David Brian Hackney MD, Presenter: Nothing to Disclose

1) Interpret MR Images of degenerative disk disease in terms of biochemical and biophysical changes in the disk. These include: characterizing hydration of the disk and inferring the content of disk macromolecules, primarily glycosoaminoglycan and collagen. 2) Use MR Imaging to observe or estimate in the degenerating disk: fixed charge density, water content, and MR relaxation rate parameters. 3) Apply these principles to interpret spine MR studies in terms of the stage of degenerative disk disease and the severity biomechanical derangement.

The disk converts axial load to tensile strain on the annulus and fiber insertion on endplate and absorbs shock. It consists of cartilaginous endplates, an amorphous nucleus pulposus, and a highly collagenous annulus fibrosus. The annulus is a series of collagen lamellae oriented 60 degrees off the long axis of the spine, with fiber orientation alternating in adjacent layers. The annulus is largely composed of Type I and Type II cartilage. In the nucleus a collagen network traps the proteoglycan aggrecan. Disk neutrality is maintained with elevated sodium content in the extracellular space matching the abundant negative charge of the GAG side-chains of aggrecan. This ionic content elevates the osmotic pressure and thus the water content. The disk composition and structure change in normal aging, and accelerate in degeneration. The proteoglycan content and water content decrease from very high values in infants, to lower values in adults, and progressively lower levels still in degeneration. Hydration and proteoglycan content are closely related in normal and degenerated disk. Water content of the disk gradually decreases under load. This leads to well-established diurnal variation in water content of the disk, highest after arising from sleep and decreasing during the day. The relationship between load and water content appears directly dependent on disk composition, but is not independently determined by stage of degeneration. The familiar high signal on T2 weighted MR images reflects water content, and proteoglycan content. Attempts to resolve which of these is most responsible for the signal characteristics of the disk have yielded conflicting results, perhaps due the the high collinearity among them. Water content changes in response to loads of daily living, and the signal intensity and height of the disk decrease during the day. Better understanding of the degenerative process will aid interpretation of MR studies and assessment of disk response to therapy.

Hackney, D, Advanced Imaging of Disk Degeneration.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9001322.html