Magnetic resonance tractography of the brachial plexus: step-by-step

Abstract

Background: Magnetic resonance (MR) tractography of the brachial plexus (BP) is challenging due to different factors such as motion artifacts, pulsation artifacts, signal-to-noise ratio, spatial resolution; eddy currents induced geometric distortions, sequence parameters and choice of used coils. Notably challenging is the separation of the peripheral nerve bundles and skeletal muscles as both structures have similar fractional anisotropy values. We proposed an algorithm for robust visualization and assessment of BP root bundles using the segmentation of the spinal cord (SSC, C4-T1) as seed points for the initial starting area for the fibre tracking algorithm.

Methods: Twenty-seven healthy volunteers and four patients with root avulsions underwent magnetic resonance imaging (MRI) examinations on a 3T MR scanner with optimized measurement protocols for diffusion-weighted images and coronal T2 weighted 3D short-term inversion recovery sampling perfection with application optimized contrast using varying flip angle evaluation sequences used for BP fibre reconstruction and MR neurography (MRN). The fibre bundles reconstruction was optimized in terms of eliminating the skeletal muscle fibres contamination using the SSC and the tracking threshold of the normalized quantitative anisotropy (NQA) on reconstruction of the BP. In our study, the NQA parameter has been used for fiber tracking instead of fractional anisotropy (FA). The diffusion data were processed in individual C4-T1 root bundles using the generalized q-sampling imaging (GQI) algorithm. Calculated diffusion parameters were statistically analysed using the two-sample t-test. The MRN was performed in MedINRIA and post-processed using the maximum intensity projection (MIP) method to demonstrate BP root bundles in multiple planes.

Results: In control subjects, no significant effect of laterality in diffusion parameters was found (P>0.05) in the BP. In the central part of the BP, a significant difference between control subjects and patients at P=0.02 was found in the NQA values. Other diffusion parameters were not significantly different.

Conclusions: Using NQA instead of FA in the proposed algorithm allowed for a better separation of muscle and root nerve bundles. The presented algorithm yields a high quality reconstruction of the BP bundles that may be helpful both in research and clinical practice.

Keywords: Diffusion tensor imaging (DTI); brachial plexus; generalized q-sampling imaging algorithm (GQI algorithm); magnetic resonance neurography (MRN); magnetic resonance tractography (MRT).

Figure 1

The diffusion ODF obtained from q-sampling imaging. (A) The ODF map; (B) details of zoomed voxels with ODF shape and orientations of multiple fibre population. The directions of the ODF are pseudo-colored: red in the left-right direction (L-R), blue in the superior-inferior direction (S-I), and green in the anterior-posterior (A-P) direction. ODF, orientation distribution function.

Figure 2

Effects of the NQA and angular threshold on the reconstruction of the fibre bundles. (A) Sagittal view; (B) SSC used as a ROI for the initial starting area for the fiber tracking algorithm; (C) the dependency of fiber tracking results on the NQA (e.g., 0.1) and angular thresholds; multiple fiber bundles (C4-T1) are visible with a decreasing NQA threshold. The best reconstruction (green circled image) of the brachial plexus requires an NQA threshold 0.03 (or 0.02) and angular threshold 30° to 50° (two last columns). Directional colors show the local orientation of the ODF and the fiber tracts: red in the left-right direction (L-R), blue in the superior-inferior direction (S-I), and green in the anterior-posterior (A-P) direction. NQA, normalized quantitative anisotropy; ODF, orientation distribution function; SSC, segmented spinal cord; ROI, region of interest.

Figure 3

A block scheme of the fiber tracking workflow describing the proposed algorithm for magnetic resonance tractography of the BP. BP, brachial plexus; ODF, orientation distribution function; FA, fractional anisotropy; MD, mean diffusivity; AD, axial diffusivity; RD, radial diffusivity; NQA, normalized quantitative anisotropy; DSI Studio, freeware tool for magnetic resonance tractography; SSC, segmented spinal cord; SC, spinal cord.

Figure 4

Reconstruction of the root bundles C4-T1 in detail in a 24-year-old healthy man. Figure parts (A) to (J) show individual reconstruction steps: (A) SSC was used as seed points for the initial starting area for the fibre tracking algorithm within these seeding regions at subvoxel resolution; (B) reconstruction of the major pathway roots of the right BP (C4-T1) using the SSC with NQA of 0.03 and angular thresholding of 40°; (C,D) selection of the right BP using shortcut Ctrl + S; (E,F) selection of the left BP; (G-J) central part BP selection using shortcut Ctrl + D. Directional colors show the local orientation of the orientation distribution function and the fiber tracts: red in the left-right direction (L-R), blue in the superior-inferior direction (S-I), and green in the anterior-posterior (A-P) direction. SSC, segmented spinal cord; BP, brachial plexus; ODF, orientation distribution function map; ROI, regions of interest; L, left; R, right; SC, spinal cord; NQA, normalized quantitative anisotropy.

Figure 5

Representative MRT images and corresponding results of MRN reconstruction from a coronal 3D STIR SPACE images. (A) MRT acquired in a 25-year-old healthy man shows the architectural configuration of the reconstructed fibre bundles (C4-T1). (B) Corresponding MRN reconstruction. (C) The central part of the brachial plexus (C4-T1) of the healthy man. (D) MRT acquired in a 30-year-old patient with nerve root avulsions. (E) Corresponding MRN reconstruction. Three asterisks show nerve root avulsions (C7-T1). (F) The central part of the brachial plexus (C4-T1) of the patient. Directional colors (A, D) show the local orientation of fiber tracts: red in the left-right direction (L-R), blue in the superior-inferior direction (S-I), and green in the anterior-posterior (A-P) direction. The color coding of the magnitude of NQA in the central part of the brachial plexus shows color differences of fiber tracking results between a healthy subject (C) and a patient (F). MRT, magnetic resonance tractography; MRN, magnetic resonance neurography; NQA, normalized quantitative anisotropy; STIR, short-tau inversion recovery; SPACE, sampling perfection with application optimized contrast using varying flip angle evaluation.

Figure 6

The results of the two-sample t-tests are depicted in graphical form (box plot) that shows a significant difference between control subjects and patients at P=0.02 in NQA values in the central part of the BP without root bundles (C4-T1). DTI-based metrics (FA, MD, AD, and RD) were not significantly different. NQA, normalized quantitative anisotropy; BP, brachial plexus; FA, fractional anisotropy; MD, mean diffusivity; AD, axial diffusivity; RD, radial diffusivity.

Figure 7

A representative contrast-enhanced MRN with MRA in a 37-year-old patient with left infraclavicular injuries of the brachial plexus. (A) The result of 3D CT of humeral fracture. (B-D) Neurosurgical reconstructions of RN and MCN. (E) The result of non-enhanced MRN. (F) Contrast-enhanced MRN. (G) Merged contrast-enhanced MRN and MRA images after neurosurgical reconstruction; it shows the benefit of contrast-enhanced MRN. MRN, magnetic resonance neurography; MRA, magnetic resonance angiography; CT, computed tomography; RN, radial nerves; MCN, musculocutaneus nerves.