DTI of the Visual Pathway - White Matter Tracts and Cerebral Lesions

Summary

Diffusion tensor imaging (DTI) was performed to try to depict the major parts of the visual pathway. The goal was to use an FDA approved standard commercial workstation which could be used for everyday routine in order to try to reduce postoperative damage of the visual pathway in the patients.

Abstract

DTI is a technique that identifies white matter tracts (WMT) non-invasively in healthy and non-healthy patients using diffusion measurements. Similar to visual pathways (VP), WMT are not visible with classical MRI or intra-operatively with microscope. DTI will help neurosurgeons to prevent destruction of the VP while removing lesions adjacent to this WMT. We have performed DTI on fifty patients before and after surgery between March 2012 to January 2014. To navigate we used a 3DT1-weighted sequence. Additionally, we performed a T2-weighted and DTI-sequences. The parameters used were, FOV: 200 x 200 mm, slice thickness: 2 mm, and acquisition matrix: 96 x 96 yielding nearly isotropic voxels of 2 x 2 x 2 mm. Axial MRI was carried out using a 32 gradient direction and one b0-image. We used Echo-Planar-Imaging (EPI) and ASSET parallel imaging with an acceleration factor of 2 and b-value of 800 s/mm². The scanning time was less than 9 min.

The DTI-data obtained were processed using a FDA approved surgical navigation system program which uses a straightforward fiber-tracking approach known as fiber assignment by continuous tracking (FACT). This is based on the propagation of lines between regions of interest (ROI) which is defined by a physician. A maximum angle of 50, FA start value of 0.10 and ADC stop value of 0.20 mm²/s were the parameters used for tractography.

There are some limitations to this technique. The limited acquisition time frame enforces trade-offs in the image quality. Another important point not to be neglected is the brain shift during surgery. As for the latter intra-operative MRI might be helpful. Furthermore the risk of false positive or false negative tracts needs to be taken into account which might compromise the final results.

Introduction

Diffusion tensor imaging (DTI) is used to portray WMT non-invasively in the human brain¹. It has been used in the past decade to reduce the risk of harming eloquent areas of the brain during surgery¹.

DTI was performed in fifty patients between March 2012 and January 2014 to portray the visual pathway. DTI might improve preservation of eloquent areas of the brain during surgery by providing important information about the anatomical location of white matter tracts. It has been incorporated into strategic planning for resection of complex brain lesions¹. However, the portrayal of the visual pathway remains a ....

Protocol

NOTE: This protocol follows the guidelines of the Centre Hospitalier de Luxembourg in Luxembourg.

1. Preparation of Diffusion Tensor Imaging for the Visual Pathway for Neurosurgery and Follow Up

1. Perform an MRI-scan at least one day before surgery strictly axial using 32 gradient directions and one b0-image. Keep in close touch with the neuroradiology unit at any moment.
   NOTE: Make clear to the neuroradiologist that the images after surgery are the same as those before the operation.
2. Using a 3-Tesla MRI, perform a 3DT1-weighted and DTI-sequence scans. Perform a 3DT1-weighted sequence af....

Results

This protocol enables the physician to portray adequately the major parts of the VP. It can be used with a little amount of time in order to prevent damages in patients with cerebral lesions next to eloquent areas. Postoperative controls show also good results. VP is portrayed in Figure 7 after the patient was operated from a glioblastoma. Figure 2 shows the VP after recurrence of a glioblastoma. The authors recognize the fact of the difficulties presented by this protocol to depict the .......

Discussion

DTI is a technique enabling the neurosurgeon to visualize white matter tracts in vivo⁸. The visual pathway is one of these tracts. Although this method provides physicians with new possibilities regarding the treatment of patients with lesions concerning eloquent regions of the brain we have to say that some limitations of this technique do still exist. The first and most obvious challenge is brain shift, which remains an issue under investigation⁴. After opening the dura mater and after ma.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
3-Tesla-MRI	General Electric	Signa LX version 9.1
Surgical Navigation System Program	Medtronic	9734478
Surgical Navigation System Program	Medtronic	4500810331  20016318