Fascicular ratio: a new parameter to evaluate peripheral nerve pathology on magnetic resonance imaging: a feasibility study on a 3T MRI system

Abstract

The objective of the study was to define and quantitatively evaluate the fascicular ratio (FR) on magnetic resonance imaging (MRI) in patients with peripheral neuropathies compared with healthy controls. Forty control subjects (20 women, 20 men; age, 44.6 ± 13.4 years) and 40 patients with peripheral neuropathy (22 women, 18 men; age, 50.3 ± 10.2 years) were examined with a standard 3T MRI protocol. With customized software (with semiautomatic and automatic interface), the hypointense and hyperintense areas of the peripheral nerves corresponding to fascicular and nonfascicular tissue were examined on T1-weighted sequences. The ratio of fascicular pixels to total pixels was called FR. Correlation with FR calculated on high-resolution ultrasound was performed. The statistical analysis included the Mann-Whitney U test of controls versus patients, the receiver operating characteristic (ROC) analysis, and the subgroup analysis of patients according to etiologies of neuropathy. Intraobserver and interobserver agreement was calculated based on the evaluation made by 3 readers. Finally, a complete automatic evaluation was performed. On MRI, FRs were significantly increased in patients compared with controls (FR, 76.7 ± 15.1 vs 56 ± 12.3; P < 0.0001 for the semiautomatic interface; and FR 66.3 ± 17.5 vs 47.8 ± 18.4; P < 0.0001 for the automatic interface). The increase in FR was caused mainly by an increase in the hypointense part of the nerve. This observation was valid for all causes of neuropathies. ROC analysis found an area under the curve of 0.75 (95% confidence interval, 0.44-0.81) for FR to discriminate neuropathy from control. The correlation coefficient between MRI and ultrasound was significant (r = 0.49; 95% confidence interval for r, 0.21-0.70; P = 0.012). With the semiautomated evaluation, the mean intraobserver agreement was good (K = 0.86). The interobserver agreements were also good (reader 1 vs reader 2, k = 0.71; reader 2 vs reader 3, k = 0.78; reader 3 vs reader 1, k = 0.71). There were no statistically significant differences between the results obtained using the 2 methods. FR calculation on MRI is feasible, and it may be used in adjunct to standard MRI evaluation in peripheral nerve disorders.

FIGURE 1

Quantitative assessment of the FR with the semiautomatic or interactive method. In this figure, the computer interface is shown. The nerve is identified within a region of interest. In the semiautomatic version, the radiologist adjusts the density threshold to separate fascicle and nonfascicles. The FR is read into the blue circle. In the fully automated method, there is no need to adjust the threshold. In this figure, artificially modified for visual purposes, the FR is 45%. FR = fascicular ratio.

FIGURE 2

Quantitative analysis of FR. Regions of interest were drawn around the epineurial contour of peripheral nerves in axial T1-weighted images (green line). Quantitative analysis was performed on every slice, extremes excluded. On MRI images, the software, after threshold identification, differentiated the fascicles (hypointense on T1-weighted sequences) and the perifascicular tissue (hyperintense on T1-weighted sequences) that resembles the signal of fat. The FR was calculated based on the volume of the fascicles in relation to the total volume of the nerve. The pathological nerve is a tibial nerve. The results are reported in the text and tables. FR = fascicular ratio, MRI = magnetic resonance imaging.