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

The overall goal of this procedure is to map intra cerebral networks in patients with brain tumors. This is accomplished by first acquiring diffusion tensor imaging, or DTI, along with structural imaging for surgical planning. The second step is to use software to perform fiber tracking with the acquired data.

Next, the data is integrated into a format that can be used in the neuro navigation system. The final step is the transfer of the data to the navigation system in the operating room. Ultimately, DTI fiber tracking is used to show white matter tracks intraoperatively in order to help reduce postoperative morbidity.

The main advantage of the presented protocol compared to more sophisticated research technologies is it's durability for everyday routine use. This method can help answer key questions in the field of neurosurgery regarding spatial relationships between cerebral lesions and wide matter tracts. Besides the fact that this method of visualization of the visual pathway helps us in this special kind of anatomy, the method with a very well applicable clinical protocol is of very high value for each and any brain tumor operation, but also for stereotactic functional interventions in neurosurgery.

Generally, for individuals who are new in this field of work, we have to say that they may have problems in the very beginning due to the very complex software, and they have to pass a steep learning curve. Using a three Tesla MRI perform all scans for surgical planning at least one day prior to the patient's scheduled surgery. Acquire the diffusion tensor imaging or DTI scan using strictly axial slices, 32 gradient directions and one B zero image.

Also, acquire a three dimensional T one weighted sequence, as well as a high resolution T two weighted structural image. Also, obtain the three DT one image after surgery, assuring that all image parameters are the same as those used before the operation. Begin by opening the surgical navigation system program such as Medtronic, stealth vis as seen here.

Then import the MRI DICOM images for the patient. Repeat this procedure three times for all the sequences acquired. Click add to view and add every sequence separately.

Do not select proceed with view. Next, click on tools, then on open DTI tensor preparation, and a new window will appear. First, perform gradient assignment and change the B value from 1000 to 800 seconds per millimeter squared.

Adjust the threshold on the top right of the window, either manually by simply entering a number or by moving the cursor. 20 is a suggested value. Next, perform gradient registration.

First click on all auto, which will take up to five minutes to perform. Then click verify all registrations and verify the registrations in order to continue. Now perform co-registration of all the acquired images coregister, the structural MR scans and the DTIB zero images manually as automatic co-registration may not be satisfactory.

Then select, verify all registrations. Finally, perform the tensor computation. Make sure that the fa, DEC, and a DC selections are all turned on in the software.

Then click on compute. Once the computation is complete, be sure to save all the data before continuing with fiber tracking. Note that anatomic knowledge of the visual pathway is very important for optimal fiber tracking results, and hence, a physician should be consulted when selecting landmarks.

Begin with the healthy hemisphere. By finding the three anatomical landmarks the fibers should pass through. First, locate the optic chiasm location.

A region of interest or ROI can be used here as a starting point. Or alternatively, the region can be segmented manually set the maximum angle to 50 degrees since the risk of false tracks will rise. If the angle is too high, then track the fibers either from the region of interest or from the segmented area, or both.

The tracked fibers should reach the left gen nucleus or LGN, which is the second anatomical landmark of the visual pathway. Next, the LGN can be segmented with the same method used for the optic chiasm, and then the fibers can alternatively be tracked back to the optic chiasm from this location. Next segment, the visual cortex.

This segmentation may take some time as the anatomical image typically contains many slices. Then track the fibers from the visual cortex to the LGN, or it is possible to track them from the LGN to the visual cortex as well. Repeat these steps for the other hemisphere containing the lesion.Note.

If the visual cortex is invaded by a tumor or edema, then use a region of interest in place of a segmented area and then let the fibers run in the direction of the LGN. Also, segment the cerebral lesion and the surrounding edema. Assign a color for every segmented area or lesion in order to easily distinguish them from one another.

Be sure to save the procedure after each segmentation step. Once all segmentation and fiber tracking is completed, export the data locally. It is possible to export it to the operating room directly, but this is not recommended.

Export as 3D objects, making sure to export only the navigation exam, not the hybrid exam for the best quality images. Next, enter the software's cranial menu. Choose the patient name and select the stealth merge option.

Be sure to choose the three DT one weighted images as the reference exam. Then create a 3D model and include all segmentations and white matter tracks that were previously exported. Finally, import the data into the operation room neuro navigation system for use during the resection.

Here we can see fiber tracking results in yellow for the visual pathway. In a patient with glioblastoma recurrence, the tumor is segmented in red while the surrounding edema is segmented in purple. In this pre-surgical glioblastoma patient, the visual pathway fiber tracks in yellow appear to be displaced by the adjacent tumor.

This image shows visual pathway fiber tracks after glioblastoma resection. The dark pink represents the cavity of the tumor and the edema in purple is adjacent to the visual pathway. The additional MRI scanning time is less than nine minutes, and that makes this protocol available for everyday use.

While attempting this procedure, it is important to keep in mind that fiber tracking can provide useful additional information to the surgeon, but should never be relied on as the only source of information for surgical decision making. This technique of DTI fiber tracking and their integration into the navigational system together with electrophysiological measuring methods intraoperatively, such as visual evoke potential measuring or other recording techniques will be of very high value for brain surgery in many fields in the future. After watching this video, you should have a good understanding of how to perform DTI fiber tracking in healthy subjects and in patients who require surgery adjacent sent to this white metre.