Assessment of glycosaminoglycan concentration changes in the intervertebral disc via chemical exchange saturation transfer

In this study, it is shown that the chemical exchange saturation transfer (CEST) method for hydroxyl protons can be used to detect changes in glycosaminoglycan (GAG) concentration in the intervertebral disc. The method, termed gagCEST, was demonstrated ex vivo by correlating the CEST effect with the fixed charge density (FCD) of the nucleus pulposus (NP), as well as by correlating tissue CEST images with their corresponding ²³Na images. Incubation of five NP samples with trypsin produced samples with varying GAG content (n=19). A good correlation was found between the -OH CEST effect and FCD, as well as with the N-acetyl signal amplitude. gagCEST imagesinvitro further illustrated the amount of detail obtainable from this contrast mechanism when compared with conventional imaging. The large concentration of GAG and the relatively long T ₁ of water in NP make the method sensitive, in particular, for the assessment of GAG depletion in this tissue. It is the loss of GAG in NP that indicates the early stage of disc degeneration.