Introduction/Purpose

DTI is the most important MR functional sequence in peripheral nerve study. However, due to the very small size of small animal’s nerve, DTI imaging is a difficulty in nerve MR study. Although readout-segmented echo planar imaging DTI (RS-DTI) was considered to correct artifacts and improve resolution clinically¹, its performance in the peripheral nerves of small animals was unknown. Our study compared single-shot echo-planar imaging DTI (SS-DTI) in rat sciatic nerve with RS-DTI to correlate between DTI parameters and fiber histopathology, to clarify which DTI is more superior for small animal peripheral nerve study and to provide reference values for the DTI study of tiny nerves of the human body.

Materials and Methods

Eight healthy adult male Sprague-Dawley rats were anaesthetized to deep sleep (7% chloralhydrate, 5mL/kg, intra-peritoneal injection) and scanned at 3T (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with SS-EPI and RS-EPI (seven segments) DTI (resolution: 0.7×0.7×1.5 mm³; b-values: 0, 800 s/mm²; and diffusion directions: 20), as shown in Table 1. During the examination, rat limbs were attached to the animal coil (8-ch animal coil, Suzhou Medcoil Healthcare Co.,Ltd.) by proof fabric to further prevent movement.
Image quality were evaluated by two independent readers (C.Y.Y., radiologist with 10 years of experience in DTI of the nerve; P.Z.X., with 3 years of experience in DTI of the nerve) on DWI b=800 s/mm² image. Morphology of the sciatic nerve, distortions of the nearby femur, muscles, and homogeneous of neuromuscular were evaluated and scored, good performance with 3 points and poor performance with 1 point. Standard DTI parameters of FA (Fractional Anisotropy), AD (Axial Diffusivity), RD(Radial Diffusivity), and ADC(Apparent Diffusion Coefficient) values were acquired from the workstation singo.via.
The rats were sacrificed after MR imaging. The middle stumps of the nerves were harvested and stained with toluidine blue. The toluidine blue slices were analyzed morphometrically by ImageJ software (https://imagej.net) for histological quantification. The degree of correlation between the DTI parameters and histopathological parameters (the percentage of axon area, percentage of myelin area, thickness of myelin, diameter of myelinated fiber) was calculated by using the Pearson correlation coefficient and compared by the modified Fisher Z-transform respectively.

Results

The image quality scores of SS-EPI were higher than those of RS-EPI in the blurring of the nerve margin (P= 0.008), artifacts of the nerve (P= 0.008), and homogeneity of the neuromuscular region (P= 0.007) (Figure 1, Figure 2). A higher FA (0.661 ± 0.010 vs. 0.607 ± 0.009, P<0.001) and lower RD (0.593 um²/ms ± 0.020 vs. 0.693 um²/ms ± 0.018, P<0.001) were found for SS-EPI compared to RS-EPI. As shown in Table 2 and Figure 3, for both sequences, FA was correlated with all of the histopathological parameters (r=0.913 to 0.963 for SS-EPI, and r=0.741 to 0.897 for RS-EPI, P=<0.001~0.036), except the diameter of myelinated fibers for RS-EPI (r=0.623, P= 0.099); and RD was correlated with all of the histopathological parameters (r= -0.886 to -0.948 for SS-EPI, and r= -0.709 to -0.812 for RS-EP, P=<0.001~0.049), except the percentage of the axonal area for RS-EPI (r= -0.691, P=0.058). The overall correlation coefficients of FA and RD obtained with SS-EPI were numerically higher than those obtained with RS-EPI, and the coefficients between FA and myelin thickness (P=0.027), FA and myelinated fiber diameter (P=0.036), and RD and myelin thickness (P=0.05) for SS-EPI were statistically higher than those for RS-EPI.

Discussion and Conclusion

We found significantly higher image quality of rat sciatic nerve DW imaging based on single-shot echo-planar imaging (SS-EPI) compared with readout-segmented echo-planar imaging (RS-EPI). The SS-EPI derived DTI showed a stronger linear correlation with histopathological parameters than RS-EPI derived DTI parameters. This may be because RS-EPI is extremely sensitive to motion artifacts². Even the invisible trembling after anesthesia can lead to motion artifacts, especially in tiny structures like sciatic nerve. What’s more, SS-EPI has a much shorter scanning time than RS-EPI in our study (4:06min VS 14:12min), reducing the risk of motion artifacts. Secondly, the main advantage of RS-EPI compared to SS-EPI is the significant reduction of geometric distortion caused by susceptibility artifacts^{3, 4}, especially in regions prone to distortion, such as the orbit, skull base, and posterior fossa. However, in areas with less susceptibility artifacts such as the sciatic nerve (located in the deep surface of the muscle and has a small difference in magnetization rate with the adjacent tissues), the main advantage of RS-EPI is not obvious anymore³. FA and RD value are sensitive biomarkers to detect the minor changes of myelin^{5, 6}. Our study indicated that SS-EPI can not only acquire better image quality, but possess stronger correlation with myelin parameters compared to RS-EPI. However, RS-EPI with various readout segmentations did not investigate in this study. Less readout segmentation with short acquisition time may reduce the motion artifact and shows high performance for the rat sciatic nerve, which need to be investigated in our future study.
In conclusion, for the rat sciatic nerve DW imaging, SS-EPI sequence has significantly higher image quality compared with RS-EPI sequence. The FA and RD derived from the SS-EPI sequence might be more sensitive and quantitative biomarkers to detect the histopathological change of rat sciatic nerve.

References

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