Pathophysiology of the intervertebral disc and the challenges for MRI

Through its ability to make relatively noninvasive and repeatable measurements, MRI has a great deal to offer, not only to clinical diagnosis of intervertebral disc disorders but also as a tool for basic research into disc physiology and the etiology of disc degeneration. In this brief review we outline the structure of the disc, the composition and organization of its macromolecules, and the changes that occur during disc degeneration, attempting to summarize features that have been or could become targets of MRI characterization. It is important to recognize, however, the fundamental limitation that most of the changes so far observed in MRI are consequences of alterations in cellular metabolism that occurred months to years previously and provide little insight into the current functional status of the tissue. There is therefore a need to develop MR techniques that directly characterize cellular activity and factors such as nutrient delivery on which it is critically dependent. We therefore briefly review cellular energy metabolism and nutrient transport into the avascular disc and consider the ability of MRI to reveal information about such processes. As a corollary of this discussion we also consider the constraints that the unusual transport properties of the disc impose on the delivery of contrast agents to the disc, since an understanding of these limitations is central to interpretation of the resulting images.