Reliability and reproducibility of sciatic nerve magnetization transfer imaging and T2 relaxometry

Abstract

Objectives: To assess the interreader and test-retest reliability of magnetization transfer imaging (MTI) and T2 relaxometry in sciatic nerve MR neurography (MRN).

Materials and methods: In this prospective study, 21 healthy volunteers were examined three times on separate days by a standardized MRN protocol at 3 Tesla, consisting of an MTI sequence, a multi-echo T2 relaxometry sequence, and a high-resolution T2-weighted sequence. Magnetization transfer ratio (MTR), T2 relaxation time, and proton spin density (PSD) of the sciatic nerve were assessed by two independent observers, and both interreader and test-retest reliability for all readout parameters were reported by intraclass correlation coefficients (ICCs) and standard error of measurement (SEM).

Results: For the sciatic nerve, overall mean ± standard deviation MTR was 26.75 ± 3.5%, T2 was 64.54 ± 8.2 ms, and PSD was 340.93 ± 78.8. ICCs ranged between 0.81 (MTR) and 0.94 (PSD) for interreader reliability and between 0.75 (MTR) and 0.94 (PSD) for test-retest reliability. SEM for interreader reliability was 1.7% for MTR, 2.67 ms for T2, and 21.3 for PSD. SEM for test-retest reliability was 1.7% for MTR, 2.66 ms for T2, and 20.1 for PSD.

Conclusions: MTI and T2 relaxometry of the sciatic nerve are reliable and reproducible. The values of measurement imprecision reported here may serve as a guide for correct interpretation of quantitative MRN biomarkers in future studies.

Key points: • Magnetization transfer imaging (MTI) and T2 relaxometry of the sciatic nerve are reliable and reproducible. • The imprecision that is unavoidably associated with different scans or different readers can be estimated by the here presented SEM values for the biomarkers T2, PSD, and MTR. • These values may serve as a guide for correct interpretation of quantitative MRN biomarkers in future studies and possible clinical applications.

Keywords: Biomarkers; Magnetic resonance imaging; Observer variation; Peripheral nervous system; Reproducibility of results.

Fig. 1

Study design flowchart. Twenty-one healthy participants underwent repeated MR neurography of their sciatic nerve on three separate days, each covering the exact same anatomical region. Image analysis and postprocessing were performed by two independent readers. Interreader agreement and test-retest reproducibility were statistically assessed for each of the biomarkers transverse relaxation time (T2), proton spin density (PSD), and magnetization transfer ratio (MTR)

Fig. 2

Image postprocessing. Nerve segmentation was performed by each reader independently by delineating the tibial portion of the sciatic nerve using the freehand region of interest (ROI) tool in Osirix in the T2-turbo spin-echo (TSE) sequence (a). Subsequently, ROIs were transferred onto co-registered gradient-echo sequences for magnetization transfer imaging (MTI) (b), and multi-echo sequences for T2-relaxometry (c) and quantitative readout parameters were assessed. MTR = magnetization transfer ratio, TE = echo time, S₀ = MR signal intensity without saturation pulse, S₁ = MR signal intensity with saturation pulse

Fig. 3

Descriptive statistics for both readers and all three scans. Values are illustrated as boxplots with whiskers to visualize measurement distribution (box showing the 25th to 75th percentiles with a line at the median, whiskers indicate the minimum and maximum data values). MTR = magnetization transfer ratio, T2 = transverse relaxation time, PSD = proton spin density

Fig. 4

Bland-Altman plots for assessment of test-retest reliability of magnetization transfer ratio (MTR) (a), transverse relaxation time T2 (b), and proton spin density (PSD) (c) for both observers and all three scans, respectively. The black continuous line represents the mean of all differences (bias), grey dotted lines show the 95% limits of agreement

Fig. 5

Bland-Altman plots for assessment of interreader reliability of magnetization transfer ratio (MTR) (a), transverse relaxation time T2 (b), and proton spin density (PSD) (c) of the sciatic nerve. The black continuous line represents the mean of all differences (bias), grey dotted lines show the 95% limits of agreement