Role of Diffusion MRI Tractography in Endoscopic Endonasal Skull Base Surgery

We present a protocol to integrate diffusion MRI tractography in patient work-up to endoscopic endonasal surgery for a skull base tumor. The methods for adopting these neuroimaging studies in the pre- and intra-operative phases are described.

Transcript

Using an integrated neuroimaging and neurosurgery protocol, it is possible to merge different expertise in a synergic framework to tailor a patient-specific tumor resection surgery. Using MRI tractography, it is possible to visualize white matter tract dislocation and tumor distance. Its versatility in glioma surgery has been established and can also be applied in drug-resistant focal epilepsy.

The integration of advanced neuroimaging techniques in endoscopic endonasal surgery for pituitary, diencephalic, and skull-based tumors is effective at increasing surgical safety, reducing the complications, and improving patient outcomes and quality of life. MRI tractography combined with task fMRI allows the monitoring of brain structural and functional reorganization after surgery. In addition, correlation with clinical outcomes are useful for clinical and research proposals.

Both endoscopic endonasal surgery and advanced neuroimaging require a long training period. We suggest an observership or a fellowship in academic tertiary referral centers at which these techniques are being implemented. Through visible demonstration, it's possible to make the steps of this method that have not been standardized yet and to clarify how to integrate different expertise.

Using a standardized multimodal MRI protocol high-field scanner, acquire high resolution and volumetric anatomical sequences using T1-weighted pre and post-gadolinium contrast agent administration and FLAIR T2-weighted imaging. Acquire continuous sagittal slices providing an isotropic resolution of a one by one by one cubic millimeter scanning time of about five minutes per sequence. Acquire a high-resolution T2-weighted sequence to localize the tumor area for cranial nerve visualization with the volumetric constructive interference and steady-state voxel dimension of 0.5 by 0.5 by 0.5 cubic millimeters and a scanning time of about nine minutes.

Acquire diffusion-weighted sequences using single shot echo planar images, a two by two by two cubic millimeter voxel dimension, 64 magnetic gradient directions with a 2, 000 second per square millimeter B value, 98 millisecond echo time, and 4, 300 millisecond relaxation time. Acquire five volumes with a null B value at the beginning of the diffusion-weighted acquisition with the phase encoding direction set to anterior-posterior and a scanning time of five minutes. Then acquire three volumes with a null B value, but reversed posterior-anterior phase encoding direction to correct any imaging distortions due to the echo planar image acquisition and a scanning time of 42 seconds.

Continuous near axial slices will be acquired. For segmentation of the tumor, load the images into the ITK-SNAP software and inspect the tumor in the t1. nii, flair.

nii, and t1_contrast. nii images. Then select the anatomical plane to follow when drawing the lesion.

For tractographic analysis of the segmented tumor, run the fsl-dtifit function to model diffusivity in the different spatial directions and obtain the fa. nii, md. ii, and v1.

nii diffusion tensor maps. Evaluate the diffusion tensor imaging maps to assess any abnormal diffusivity values that may occur in the presence of tumor edema or infiltration and select the seed_image and include options based on a priori anatomical knowledge to adopt a seed target approach. Then manually draw regions of interest to set the seed or target for tractography.

For an accurate description of the diffusion tensor imaging parameters, use long tract algorithm such as the MATLAB-based algorithm that model the surface tract geometry with the Laplacian operator properties. To visualize the 3D volume rendering, in the Surf Ice software, click file and open in the command panel and select the obj file. Before scheduling the procedure, perform a neurological physical examination with a collection of anamnestic information about weight gain, the sensation of hunger, continuous monitoring of the rectal temperature every two minutes for 24 hours, and a 24-hour sleep/wake cycle recording.

Based on the results of tumor segmentation and the relationship with the functional eloquent neural structures, discuss the patient candidacy for surgery in a collegial team meeting to determine the most appropriate surgical approach. After selecting the surgical corridor with the most minimal risk of injury to the neural structures, define the safe resection area for each case, localizing the critical neural structure under which the proximity the resection must be arrested to avoid permanent damage. Then merge the most relevant MRI sequences and import the sequences, including the tractography reconstructions, into the operative phase neuro navigation system.

Before beginning the procedure, select the brain surgery electromagnetic registration modality. Register the neuro navigation system on the patient, adopting a free tracking technique or external markers and control the accuracy of the achieved registration, checking the position of the external markers on the imported MRI. When the patient is ready, use a zero degree endoscope to harvest the nasoseptal flap.

Next, perform an anterior sphenoidectomy and a posterior septostomy and ethmoidectomy, preserving the middle turbinate as possible. Open the cellar and tuberculum bones. After coagulation of the superior intracavernous sinus, make an H-shaped incision in the dura layer.

Leave the tumor by the arachnoidal plane and centrally de-bulk the tumor. Remove the tumor capsule from the surrounding diencephalic neural structures and use angled optics to explore the surgical cavity for any remaining pieces of tumor. When all of the tumor has been removed, use an intradural intracranial layer of dural substitute to close the osteo-meningeal opening.

Then place an extradural intracranial layer of dural substitute scaffolded with abdominal fat and eventually bone and cover the closure with the nasoseptal flap. In this representative patient, brain MRI revealed a suprasellar tumor occupying the optochiasmatic cistern and invading the third ventricle with an irregular polycystic morphology. The optic pathway tractography and bilateral optic cranial nerves were reconstructed, but susceptibility artifacts within the interface between the brain bones and blood vessels did not allow a complete reconstruction of the fibers connecting the optic chiasm to the optic nerves.

Investigation of the pyramidal tract diffusivity profile and a long tract diffusion tensor imaging map statistics showed the presence of a focal FLAIR T2-weighted hyperintensity at the level of the right posterior limb of the internal capsule, corresponding to a 5%increase of the right mean diffusivity measure compared to the left side. Using an endoscopic endonasal extended transplant transtuberculum approach, the tumor was centrally de-bulked in conjunction with the draining of its cystic component. The craniopharyngioma was then able to be progressively detached from the neural structures to adopt the arachnoid as a cleavage plane.

At the end of the surgery, complete tumor removal with preservation of the hypothalamic anatomy was achieved. The repair of the osteo-dural defect was then performed using abdominal fat and the nasoseptal flap. Three months after the surgery, a complete tumor removal with no remnant or recurrence was observed.

In the preoperative workup, the most relevant steps are:an accurate diffusion-weighted sequences acquisition and tumor segmentation. During surgery, the key point is an accurate identification of the neural structures. The visualization of the neural structures provided by this method can be adopted for all skull-based regions, reducing the risk of permanent disabilities for many other tumors.

The tractographic reconstruction of cranial nerves and neuro pathways can facilitate our understanding of the relationship between tumors and the structures, potentially providing an innovative outcome predictor for patient symptoms.

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Journal (Neuroscience)

Journal (Neuroscience)

Role of Diffusion MRI Tractography in Endoscopic Endonasal Skull Base Surgery

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