Technical note Assessment of interstitial water content of articular cartilage with T 1 relaxation Jatta E. Berberat a, ⁎ , Mikko J. Nissi a , Jukka S. Jurvelin a,b , Miika T. Nieminen c,d a Department of Physics, Kuopio University, 70211 Kuopio, Finland b Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, 70211 Kuopio, Finland c Department of Diagnostic Radiology, Oulu University Hospital, 90029 Oulu, Finland d Department of Radiology, Oulu University, 90029 Oulu, Finland Received 19 June 2008; revised 2 September 2008; accepted 25 September 2008 Abstract The interstitial water content typically increases in the early degeneration of articular cartilage. Previously, T 2 relaxation has been related to water content, yet it is known to be strongly affected by the collagen orientation. Articular cartilage plugs from the bovine patella, femur and tibia (N=20) were mapped for T 1 and T 2 at 9.4 T to test the ability of T 1 relaxation to reflect cartilage water content. As a reference, water and proteoglycan (PG) contents were determined. Significant (Pb.01) linear associations were demonstrated between the relaxation rates and tissue water content (R 1 : r =-.81, R 2 : r =-.60) and PG content (R 1 : r =.75). After adjustment for the tissue water content, partial correlation analysis did not show significant associations between the relaxation rates and tissue PG content. After the effect of PGs was removed, significant (Pb.05) linear correlation between the relaxation rates and tissue water content (R 1 : r =-.48, R 2 : r =-.50) was observed. Thus, the spin-lattice relaxation rate is proposed to provide a biomarker for water content in articular cartilage. © 2009 Elsevier Inc. All rights reserved. Keywords: Water content; T 1 ; Cartilage 1. Introduction Articular cartilage consists of the collagen network (15– 22% by wet weight), proteoglycans (PGs) (4–7%) and water (60–80%) [1]. The hydrophilic nature of cartilage PGs creates a swelling pressure that is restricted by the collagenous network. The water content of healthy cartilage varies between the joint surfaces [2] and decreases from the surface to deep cartilage [3–5]. In degenerative joint diseases, such as osteoarthritis (OA), the extracellular cartilage matrix is disrupted. This leads to an increase in cartilage water content as the collagenous network poorly resists the swelling pressure. In early OA, water content is anticipated to increase about 10% [6,7]. Besides a general increase in the water content, changes in the water distribution across the thickness of cartilage take place [3]. Thus, the measurement of water content can provide sensitive information on the degree of early cartilage degeneration [8,9]. Previously, water content of cartilage has been estimated through the proton density (PD) imaging [10] and T 2 relaxation time measurements [11]. The first approach requires an external phantom for calibration and requires a T 2 measurement if compensation for T 2 effects is desired. T 2 relaxation time, besides reflecting the water content, is significantly dependent also on the orientation of the collagen network through interactions between the water bound to collagen fibrils, a phenomenon known as the magic angle effect [12–15]. Given the limitations of the PD or T 2 - based approaches, more robust MRI methods are required for determining the cartilage water content. In this study, we aimed to reveal (a) whether T 1 relaxation could serve as a surrogate marker for the interstitial water content and (b) the possible association between the relaxation rates and the PG content in articular cartilage. Available online at www.sciencedirect.com Magnetic Resonance Imaging 27 (2009) 727 – 732 ⁎ Corresponding author. Department of Physics, University of Kuopio, POB 1627, 70211 Kuopio, Finland. Tel.: +358 41 79 277 6232; fax: +358 8 315 2112. E-mail address: jatta.kurkijarvi@uku.fi (J.E. Berberat). 0730-725X/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.mri.2008.09.005