MR T2 Relaxation Time Measurements for Cartilage and Menisci

Abstract

IntroductionMagnetic Resonance Imaging (MRI) can be used not only for morphologic but also for quantitative assessment of knee cartilage. Quantitative T1rho and T2 relaxation time measurements and dGEMRIC (delayed Gadolinium enhanced MRI of the cartilage) have emerged as potential cartilage biomarkers to assess early degenerative disease. This chapter focuses on the T2-technique and clinical applications of hyaline cartilage and meniscal T2 relaxation time measurements in particular at the knee.The chapter is structured into four parts: After a brief background section, image acquisition and processing are outlined. Subsequently, image analysis and clinical applications are presented. BackgroundEarly stages of knee osteoarthritis (OA) are characterized by degradation of components of the extracellular matrix of the cartilage [1,2]. The proteoglycan content decreases and the collagen network shows signs of disorganization. The latter induces higher water mobility and consequentially increased water content within the cartilage. These initial cartilage changes are subclinical, i.e., individuals do not have symptoms of OA. T1rho and T2 relaxation times are affected by these pathophysiological processes [3,4]. Whereas T1rho relaxation time is more sensitive to the proteoglycan content of the cartilage, T2 relaxation time is more sensitive to cartilage hydration and orientation and integrity of the collagen network. Therefore, T1rho and T2 are noninvasive biomarkers for cartilage degeneration. These biomarkers may be used to detect early stages of the disease, quantitatively assess disease severity, sensitively monitor disease progression, and monitor OA therapy. Since they are sensitive to tissue components of cartilage that change before morphologic signs are detectable, they are particularly suited for the early disease stages, which are not accessible with standard morphological MR imaging [5]. In one of the early studies, Dardzinski et al. analyzed T2 measurements in asymptomatic volunteers and found a reproducible pattern of increasing T2 that was proportional to the known spatial variation in cartilage water [6]. Based on these findings, the authors postulated that these regional T2 differences were secondary to the restricted mobility of cartilage water within an anisotropic solid matrix. In a subsequent clinical study, Mosher et al. studied asymptomatic volunteers and individuals with symptoms of patellar chondromalacia using quantitative T2 maps of patellar cartilage and found an asymptomatic increase in T2 with aging as well as an increase in cartilage T2 from the radial zone to the articular surface [7]. Based on these studies and findings, investigators adopted T2 relaxation time measurements to assess cartilage degeneration in clinical studies, which culminated in implementing T2 relaxation time measurements in the NIH sponsored Osteoarthritis Initiative (OAI) multicenter trial. Image Acquisition and Processing Imaging Sequences and Field StrengthQuantitative T2 relaxation time measurements in vivo are t...