Evaluating SPIO-labelled cell MR efficiency by three-dimensional quantitative T2* MRI

An in vitro MR-assay for superparamagnetic iron oxide (SPIO) particle cell labelling assessment via three-dimensional quantitative T(2) (*) MR microscopy was proposed. On high-resolution images, and due to the high susceptibility difference between the particles and the surrounding medium, SPIO internalized in cells induces signal loss which may be counted and measured on T(2) (*) maps. The increase in both labelled cell percentage and the average perturbation volume with an added amount of iron in the incubation medium proved that intracellular iron uptake is dependent upon the initial concentration of incubation iron. It also proved that the observed increases in total cellular iron uptake measured by inductively coupled plasma optical emission spectroscopy are due to both an increase in the iron mass per cell and also an increase in labelled cell concentration. MR results were compared with Prussian blue staining histology. The sensitivity of the MR methodology was then used to distinguish labelling differences for two different types of particle coating. The MRI-assay we proposed is a compulsory tool to optimize labelling efficiency in order to improve in vivo cell detection. Key parameters for detection, such as the percentage of cell labelling, the effect on the image for a given amount of internalized iron and labelling distribution among a cell population, are easily obtained. The comparison of different contrast agents for labelling one cell type, the assessment of one type of contrast agent for labelling different cell types and/or the evaluation of labelling strategies, are possible without having recourse to classical methods, and provide improved accuracy, since the principle is based on intracellular relaxivity.

Volumetric assessment of myocardial viability in rats using 3D double contrast enhanced T1 and T2-weighted MRI

OBJECTIVE

Volumetric evaluation of the myocardial viability post-infarction in rats using 3D in vivo MR imaging at 7 T using injection of an extracellular paramagnetic contrast agent and intravascular superparamagnetic iron oxide nanoparticles in the same imaging session.

MATERIALS AND METHODS

Five hours after induction of permanent myocardial infarction in rats (n=6), 3D in vivo T1- and T2-weighted MR Imaging was performed prior to and after Gd-DOTA injection (0.2 mmol/kg) and prior to and after nanoparticle injection (5 mg Fe/kg) to assess infarct size and myocardial viability.

RESULTS

3D MR Imaging using a successive contrast agent injection showed a difference of infarct size after Gd-DOTA injection on T1-weighted images compared to the one measured on T2-weighted images after Gd-DOTA and nanoparticle injection.

CONCLUSION

The use of 3D T1- and T2-weighted MR Imaging using a double contrast agents protocol made possible the accurate characterization of myocardial infarction volume and allowed the detection of myocardial viability post-infarction in rats.

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

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.