The year(s) of the contrast agent - micro-MRI in the new millennium.

The beginning of the new millennium has been a dynamic time for the field of magnetic resonance imaging (MRI). Exciting recent advances have been made at all levels of imaging, ranging from the visualization of single cells to rodents, birds and the human brain. Many of these techniques employ contrast agents to visualize the movement or activity of cells or organs in vivo; examples of this include the observation of stem cell migration, the tracking of labeled T cells, and the visualization of the events of gastrulation in developing Xenopus embryos. Other advances include improved techniques for elucidating white matter tracts in brain by either monitoring the diffusion of water along the tracts or tracing active neuronal tracts in vivo with Mn(2+) ions. Imaging of the immune system presents two dramatically different challenges: imaging most if not all of the body to follow cell trafficking, and imaging at cellular resolution to follow key intercellular and intracellular events.