Researchers have combined two technologies to obtain high-resolution images of cartilage, an achievement that could make possible improved detection and tracking of osteoarthritis.Microcomputed tomography previously has been ineffective for imaging soft tissue such as cartilage. By introducing Hexabrix -- an x-ray-absorbing contrast agent with a negative charge -- into the tissue and then performing the scan, researchers from Georgia Institute of Technology in Atlanta obtained detailed images that show the composition of the connective tissue. The cartilage-imaging technique shows (upper left) an intact rabbit knee; (upper right) a segmented rabbit knee with cartilage (gold) and bone (red); and (lower center) the range of cartilage-layer thickness in the rabbit knee. The noninvasive imaging technique, known as equilibrium partitioning of an ionic contrast agent via microCT, was tested in vitro to monitor the degradation of bovine cartilage cores and in situ to examine the thin layer of cartilage in a rabbit knee. The technique allowed the researchers to see the distribution of molecules known as proteoglycans throughout the tissues. Unhealthy cartilage gradually loses these molecules, whereas healthy tissue maintains a normal distribution. As described in the Dec. 19 issue of PNAS, they calculated the number of molecules in different parts of the cartilage and measured its thickness. Senior author Marc E. Levenston, an associate professor of mechanical engineering, said that small-animal in vivo testing is planned in future research to monitor the progression of osteoarthritis. If successful, the imaging technique could eventually provide pharmaceutical researchers with valuable information on the effects of drugs designed to combat the degenerative disease.