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.
“By detecting PG content and distribution, the technique reveals information about both the thickness and composition of the cartilage -- important factors for monitoring the progression and treatment of osteoarthritis,” said Associate Professor Marc Levenston in Georgia Tech’s George W. Woodruff School of Mechanical Engineering.
He and Associate Professor Robert Guldberg, also in the 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.
“This technique will allow pharmaceutical researchers to obtain more detailed information about the effects of new drugs and other treatment strategies for treating osteoarthritis,” Levenston said.
A report on the research will be published Dec. 19 in the journal Proceedings of the National Academy of Sciences and appeared in the online early edition on Dec. 4. The National Science Foundation, National Institute of Arthritis and Musculoskeletal and Skin Disorders, and the Arthritis Foundation funded the work.
Experiments conducted by Ph.D. student Ashley Palmer 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.
In follow-on research funded by a new, two-year grant from the National Institutes of Health, the researchers will gather additional quantitative data and use the technique to examine the very thin cartilage of rat knee joints. Researchers will nondestructively evaluate osteoarthritis progression and then attempt to use this approach to monitor cartilage changes over time in vivo, or inside the same live animals.
“Ultimately, if we can monitor cartilage changes with good resolution and do it with little or no invasion of the tissue in live animals, then we can track osteoarthritis progression and the effects of drug therapy or other treatments over time,” Guldberg said.
Researchers have already addressed a significant technical hurdle in making the imaging technique feasible. They researched several contrast agents and tried two others before choosing Hexabrix™, which is approved by the Food and Drug Administration for use as a contrast agent for various imaging procedures in humans. When diluted, it produced an X-ray signal that allowed distinction of bone from cartilage.
“The ability to separate bone from cartilage in the microCT scan is a big deal,” Guldberg said. “It suggests that this technique may work in vivo.”
But dilution reduces the contrast agent’s sensitivity and therefore the technique’s PG-monitoring capability, the authors write in their paper. “In this next phase of research, we hope to find a one-shot concentration of the contrast agent that works for analyzing both cartilage thickness and composition,” said Levenston, the senior author on the paper.
In addition, the researchers must address technical issues involving the in vivo delivery and retention of a sufficient volume and concentration of the contrast agent, they note in the paper.
“But even if the technique only works for in vitro studies, it still provides useful quantitative, high-resolution, 3D images that researchers can use to nondestructively monitor cartilage degeneration and even regeneration in small animal models,” Guldberg said.
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