Optically Pinpointing Magnetic Nanoparticles within Biological Tissue

Abstract

A emerging imaging technology known as optical coherence tomography (OCT) provides microscopic in vivo imaging by using interferometry to detect light reflected from deep tissue structures. OCT imaging has been adopted by medical practitioners in many areas, including ophthalmology, cardiology and gastroenterology. Currently, there is an intense search for stains or contrast agents to use with OCT, because conventional markers such as fluorescent dyes emit incoherent light, for which there is no OCT interference signal.1 Magnetic iron oxides, such as carbohydrate-coated magnetite (Fe3O4) nanoparticles, have recently been FDAapproved as human injectable contrast agents for MRI.2 This class of particles is highly responsive to a magnetic field gradient, and at lower frequencies than those used in MRI (10 to 100 Hz), they can be mechanically modulated or “wiggled” within the tissue microenvironment. The magnetomotive “wiggling” exhibits a unique optical light scattering signature when probed using OCT. Our group developed a technique in which a modulated magnetic field is applied during OCT imaging, dubbed magnetomotive OCT (MMOCT).3 Using MMOCT, magnetic iron oxides are pinpointed within a standard OCT image, requiring only the addition of a small electromagnet to the imaging system. The basic MMOCT technique involves querying the tissue both before and after application of the magnetic field gradient. By comparing the OCT data before and after, differences are attributed to a magnetic field-specific reaction, thus pinpointing the locations of the magnetic particles. This concept was first demonstrated in tissue scaffolds containing macrophage cells labeled with magnetic particles, allowing one to distinguish labeled and unlabeled cells within a thick (1.5 mm deep) sample.3 However, the of Illinois at Urbana-Champaign, Beckman Institute for Advanced Science & Technology in Urbana, Ill.