MR T1ρ Relaxation Time Quantification in Cartilage

Abstract

Osteoarthritis (OA) is characterized by the progressive loss of hyaline articular cartilage. MRI has been widely applied to visualize cartilage directly. However, conventional MRI is limited to showing cartilage morphological changes at a stage when cartilage is already irreversibly lost. Standard cartilage-dedicated MR techniques include fat-saturated T2-weighted, proton density-weighted fast spin echo (FSE) sequences and T1-weighted spoiled gradient echo (SPGR) sequences. These sequences are inconclusive in quantifying early degenerative changes of the cartilage matrix [1]. As discussed in detail in Chap. 1, hyaline articular cartilage is composed by few chondrocytes surrounded by a large extracellular matrix (ECM). The ECM is composed primarily by water and two groups of macromolecules: proteoglycan and collagen fibers. Early events in the development of cartilage matrix breakdown include the loss of proteoglycans, changes in water content, and molecular-level changes in collagen [2]. Early diagnosis of cartilage degeneration would require the ability to noninvasively detect changes in proteoglycan concentration and collagen integrity before gross morphologic changes occur. This capability of early diagnosis would be valuable for preventing the progression of disease and for therapeutic monitoring. MR T1ρ relaxation time quantification in cartilage has been proposed as a promising tool towards this goal, and T1ρ quantification techniques have been developed in the past decade for imaging early biochemical changes in cartilage matrix. The major clinical applications have been in osteoarthritis (OA) and acutely injured knees that have a high risk of developing OA. In this chapter, we will discuss the basic principles of T1ρ relaxation mechanism, technical development, and clinical applications of this novel technique.