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A Mn[Cl.sub.2]-based MR signal intensity linear response phantom.

Article Excerpt
Age-related intervertebral disk (IVD) degeneration is a major contributor to spinal morbidity in modern society) Previous authors have suggested that physiological (ie, nontraumatic) degenerative changes in the IVDs likely begin as early as the second decade of life and progress until death. (2) While degradation of proteoglycan metabolism and expression pathways have been heavily implicated in these changes, (3) the precise mechanism by which the process occurs remains unclear. (4) Independent of the finite molecular interactions, a clear link has been made between progressive degeneration of the IVDs and a reduction in relative disk water content. (2,5-7)

Orthopedic and biomechanical research using a variety of ex vivo and postmortem models has established a strong positive correlation between IVD hydration (independent of age) and physiological spinal performance. (8-10) While such a link suggests the potential appropriateness of rehydrating therapies and interventions to promote or restore function in otherwise degenerate disks, there currently are no universally accepted means for noninvasive description and quantification of the fluid content of in vivo fibrocartilage. Thus, longitudinal assessment of the efficacy of chondro-promoting treatment regimens has been difficult and has relied heavily on subjective patient outcome reporting. Such evaluation is further complicated by the high degree of both within-patient and between-patient variation in the magnetic resonance (MR) signal presentation of the lumbar IVD. Such variability prevents the broad application of standardized ("normal") reference intervals for identifying abnormality.

Research, clinical and anecdotal evidence has long suggested a direct correlation between observed MR signal intensity (SI) and tissue hydration. (11) Such a premise is supported by basic time-relaxation characteristics: Structures rich in hydrogen ions (ie, water) appear brighter under T2-weighted imaging conditions and, conversely, structures low in hydrogen ion content appear darker. (11) Despite this subjective MR phenomenon, there remains no accepted noninvasive model for determining relative water content of tissue in vivo.

Manganese chloride (Mn[Cl.sub.2]) has been described previously as an appropriate signal-enhancing material for use in clinical studies of physiological function (12-15) and its basic application as a contrast-variable MR imaging phantom also has been explored. (15,16) Manganese chloride is a relatively inexpensive and widely available inorganic compound that is safe to handle, easy to store and has a high degree of phantom reproducibility. (15) In an aqueous state (ie, bound or free fluid) Mn[Cl.sub.2] displays basic paramagnetic polarity (17) and appears to exhibit the somewhat unusual characteristic of being concentration-dependent signal enhancing under both basic T1- and T2-weighted imaging conditions. Collectively, these attributes make Mn[Cl.sub.2] an ideal agent for a contrast-variable MR phantom model and suggest that Mn[Cl.sub.2] might be a useful and appropriate calibration tool for assessing tissue hydration.

In the...