A simplified approach to define cervical vertebral levels in spinal cord MRI studies

Abstract

Background and purpose: Spinal cord (SC) cross-sectional areas (CSAs) assessed with MRI have proven to be extremely valuable imaging markers in several diseases. Among the challenges is the delineation of vertebral levels to determine level-dependent changes in cord atrophy. With this study, we aimed to (1) test the hypothesis that there is proportionality in the position of the first six intervertebral discs and the length of the upper portion of the SC and (2) show that a proportionality approach can simplify the CSA assessment across vertebrae offering good reliability.

Methods: Forty-six volunteers underwent standard T2-weighted and T1-weighted cervical SC MRI acquisitions. The distance between the obex and the intervertebral discs (from C2-C3 to T1-T2) was measured on the T2-weighted acquisitions of the entire cohort. In a test-retest experiment on 12 subjects, the % disc position values were used to define vertebral levels, and a comparison was performed with manual vertebrae assignment in terms of mean CSA and its coefficient of variation.

Results: The mean upper cord length for the cohort was 144.0 ± 13.1 mm. The discs' level % position in the upper cord was found to be fairly consistent, with standard deviations of 0.8%-1.7%. The mean vertebral CSA obtained with the proportionality method was substantially equivalent to the manual approach in terms of mean CSA values and test-retest reliability.

Conclusions: With this study, we propose a proportionality method for the assignment of cervical SC vertebral levels that can simplify the processing of MRI datasets in the context of CSA measurements.

Keywords: cross‐sectional area; spinal cord MRI; spinal cord atrophy; vertebral levels.

FIGURE 1

Example of measurement performed on the T2‐weighted images (A: raw image, B: straightened image). In Panel A, distances measured along the centerline are reported with different colors (green: obex/C2‐C3; pink: C2‐C3/C3‐C4; yellow: C3‐C4/C4‐C5; blue: C4‐C5/C5‐C6; light blue: C5‐C6/C6‐C7; red: C6‐C7/C7‐T1; white: C7‐T1/T1‐T2), while in Panel B, the green dotted line represents the slice including the obex, the white dotted line represents the slice of the T1‐T2 disc, and the orange dotted lines mark slices correspondent to the other intervertebral discs.

FIGURE 2

Comparison of vertebral levels position (% relative to the obex to T1‐T2 length), as measured along the centerline on the raw T2‐weighted (T2‐w) images (y‐axis) and on the straightened T2‐w images (x‐axis) for the entire cohort of 46 subjects.

FIGURE 3

Correlations between mean cross‐sectional area (expressed as squared millimeters) obtained for the 12 subjects on the five vertebral bodies with the manual method (method1) and the proportionality method (method2). Left: test session; right: retest session. The 95% interval level is indicated as an ellipsoid.

FIGURE 4

Bland‐Altman plots for average vertebral cross‐sectional area obtained for the 12 subjects in the test session (retest session plots are similar and not reported). Data are expressed as square millimeters. Continuous red lines represent mean differences, while dashed red lines delimit the upper and lower 95% interval. Method1: manual method; method2: proportionality method.