Jean-Sébastien Raynaud

Evaluation of Matrix Metalloproteinases in Atherosclerosis Using a Novel Noninvasive Imaging Approach

Objective—Despite great advances in our knowledge, atherosclerosis continues to kill more people than any other disease in the Western world. This is because our means of identifying truly vulnerable patients is limited. Prediction of atherosclerotic plaque rupture may be addressed by MRI of activated matrix metalloproteinases (MMPs), a family of enzymes that have been implicated in the vulnerability of plaques prone to rupture. This study evaluated the ability of the novel gadolinium-based MRI contrast agent P947 to target MMPs in atherosclerotic plaques. Methods and Results—The affinity of P947 toward activated MMPs was demonstrated in vitro. The affinity and specificity of P947 toward matrix metalloproteinase (MMP)-rich plaques was evaluated both in vivo using ApoE−/− mice and ex vivo in hyperlipidemic rabbits. Gadolinium content quantification and MRI showed a preferential accumulation of P947 in atherosclerotic lesions compared with the nontargeted reference compound, Gd-DOTA. The ex vivo assay on rabbit plaques revealed a higher uptake of P947. Moreover, using human carotid artery endarterectomy specimens, P947 facilitated discrimination between histologically defined MMP-rich and MMP-poor plaques. An in vivo MRI investigation in mice revealed that P947 greatly improved the ability to visualize and delineate atherosclerotic plaques. Conclusions—P947 may be a useful tool for the detection and characterization of the MMP-rich atherosclerotic plaques.

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. Magn Reson Med, 2006.

Ultra-sensitive molecular MRI of cerebrovascular cell activation enables early detection of chronic central nervous system disorders

Since endothelial cells can be targeted by large contrast-carrying particles, molecular imaging of cerebrovascular cell activation is highly promising to evaluate the underlying inflammation of the central nervous system (CNS). In this study, we aimed to demonstrate that molecular magnetic resonance imaging (MRI) of cerebrovascular cell activation can reveal CNS disorders in the absence of visible lesions and symptoms. To this aim, we optimized contrast carrying particles targeting vascular cell adhesion molecule-1 and MRI protocols through both in vitro and in vivo experiments. Although, pre-contrast MRI images failed to reveal the ongoing pathology, contrast-enhanced MRI revealed hypoperfusion-triggered CNS injury in vascular dementia, unmasked amyloid-induced cerebrovascular activation in Alzheimers disease and allowed monitoring of disease activity during experimental autoimmune encephalomyelitis. Moreover, contrast-enhanced MRI revealed the cerebrovascular cell activation associated with known risk factors of CNS disorders such as peripheral inflammation, ethanol consumption, hyperglycemia and aging. By providing a dramatically higher sensitivity than previously reported methods and molecular contrast agents, the technology described in the present study opens new avenues of investigation in the field of neuroinflammation.

Tumor imaging using P866, a high-relaxivity gadolinium chelate designed for folate receptor targeting†

The objective of this study was to evaluate the potential of a high‐relaxivity macromolecular gadolinium (Gd) chelate to target folate receptors (FRs). P866 is a dimeric high‐relaxivity Gd chelate coupled to a folate moiety. Binding affinity, in vivo biodistribution studies in KB tumor‐bearing mice at 1, 4, and 24 h, and dynamic contrast‐enhanced (DCE)‐MRI (2.35 T) over 4 h were assessed. Binding and internalization of P866 through the FR was demonstrated. Due to the high molecular volume of P866, the binding affinity compared to free FA was decreased (KD = 59.3 ± 1.8 nM and 5.9 ± 0.2 nM, respectively). Tumor/muscle (T/M) uptake was 5.4 ± 1.0, 4 h after injection of 15 μmol/kg. Competition with free FA was less effective when the dose was increased due to a saturation of FR. At a dose of 5 μmol/kg, a 70% difference in signal enhancement was observed between P866 and the nonspecific reference compound, thus demonstrating the specificity of FR targeting. While this high‐relaxivity folate‐Gd chelate has demonstrated its potential capacity to target in vivo FR on tumors, the sensitivity is probably limited to a certain extent by the saturation of the FR and by the decrease in the apparent relaxivity of the internalized part of P866 in the tumor cells. Magn Reson Med 60:1337–1346, 2008.

Magnetic resonance imaging of myocardial perfusion and viability using a blood pool contrast agent.

Rationale and Objectives:A comprehensive cardiac magnetic resonance (MR) examination should comprise imaging of myocardial perfusion, viability, and the coronary arteries. Blood pool contrast agents (BPCAs) improve coronary MR angiography, whereas their potential for imaging of perfusion and viability is unknown. The abilities to noninvasively image myocardial perfusion and viability using the BPCA P792 (Guerbet, France) were tested in a closed-chest model of nonreperfused myocardial infarction in 5 pigs. Materials and Methods:Two to 3 days after instrumentation, myocardial perfusion imaging with a saturation-recovery steady-state free precession technique and viability imaging with an inversion-recovery fast low-angle shot sequence were conducted on a 1.5-T MR scanner using the extracellular contrast agents (ECCA) Gd-DOTA (0.1 mmol Gd/kg) and blood pool contrast agent (BPCA) P792 (0.013 mmol Gd/kg). Results:Perfusion defects were visualized in all pigs with good correlation between the ECCA and the BPCA (1.77 ± 1.16 cm2 vs. 1.80 ± 1.19 cm2, r = 0.959, P < 0.01). Reduced myocardial perfusion was detected using the ECCA up to 80 seconds after injection. In contrast, BPCA administration enabled visualization of perfusion defects on equilibrium perfusion imaging in all cases for 10 minutes. The size of myocardial infarction detected with viability MR imaging correlated well between the standard method (ECCA) and delayed-enhancement imaging with the BPCA (5.40 ± 3.16 versus 5.52 ± 3.13 cm3, r = 0.994, P < 0.002). Conclusions:The BPCA investigated in this study allows both reliable detection of perfusion defects on first pass and equilibrium perfusion imaging and characterization of viability after myocardial infarction. Thus, this contrast agent is suitable for a comprehensive cardiac MR examination.

Assessment of myocardial infarction in pigs using a rapid clearance blood pool contrast medium

Delayed enhancement MRI using extracellular contrast media allows reliable detection of myocardial infarction. If blood pool contrast media like P792 (Vistarem, Guerbet, France), in addition to improving coronary MR angiography, can be shown to also produce delayed enhancement in myocardial infarction they could improve the prerequisites for a comprehensive cardiac MR examination. In this study reperfused myocardial infarction in five minipigs was imaged with an inversion‐recovery fast low‐angle shot sequence using P792 (0.013 mmol Gd/kg) and the extracellular contrast medium Gd‐DOTA (Dotarem, 0.1 mmol Gd/kg, Guerbet). The infarction size determined on MRI using P792 (7.55 ± 2.31 cm2) highly correlated both with histomorphometry (7.81 ± 2.18 cm2, r = 0.991, P < 0.002) and with MRI using Gd‐DOTA (7.85 ± 2.35 cm2, r = 0.978, P < 0.005). Bland‐Altman analysis showed that the limit of agreement of MRI using P792 compared to histomorphometry was 3.3 ± 7.6% of the infarction size. The contrast‐to‐noise ratio between infarcted and remote myocardium was not significantly different between Gd‐DOTA (5.9 ± 2.4) and P792 (4.4 ± 1.1, P = 0.5). The blood pool contrast medium P792 allows reliable assessment of viability with good contrast and accuracy. Magn Reson Med 51:703–709, 2004.

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.

Muscular transverse relaxation time measurement by magnetic resonance imaging at 4 Tesla in normal and dystrophic dy/dy and dy2j/dy2j 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.

Lack of evidence of a relationship between magnetic resonance signal intensity changes in the globus pallidus and dentate nucleus, and repeated administrations of gadoterate meglumine in children

Dear Editors, Rossi Espagnet and colleagues [1] reported significant increases of the globus pallidus-to-thalamus and the dentate nucleus-to-pons signal intensity ratios on unenhanced T1weighted brain magnetic resonance images from children exposed to multiple injections of the macrocyclic gadoliniumbased contrast agent gadoterate meglumine. However, this study has several important inconsistencies and limitations. In the Materials and Methods section, the authors wrote that each control subject was matched to a patient for age at both the first and the last MR examinations. The groups did not differ statistically at baseline but the results of the comparison at the last examination were not presented, thus precluding any interpretation of the signal intensity ratio increases. Age-dependent changes in native T1-weightedMR contrast of the brain may well account for these effects. In the Results section, the authors correlated the increases in the globus pallidus-to-thalamus and the dentate nucleus-to-pons signal intensity ratios to the number of gadolinium-based contrast agent injections. This association is not consistent with data from previous studies reporting an absence of correlation between these parameters in children after serial administrations of a linear gadolinium-based contrast agents [2, 3]. In Fig. 2, Rossi Espagnet and colleagues showed the relationships between the signal intensity ratios and the mean time intervals from the first administration. These graphs are misleading because the standard deviations of the time intervals at each injection were not presented. According to Table 1 in Rossi Espagnets publication, the mean interval between MR examinations varied from 1 day to 532 days. It is likely that the mean time intervals reported in Fig. 2 were highly heterogeneous from one patient to another or between two injections in the same patient. Short time intervals of one to several days may have resulted in signal intensity increases due to incomplete wash-out of the gadolinium-containing molecules from the brain, and possibly to a significant signal intensity ratio increase in some patients. The authors excluded the effect of a history of radiation therapy to the brain as a possible cause for the signal intensity ratio increases. However, it is probable that these patients with brain tumors underwent additional radiotherapy sessions during the study period. Blood–brain barrier disruption induced by radiotherapy may contribute to signal intensity increases following macrocyclic gadoliniumbased contrast agent injection. Some of the data reported by the authors in the Results section are not consistent with those that they presented graphically. The authors wrote that the globus pallidus-to-thalamus signal intensity mean value at first MR examination was 1.06 ±0.04 whereas it was rather equal to 1.048 according to Fig. 2. Such a mistake may have affected the statistical analyses. Moreover, Flood and colleagues [3] found that the dentate nucleus-to-pons signal intensity ratio difference between the last and first MR examinations in children exposed to a linear gadolinium-based contrast agent was 1.035–0.995=0.04 [3]. In the present study, the difference was 1.02–0.95=0.07. It is difficult to understand how gadoterate meglumine could have triggered a greater signal intensity difference than a linear gadolinium-based contrast agent without inducing any visible enhancement. Altogether, this exploratory study presented major inconsistencies that could have biased the interpretation of the results. * Eric Lancelot eric.lancelot@guerbet-group.com

Global strategy to extract automatically relevant subdominant perfusion information: application to skeletal muscle NMR imaging with arterial spin labeling

This paper describes a global strategy of image processing to automatically extract perfusion information, when it is not the predominant information in the sequence of images. It is applied to MR perfusion studies of skeletal muscles, acquired with arterial spin tagging sequences. First the dynamic images are registered, then methods based on factor analysis are applied to discriminate between highly perfused regions and low perfused regions. Finally a functional segmentation is achieved on the parametric images, which are computed by factor analysis. An example of the whole processing is given, which shows the possible functional discrimination between different regions inside one muscle. The effect of misregistration on the final segmentation is studied. The method is proved to be efficient provided that image data could be either well registered or discarded.