About Cellular and Molecular Imaging
Cellular MRI is a newly emerging field of imaging that combines ultra-high resolution MRI with the use of sensitive and specific cell labeling agents for the direct imaging of cells. The majority of research in this area has utilized magnetic nanoparticles (MNP) because of their greater sensitivity compared to Gadolinium-based contrast agents. In experimental cellular MRI, cells are typically loaded with MNP prior to their injection or implantation. The resulting signal hypo-intensities can be tracked in vivo providing information about the presence, location and migration of the iron-labeled cells. Various cell types have been tracked in vivo over several weeks and multiple cell divisions and with minimal impact on cell function.
A number of different iron based magnetic nanoparticles (MNP) are available for research including ultra-small superparamagnetic nanoparticles (USPIO, 5-10nm), standard superparamagnetic nanoparticles (SPIO, 80-120nm) and micron-sized superparamagnetic nanoparticles (MPIO, 0.9 micron and up). There are two main strategies for labeling cells with MNP: in vivo ‘active’ labeling or in vitro labeling prior to injection or implantation of cells. When MNP are administered IV for active labeling MR images are typically acquired 24 hours after the administration on MNP. The presence of this magnetic label in cells causes a distortion in the magnetic field and leads to signal hypo-intensities (negative contrast) in images sensitive to this change (T2- or T2*-weighted images). The region of signal loss is the image typically much larger than the actual area occupied by the cells, an effect known as the ‘blooming’ artifact.
Targeted cellular MRI is a rapidly expanding field of research. “Targeted” MRI contrast agents represent a class of imaging probes that are conjugated to specific antibodies, which are over-expressed on cell surface proteins. Targeting agents can be designed to concentrate at cell targets promoting accumulation of the MNP in target cells thereby increasing MRI sensitivity. These agents are also designed to target specific cells (most often cancer or immune cells), improving MRI specificity.
Some examples of targeting agents used to date for targeted MRI of cancer include: luteinizing hormone releasing hormone (LHRH-) and Her2/neu-conjugated MNP for breast cancer metastases, and folate receptor targeted MNP and RGD-MNP for imaging angiogenesis in tumors.
