Contrast Agent Promising in Osteoarthritis

Photo: Rabbit knees viewd by new technique

Microcomputed tomography (microCT) which yields three-dimensional X-ray images with a resolution 100 times higher than clinical CT scans is commonly used to image bone for osteoporosis research but has not been useful for imaging soft biological tissues such as cartilage. These tissues simply don't interfere with the microCT's X-rays as they pass through a sample, and therefore don't show up on scans.

But by combining microCT with an X-ray-absorbing contrast agent that has a negative charge, researchers at the Georgia Institute of Technology were able to image the distribution of negatively charged molecules called proteoglycans (PGs). These molecules are critical to the proper functioning of cartilage.

Associate Professors Marc Levenston and Robert Guldberg from Georgia Tech's George W. Woodruff School of Mechanical Engineering collaborated to establish and validate the principle of the technique, dubbed Equilibrium Partitioning of an Ionic Contrast agent-microCT, or EPIC-microCT. Then they applied the technique in vitro to monitor the degradation of bovine cartilage cores and to visualize the thin layer of cartilage in an intact rabbit knee.

Experiments established the principles and protocol of EPIC-microCT. Researchers first immersed cartilage samples in the contrast agent solution and waited for the agent to diffuse into the tissue. Tissue with fewer negatively charged PGs absorbed more of the negatively charged contrast agent, and tissue with a higher PG concentration repelled it.

Researchers then used EPIC-microCT to detect the concentrations of the contrast agent, which allowed them to calculate the amount of PGs in different parts of the cartilage. Because degrading cartilage loses PGs over time, researchers could monitor the progression of tissue changes. In addition, differences in the X-ray signal of cartilage and bone allowed researchers to isolate the cartilage layer on a rabbit joint and determine its thickness, indicating that this technique also can be used to measure tissue thinning during disease progression.; Source: Georgia Institute of Technology