The use of microgel iron oxide nanoparticles in studies of magnetic resonance relaxation and endothelial progenitor cell labelling.

In vivo tracking of stem cells after transplantation is crucial for understanding cell-fate and therapeutic efficacy. By labelling stem cells with magnetic particles, they can be tracked by Magnetic Resonance Imaging (MRI). We previously demonstrated that microgel iron oxide nanoparticle (MGIO) provide superior tracking sensitivity over commercially available particles. Here, we describe the synthesis of MGIO and report on their morphology, hydrodynamic diameters (87-766 nm), iron oxide weight content (up to 82%) and magnetization characteristics (M(s)=52.9 Am(2)/kg, M(R)=0.061 Am(2)/kg and H(c)=0.672 A/m). Their MR relaxation characteristics are comparable to those of theoretical models and represent the first such correlation between model and real particles of varying diameters. A labelling study of primary endothelial progenitor cells also confirms that MGIO is an efficient label regardless of cell type. The facile synthesis of MGIO makes it a useful tool for the studying of relaxation induced by magnetic particles and cellular tracking by MRI.

Microgel iron oxide nanoparticles for tracking human fetal mesenchymal stem cells through magnetic resonance imaging.

Stem cell transplantation for regenerative medicine has made significant progress in various injury models, with the development of modalities to track stem cell fate and migration post-transplantation being currently pursued rigorously. Magnetic resonance imaging (MRI) allows serial high-resolution in vivo detection of transplanted stem cells labeled with iron oxide particles, but has been hampered by low labeling efficiencies. Here, we describe the use of microgel iron oxide (MGIO) particles of diameters spanning 100-750 nm for labeling human fetal mesenchymal stem cells (hfMSCs) for MRI tracking. We found that MGIO particle uptake by hfMSCs was size dependent, with 600-nm MGIO (M600) particles demonstrating three- to sixfold higher iron loading than the clinical particle ferucarbotran (33-263 versus 9.6-42.0 pg iron/hfMSC; p < .001). Cell labeling with either M600 particles or ferucarbotran did not affect either cellular proliferation or tri-lineage differentiation into osteoblasts, adipocytes, and chondrocytes, despite differences in gene expression on a genome-wide microarray analysis. Cell tracking in a rat photothrombotic stroke model using a clinical 1.5-T MRI scanner demonstrated the migration of labeled hfMSCs from the contralateral cortex to the stroke injury, with M600 particles achieving a five- to sevenfold higher sensitivity for MRI detection than ferucarbotran (p < .05). However, model-related cellular necrosis and acute inflammation limited the survival of hfMSCs beyond 5-12 days. The use of M600 particles allowed high detection sensitivity with low cellular toxicity to be achieved through a simple incubation protocol, and may thus be useful for cellular tracking using standard clinical MRI scanners.

In vivo assessment of mouse hindleg intramyocellular lipids by 1H-MR spectroscopy.

RATIONALE AND OBJECTIVES: (1)H-magnetic resonance spectroscopy ((1)H-MRS) has proved to be the sole in vivo technique able to measure intramyocellular lipids (IMCL) in both humans and animals. Mouse models are now widely used for physiologic studies and drug discovery. However, IMCL assessment using (1)H-MRS is hindered in this animal model by the small muscle size and strong contamination from the extramyocellular lipid (EMCL) signal. The objective of this study was to the use of (1)H-MRS for IMCL quantification in mice at different ages. MATERIALS AND METHODS: Noninvasive IMCL quantification was performed at 7 T in tibialis anterior (TA) muscles of healthy male C57/BL6 mice (n = 9; age, 13.6 +/- 1 months), db/db mice (n = 4), and their C57BL/KSJ control littermates (n = 4) at 7 and 17 weeks of age. RESULTS: The IMCL content of diabetic mice TA was significantly higher than their littermates (2.41 +/- 0.5 vs. 1.21 +/- 0.35, P < .01). An age effect was observed, with TA IMCL levels being lower in older than younger control mice, but increasing between 7 and 17 weeks in the db/db mice. CONCLUSIONS: The feasibility of (1)H-MRS spectroscopy was demonstrated in mice muscle, despite its small size, and used to assess IMCL content in db/db mice.

Biomimetic MRI contrast agent for imaging of inflammation in atherosclerotic plaque of ApoE-/- mice: a pilot study.

OBJECTIVE: Atherosclerosis involves an inflammatory process characterized by cellular and molecular responses. A slow-clearance blood-pool paramagnetic agent (CMD-A2-Gd-DOTA: P717) chemically modified to create a functionalized product (F-P717) for targeting inflammation in vessel walls was evaluated in vivo in mice. METHODS AND RESULTS: Carboxylate and sulfate groups were grafted onto the macromolecular paramagnetic Gd-DOTA-dextran backbone. Products were also fluorescently labeled with rhodamine isothiocyanate. Pre- and postcontrast MRI was performed on a 2-Tesla magnet in ApoE-/- and control C57BL/6 mice after P717 or F-P717 injection at a dose of 60 micromol Gd/kg. Axial T1-weighted images of the abdominal aorta were obtained using a 2D multislice spin-echo sequence. F-P717 significantly enhanced the magnetic resonance imaging (MRI) signal in the abdominal aortic wall of ApoE-/- mice (>50% signal-to-noise ratio increase between 10 and 30 minutes), but not of control mice. P717 produced only moderate (<20%) MRI signal enhancement within the same time frame. The MRI data were correlated to histopathology. Immunofluorescence in ApoE-/- mice colocalized F-P717 but not P717 with the inflammatory area revealed by P-selectin labeling. CONCLUSION: This study demonstrates the efficacy of F-P717 as a new molecular imaging agent for noninvasive in vivo MRI location of inflammatory vascular tree lesions in ApoE-/- mice.

Effects of chloramphenicol on brain energy metabolism using 31P spectroscopy: influences on sleep-wake states in rat.

Effects of chloramphenicol (antibiotic inhibiting complex-1 of respiratory chain) and thioamphenicol (TAP, a structural analog of CAP inactive on complex-1) were examined on cerebral energy metabolites and sleep-wake cycle architecture in rat. In the first group, animals were chronically equipped with a cranial surface resonator and (31)P spectroscopic measurements were performed using a 2 T magnetic resonance spectrometer (operating frequency 34.46 MHz). CAP administration (400 mg/kg, tail vein, light period) induced deficits in phosphocreatine (-30%, p < 0.01) and ATP (-40%, p < 0.01), whereas TAP (400 mg/kg) had no effect. In the second group, animals were chronically implanted with polygraphic electrodes for EEG and electromyogram recordings. CAP administered intraperitoneally at light-onset reduced rapid-eye movement (REM) sleep (-60% in the first 6 h of light period, p < 0.01), increased waking state (+65% in the first 6 h of light period, p < 0.01), and slightly affected slow-wave sleep (SWS). During waking state, theta and sigma power bands of the EEG were, respectively, increased and decreased (p < 0.05). During SWS, delta power band was reinforced (p < 0.05), while theta, alpha, and sigma bands were decreased (p < 0.05). No changes occurred during REM sleep. TAP had no effect on sleep-wake states and spectral components of the EEG. Overall, these data indicate that REM sleep occurrence is linked to an aerobic production of ATP.

High-resolution contrast-enhanced MRI of atherosclerosis with digital cardiac and respiratory gating in mice.

Atherosclerosis initially develops predominantly at the aortic root and carotid origin, where effective visualization in mice requires efficient cardiac and respiratory gating. The present study sought to first compare the high-resolution MRI gating performance of two digital gating strategies using: 1) separate cardiac and respiratory signals (double-sensor); and 2) a single-sensor cardiorespiratory signal (ECG demodulation), and second, to apply an optimized processing technique to dynamic contrast-enhanced (CE) carotid origin vessel-wall imaging in mice. High-resolution MR mouse heart and aortic arch images were acquired by ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (MathWorks, USA). Double-sensor gating used a respiratory sensor while single-sensor gating used breathing-modulated ECG to generate a demodulated respiratory signal. Pre- and postcontrast T(1)-weighted images were acquired to evaluate vessel-wall enhancement with a gadolinium blood-pool agent (P792; Guerbet, France) at the carotid origin in vivo in ApoE(-/-) and C57BL/6 mice, using the optimized cardiorespiratory gating processing technique. Both strategies provided images with improved spatial resolution, less artifacts, and 100% correct transistor-to-transistor logic (TTL) signals. Image quality allowed vessel-wall enhancement measurement in all the ApoE(-/-) mice, with maximal (32%) enhancement 27 min postinjection. The study demonstrated the efficiency of both cardiorespiratory gating strategies for dynamic contrast-enhanced vessel-wall imaging.

Real-time gating system for mouse cardiovascular MR imaging.

Mouse cardiac MR gating using ECG is affected by the hostile MR environment. It requires appropriate signal processing and correct QRS detection, but gating software methods are currently limited. In this study we sought to demonstrate the feasibility of digital real-time automatically updated gating methods, based on optimizing a signal-processing technique for different mouse strains. High-resolution MR images of mouse hearts and aortic arches were acquired using a chain consisting of ECG signal detection, digital signal processing, and gating signal generation modeled using Simulink (The MathWorks, Inc., Natick, MA, USA). The signal-processing algorithms used were respectively low-pass filtering, nonlinear passband, and wavelet decomposition. Both updated and nonupdated gating signal generation methods were tested. Noise reduction was assessed by comparison of the ECG signal-to-noise ratio (SNR) before and after each processing step. Gating performance was assessed by measuring QRS detection accuracy before and after online trigger-level adjustments. Low-pass filtering with trigger-level adjustment gave the best performance for mouse cardiovascular imaging using gradient-echo (GE), spin-echo (SE), and fast SE (FSE) sequences with minimum induced delay and maximum gating efficiency (99% sensitivity and R-peak detection). This simple digital gating interface will allow various gating strategies to be optimized for cardiovascular MR explorations in mice.

Reference signal extraction from corrupted ECG using wavelet decomposition for MRI sequence triggering: application to small animals.

BACKGROUND: Present developments in Nuclear Magnetic Resonance (NMR) imaging techniques strive for improved spatial and temporal resolution performances. However, trying to achieve the shortest gradient rising time with high intensity gradients has its drawbacks: It generates high amplitude noises that get superimposed on the simultaneously recorded electrophysiological signals, needed to synchronize moving organ images. Consequently, new strategies have to be developed for processing these collected signals during Magnetic Resonance Imaging (MRI) examinations. The aim of this work is to extract an efficient reference signal, from an electrocardiogram (ECG) that was contaminated by the NMR artefacts. This may be used for image triggering and/or cardiac rhythm monitoring. METHODS: Our method, based on sub-band decomposition using wavelet filters, is tested on various ECG signals recorded during three imaging sequences: Gradient Echo (GE), Fast Spin Echo (FSE) and Inversion Recovery with Spin Echo (IRSE). In order to define the most adapted wavelet functions to use according to the excitation protocols, noise generated by each imaging sequence is recorded and analysed. After exploring noise models along with information found in the literature, a group of 14 wavelets, members of three families (Daubechies, Coiflets, Symlets), is selected for the study. The extraction process is carried out by decomposing the contaminated ECG signals into 8 scales using a given wavelet function, then combining the sub-bands necessary for cardiac synchronization, i.e. those containing the essential part of the QRS energy, to construct a reference signal. RESULTS: The efficiency of the presented method has been tested on a group of quite representative signals containing: highly contaminated (mean SNR<--5 dB) simulated ECGs that replicate normal and pathological human heart beats, as well as some pathological and healthy rodents' actual ECG records. Despite the weak SNR of the contaminated ECG, the performances were quite satisfactory. When comparing the wavelet performances, one may notice that for a given sequence, some wavelets are more efficient for processing than others; for GE, FSE and IRSE sequence, good synchronisation condition is accomplished with coif5, sym8, and sym4 respectively. CONCLUSION: Sub-band decomposition proved to be very suitable for extracting a reference signal from a corrupted ECG for MRI triggering. An appropriate choice of the wavelet function, in accordance with the image sequence type, could considerably improve the quality of the reference signal for better image synchronization.

Sucrose solution freezing studied by magnetic resonance imaging.

Ice formation of a 20% w/v sucrose solution was monitored during the freezing process by magnetic resonance imaging (MRI). An original experimental setup was designed with oil as a cooling fluid that allows accurate control of the temperature. The NMR signal intensity of particular sampled volumes was observed during the entire cooling period, from 0 to -50 degrees C, showing a peak characteristic to a transition before the loss of the signal. Moreover, spatial ice distribution of the frozen matrix was observed by high resolution MRI with an isotropic resolution of 78x78x78microm(3). MRI has proved to be a novel technique for determining the glass transition temperature of frozen sucrose solutions, in the concentration range where calorimetric measurements are not feasible.

In vivo magnetic resonance imaging of large spontaneous aortic aneurysms in old apolipoprotein E-deficient mice.

UNLABELLED: Old ApoE-deficient mice were studied in vivo by magnetic resonance imaging (MRI) to prospectively evaluate vascular remodeling associated with atherosclerotic lesions. MATERIAL AND METHODS: Old female ApoE-/- mice on a normal diet were followed by MRI at 2 Tesla for a 3-month period and killed for histopathology. Aortic dimensions were measured and compared. RESULTS: High-quality in vivo MR images were obtained at 2 Tesla with in plane spatial resolution of 86 X 86 microm2. On MRI, aortic lumen enlargement (>1.5-fold dilation) was seen in 10 of 13 mice, located predominantly in the suprarenal portion of the aorta. The mean maximal diameter of the aneurysms and of the aorta above and below the aneurysm were, respectively, 1.12 +/- 0.32 mm and 0.53 +/- 0.08 mm by MRI and 1.3+/- 0.41 mm and 0.55 +/- 0.15 mm by histology. Matched histologic cross-sections of the aortic wall showed medial degradation with rupture of the internal elastic lamina at multiple sites, associated with fibrolipidic plaque containing cholesterol crystals. CONCLUSIONS: Aortic lumen enlargement was diagnosed in old ApoE-/- mice at sites with advanced atherosclerotic plaques. MRI has potential both as an in vivo imaging technique for screening mouse models for vascular wall pathology and to follow arterial remodeling associated with the disease progression.

Kinetics of anaerobic metabolism in human skeletal muscle: influence of repetitive high-intensity exercise on sedentary dominant and non-dominant forearm. A 31P NMR study.

Potential differences were assessed between the dominant (D) and non-dominant (ND) forearms of sedentary subjects during anaerobic exercise. Subjects performed voluntary concentric contractions of D and ND forearm muscle during a series of three high-intensity (60% of the maximal voluntary contraction force (MVC)) exercise bouts. The time-dependent changes in intracellular pH (pH(i)), Pi, and PCr concentrations, and their relation to muscular work were examined using 31P magnetic resonance spectroscopy (MRS) techniques, and revealed that D forearm metabolic kinetics in sedentary individuals are improved during repetitive high-intensity exercise compared to their respective ND forearm muscle. We postulate that the more regular and preferential utilization of the D limb leads to a "trained-like" condition.

Retrospective intra-scan motion correction.

This paper analyzes the effects of intra-scan motion and demonstrates the possibility of correcting them directly in k-space with a new automatic retrospective method. The method is presented for series of 2D acquisitions with Cartesian sampling. Using a reference k-space acquisition (corrected for translations) within the series, intra-scan motion parameters are accurately estimated for each trajectory in k-space of each data set in the series resulting in pseudo-random sample positions. The images are reconstructed with a Bayesian estimator that can handle sparse arbitrary sampling in k-space and reduces intra-scan rotation artefacts to the noise level. The method has been assessed by means of a Monte Carlo study on axial brain images for different signal-to-noise ratios. The accuracy of motion estimates is better than 0.1 degrees for rotation, and 0.1 and 0.05 pixel, respectively, for translation along the read and phase directions for signal-to-noise ratios higher than 6 of the signals on each trajectory. An example of reconstruction from experimental data corrupted by head motion is also given.

High-resolution magnetic resonance imaging at 2 Tesla: potential for atherosclerotic lesions exploration in the apolipoprotein E knockout mouse.

INTRODUCTION: The aim of the present study was to evaluate the potential of high-resolution MRI at 2 Tesla (T) for direct noninvasive imaging of the aortic wall in a mouse model of atherosclerosis. MATERIAL AND METHODS: A specific mouse antenna was developed and sequence parameters were adjusted. T(1)- and T2-weighted images of abdominal aorta were obtained at 2 T with a spatial resolution of 86 x 86 x 800 microm3 in vivo. With a dedicated small coil, ex vivo MRI of the aorta was performed with a spatial resolution of 54 x 54 x 520 microm3. RESULTS: In vivo, the aortic wall was clearly defined on T(2)-weighted images in 15 of 16 mice: along the aorta the lumen circumference ranged from 1.07 to 3.61 mm and mean wall thickness from 0.11 to 0.67 mm. In vivo measurements of plaque distribution were confirmed by ex vivo MR imaging and by histology, with a good correlation with histology regarding lumen circumference (r = 0.94) and wall thickness (r = 0.97). CONCLUSION: Magnetic resonance imaging at 2 T to analyze in vivo atherosclerotic lesions in mice is possible with a spatial resolution of 86 x 86 x 800 microm3 and thus can be used for noninvasive follow-up in evaluation of new drugs.

Trabecular alveolar bone in the human mandible: a dual-energy x-ray absorptiometry study.

OBJECTIVE: The purpose of this study was to evaluate the potential use of dual energy x-ray absorptiometry for the assessment of bone mineral content and bone mineral density before implant placement. MATERIALS AND METHODS: The material examined consisted of 63 mandibular bone specimens cut from 21 fresh cadavers (11 men; 10 women). Three specimens were cut per cadaver in the incisal, premolar, and molar regions. Three regions of interest (G, R1, and R2) were delineated. The global bone specimen (G) consisted of the whole specimen (ie, both cortical and trabecular bone). R1 and R2 were delineated in the trabecular bone of the alveolar process. RESULTS: In all subjects, the combined bone mineral content of the whole mandible specimens (global bone mineral content) was significantly correlated with age. The difference between the mean bone mineral densities of the male and female mandibles was found to be significant for G (P = .009). The mean bone mineral densities of dentate and edentulous specimens were also found to be significantly different for G and R1, respectively (P = .001 and P = .02), but not for R2. A positive correlation could be detected among the mean bone mineral density of G and R1, G and R2, and R1 and R2 of (1) male and female specimens, (2) dentate and edentulous specimens, and (3) incisal, premolar, and molar specimens. CONCLUSIONS: The intra-alveolar trabecular bone of these 21 mandibles is affected by the same local and systemic influences as cortical bone, whereas the infra-alveolar trabecular bone is mostly sensitive to dental status. The cortical and trabecular bone of the 10 mandibles from women is more sensitive to systemic influences, whereas that from men is more sensitive to local influences. This is somewhat in agreement with some studies that found an association between osteoporosis and oral bone loss, which is a metric measure.

[NMR radiofrequency microcoil design: usefulness of electromagnetic simulation]. / Conception de microbobines radiofréquence pour la RMN: apport d'une simulation électromagnétique.

The extraction of the Nuclear Magnetic Resonance (NMR) spectra of samples having smaller and smaller volumes is a real challenge. Either these reductions of volume are dictated by the difficulties of production of sufficiently large samples or by necessities of miniaturisation of the analysing system, in both cases a careful design of the radiofrequency coil, ensuring an optimum reception of the NMR signal, is required. We have also evaluated the usefulness of electromagnetic simulation software for the design and optimisation of these radio-frequency coils, which are more and more used in biology and health research projects.