Evaluation of the Spatial Resolution with 1.5–4 Tesla in a Stenosis Model

Three-dimensional reconstruction of a vascular network by dynamic tracking of magnetite nanoparticles

PURPOSE

Visualization of small blood vessels feeding tumor sites provides important information on the tumors and their microenvironment. This information plays an important role in targeted drug therapies using magnetic gradients. However, capabilities of current clinical imaging modalities may be insufficient to resolve complex microvascular networks. The purpose of this study is to map the vascular network, 3D, based on the magnetic susceptibility contrast.

METHODS

Magnetic particles induce an inhomogeneity in the MRI's magnetic field in an order much larger than their real size. This is an approach to compensate the spatial resolution insufficiency of a clinical MR scanner. Micron-sized agglomerations of magnetite nanoparticles were injected in a 3D phantom vascular network, and a fast multislice, multiacquisition MR sequence was applied to track the agglomerations along their trajectories. The experiment was performed twice for two different imaging planes: coronal and transversal. The susceptibility artifact in the images indicated the presence and the position of the agglomerations. The calculated positions through multiple images were assembled to build up the 3D distribution of the vascular network.

RESULTS

The calculated points were compared with the centerline of the channels, extracted from the 3D reference image, to determine the absolute measurement error. The mean error was measured to be approximately half of the pixel's size. It was found that the positioning error on the axis perpendicular to the imaging slice was nearly twice as high as on the imaging plane axes due to the slice thickness. In order to compensate for the lack of resolution on the perpendicular axis, the reconstruction was performed using a combination of coronal and transversal data. The combination of the coordinates led to a significant decrease in the mean measurement error at each segment in the vascular network (p < 0.001).

CONCLUSIONS

A method for 3D reconstruction of a microvascular network based on the susceptibility contrast in MRI and using a clinical scanner and a commercial receiver coil was proposed. The method presents a novel approach for reconstruction of vascular networks using the susceptibility effect. The proposed method may be applied to resolve vascular networks at a micrometric scale.

MRI visualization of a single 15 µm navigable imaging agent and future microrobot

In magnetic resonance imaging (MRI), the susceptibility-based contrast provides a way to amplify the effects of a magnetic microparticle, whereas its volume is largely inferior to the spatial resolution of the system. This concept presents an approach to visualization by means of susceptibility artifact using ferromagnetic microparticles. In this work, the amount of the susceptibility artifact was investigated using a simulation model and in vitro experiments on stainless steel microspheres measuring 40, 20 and 15 microm in diameter. The results showed that using a clinical MRI system, a single 15 microm microsphere is detectable in gradient-echo scans. The extent of the susceptibility artifact was found to be related to the scan parameters and the particles' sizes. Since the same ferromagnetic microparticle can be used for MRI-based propulsion, these results suggest several potential applications for navigable agents and microrobots involved in therapy, diagnostics, and imaging inside the microvascular network of the human body.