Multicomponent T2 analysis of articular cartilage with synovial fluid partial volume correction

The extracellular matrix of articular cartilage is a complex network of type II collagen, proteoglycan, and water.1 Quantitative magnetic resonance imaging (MRI) methods have been used to evaluate the composition and ultrastructure of articular cartilage.2 Among these techniques, the transverse relaxation time constant (T2) has been the most extensively studied and has been shown to be sensitive for detecting cartilage degeneration.3,4 However, T2 relaxation time is a nonspecific parameter influenced by multiple factors including tissue anisotropy, hydration, and macromolecule content.5–7 Multicomponent T2 mapping methods have been proposed to improve the specificity of cartilage T2 analysis.8–10 Previous studies using nuclear magnetic resonance (NMR) have identified three water components within cartilage, which were assigned to extremely fast-relaxing water bound to collagen (T2 = 2 msec, fraction = 6%), fast-relaxing water bound to proteoglycan (T2 = 25 msec, fraction = 15%), and slow-relaxing bulk water (T2 = 96 msec, fraction = 79%).8 Multicomponent T2 analysis is typically performed using multiecho spin-echo techniques.8–10 Previous studies have shown that the fraction of the fast-relaxing water component (FF) is a sensitive and specific measure of the proteoglycan content of cartilage and that the T2 relaxation time of both the fast-relaxing (T2F) and slow-relaxing (T2S) water components change with enzymatic degradation of cartilage.8,9 However, spin-echo techniques require a long acquisition time, which have limited cartilage assessment to a single slice of ex vivo specimens. An alternative method called Multicomponent Driven Equilibrium Single Pulse Observation (mcDESPOT) has recently been proposed to accelerate multicomponent T2 analysis using rapid 3D steady-state imaging.11,12 mcDESPOT can be used to characterize the fast-and slow-relaxing water components of articular cartilage with complete anatomic coverage of the human knee joint at 3.0T in a clinically feasible scan time.12–14 One confounding factor in the two-pool mcDESPOT analysis is synovial fluid partial volume averaging with superficial cartilage. The extremely slow-relaxing synovial fluid water (T2 in the range of seconds) can cause a significant increase in T2 relaxation time as well as an underestimation of the water fraction of the fast-relaxing water component.15 Thus, there is a need to account for synovial fluid in the mcDESPOT model to better assess superficial cartilage and to avoid biased multicomponent T2 measurements. Three-pool models have been recently proposed for mcDESPOT15 and quantitative magnetization transfer (MT) imaging16 to improve the characterization of neural tissues in the presence of partial volume averaging with cerebrospinal fluid (CSF). Specifically, a mcDESPOT three-pool method introduces a third nonexchanging water component to model the partial volume averaging effects of CSF, which has been shown to be effective at correcting CSF contamination in brain imaging.15 This study was performed to investigate the use of the three-pool mcDESPOT model to account for the confounding effects of synovial fluid on multicomponent T2 analysis of the articular cartilage of the knee joint in human subjects.