Hasan Alsaid

Rapid-Clearance Iron Nanoparticles for Inflammation Imaging of Atherosclerotic Plaque: Initial Experience in Animal Model

PURPOSE To evaluate the use of a recently developed fast-clearing ultrasmall superparamagnetic iron oxide (USPIO) for detection of vascular inflammation in atherosclerotic plaque. MATERIALS AND METHODS The study protocol was approved by the animal experimentation ethics committee. A recently introduced USPIO, P904, and a reference-standard USPIO, ferumoxtran-10, were tested in a rabbit model of induced aortic atherosclerosis. In vivo magnetic resonance (MR) angiography and T2*-weighted plaque MR imaging were performed at baseline and after administration of P904 and ferumoxtran-10 (administered dose for both, 1000 micromol of iron per kilogram of body weight) in 26 hyperlipidemic New Zealand white rabbits. The variation in vessel wall area over time was evaluated with nonparametric testing. Ex vivo MR imaging findings were compared with iron content at linear regression analysis. RESULTS With in vivo MR imaging, plaque analysis was possible as early as 24 hours after P904 injection. The authors observed a 27.75% increase in vessel wall area due to susceptibility artifacts on day 2 (P = .04) and a 38.81% increase on day 3 (P = .04) after P904 administration compared with a 44.5% increase in vessel wall area on day 7 (P = .04) and a 34.8% increase on day 10 (P = .22) after ferumoxtran-10 administration. These susceptibility artifacts were correlated with intraplaque iron uptake in the corresponding histologic slices. The number of pixels with signal loss on the ex vivo MR images was linearly correlated with the logarithm of the iron concentration (P = .0001; R(2) = 0.93). CONCLUSION Plaque inflammation in rabbits can be detected earlier with P904 than with ferumoxtran-10 owing to the faster blood pharmacokinetics and the early uptake of P904 in the reticuloendothelial system. SUPPLEMENTAL MATERIAL http://radiology.rsnajnls.org/cgi/content/full/252/2/401/DC1.

Assessment of age modulated vascular inflammation in ApoE-/- mice by USPIO-enhanced magnetic resonance imaging.

Objective:Inflammation within atherosclerotic lesions increases the risk for plaque rupture and thrombosis. A functional approach to plaque analysis is the intravenous administration of ultrasmall superparamagnetic particles of iron oxide (USPIO) that enables visualization of macrophages residing in the plaques. In this study, we sought to characterize the age-related inflammatory status associated with atherosclerosis lesion progression in ApoE−/− mice using USPIO-enhanced magnetic resonance imaging (MRI). Materials and Methods:A total of 24 ApoE −/− mice were divided in 4 groups (N = 6) and were given a high cholesterol diet from 6 weeks of age to the end of the protocol. One group per MR time point was investigated at 10, 16, 24, and 34 weeks of age. Each MR examination was performed on a 4.7 T scanner and consisted of baseline and 48 hours post-USPIO administration imaging sessions. P904, a USPIO contrast agent (Guerbet, Paris, France) with a potential for plaque macrophage targeting, was used.Vessel wall area measurements were performed on high resolution spin echo transverse images. Multi-echo gradient-echo images acquired with the same geometry were used to calculate T2* maps of the vessel wall using a pixel-by-pixel monoexponential fit. A one-way analysis of variance was performed to characterize the temporal variation of vessel wall area, susceptibility artifact area, baseline, and post-USPIO T2* values. MR measurements were correlated with the histologic findings. Results:A significant increase was found in the aortic wall area from 1.4 ± 0.2 at 10 weeks to 2.0 ± 0.3 mm2 at 34 weeks of age (P < 0.05). Concerning the post-USPIO MRI, signal loss regions, with patterns spanning from focal to the complete disappearance of the vessel wall, were observed on all postcontrast images. A significant increase in the size of the susceptibility artifact was observed from 0.5 ± 0.2 to 2.4 ± 1.0 at 24 weeks (P < 0.05) and to 2.0 ± 0.9 mm2 at 34 weeks (P < 0.05).The T2* values calculated on the 48 hours post-USPIO images were shorter compared with baseline. The decrease was 34% ± 16% at 10 weeks, 57% ± 11% at 16 weeks, 57% ± 16% at 24 weeks, and 48% ± 13% at 34 weeks.The Pearsons correlation test between measurement of aortic wall area performed on both MR images and histologic analysis showed a statistically significant correlation (r = 0.695 and P < 0.05). A correlation was also obtained between the signal loss area and the macrophages covered area (r = 0.68 and P < 0.05). Conclusions:This study demonstrated the feasibility of USPIO-enhanced MRI in assessing the inflammatory status related to the temporal progression of the atherosclerosis plaque in ApoE−/− transgenic mice model of atherosclerosis. In our experimental conditions, the vascular inflammation peak, for the ApoE−/− mice feeding high-fat/high-cholesterol diet is measured between 16 and 24 weeks of age.

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 &mgr;mol 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.

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 T1‐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. Magn Reson Med, 2007.

Longitudinal 3He and proton imaging of magnetite biodistribution in a rat model of instilled nanoparticles

Epidemiological and toxicological studies have provided evidence that accidentally inhaled nanosize ultrafine particles can induce chronic or acute health damage. MRI, being noninvasive, is able to assess the biodistribution and clearance of magnetically labeled nanoparticles induced by instillation or inhalation. We report 3He and proton MRI follow‐up of lung, liver, spleen, and kidney distribution of USPIO (ultrasmall superparamagnetic iron oxide) in a rat model. The sensitivity of the imaging technique to various concentrations of instilled magnetite suspension was first assessed in vivo (n = 12). A 2‐week longitudinal imaging study was then performed on animals (n = 7) instilled with a 0.5 mg magnetite solution. Hypointense and void signal regions associated with intrapulmonary USPIO were observed in the 3He ventilation images throughout the study, whereas no USPIO‐related proton signal intensity changes were found. Intrapulmonary magnetite nanoparticle confinement was confirmed by ex vivo iron assay and histological analysis. This study demonstrates that combined 3He and proton MRI enables noninvasive assessment of the distribution and clearance of magnetically labeled instilled nanoparticles. Magn Reson Med 59:1298–1303, 2008.

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. Magn Reson Med 57:29–39, 2007.

M1-Activated Macrophages Migration, A Marker of Aortic Atheroma Progression: A Preclinical MRI Study in Mice

Background:M1-activated Macrophages (M1M) play a major role in atherosclerotic lesions of aortic arch, promoting proinflammatory response. In vivo trafficking of M1M in aortic plaques is therefore critical. Methods:M1M from bone marrow cell culture were magnetically labeled, using iron nanoparticles, intravenously injected and followed up with 3 day magnetic resonance imaging (MRI) in mice developing macrophage-laden atheroma (ApoE2 knock-in mice). M1M recruitment in aortic arch lesions was assessed both by MRI and histology. Results:In all ApoE2 knock-in mice injected with labeled cells, high resolution MRI showed localized signal loss regions in the thickened aortic wall, with a maximal effect at day 2 (−34% ± 7.3% P < 0.001 compared with baseline). This was confirmed with Prussian blue (iron) staining and corresponded to M1M (Major Histo-compatibility Complex II positive). Clear different intraplaque and adventitial dynamic distribution profiles of labeled cells were observed during the 3 days. Conclusion:M1M dynamic MRI is a promising marker to noninvasively assess the macrophage trafficking underlying aortic arch plaque progression.

In vivo cardiac anatomical and functional effects of wheel running in mice by magnetic resonance imaging.

Physical activity is frequently used as a strategy to decrease pathogenesis and improve outcomes in chronic pathologies such as metabolic or cardiac diseases. In mice, it has been shown that voluntary wheel running (VWR) could induce an aerobic training effect and may provide a means of exploring the relationship between physical activity and the progression of pathology, or the effect of a drug on locomotor activity. To the best of our knowledge, in vivo magnetic resonance imaging (MRI) and other non-invasive methods had not been investigated for training evaluation in mice; therefore, it was proposed to test an MRI method coupled with a cardiorespiratory gating system on C57Bl/6 mice for in vivo heart anatomical and functional characterization in both trained and untrained animals. Twenty mice were either assigned to a 12-week VWR program or to a control group (CON – no wheel in the cage). At week 12, MRI scans showed an increase in the left ventricular (LV) wall mass in the VWR group compared with the CON group. The ex vivo measurements also found an increase in the heart and LV weight, as well as an increase in oxidative enzyme activities (i.e. cytochrome c oxidase [COx] in the soleus). In addition, correlations have been observed between ex vivo LV/body weight ratio, COx activity in the soleus and in vivo MRI LV wall mass/body weight. In conclusion, mouse cardiac MRI methods coupled with a cardio-respiratory gating system are sufficiently effective and feasible for non-invasive, training-induced heart hypertrophy characterization, and may be used for longitudinal training level follow-up in mouse models of cardiovascular and metabolic diseases.

Biomimetic Mri Contrast Agent for Imaging of Inflammation in Atherosclerotic Plaque of Apoe: A Pilot Study−/−: A Pilot Study 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 &mgr;mol 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.

Biomimetic MRI Contrast Agent for Imaging of Inflammation in Atherosclerotic Plaque of ApoE / Mice

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 &mgr;mol 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.