Detection of vascular cell adhesion molecule-1 expression with USPIO-enhanced molecular MRI in a mouse model of cerebral ischemia.

Vascular damage plays a critical role after stroke, leading notably to edema, hemorrhages and stroke recurrence. Tools to characterize the vascular lesion are thus a real medical need. In this context, the specific nanoparticular contrast agent P03011, an USPIO (ultrasmall superparamagnetic iron oxide) conjugated to a peptide that targets VCAM-1 (vascular cell adhesion molecule-1), was developed to detect this major component of the vascular inflammatory response. This study aimed to make the proof of concept of the capacity of this targeted USPIO to detect VCAM-1 with MRI after cerebral ischemia in mouse. The time course of VCAM-1 expression was first examined by immunohistochemistry in our model of cerebral ischemia-reperfusion. Secondly, P03011 or nontargeted USPIO P03007 were injected 5 h after ischemia (100 µmol iron kg⁻¹; i.v.) and in vivo and ex vivo MRI were performed 24 h after ischemia onset. Double labeling immunofluorescence was then performed on brain slices in order to detect both USPIO and VCAM-1. VCAM-1 expression was significantly up-regulated 24 h after ischemia in our model. In animals receiving P03011, both in vivo and ex vivo MRI performed 24 h after ischemia onset showed hypointense foci which could correspond to iron particles. Histological analysis showed a co-localization of the targeted USPIO and VCAM-1. This study demonstrates that VCAM-1 detection is possible with the USPIO P03011 in a model of cerebral ischemia. This kind of contrast agent could be an interesting clinical tool to characterize ischemic lesions in terms of vascular damage.

[In vivo imaging for biodistribution and metabolism evaluations of new chemical entities]. / Applications de l'imagerie pour l'étude de la biodistribution et du métabolisme de nouvelles molécules.

Due to numerous technical developments, in vivo imaging is suitable for pharmacokinetic and metabolism studies of new chemical entities as well as for evaluating their pharmacological or biological effects. MRI, nuclear medicine, X-Ray, ultrasound and optical imaging are available for both clinical and experimental imaging with even higher performance. For all these imaging modalities, diagnostic agents are useful to improve contrast and specificity. Specific targeting of biological events is addressed by molecular imaging. From a pharmacodynamic perspective, radiolabeling of a new chemical entity allows in vivo visualization quantitative measure of its biodistribution, its elimination and its specific molecular binding. Non-invasive imaging methods are useful for longitudinal investigations of biological changes. Based on nanotechnologies, specificity of drug delivery can be monitored by imaging. New developments in hybrid imaging technologies as well as multimodal contrast agents reinforce in vivo experimental and clinical proof of mechanism of new chemical entities.

Magnetic resonance imaging of ruptured plaques in the rabbit with ultrasmall superparamagnetic particles of iron oxide.

BACKGROUND: Magnetic resonance imaging (MRI) enhanced with ultrasmall superparamagnetic particles of iron oxide (USPIO) has previously been evaluated in hyperlipidemic rabbits. The aim of this study was therefore to compare USPIO in ruptured and non-ruptured arteries in an atherosclerotic rabbit model. METHODS: Atherosclerotic-like lesions were induced by the combination of endothelial abrasion and high-cholesterol diet in iliac rabbit arteries (n = 16). Rupture of atherosclerotic lesions was realized by oversized balloon angioplasty in one iliac artery, whereas the contralateral artery was used as control. USPIO (ferumoxtran-10: 1 mmol Fe/kg) was administered immediately (n = 10) or 28 days (n = 6) after injury. MRI and histological analysis were performed 7 and 35 days after injury and in control arteries. RESULTS: In vivo MRI analysis showed extended susceptibility artifact with transluminal signal loss in all ruptured arteries 7 days after injury. In contrast, hyposignal was reduced 35 days following injury (i.e. after healing), and absent in non-ruptured arteries. Similarly, histological analysis of iron uptake was significantly increased 7 days after injury compared to healed-ruptured and control arteries. CONCLUSIONS: Accumulation ofUSPIO is significantly increased in ruptured as compared to non-ruptured arteries in the atherosclerotic rabbit model.

Influence of vascular filling and perfusion on BOLD contrast during reactive hyperemia in human skeletal muscle.

Mechanisms generating BOLD contrast are complex and depend on parameters that are prone to large variations, in particular in skeletal muscle. Here, we simultaneously measured perfusion by ASL, and BOLD response in the calf muscle of 6 healthy volunteers during post-ischemic reactive hyperemia. We tested whether the relation between the two was altered for varying degrees of leg vascular replenishment induced by prior positioning of the leg at different heights relative to the heart. We found that the BOLD response depended on perfusion, but also on the degree of repletion of leg blood vessels. We conclude that simultaneous determination of perfusion by ASL is important to identify the mechanisms underlying BOLD contrast in the skeletal muscle.

Metabolic and vascular support for the role of myoglobin in humans: a multiparametric NMR study.

In human muscle the role of myoglobin (Mb) and its relationship to factors such as muscle perfusion and metabolic capacity are not well understood. We utilized nuclear magnetic resonance (NMR) to simultaneously study the Mb concentration ([Mb]), perfusion, and metabolic characteristics in calf muscles of athletes trained long term for either sprint or endurance running after plantar flexion exercise and cuff ischemia. The acquisitions for (1)H assessment of Mb desaturation and concentration, arterial spin labeling measurement of muscle perfusion, and (31)P spectroscopy to monitor high-energy phosphate metabolites were interleaved in a 4-T magnet. The endurance-trained runners had a significantly elevated [Mb] (0.28 +/- 0.06 vs. 0.20 +/- 0.03 mmol/kg). The time constant of creatine rephosphorylation (tauPCr), an indicator of oxidative capacity, was both shorter in the endurance-trained group (34 +/- 6 vs. 64 +/- 20 s) and negatively correlated with [Mb] across all subjects (r = 0.58). The time to reach maximal perfusion after cuff release was also both shorter in the endurance-trained group (306 +/- 74 vs. 560 +/- 240 s) and negatively correlated with [Mb] (r = 0.56). Finally, Mb reoxygenation rate tended to be higher in the endurance-trained group and was positively correlated with tauPCr (r = 0.75). In summary, these NMR data reveal that [Mb] is increased in human muscle with a high oxidative capacity and a highly responsive vasculature, and the rate at which Mb resaturates is well correlated with the rephosphorylation rate of Cr, each of which support a teleological role for Mb in O(2) transport within highly oxidative human skeletal muscle.

A comparison of voluntary and electrically induced contractions by interleaved 1H- and 31P-NMRS in humans.

Skeletal muscle voluntary contractions (VC) and electrical stimulations (ES) were compared in eight healthy men. High-energy phosphates and myoglobin oxygenation were simultaneously monitored in the quadriceps by interleaved (1)H- and (31)P-NMR spectroscopy. For the VC protocol, subjects performed five or six bouts of 5 min with a workload increment of 10% of maximal voluntary torque (MVT) at each step. The ES protocol consisted of a 13-min exercise with a load corresponding to 10% MVT. For both protocols, exercise consisted of 6-s isometric contractions and 6-s rest cycles. For an identical mechanical level (10% MVT), ES induced larger changes than VC in the P(i)-to-phosphocreatine ratio [1.38 +/- 1.14 (ES) vs. 0.13 +/- 0.04 (VC)], pH [6.69 +/- 0.11 (ES) vs. 7.04 +/- 0.07 (VC)] and myoglobin desaturation [43 +/- 15.9 (ES) vs. 6.1 +/- 4.6% (VC)]. ES activated the muscle facing the NMR coil to a greater extent than did VCs when evaluated under identical technical conditions. This metabolic pattern can be interpreted in terms of specific temporal and spatial muscle cell recruitment. Furthermore, at identical levels of energy charge, the muscle was more acidotic and cytoplasm appeared more oxygenated during ES than during VC. These results are in accordance with a preferential recruitment of type II fibers and a relative muscle hyperperfusion during ES.

Coexistence of liquid and solid phases in flowing soft-glassy materials.

Magnetic-resonance-imaging rheometrical experiments show that concentrated suspensions or emulsions cannot flow steadily at a uniform rate smaller than a critical value (gamma(c)). As a result, a "liquid" region (sheared rapidly, i.e., at a rate larger than gamma(c)) and a "solid" region (static) coexist. The behavior of the fluid in the liquid region follows a simple power-law model, while the extent of the solid region increases with the degree of jamming of the material.

Determination of skeletal muscle perfusion using arterial spin labeling NMRI: validation by comparison with venous occlusion plethysmography.

T(1)-based determination of perfusion was performed with the high temporal and spatial resolution that monitoring of exercise physiology requires. As no data were available on the validation of this approach in human muscles, T(1)-based NMRI of perfusion was compared to standard strain-gauge venous occlusion plethysmography performed simultaneously within a 4 T magnet. Two different situations were investigated in 21 healthy young volunteers: 1) a 5-min ischemia of the leg, or 2) a 2-3 min ischemic exercise consisting of a plantar flexion on an amagnetic ergometer. Leg perfusion was monitored over 5-15 min of the recovery phase, after the air-cuff arterial occlusion had been released. The interesting features of the sequence were the use of a saturation-recovery module for the introduction of a T(1) modulation and of single-shot spin echo for imaging. Spatial resolution was 1.7 x 2.0 mm and temporal resolution was 2 s. For data analysis, ROIs were traced on different muscles and perfusion was calculated from the differences in muscle signal intensity in successive images. To allow comparison with the global measurement of perfusion by plethysmography, the T(1)-based NMR measurements in exercising muscles were rescaled to the leg cross-section. The perfusion measurements obtained by plethysmography and NMRI were in close agreement with a correlation coefficient between 0.87 and 0.92. This indicates that pulsed arterial techniques provide determination of muscle perfusion not only with superior spatial and temporal resolution but also with exactitude.

Muscular transverse relaxation time measurement by magnetic resonance imaging at 4 Tesla in normal and dystrophic dy/dy and dy(2j)/dy(2j) mice.

Muscular transverse relaxation times values were measured in vivo in normal mice (strain C57BL6/J, n=14) and in murine models of human congenital muscular dystrophy (dy/dy, n=9; dy(2j)/dy(2j), n=8). A single-slice multi-echo sequence was used. Gastrocnemius/soleus complex, thigh and buttock muscles were studied. Muscular transverse relaxation times values were compared between different muscle groups in each type of animal and between animal groups. Differences were observed between normal and dy(2j)/dy(2j) mice from 3 to 12 weeks of age, and between normal and dy/dy mice at 6 weeks. In specific age ranges, the values of muscular transverse relaxation times in two dystrophic models are different from those in normal mice, and could thus be used as an index of modifications in dystrophic muscle to evaluate therapies.

Characterization of atherosclerotic plaque components by high resolution quantitative MR and US imaging.

High resolution MRI at 3 T and US imaging at 50 MHz were used for atherosclerotic plaque characterization. For 14 excised segments of human arteries, conventional MR and US images, quantitative MR T2 maps, US integrated attenuation (IA) maps, and histologic sections were produced and compared. The MR T2 and US attenuation mean values estimated in selected regions of interest were related with tissue type as identified on histologic sections. Significant distinction between media or collagen and lipid or collagen lipidic plaque was achieved with both techniques (MR: P < .001; US: P < .01). Significant distinction was obtained between media and collagen (P < .0001) and between iliac and aortic media (P < .05) with MR T2 but not with IA. MR and US native and parametric images, with different sensitivities to tissue type, provide complementary information useful for quantitative plaque characterization.

US backscatter and attenuation 30 to 50 MHz and MR T2 at 3 Tesla for differentiation of atherosclerotic artery constituents in vitro.

This study compares quantitative characterization of atherosclerotic artery constituents by high resolution estimates of ultrasonic attenuation, ultrasonic attenuation-compensated backscatter, and magnetic resonance transverse relaxation time. Atherosclerotic human arteries were studied in vitro at 37°C. Backscattered radio frequency signals were acquired with a 50 MHz backscatter acoustic microscope. Ultrasonic parametric images were constructed from the integrated (30 to 50 MHz) backscatter and attenuation obtained using FFT methods with diffraction correction and a multinarrow-band attenuation algorithm. Parametric magnetic resonance images were constructed from calculated values of the transverse relaxation time T2 determined from an 8 echo-single-slice sequence at 3 Tesla. In a total of 54 regions of interest, average values of integrated attenuation, integrated backscatter compensated for the attenuation between the artery surface and the scattering volume, and the transverse relaxation time were correlated with local tissue composition as assessed by histology. Results show that ultrasound and magnetic resonance techniques offer complementary approaches for characterization of plaque composition.