such a rise may be correlated with the increased bound water measured in cold-acclimated N. rhenorhodanensis, and may be linked to the survival of this species when it was inoculated. The ecological significance of the survival of the aquatic subterranean crustacean to inoculative freezing is paradoxical, as temperature is currently highly buffered in its habitat. However, we assume that past geographical distribution and resulting life history traits of N. rhenorhodanensis are key parameters in the current cold-hardiness of the species.

Mechanical properties of glenoid cancellous bone.

BACKGROUND Loosening of the glenoid component in total shoulder arthroplasty is the main late complication of this procedure; it may be assumed that it is highly dependent on the quality of the glenoid cancellous bone. Very little is known about the mechanical properties of this cancellous bone. The aim of this study was to determine these properties (Youngs modulus and strength) as well as bone density in different parts of the glenoid cancellous bone to assess their variations. METHODS Eleven scapulas were obtained from six fresh-frozen, unembalmed human cadavers. Eighty-two cubic cancellous bone specimens were extracted and tested using a uniaxial compression test; then the specimens were defatted and correlations with bone density were determined. FINDINGS The study showed significant differences in the mechanical properties with anatomic location and directions of loading. Youngs modulus and strength were found to be significantly higher at the posterior part of the glenoid with the weakest properties at the antero-inferior part. Cancellous bone was found to be anisotropic with higher mechanical properties in the latero-medial direction perpendicular to the articular surface of the glenoid. The apparent density was on average equal to 0.29 g/cm(3) with the higher values at the posterior and superior part of the glenoid. Good correlation between apparent density and elastic modulus was found only in the sagittal planes but not in the coronal and axial plane. INTERPRETATION The mechanical properties determined in this study showed the anisotropy of the glenoid cancellous bone; values of these properties could provide input data for finite element method analyses in shoulder prosthesis designs.