Magnetic Resonance Imaging for the assessment of degenerative disc disease of the lumbar spine
Advances in Magnetic Resonance Imaging for the assessment of degenerative disc disease of the lumbar spine
From: Semin Spine Surg. 2007 June; 19(2): 65–71
The intervertebral disc is characterized by a tension-resisting annulus fibrosus, and a compression-resisting nucleus pulposus composed largely of proteoglycan. Both the annulus and the nucleus function in concert to provide the disc with mechanical stability. Early disc degeneration begins in the nucleus with proteoglycan depletion. Quantitative MRI techniques have been developed to non-invasively quantify the earliest degenerative changes that occur within the disc. Our ability to identify and quantify these early biochemical changes will provide a better understanding of the pathophysiology of disc degeneration and facilitate the study of interventions that aim to halt or reverse the degenerative process.
Degenerative disc disease of the intervertebral disc is the most common cause of back-related disability among North American adults. This sometimes debilitating condition affects nearly 12 million people in the United States, and may generate direct and indirect costs exceeding 50 billion dollars annually in health-related expenditures. The radiographic evaluation of patients with degenerative disc disease often begins with plain film radiography and a standard T1- and T2-weighted MRI to assess for structural changes within the nucleus and annulus indicative of disc degeneration including a loss of T2-weighted MRI signal, loss of disc height, disc bulge or herniation, posterior element arthrosis, stenosis, and potential vertebral body compromise. While standard MRI is able to detect these later stage developments, it is not able to provide a quantitative measure of the early changes that characterize early degenerative disc disease. This limitation has led to the search for quantitative, non-invasive measures to evaluate the earliest changes involved in the initiation of the degenerative cascade. Such an imaging tool will be important for the evaluation of the patients with early degenerative disc disease, and also in the assessment of disc regenerative or restorative technologies that aim to halt or reverse the degenerative process.
The intervertebral disc has 3 sub-structures: the annulus fibrosus, which envelops the gel-like nucleus pulposus, and the endplates. The proteoglycan rich nucleus pulposus is comprised of a network of randomly distributed collagen fibrils in a hydrated extra-fibrillar matrix. In contrast, the annulus fibrosus is highly organized. It has 15–40 lamellae (layers) enveloping the nucleus pulposus. In each lamella, collagen fibers are positioned at a 30° angle to the axial axis of the spine. 70–80% of the nucleus pulposus and about 65% of the annulus fibrosus consist of water. After water is removed, an intervertebral disc is composed of collagens (50–70%), proteoglycans (10–50%), and other non-collagenous proteins (up to 25%). Collagen in the annulus fibrosus gives the intervertebral disc its tensile strength. The glycosaminoglycan branches of the proteoglycan are negatively charged, and they bind water. This water-binding property of glycosaminoglycan gives an intervertebral disc its hydrostatic pressure, which resists loading.
The initial stage of degenerative disc disease is marked by proteoglycan degradation and subsequently a reduced capacity of the nucleus pulposus to bind water. In later stages of the disease, morphological changes such as a loss of disc height, annular tears and rim lesions, and osteophyte formation materialize. Previous studies have implicated degenerative disc disease with low back pain. During the intial proteoglycan degradation, the proteoglycans of nucleus pulposus breakdown to clusters of short aggregated and non-aggregated molecules, the glycosaminoglycan content decreases, and the nucleus pulposus’s capacity to bind water reduces. The increased modulus and decreased hydrostatic pressure triggers the nucleus pulposus to undergo a phase change, from a fluid-like material to a more solid-like material. Mechanical function of degenerated motion segments is compromised in all loading conditions. The current standard surgical treatment for lower back pain with advanced intervertebral disc degeneration is lumbar spinal fusion. However, if degenerative disc disease can be detected at an earlier stage, the condition may benefit from emerging alternative treatments (e.g., nucleus replacement, total disc arthroplasty, cell therapy, growth factor therapy). Evaluation of the viability of potential degenerative disc disease therapies and longitudinal monitoring of degenerative disc disease progression demands an objective and quantitative imaging strategy, one that is sensitive to the biochemical changes in the early stages of degenerative disc disease. In this section, we will begin with a brief overview of conventional diagnostic quantitative MRI strategies (T1 and T2 relaxation mapping, dGEMRIC), followed by emerging novel quantitative MRI techniques (sodium, magic echo, and T 1?) that show promise as noninvasive, accurate diagnostic imaging tools for the quantitative assessment of early stage degenerative disc disease.
Standard T1 and T2 relaxation mapping are capable of detecting biochemical changes in the nucleus pulposus but have limitations, including increased scan time, limited dynamic range, and a lack of consistency between in vivo and in vitro values. Recent advancements in MRI, such as sodium MRI, magic echo, and T1? MRI, have been shown to be even more sensitive to PG degradation in the nucleus pulposus. As these quantitative MRI methods are still in the research phase, their efficacy still needs to be more fully evaluated in a clinical setting. However, preliminary clinical data on sodium, magic echo, and T1? MRI have demonstrated great potential to become noninvasive quantitative diagnostic tools for early degenerative disc disease.
