B. Denizot

High field magnetic resonance imaging evaluation of superparamagnetic iron oxide nanoparticles in a permanent rat myocardial infarction.

Chapon C, Franconi F, Lemaire L, et al. High field magnetic resonance imaging evaluation of superparamagnetic iron oxide nanoparticles in a permanent rat myocardial infarction. Invest Radiol 2003;38:141–146. Rationale and Objectives. The purpose of this study was to evaluate superparamagnetic iron oxide (SPIO) nanoparticles to discriminate infarcted from normal tissue after myocardial infarction using high field MR imaging (7 tesla). Materials and Methods. Permanent myocardial infarction was induced in rats. SPIO nanoparticles (1 mg Fe/kg) were assessed with T1-weighted gradient echo sequence to visualize the myocardial infarction 48 hours after ligature (n = 6). Furthermore, MR Imaging was performed using a T2-weighted RARE sequence and nanoparticles were injected (5 or 10 mg Fe/kg) on 36 rats 5, 24 or 48 hours after infarction. Results. No changes in contrast between normal and infarcted myocardium was observed after nanoparticle injection on T1-weighted images. However, nanoparticles induced a significant contrast increase between normal and infarcted myocardium on T2-weighted images whatever the delay between infarction and imaging (2.99 ± 1.66 preinjection vs. 7.82 ± 1.96 after SPIO injection at a dose of 5 mg Fe/kg 5 hours postinfarction, P = 0.0001). Conclusions. Nanoparticle injection made it possible to discriminate normal from infarcted myocardium on T2-weighted images. However, the high magnetic field prevented the visualization of the T1 effect of SPIO nanoparticles.

Early detection of liposome brain localization in rat experimental allergic encephalomyelitis.

Abstract Blood-brain barrier (BBB) permeability increases prior to the development of clinical signs in early-stage multiple sclerosis (MS). Detection of subtle changes would thus be helpful for diagnostic purposes and rapid therapeutic decisions before new episodes. Since multiple sclerosis and experimental allergic encephalomyelitis (EAE) have numerous common features, in particular BBB-permeability characteristics, and since we have previously shown that BBB localization is disturbed by tumors, embolism, and mannitol injection, we investigated BBB-liposome permeability in an EAE rat model. Twenty young male Lewis rats received a single intradermal inoculation of guinea-pig spinal cord. The effect of the Freund’s adjuvant and spinal cord alone on brain permeability were also assessed. In order to compare solution permeability and liposome localization, radioactive liposomes and, 1 h later, 99mTc-DTPA were injected intravenously. Scintigraphic acquisitions were obtained to follow the biodistribution of radioactivity in the whole body. Each rat was subjected to a first examination before inoculation and then every two days until completion and may be considered as its own control. EAE induced a previously unreported increase in global-body permeability, probably due to inflammation. Liposome brain localization and brain/heart ratio were significantly different between normal animals and those with early-stage EAE (before appearance of clinical signs) and distinguished between different disease stages in clinically patent EAE. The index of disease progression was modified earlier than with 99mTc-DTPA injection. One explanation may be particle pick-up by circulating macrophages, which cross the BBB during this pathology. For clinical applications, experiments must be confirmed on models more reliable for human multiple sclerosis.

Investigation of blood-brain barrier permeability to magnetite-dextran nanoparticles (MD3) after osmotic disruption in rats

The permeability of experimentally disrupted blood-brain barrier (BBB) to superparamagnetic nanoparticles (MD3) was studied in rats. BBB opening was induced by intracarotid injection of mannitol. One hundred eighty rats were used for the study. Rats were examined at two time points, 30 minutes and 12 hours after intracarotid mannitol injection. Different preparations intravenously injected 30 minutes before rat sacrifice were used for characterization of BBB disruption. BBB integrity was determined with99mTc-diethylenetriamine pentaacetic acid (DTPA) and99mTc-albumin. Iron oxide-glucose particles (12-nm mean diameter),99mTc-labeled lecithin-cholesterol liposomes of three different sizes (50, 100, and 200 nm), and polyethylene glycol (PEG)-coated99mTc liposomes (50 nm) were used for investigations of the dependence of BBB permeability on particle system size or surface. Magnetite-dextran nanoparticles (MD3) were evaluated as superparamagnetic contrast agent to monitor with magnetic resonance imaging (MRI) the BBB breakdown.In vitro T1 and T2 relaxation times of the brain tissue were measured at 40 MHz and 37°C, and T2-weighted MR images were acquired at 0.5 T. After intracarotid mannitol infusion, as expected, the BBB breakdown was immediate and temporary as judged by soluble molecule diffusion. MD3 nanoparticles crossed the BBB 12 hours after intravenous mannitol injection, at a time when brain permeability for molecules or small particles returns to normal. Magnetite crystals were found in cytoplasmic vesicles of glial cells. On MRI, signal intensity decreased after injection of MD3, even 12 hours after mannitol injection. This particularity could be useful in the study of focal pathological lesions accompanied by BBB permeability modifications. In such conditions, superparamagnetic particle contrast agents could be caught by the BBB, allowing the observation of impaired BBB areas without detectable cellular lesions.

Quality assessment of compressed cardiac MRI. Effect of lossy compression on computerized physiological parameters

With the increasing number of medical imaging modalities, compressing digital medical images can facilitate their transmission and storage. This paper evaluates and compares two lossy compression methods: JPEG and wavelet, applied to the end diastolic phase slices of a cardiac MRI acquisition. First, the qualitative image quality was evaluated with clinical criteria and was performed by radiologists. The compression image quality was then not judged satisfactory over 22:1 for JPEG and 32:1 for wavelets. Secondly, we evaluated the effect of these lossy methods on physiological parameters, such as left ventricular mass and end-diastolic volume, calculated by an automated segmentation algorithm. The wavelet coding allows a compression ratio of 67:1, while JPEG is limited to 30:1. The wavelet compression performance was superior to JPEG at high and low compression ratios. We conclude that limiting the compression ratios to those defined by the psychovisual assessment secures the validity of physiological parameters in cardiac MRI.