Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage

In this article, both sodium magnetic resonance (MR) and T-1 rho relaxation mapping aimed at measuring molecular changes in cartilage for the diagnostic imaging of osteoarthritis are reviewed. First, an introduction to structure of cartilage, its degeneration in osteoarthritis (OA) and an outline of diagnostic imaging methods in quantifying molecular changes and early diagnostic aspects of cartilage degeneration are described. The sodium MRI section begins with a brief overview of the theory of sodium NMR of biological tissues and is followed by a section on multiple quantum filters that can be used to quantify both bi-exponential relaxation and residual quadrupolar interaction. Specifically, (i) the rationale behind the use of sodium MRI in quantifying proteoglycan (PG) changes, (ii) validation studies using biochemical assays, (iii) studies on human OA specimens, (iv) results on animal models and (v) clinical imaging protocols are reviewed. Results demonstrating the feasibility of quantifying PG in OA patients and comparison with that in healthy subjects are also presented. The section concludes with the discussion of advantages and potential issues with sodium MRI and the impact of new technological advancements (e.g. ultra-high field scanners and parallel imaging methods). In the theory section on T-1 rho, a brief description of (i) principles of measuring T-1 rho relaxation, (ii) pulse sequences for computing T-1 rho relaxation maps, (iii) issues regarding radio frequency power deposition, (iv) mechanisms that contribute to T-1 rho in biological tissues and (v) effects of exchange and dipolar interaction on T-1 rho dispersion are discussed. Correlation of T-1 rho relaxation rate with macromolecular content and biomechanical properties in cartilage specimens subjected to trypsin and cytokine-induced glycosaminoglycan depletion and validation against biochemical assay and histopathology are presented. Experimental T-1 rho data from osteoarthritic specimens, animal models, healthy human subjects and as well from osteoarthritic patients are provided. The current status of T-1 rho relaxation mapping of cartilage and future directions is also discussed. Copyright (C) 2006 John Wiley & Sons, Ltd.