InitRech 2015/2016, sujet 9

Synthesis of the article :

Brain stimulation in neurosurgery procedure involve the use of deeply implanted electrodes into the brain. The target depends of the procedure but many rules exists to ensure that the trajectory of the electrodes will ensure the result as well as the patient's safety.

But a problem remain unsolved in those procedures: the shift of brain tissues after the skull drilling.

Currently, there are many models available from the most simple: mass-spring-damper model, not very accurate but low computational time. To the most used FEM, which can be set with a lot of parameters. So based on those models, monitoring of the patient ans set of rules, surgeons can plan the best way to implant the electrodes.

Therefore this process remains quite long and difficult that's why they developed a an automatic planning system including simulations.

There solution remains in the use of models and simulations of the brain which mean a study of the brain deformations, the cerebrospinal fluid model and limit conditions coupled with a trajectory planning in two phases : static environment and dynamic environment. Once in dynamic environment they compute the feasible insertion zone, and then the optimized trajectory for a given point.

The experimental result remain from a test on a patient-specific 3D model and shown many things: -the computational time is compatible with a clinical use. -even if the trajectory found in dynamical model are more “dangerous” it's still acceptable -in case of brain shift, the safest zones for insertion don't overlap those in case of no shift.

In conclusion the brain shift prediction are pretty accurate by removing many point who becomes dangerous, and even if the system can be improved, the result are already here.

Core contribution:

As said before, this contribution remain in two mains parts: model and simulation of the brain and trajectory planning for the needle. So first, to define the best model to brain itself many hypothesis are set in order to work with the appropriate tools and stick with the reality. The main points are: the brain is subject of the law of continuum mechanics, so numerical method describe it as finite set of element to solve motion equation. And the other one is the fact that the process take place at a very low velocity so this is a quasi-static problem with a multiple state at equilibrium. To solve this the first-order linearization is use at each step. And parameters remain from biological behavior with the appropriate law.

The trajectory planning itself is solve as an optimization problem with hard and soft constraint. The difference between static model and dynamic one is represented by the intensity of shift, so the best trajectory must pass the constraint test whatever the intensity of the shift between a min and a max.

Possible incoming applications:

The main application of this proceed is obviously to increase the security when performing deep brain stimulation for cure many diseases (Parkinson, dystonia) or tremors from car crash accident for instance as it has a very low computational time. Therefore clinical validation are still required. But we can pursue the research to increase to increase the quality of the result and even maybe develop a robot who may be able make the surgery with real-time processing between his sensors and the algorithm.

Furthermore, the difference between the algorithm information and those bring by the sensor may allow to improve it and get a better understand of the brain behavior. Maybe some other surgery procedure may face to the same type of behavior like those of the vertebral spine which need a lot of accuracy.

On the other way, it should be possible to apply those simulation in other case. Indeed, this algorithm have been developed with law of biological model but other parameters may allow to simulate the same type of pressure losses in other case such as sounding balloon composed of two balloon and simulate the effect of hole in the first one and consequences on the second one. Or in an inflatable space habits such as the recent one test on ISS: BEAM.

In conclusion, once this system will be approved by clinical validation, it may quickly improve decease the rate of failure and by the way, side effect of those type of procedure. And it should be even more true for urgent procedures where doctors cannot spend time to analyze the patient's brain.