The Appropriateness Of Measuring Age-related Disc Degeneration On The Mid-sagittal Lumbar MR Image

Disc degeneration has long been regarded as one of the primary contributors to back pain.Yet,to date a standardized measure for disc degeneration is absent.In the history of spine research,dozens of histological and radiographic approaches have been used to evaluate disc degeneration.Currently,magnetic resonance(MR)is the best imaging modality for clinical investigations of the disc.Yet,a number of spinal phenotypes,such as disc signal intensity,disc height,disc bulging,vertebral osteophytes,and endplate irregularity,have been used to measure disc degeneration,either separately or jointly.There are also a few MRI-based visual classification systems.Among them,the most commonly used perhaps is the Pfirrmann scale.This grading,however,is based on subjective judgments of many perspectives of the disc,and sometimes is difficult to distinguish one stage of disc degeneration from another.On the other hand,the ordinal nature of this scale also limits its sensitivity to detect changes of disc degeneration in longitudinal studies,particularly for the elderly spines.To improve measures of disc degeneration,quantitative measurements were introduced to facilitate scientific research of the disc.Yet,quantitative measurements of disc degeneration were typically acquired from a single mid-sagittal MRI.Given the complicated context of disc degeneration,primary MR findings of disc degeneration may not necessarily present on the mid-sagittal MRI.For example,disc herniation,a severe form of disc degeneration,typically occurs at the posterolateral region of a disc rather than at the postero-central region.In clinical practice,we have also noticed that disc signal on para-sagittal MRIs seems to be lower than that on the mid-sagittal MRI.Aims:This study investigated the appropriateness of using the mid-sagittal lumbar MRI to measure various phenotypes of age-related disc degeneration.Methods:Lumbar spine MRIs were randomly selected from the subjects in Hangzhou Lumbar Study.An image analysis program Spine Explorer was used to obtain quantitative measurements for disc height,bulging and signal on para-and mid-sagittal T2-weighted MRIs.Measurements on para-and mid-sagittal MRIs,and their associations with age were compared.Results:Lumbar spine MRIs of 66 subjects(mean age 50.3 years,standard deviation 16.5 years,range 22-84 years)were selected and studied form 332 residents in Hangzhou Lumbar Study.Mean disc height and disc height index measurements acquired on the mid-sagittal MRI were statistically significantly greater than that acquired on the para-sagittal MRI(P<0.001).For posterior disc bulging measurements,these acquired on the mid-sagittal MRI were greater than that on the para-sagittal image for both upper and lower lumbar regions(P<0.001).However,anterior disc bulging measurements acquired on the para-sagittal MRIs were statistically greater in the upper lumbar region(P=0.013)but less in the lower lumbar region than those acquired on the mid-sagittal MRI(P=0.019).All disc signal intensity measurements acquired on the mid-sagittal MRI were statistically greater than those acquired on the para-sagittal MRIs(P<0.001).Disc height measurements were not linearly associated with age.Greater age was correlated to greater anterior(r=0.45,P<0.001)and posterior(r=0.33,P<0.01)bulging on para-sagittal MRIs,but not posterior disc bulging on the mid-sagittal MRI(r=-0.10,P>0.05).Disc signal intensity measurements on the mid-sagittal MRI had stronger correlations with age than those on para-sagittal MRIs.Mean and standard deviation of disc signal intensity acquired on the mid-sagittal MRI had the strongest correlations with age among all measures of disc degeneration studied(r=-0.50,-0.67 respectively,P<0.001 for both).Conclusion:Although severe disc narrowing clearly is a sign of severe disc degeneration,disc height was not linearly associated with age,neither on mid-sagittal image nor on para-sagittal images.Disc signal measurements acquired on the mid-sagittal MRI were reliable and had strong correlations with age and thus,can be used as an appropriate measure of disc degeneration.Disc bulging had better be measured on para-sagittal MRIs and disc signal should be measured on mid-sagittal MRIs.