A new class of contrast agents designed for imaging cartilage during computed tomography scans promises to be several times more sensitive than current agents and to generate clearer pictures of the state of cartilage in joints (J. Am. Chem. Soc., DOI: 10.1021/ja9053306). Unlike most contrast agents, which are anionic and give only limited information about the cartilage itself, the agents developed by Mark W. Grinstaff of Boston University and colleagues are cationic. The group hypothesized that a cationic molecule would be more attracted to a key anionic component of cartilage: heavily sulfated and carboxylated polysaccharides known as glycosaminoglycans (GAGs). In humans, declining GAG levels signal development of osteoarthritis and other cartilage trauma. The researchers synthesized three substituted iodobenzenes bearing cationic ammonium groups, which are analogs of commercially available anionic contrast agents. When they tested the new compounds on rabbit femurs, they found that the cationic agents are attracted to GAGs and yield sharper images than the anionic agents. Better quantitative information about the state of cartilage should aid diagnosis and treatment of osteoarthritis and help monitor the effectiveness of drugs and tissue engineering, the researchers write.