# Diffusion Tensor Imaging of a Median Nerve by Magnetic Resonance: A Pilot Study

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{−9}m

^{2}/s in the intrafascicular region, somewhat lower values of 0.27 and 0.95 × 10

^{−9}m

^{2}/s in the perineurium region and close to isotropic with very slow diffusion (0.15 and 0.05 × 10

^{−9}m

^{2}/s) in the epineurium region.

## 1. Introduction

## 2. Theory

## 3. Materials and Methods

#### 3.1. Nerve Samples

#### 3.2. Diffusion Tensor Imaging

_{0}= 0.26 T/m, b = 1150 s/mm

^{2}in 19 different gradient directions and one image with b = 0, field of view (FOV): 9 × 4.5 × 10 mm

^{3}, imaging matrix 256 × 128 × 16, signal averages: 4, scan time: 1 day and 16 h. Image resolution was equal to 35 μm along the in-plane directions and the slice thickness was equal to 625 μm. Directions of 19 different diffusion gradients are shown in Table 1; all diffusion gradients had identical amplitudes of G

_{0}and therefore the same b-value.

#### 3.3. DTI Calculation and Image Processing

_{1}, D

_{2}, D

_{3}, their corresponding eigen vectors, mean diffusivity MD and the fractional anisotropy FA were calculated by C-code software written specifically for this study by the authors. In this software, the solution of Equations (14)–(18) was implemented using the numerical methods [19]. The calculated maps were analyzed for mean values and their errors by ImageJ digital image processing program (NIH, Bethesda, MD, USA). The illustration of DTI data by diffusion ellipsoids was carried out by utilizing the ray-tracing POV-Ray software (open source) for creating three-dimensional (3D) graphics. More details about this software along with its code are included in the Supplementary Materials.

## 4. Results

^{2}, however, with different diffusion gradient directions that are given in Table 1. The first image has no diffusion weight (b = 0) and has on average more than double the signal compared to the images with diffusion weight. The signal reduction is especially apparent in the intrafascicular region, while it is practically negligible in the epineurium region. Higher signal reduction coincides with the regions of faster diffusion (Table 2).

_{1}, D

_{2}, D

_{3}, mean diffusivity MD and fractional anisotropy FA. From these images, it can be inferred that the first eigenvalue D

_{1}is on average considerably larger than the second and the third eigenvalue, D

_{2}and D

_{3}. These two eigenvalues are also closer in values to each other. Eigenvalue D

_{1}is the largest in the intrafascicular region, which has also the highest FA. Lower values of D

_{1}and also of FA were found in the perineurium region. The values of eigenvalues D

_{2}and D

_{3}were found to be higher in the perineurium than in the intrafascicular region. Diffusion values (of all eigenvalues) were found to be the lowest and practically below the detection threshold in the epineurium region.

_{1}, D

_{2}, D

_{3}, respectively. The first eigenvector ${\stackrel{\rightharpoonup}{e}}_{1}$ has direction of the fastest diffusion, i.e., of eigenvalue D

_{1}. In Figure 4, the top row scale defines the x, y and z components of the first eigenvector ${\stackrel{\rightharpoonup}{e}}_{1}$ by red-, green- and blue-scaled images, while in the bottom segment illustrates the composite RBG image of all three previous images. In this composite image, the blue color dominates, especially in the intrafascicular region. This indicates that the diffusion in these regions is the fastest along the z-direction, i.e., along nerve fibers, which is an expected result.

_{1}, D

_{2}, D

_{3}, mean diffusivity MD and fractional anisotropy FA from the corresponding DWI signals shown in Figure 2. The obtained results are presented in terms of means and standard deviations in Table 2. From these results, it can be seen that in the intrafascicular region, D

_{1}value of 1.00 × 10

^{−9}m

^{2}/s was 67% higher than D

_{2}and 85% higher than D

_{3}. The intrafascicular region has also the highest FA. An almost identical eigenvalue D

_{1}, however, only 22% higher than D

_{2}and 85% higher than D

_{3}, was obtained for the perineurium region. The perineurium region has therefore slightly lower FA than the intrafascicular region, while the MDs of both regions are quite similar 0.71 × 10

^{−9}m

^{2}/s vs. 0.77 × 10

^{−9}m

^{2}/s (intrafascicular region vs. perineurium). The ROI analysis verified that the epineurium region has the slowest diffusion of all three analyzed regions. Its MD of 0.05 × 10

^{−9}m

^{2}/s is approximately 15-times lower than the MDs of the intrafascicular and epineurium regions. FA of the epineurium region of 0.15 is also the lowest of all three regions. This value is close to the isotropic diffusion considering the influence of noise in the measured FA.

## 5. Discussion

_{1}, D

_{2}, D

_{3}, mean diffusivity (MD) and the fractional anisotropy (FA), for the three different anatomical regions of the nerve, i.e., intrafascicular, perineurium and epineurium regions, were extracted. Since this was a pilot study, all the research was concluded on an individual median nerve sample from a single donor. Thus, the results obtained are mainly orientational and therefore do not include the possible intra- and inter-subject differences.

^{2}, respectively. However, DW signals of the regions in Figure 6 had, for the square root of the number of pixels in the region, higher SNRs, e.g., on average 16-times higher for the intrafascicular region. The DTI parameters obtained by calculating the DW signals from the regions (Table 2) are therefore different and more accurate than the corresponding values obtained as region averages of DTI maps (Figure 3). The median nerve is the biggest nerve of the upper extremity, while radial and ulnar nerves are smaller in comparison to it in the upper arm, both nerves could be depicted with similar parameters as described above [23]. However, branches of these nerves are considerably smaller and would therefore require imaging with even higher spatial resolution, which would inevitably lead to the reduction in SNR for DW images and thus render DTI less accurate, or on the contrary, longer scan times due to more signal averaging to maintain the accuracy of DTI.

## 6. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Imaging pulse sequence that was used for DTI. The sequence is based on the standard 3D spin–echo sequence with the addition of diffusion gradients.

**Figure 2.**Diffusion-weighted images of the median nerve that correspond to the diffusion gradients in Table 1. Images were acquired with in-plane resolution of 35 μm in central transversal 0.6 mm thick slice across the nerve. The diffusion-weighting parameter b was equal to 1150 s/mm

^{2}.

**Figure 3.**Eigen values of diffusion tensor D

_{1}, D

_{2}and D

_{3}(first row) and mean diffusivity MD and fractional anisotropy FA (second row) of the median nerve. The maps were calculated from the DWI data in Figure 2.

**Figure 4.**Components of the first eigenvector shown by absolute values for e

_{1x}(red), e

_{1y}(green) and e

_{1z}(blue). The maps of components were used to calculate a composite (RGB) image where pixel color indicates the eigenvector orientation.

**Figure 5.**Diffusion tensor in pixels of selected slice represented by ellipsoids. Axes of ellipsoids are proportional to diffusion eigenvalues, while direction of the axes (orientations of the ellipsoids) corresponds to the directions of eigenvectors. Color of the ellipsoids is determined by the composite RGB color as described in Figure 4.

**Figure 6.**Selected regions of interest (ROIs) in intrafascicular (

**a**), perineurium (

**b**) and epineurium (

**c**) anatomical regions of the median nerve in which the DTI parameters shown in Table 2 were calculated. Note that ROIs for the perineurium regions are only curved lines, one pixel (35 μm) thick.

**Table 1.**List of 19 different diffusion magnetic field gradients that were used for DTI. All the gradients have identical amplitudes of G

_{0}but different directions.

Number | G_{x}/G_{0} | G_{y}/G_{0} | G_{z}/G_{0} |
---|---|---|---|

1 | −0.0620 | 0.2380 | 0.9693 |

2 | 0.6092 | 0.7125 | 0.3483 |

3 | −0.7878 | 0.3181 | −0.5274 |

4 | 0.0636 | 0.1273 | 0.9898 |

5 | −0.8853 | −0.0582 | 0.4613 |

6 | −0.1428 | −0.8146 | 0.5622 |

7 | 0.4337 | −0.8094 | −0.3959 |

8 | 0.2289 | −0.4735 | 0.8505 |

9 | −0.6445 | −0.2860 | −0.7091 |

10 | −0.4461 | 0.3051 | 0.8414 |

11 | 0.0199 | −0.9966 | −0.0799 |

12 | −0.7535 | −0.5805 | 0.3086 |

13 | −0.8716 | 0.4899 | −0.0185 |

14 | −0.4636 | 0.7949 | −0.3914 |

15 | −0.7682 | 0.4252 | 0.4787 |

16 | −0.1220 | −0.6652 | −0.7367 |

17 | −0.9632 | −0.1645 | −0.2125 |

18 | 0.3718 | 0.3251 | −0.8696 |

19 | −0.4298 | −0.8959 | 0.1125 |

**Table 2.**DTI parameters and their average for different ROIs in intrafascicular, perineurium and epineurium anatomical regions of the median nerve.

Intrafascicular region | |||||||

Number | Area [mm ^{2}] | Signal [A.U.] | D_{1} | D_{2} | D_{3} | MD | FA [0–1] |

[10^{−9} m^{2}/s] | |||||||

1 | 0.22 | 420 | 1.07 | 0.60 | 0.51 | 0.73 | 0.39 |

2 | 0.53 | 477 | 1.00 | 0.63 | 0.59 | 0.74 | 0.29 |

3 | 0.60 | 511 | 0.92 | 0.64 | 0.56 | 0.71 | 0.27 |

4 | 0.31 | 433 | 1.05 | 0.57 | 0.50 | 0.71 | 0.40 |

5 | 0.11 | 472 | 1.05 | 0.59 | 0.55 | 0.73 | 0.36 |

6 | 0.23 | 473 | 0.92 | 0.62 | 0.56 | 0.70 | 0.27 |

7 | 0.19 | 494 | 0.96 | 0.58 | 0.53 | 0.69 | 0.32 |

469 ± 32 | 1.00 ± 0.06 | 0.60 ± 0.02 | 0.54 ± 0.03 | 0.71 ± 0.02 | 0.33 ± 0.06 | ||

Perineurium region | |||||||

Number | Area [mm ^{2}] | Signal [A.U.] | D_{1} | D_{2} | D_{3} | MD | FA [0–1] |

[10^{−9} m^{2}/s] | |||||||

1 | 0.06 | 641 | 0.92 | 0.67 | 0.43 | 0.68 | 0.35 |

2 | 0.08 | 672 | 1.09 | 0.76 | 0.70 | 0.85 | 0.25 |

3 | 0.04 | 708 | 1.04 | 0.96 | 0.56 | 0.85 | 0.29 |

4 | 0.03 | 795 | 0.94 | 0.87 | 0.53 | 0.78 | 0.27 |

5 | 0.04 | 625 | 0.84 | 0.67 | 0.46 | 0.66 | 0.29 |

6 | 0.02 | 676 | 0.95 | 0.76 | 0.60 | 0.77 | 0.23 |

7 | 0.02 | 692 | 1.05 | 0.93 | 0.46 | 0.81 | 0.36 |

687 ± 56 | 0.98 ± 0.09 | 0.80 ± 0.11 | 0.53 ± 0.09 | 0.77 ± 0.08 | 0.29 ± 0.05 | ||

Epineurium region | |||||||

Number | Area [mm ^{2}] | Signal [A.U.] | D_{1} | D_{2} | D_{3} | MD | FA [0–1] |

[10^{−9} m^{2}/s] | |||||||

1 | 0.32 | 1564 | 0.09 | 0.08 | 0.06 | 0.08 | 0.19 |

2 | 0.07 | 1442 | 0.04 | 0.03 | 0.03 | 0.03 | 0.20 |

3 | 0.06 | 1308 | 0.07 | 0.06 | 0.05 | 0.06 | 0.15 |

4 | 0.23 | 1551 | 0.04 | 0.03 | 0.03 | 0.03 | 0.12 |

5 | 0.07 | 1495 | 0.05 | 0.05 | 0.04 | 0.05 | 0.09 |

6 | 0.04 | 1383 | 0.04 | 0.03 | 0.03 | 0.03 | 0.20 |

7 | 0.05 | 1338 | 0.05 | 0.04 | 0.04 | 0.04 | 0.13 |

1440 ± 101 | 0.05 ± 0.02 | 0.05 ± 0.02 | 0.04 ± 0.01 | 0.05 ± 0.02 | 0.15 ± 0.04 |

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**MDPI and ACS Style**

Awais, K.; Snoj, Ž.; Cvetko, E.; Serša, I.
Diffusion Tensor Imaging of a Median Nerve by Magnetic Resonance: A Pilot Study. *Life* **2022**, *12*, 748.
https://doi.org/10.3390/life12050748

**AMA Style**

Awais K, Snoj Ž, Cvetko E, Serša I.
Diffusion Tensor Imaging of a Median Nerve by Magnetic Resonance: A Pilot Study. *Life*. 2022; 12(5):748.
https://doi.org/10.3390/life12050748

**Chicago/Turabian Style**

Awais, Kanza, Žiga Snoj, Erika Cvetko, and Igor Serša.
2022. "Diffusion Tensor Imaging of a Median Nerve by Magnetic Resonance: A Pilot Study" *Life* 12, no. 5: 748.
https://doi.org/10.3390/life12050748