Advanced Imaging of Cartilage Repair
Siegfried Trattnig, MD, Vienna General Hospital
The macromolecular network of normal healthy cartilage consists mainly of collagen and proteoglycans. Normally, the collagen network is highly organized, serves as the tissue’s structural framework, and is the principal source of tensile and shear strength. Glycosaminoglycans (GAGs) are repeating disaccharides with carboxyl and sulfate groups attached to the larger aggrecan molecule(proteoglycan) that is part of the extracellular matrix network of cartilage. GAG molecules possess considerable net negative charge and confer compressive strength to the cartilage. In all cartilage repair techniques the main goal is to restore this extracellular macromolecular network of collagen and GAG content. Several MR imaging techniques are available that enable to selectively demonstrate and quantify the GAG components and/or the collagen fiber network of the extracellular matrix and are usually summarized as “compositional imaging” of cartilage.
The GAG specific MR methods comprise dGEMRIC, gagCEST and sodium imaging and in this presentation the clinical usefulness and challenges of these techniques will be discussed. The collagen fiber network and water content specific method is T2 mapping, which also has benefits and challenges, however the new field of Radiomics by careful feature extraction in gray level co-occurence matrix (GLCM) algorithm and consecutive texture analysis can significantly enhance the sensitivity and specificity of T2 mapping in case of cartilage repair tissue, since GLCM features serve as a quantitative marker for collagen fiber organization and a powerful tool for analyzing the texture organization. Successful cartilage transplant development is characterized by continuous increase of tissue organization (predominantly collagen).