Articular Cartilage (AC) is mainly composed of an extracellular matrix which consists of collagen and proteoglycans. Proteoglycans (PG) are associated with a locally high concentration of sodium ions. Early stages of osteoarthritis are associated with the loss of proteoglycans. Loss of proteoglycans results in the proportional loss of sodium ions. Thus sodium spectroscopy and imaging are well suited for studying PG in cartilage. Due to the interaction with slowly tumbling PG macromolecules, sodium ions exhibit biexponential relaxation. In the presence of biexponential relaxation, multiple quantum coherences can be created. These multiple quantum coherences enable one to study bound pools of sodium in AC. We have implemented single (SQ) and triple quantum (TQ) sodium imaging on bovine articular cartilage. The triple quantum sequence has been tested on a phantom and then on a bovine specimen of AC. The TQ and SQ images of AC are compared. The TQ image exclusively shows signals from articular cartilage and signals from bone and saline/synovium are absent. This clearly demonstrates the feasibility of investigating the sodium pool that is bound to proteoglycans and thereby provide a probe for measuring changes in proteoglycan concentration that occur during cartilage degeneration. In vivo TQ imaging of sodium is being developed.
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