MAGNETIC MESOPOROUS SILICA NANOPARTICLES AS POTENTIAL MRI CONTRAST AGENTS

Abstract

Magnetic resonance imaging is a non-invasive diagnostic technique that utilizes native protons in biological samples to produce three-dimensional images. MR imaging relies on the native differences in relaxation time (T1 and T2) and tissue composition (amount of lipids, water, etc. present) in order to generate images. While different tissues have varying T1/T2, the low sensitivity and distinguishability of MRI, especially when comparing diseased and healthy tissue, has necessitated the use of contrast agents (CAs).

This unique class of compounds can be administered prior to taking an image to increase the minute signal intensity differences between tissues. Contrast agents are particularly useful when trying to differentiate between healthy and diseased tissue, allowing for the visualization of lesions and small tumors not normally seen without the use of such compounds. To date two main CAs are used: gadolinium chelates and iron oxide nanoparticles. Many issues still exist for these CAs including: metal leaching, agglomeration, and metal toxicity.

Previously, the use of metal-oxo clusters grafted into polymer nanobeads, utilizing the miniemulsion technique, has been explored. The encapsulation of these clusters proved to be highly advantageous as not only did they address the issues mentioned previously, as well as many more, the magnetic properties were also vastly improved with encapsulation. Still, issues exist with this polymer nanobeads system. These nanobeads cannot be redispersed once the colloidal miniemulsion is disturbed. Furthermore the encapsulation of the metal-oxo cluster leads to several disadvantages. Although there is a homogenous metal distribution, only clusters closest to the surface are utilized leading to inefficient metal usage. The complete encapsulation also does not allow for direct metal-proton interaction, a crucial relaxation pathway, thus further limiting the relaxation efficiency of these materials.

The goal of this research is to produce new CAs using silica nanoparticles and metal-oxo clusters to address the issues associated with the polymer nanobeads. The use of mesoporous silica allows for cluster anchoring, preventing leaching, but also exposes the metal to the environment allowing for better metal-proton interactions. This introduction of a new relaxation pathway should improve upon the contrast abilities of the polymer nanobead system.