Recent advances in MRI: novel contrast agents shed light on in vivo biochemistry

A new class of contrast agents for magnetic resonance imaging (MRI) has emerged that is sensitive to biochemical activity. Enzyme-activated agents offer the possibility of mapping gene expression in living animals, whereas pH-sensitive agents are of particular interest for detection of abnormal tissue. Contrast agents selective for calcium, an important second messenger, can provide information about signal transduction events. Other efforts in contrast agent development are directed toward improving delivery by conjugating contrast agents to moieties that target certain tissues or that facilitate uptake. The development of new agents introduces a means for using MRI to probe biochemical activity in living organisms.