Brain shift modeling

Deep Brain Stimulation

alex_DBS_system_7

3D model representing a unilateral DBS system on a patient, with the three components (neurostimulator, extension and electrode)

DBS is the electrical stimulation of a specific area located deep into the brain tissue, but also refers to the surgery resulting to the stimulation. Two surgeries are necessary to implant the complete DBS system, which consists of three components: the stimulating electrode, a neurostimulator and an extension connecting the electrodes and the neurostimulator.

The electrical stimulation of a specific part of the brain can treat different diseases such as movement (Parkinson’s disease for example) and affective disorders. 94% of the DBS treating PD targeted the subthalamic nucleus (STN), 3% the GPi and 3% the VIM.

Operating protocol

  1. Medical imaging without frame
  2. Stereotactic frame placement
  3. Medical imaging with head frame
  4. Pre-surgical target planning
  5. Pre-surgical trajectory planning
  6. Prepare patient for OR, including draping
  7. Stereotactic arc fixation (to locate burr hole)
  8. Incision & making burr hole
  9. Attachment of lead fixation  base on burr hole
  10. Stereotactic arc fixation (after burr hole)
  11. Physiological confirmation of anatomical target
  12. Intra-operative clinical testing with MER system
  13. DBS lead placement
  14. Intra-operative clinical testing with DBS lead

The success of the operation relies on the electrode placement precision, which the goal is to maximize the therapeutic outcomes, and minimize the adverse effects. To do that, a pre-operative planning step determines the target coordinates to stimulate, as well as the electrode trajectory to reach it, thanks to a combination of medical images of the patient and numerical tools

Brain shift

The term brain shift describes movement and deformation of the brain in terms of its anatomical and physiological position in the skull. Brain shift can be observed after a head injury or during a neurosurgery.

During the surgery, a burr hole is drilled in the patient’s skull in order to access the brain tissue at the entry point the surgeons defined earlier. The electrode is then inserted linearly in the direction of the target with the help of an accurate robot. When the skull and dura mater are open, cerebro-spinal fluid (CSF) can leak through the hole. This fluid surrounds the brain and support its weight. A leak of CSF may cause a change of intracranial pressure, leading to a brain deformation (brain shift). This phenomenon is important as brain deformation lead to a displacement of some brain structures, in particular the structures considered during the planning (target or obstacle structures). It results in a difference between the preoperative configuration, based on which the trajectory is selected, and the intraoperative configuration. Although the target motion can be neglected because it is located in deep tissue where the magnitude of deformation is small, blood vessels can shift up to 10 mm. If a blood vessel shifts across the path of the electrode, it could lead to hemorrhage and death of the patient.

Aims

Provide a safer and a more effective surgery

Summary of past achievements

The following results are part of a PhD thesis carried out in the Shacra team from Inria, funded by the French Research Agency (ANR) through project ACouStiC (ANR 2010 BLAN 020901).

Brain-shift risk during pre-operative planning

rsikmap

The color on the skin surface indicates the distance to the blood vessels. Dark red is for dangerous trajectories, while blue is for safest trajectories.

The planning utilizes pre-operative data of the patient, mostly a MRI. This planning is based on the configuration of the brain and other structures at the moment the MRI was acquired. If the brain deforms between the pre-operative MRI and the moment to implant an electrode, the planning is obsolete and should be based on the deformed configuration. Brain shift could be a problem for the efficiency of the treatment (target displacement) and regarding to the safety of the patient (blood vessels displacement) with risks of hemorrhage. Therefore, it is necessary to take this phenomenon into account in order to ensure a safe surgery to the patient. The aim of our contributions is to provide pre-operative tools based on a biomechanical model of the brain to prevent risks due to brain shift.

Physics-based intra-operative registration

It is currently impossible to identify small brain structures in intra-operative imaging systems due to a poor contrast in the images. In particular, an intra-operative CT scan only shows an homogeneous voxel intensity for the whole brain tissue. Yet, the location ofsome structures on interest would help the surgeon by adapting the procedure in case of displacement compared to the pre-operative planning. For that, one might consider using an indirect method such as an image registration method. Our objective is to propose a method relying on a biomechanical model. In our opinion, this is the best direction to explore as brain shift is a physical phenomenon. To update the pre-operative brain configuration to the intra-operative configuration, we propose to use our physics-based deformation model. The goal is to estimate the parameters of the model leading to the intra-operative configuration via an inverse problem.

Post-operative electrode curvature

A post-operative electrode displacement and deformation may appear as the brain returns to its initial position when the subdural air introduced during surgery has resolved. This hinders the efficiency of the procedure because upward migration of the electrode may fail to correctly stimulate the subthalamic area.

We propose to add the insertion of the electrode in the brain tissue in our framework. It involves the deformation of the electrode(s) as well as its interaction with the brain tissue. The simulation of the complete procedure is then possible. After the simulation of the surgery, CSF is resolved so that brain can recover its original configuration. The interaction with the electrode(s) will lead to a curvature. Due to the fixation of the electrode on the skull, we can also observe a displacement of the tip of the electrode compared to the planned coordinates.

electrodeCurvature