Abstract
Introduction
Ultrashort echo time (UTE) imaging in combination with off-resonant saturation ratios (OSR) is increasingly used for characterization of tissue with fast signal decay, such as the annulus fibrosus (AF) as well as the upper and lower cartilaginous end plate (EP). The aim of this study was to assess effects of age, gender, and location of OSR measurement on evaluation of lumbar disc degeneration.
Materials and Methods
A total of 35 lumbar intervertebral discs of 12 fresh-frozen cadaveric specimens (median age 66 years, range 49–75 years) were examined in a whole-body MR scanner using a sagittal 3D UTE sequence with an isotropic submillimeter resolution of 0.7 mm (TE 0.07 milliseconds, TR 10.3 milliseconds, FoV 128 mm, and FA 10 degrees) with and without off-resonant saturation preparation pulse at 3 T. The off-resonance frequency varied between 1 and 5 kHz. OSRs of images with and without preparation pulse were calculated for upper and lower EP as well as for AF. Intervertebral discs were classified healthy (CO, n = 20), mild degeneration (MIL, n = 7), and severe degeneration (SEV, n = 8) concerning morphology and signal alteration in conventional MR sequences and UTE imaging. A nonparametric U-test reports level of significance. Correlation between age and OSRs is calculated using Spearman rank correlation coefficient. Effects of different off-resonance frequencies and measurement locations on lumbar disc degeneration are investigated using logistic regression and ROC analysis. Cut-off values, sensitivity, and specificity are given.
Results
Best differentiation between mild and severe degeneration was seen at an off-resonance frequency of 1 kHz for AF (sensitivity 87.5%/specificity 87.5%/cut-off 0.34) and the lower EP at 1.5 kHz (sensitivity 100%/specificity 85.7%/cut-off 0.29). Comparison of CO and MIL groups revealed best differentiation at 1 kHz for upper EP (sensitivity 100%/specificity 80%/cut-off 0.35). A reasonable correlation between age and OSRs in healthy lumbar discs was seen in AF for all investigated frequencies, ρ = 0.49 (5 kHz) to 0.61 (2 kHz), and in lower EP for frequencies above 2 kHz, ρ = 0.51 (3 kHz) to 0.71 (5 kHz). Significant differences between OSR measurements were seen in AF of healthy female and male lumbar discs for all investigated sequences and in upper EP at 1 kHz.
Conclusion
OSR assessment might be a suitable method for detection of early stage lumbar disc degeneration and should be preferably performed using off-resonance frequencies between 1 and 2 kHz. Age might influence OSR measurement in AF and lower EP. Gender seems to influence results of OSRs, especially in evaluation of AF.
Color maps of OSRs at an off-resonant frequency of 1 kHz gained by UTE imaging with and without preparation pulse in healthy (a): mildly degenerated and (b): severely degenerated discs. OSR, off-resonant saturation ratios; UTE, ultrashort echo time.