Bacteriogenic magnetic nanoparticles as magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI), which has excellent spatial resolution and soft tissue contrast, is a commonly used imaging tool for clinical disease diagnosis. MRI contrast agents are often administered to enhance the image contrast between normal and diseased tissues because of their ability to shorten the relaxation time of the surrounding water molecules. Paramagnetic gadolinium-based metal chelates have predominantly been using as T₁ MRI contrast agents in clinic. There is another class of MRI contrast agents, superparamagnetic iron oxides (SPIOs), which, instead of altering T₁, have dominant effect on decreasing the transverse relaxation time (T₂) (1). In contrast to the T₁ contrast agents that generate brighter signals, SPIOs produce darker signals, so called negative contrast. However, SPIOs have much higher molar relaxivity and are thus widely used for molecular MRI applications such as cell tracking and molecular targeting (2,3). SPIO nanoparticles that comprise mainly magnetite, the ferric form of iron oxide, can be synthesized with high uniformity in size at varied diameters or lengths.