Immobilization of iron oxide magnetic nanoparticles for enhancement of vessel wall magnetic resonance imaging--an ex vivo feasibility study

Abstract

Emerging data supports a role for negative wall remodeling in the failure of vascular interventions such as vein grafts, yet clinicians/researchers currently lack the ability to temporally/efficiently investigate adventitial surface topography/total vascular wall anatomy in vivo. We established a strategy of immobilizing commercially available iron oxide magnetic nanoparticles (Fe-NPs) onto the surface of human vein conduits to facilitate high-throughput total vascular wall demarcation with magnetic resonance (MR). Binding of activated Fe-NPs to amine groups on the surface of the veins induced a thin layer of negative contrast that differentiated the adventitia from surrounding saline signal in all MR images, enabling delineation of total wall anatomy; this was not possible in simultaneously imaged unlabeled control veins. Under the conditions of this ex vivo experiment, stable covalent binding of Fe-NPs can be achieved (dose-dependent) on human vein surface for MR detection, suggesting a potential strategy for enhancing the ability of MRI to investigate total wall adaptation and remodeling in vein graft failure.

Figure 1

Schematic representation of the immobilization of Fe-NPs on human vein. Incubation of the vein in an activated Fe-NPs solution produces binding of Fe-NPs on its surface. Particles could bind to amine groups on the cell surface as well as the extracellular matrix.

Figure 2

SEM images of the outer surface of the human veins with immobilized Fe-NPs (A and B) and without Fe-NPs (C and D).

Figure 3

SEM micrograph (left) and Fe elemental mapping using EDAX (right) of unlabeled control (top row) and Fe-NPs labeled (bottom row) surface of the human veins. EDAX was positive (colored) for Fe on the Fe-NPs labeled portions only. In all images, scale bar represents 20 μm.

Figure 4

Iron histochemistry. Perl’s Prussian blue staining images of (A) Fe-NPs labeled and (B) unlabeled human veins. Presence of Fe is confirmed by the appearance of dark blue/black colored areas (shown by red arrows in panel A).

Figure 5

T1-weighted, T2*-weighted, and PD-weighted MR images (0.3 × 0.3 × 0.5 mm³ resolution) obtained at 3 T of three segments of the same saphenous vein: a segment circumferentially labeled with activated Fe-NPs as described in the text (top row); a segment with 4 equal drops of cyanoacrylate adhesive, each containing different concentration of Fe-NPs (middle row); the unlabeled control vein segment (bottom row). A sagittal curved multiplanar reformation of the T1W acquisition is shown in the first column; the dashed vertical lines indicate the locations of the axial images shown in the other columns. The concentration of Fe-NPs shown in the middle row of the sagittal reformation is 6, 12.5, 25, and 50 μg, from left to right. All vein segments were imaged immersed in Petri dishes filled with saline and mounted on polystyrene scaffolds (white stars in images) to suspend them within the fluid.

Figure 6

Definition of ROIs and lengths used for quantitative analyses; for each axial slice analyzed, a lumen ROI was first delineated for both the labeled and control segment. Next, a wall ROI was delineated for both segments. Finally, a third ROI extending to the edge of the Fe-NP label, or a corresponding similar region in the saline, was delineated for the labeled and control vein specimens, respectively. These ROIs were used to determine the average thickness of the vessel wall, the average thickness of the Fe-NP label, and the CNR between vessel wall and surrounding label or saline as appropriate. Finally, for the dose−response experiment, the maximal width of the signal void was measured for each axial slice along a line orthogonal to the vessel wall.

Figure 7

Average wall thickness of labeled and control vein segments in each slice analyzed along the length of the specimens, and average thickness of the iron-oxide susceptibility-induced signal void around the labeled vein. Error bars indicate the standard deviation of measurements across all slices.

Figure 8

Correlation between label thickness and Fe-NP load observed in the dose−response experiment. Measurements were performed independently in both raw axial images, as well as coronal/sagital multiplanar reformations.