A method for quantitative measurement of lumbar intervertebral disc structures: an intra- and inter-rater agreement and reliability study.

Background: There is a shortage of agreement studies relevant for measuring changes over time in lumbar intervertebral disc structures. The objectives of this study were: 1) to develop a method for measurement of intervertebral disc height, anterior and posterior disc material and dural sac diameter using MRI, 2) to evaluate intra- and inter-rater agreement and reliability for the measurements included, and 3) to identify factors compromising agreement.

Methods: Measurements were performed on MRIs from 16 people with and 16 without lumbar disc herniation, purposefully chosen to represent all possible disc contours among participants in a general population study cohort. Using the new method, MRIs were measured twice by one rater and once by a second rater. Agreement on the sagittal start- and end-slice was evaluated using weighted Kappa. Length and volume measurements were conducted on available slices between intervertebral foramens, and cross-sectional areas (CSA) were calculated from length measurements and slice thickness. Results were reported as Bland and Altman's limits of agreement (LOA) and intraclass correlation coefficients (ICC).

Results: Weighted Kappa (Kw (95% CI)) for start- and end-slice were: intra-: 0.82(0.60;0.97) & 0.71(0.43;0.93); inter-rater: 0.56(0.29;0.78) & 0.60(0.35;0.81). For length measurements, LOA ranged from [-1.0;1.0] mm to [-2.0;2.3] mm for intra-; and from [-1.1; 1.4] mm to [-2.6;2.0] mm for inter-rater. For volume measurements, LOA ranged from [-293;199] mm3 to [-582;382] mm3 for intra-, and from [-17;801] mm3 to [-450;713] mm3 for inter-rater. For CSAs, LOA ranged between [-21.3; 18.8] mm2 and [-31.2; 43.7] mm2 for intra-, and between [-10.8; 16.4] mm2 and [-64.6; 27.1] mm2 for inter-rater. In general, LOA as a proportion of mean values gradually decreased with increasing size of the measured structures. Agreement was compromised by difficulties in identifying the vertebral corners, the anterior and posterior boundaries of the intervertebral disc and the dural sac posterior boundary. With two exceptions, ICCs were above 0.81.

Conclusions: Length measurements and calculated CSAs of disc morphology and dural sac diameter from MRIs showed acceptable intra- and inter-rater agreement and reliability. However, caution should be taken when measuring very small structures and defining anatomical landmarks.