Emmanuelle Grillon

Vessel size imaging

Vessel size imaging is a new method that is based on simultaneous measurement of the changes ΔR2 and ΔR  *2 in relaxation rate constants induced by the injection of an intravascular superparamagnetic contrast agent. Using the static dephasing approximation for ΔR  *2 estimation and the slow‐diffusion approximation for ΔR2 estimation, it is shown that the ratio ΔR2/ΔR  *2 can be expressed as a function of the susceptibility difference between vessels and brain tissue, the brain water diffusion coefficient, and a weighted mean of vessel sizes. Comparison of the results with 1) the Monte Carlo simulations used to quantify the relationship between tissue parameters and susceptibility contrast, 2) the experimental MRI data in the normal rat brain, and 3) the histologic data establishes the validity of this approach. This technique, which allows images of a weighted mean of the vessel size to be obtained, could be useful for in vivo studies of tumor vascularization. Magn Reson Med 45:397–408, 2001.

Xenon-129 MR imaging and spectroscopy of rat brain using arterial delivery of hyperpolarized xenon in a lipid emulsion.

Hyperpolarized 129Xe dissolved in a lipid emulsion constitutes an NMR tracer that can be injected into the blood stream, enabling blood‐flow measurement and perfusion imaging. A small volume (0.15 ml) of this tracer was injected in 1.5 s in rat carotid and 129Xe MR spectra and images were acquired at 2.35 T to evaluate the potential of this approach for cerebral studies. Xenon spectra consistently showed two resonances, at 194.5 ppm and 199.0 ppm relative to the gas peak. The signal‐to‐noise ratio (SNR) obtained for the two peaks was sufficient (ranging from 12 to 90) to follow their time courses. 2D transverse‐projection xenon images were obtained with an in‐plane resolution of 900 μm per pixel (SNR range 8–15). Histological analysis revealed no brain damage except in two rats that had received three injections. Magn Reson Med 46:208–212, 2001.

In vivo 129Xe NMR in rat brain during intra-arterial injection of hyperpolarized 129Xe dissolved in a lipid emulsion

Hyperpolarized 129Xe was dissolved in a lipid emulsion and administered to anaesthetized rats by manual injections into the carotid (approximately 1-1.5 mL in a maximum time of 30 s). During injection, 129Xe NMR brain spectra at 2.35 T were recorded over 51 s, with a repetition time of 253 ms. Two peaks assigned to dissolved 129Xe were observed (the larger at 194 +/- 1 ppm assigned to intravascular xenon and the smaller at 199 +/- 1 ppm to xenon dissolved in the brain tissue). Their kinetics revealed a rapid intensity increase, followed by a plateau (approximately 15 s duration) and then a decrease over 5 s. This behaviour was attributed to combined influences of the T1 relaxation of the tracer, of radiofrequency sampling, and of the tracer perfusion rate in rat brain. Similar kinetics were observed in experiments carried out on a simple micro-vessel phantom. An identical experimental set-up was used to acquire a series of 2D projection 129Xe images on the phantom and the rat brain.

Tissue oxygen saturation mapping with magnetic resonance imaging

A quantitative estimate of cerebral blood oxygen saturation is of critical importance in the investigation of cerebrovascular disease. While positron emission tomography can map in vivo the oxygen level in blood, it has limited availability and requires ionizing radiation. Magnetic resonance imaging (MRI) offers an alternative through the blood oxygen level-dependent contrast. Here, we describe an in vivo and non-invasive approach to map brain tissue oxygen saturation (StO2) with high spatial resolution. StO2 obtained with MRI correlated well with results from blood gas analyses for various oxygen and hematocrit challenges. In a stroke model, the hypoxic areas delineated in vivo by MRI spatially matched those observed ex vivo by pimonidazole staining. In a model of diffuse traumatic brain injury, MRI was able to detect even a reduction in StO2 that was too small to be detected by histology. In a F98 glioma model, MRI was able to map oxygenation heterogeneity. Thus, the MRI technique may improve our understanding of the pathophysiology of several brain diseases involving impaired oxygenation.

Magnetic Resonance Imaging and Fluorescence Labeling of Clinical-Grade Mesenchymal Stem Cells Without Impacting Their Phenotype: Study in a Rat Model of Stroke

Human mesenchymal stem cells (hMSCs) have strong potential for cell therapy after stroke. Tracking stem cells in vivo following a graft can provide insight into many issues regarding optimal route and/or dosing. hMSCs were labeled for magnetic resonance imaging (MRI) and histology with micrometer‐sized superparamagnetic iron oxides (M‐SPIOs) that contained a fluorophore. We assessed whether M‐SPIO labeling obtained without the use of a transfection agent induced any cell damage in clinical‐grade hMSCs and whether it may be useful for in vivo MRI studies after stroke. M‐SPIOs provided efficient intracellular hMSC labeling and did not modify cell viability, phenotype, or in vitro differentiation capacity. Following grafting in a rat model of stroke, labeled hMSCs could be detected using both in vivo MRI and fluorescent microscopy until 4 weeks following transplantation. However, whereas good label stability and unaffected hMSC viability were observed in vitro, grafted hMSCs may die and release iron particles in vivo.

Method to determine in vivo the relaxation time T1 of hyperpolarized xenon in rat brain.

The magnetic polarization of the stable 129Xe isotope may be enhanced dramatically by means of optical techniques and, in principle, hyperpolarized 129Xe MRI should allow quantitative mapping of cerebral blood flow with better spatial resolution than scintigraphic techniques. A parameter necessary for this quantitation, and not previously known, is the longitudinal relaxation time (T  1tissue ) of 129Xe in brain tissue in vivo: a method for determining this is reported. The time course of the MR signal in the brain during arterial injection of hyperpolarized 129Xe in a lipid emulsion was analyzed using an extended two‐compartment model. The model uses experimentally determined values of the RF flip angle and the T1 of 129Xe in the lipid emulsion. Measurements on rats, in vivo, at 2.35 T gave T  1tissue = 3.6 ± 2.1 sec (±SD, n = 6). This method enables quantitative mapping of cerebral blood flow. Magn Reson Med 49:1014–1018, 2003.

Measuring hemoglobin oxygen saturation during graded hypoxic hypoxia in rat striatum.

We evaluated in vivo reflectance spectroscopy of visible light as a method to assess brain tissue hemoglobin oxygen saturation in rat striatum (SstrO2). Seven anesthetized and mechanically ventilated rats were subjected to incremental reduction in the fraction of inspired oxygen (Fio 2): 0.35, 0.25, 0.15, 0.12, and 0.10, followed by a reoxygenation period (Group 1). At each episode, local changes in SstrO2 and in cerebral blood flow (LCBF) were simultaneously determined in the two striatal regions, using reflectance spectroscopy and laser Doppler flowmetry, respectively. Another group of rats (Group 2, n = 6) was also studied to measure sagittal sinus blood hemoglobin saturation (SssO2) during graded hypoxic hypoxia. Corpus striatum exhibited a significant graded decrease in SstrO2, from 38% ± 17% at Fio2 of 0.35 (control) to 16% ± 10% at Fio2 of 0.12 and to 13% ± 7% at Fio2 of 0.10 (P < 0.05), with no difference between the two hemispheres. These local changes in SstrO2 were associated with a significant graded increase in LCBF: 161% ± 26% of control values and 197% ± 34% during these 2 hypoxic episodes, respectively (P < 0.05). All local changes were fully reversed during the reoxygenation period. In Group 2, SssO2 decreased from 38% ± 8% at Fio2 of 0.35 (control) to 10% ± 3% at Fio2 of 0.10, closely related to SstrO2 decreasing in hypoxia. This study shows that reflectance spectroscopy of the visible light in rat striatum could be a possible measure of continuous changes in SstrO2. SssO2 and LCBF measurements during graded hypoxic hypoxia indicate that changes in SstrO2 reflect primarily those in brain venous oxygenation.

Laser-polarized xenon nuclear magnetic resonance, a potential tool for brain perfusion imaging: measurement of the xenon T1 in vivo.

Publisher Summary This chapter examines the use of laser-polarized xenon nuclear magnetic resonance (NMR) for brain perfusion imaging. The biophysical properties of xenon make it a good candidate for NMR tissue probing or perfusion studies. Xenon is hyperpolarized by collisional spin exchange with rubidium vapor pumped optically at 795 nm. It is found that after an injection of hyperpolarized xenon dissolved in Intralipid into the rat brain, two xenon resonances can be detected in the rat brain by NMR spectroscopy. The kinetics of the NMR signal of the xenon dissolved in the cerebral tissue then depends on three effects, including the continuing input of HP xenon by the injection, the HP xenon relaxation, and the loss of signal due to RF pulses. The available xenon magnetization at the beginning of the experiment relaxes first in the Intralipid during the time needed for the tracer to reach the brain. It is observed that the nonsaturation of the brain tissue in xenon is based on the injected volume of xenon and the anatomic and physiologic characteristics of the rat brain.

Noninvasive optical monitoring of rat brain and effects of the injection of tracers for blood flow measurements

Near infrared spectroscopy using either broad band reflectance spectrophotometry or monochromatic illumination has been carried out to monitor non invasively the changes of the concentrations of chromophores in rat brain induced by the intravenous injection of various contrast agents (indocyanine green, ultrasmall magnetic particles suspension, albumine, dextran, or saline solution alone). Depending of the wavelength and of the absorption spectrum of the injected compound the bolus can be seen either by a decrease or an increase of the transmitted light, this latter due to the induced dilution of the blood by the bolus. We suggest that this could be used to determine the arterial input function of the contrast agent needed to perform absolute cerebral blood flow imaging by nuclear magnetic resonance.