Multi-sphere Head Model Ghost Sphere Correction
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Solution Overview
Problem
Conventional multi-sphere head models in magnetoencephalography (MEG) often result in 'ghost spheres' that have a significant volume outside the brain, leading to inaccuracies in dipole localization, as they do not accurately represent the physical brain volume.
Innovation Solution
A computer-implemented method to correct ghost spheres by replacing them with replacement spheres that do not exceed the brain volume, using approaches such as selecting from families of candidate spheres constrained by the brain surface and diameter, or generating a hybrid model combining local and global spheres, to ensure accurate dipole localization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional multi-sphere modeling approaches are used to match local curvature of brain surface, then the model can be generated efficiently, but ghost spheres are created with significant volume outside the brain
Solution Approach 1:
The patent applies preliminary action by performing a global sphere fitting step before generating local spheres. The global sphere is fitted to the entire brain surface first, establishing a reference framework. Then local spheres are generated with constraints based on this global fit, preventing ghost spheres from forming in the first place. This preliminary global modeling action guides the subsequent local sphere generation to maintain accuracy.
Solution Approach 2:
The patent changes parameters by introducing constraints on sphere radius and center position based on the global sphere model. Instead of freely fitting local spheres to match curvature, the radius is limited to be less than or equal to the global sphere radius, and the center is constrained to lie within the global sphere. These parameter changes eliminate ghost spheres while preserving local curvature matching where valid.
2Ease of operation
If local spheres are fitted to match brain surface curvature at each sensor location, then dipole localization can be performed, but dipoles are incorrectly placed outside the brain volume
Solution Approach 1:
The global sphere acts as an intermediary between the brain surface geometry and the local sensor spheres. It mediates the relationship by providing a reference framework that constrains local sphere placement. The global sphere's surface serves as a boundary that local spheres must respect, ensuring they remain within valid brain volume while still enabling dipole localization at each sensor position.
3Productivity
If ghost spheres are present in the model, then the multi-sphere model can be completed for all sensors, but the physical reality of brain volume is not represented
Solution Approach 1:
The patent extracts and removes the problematic degree of freedom that allows ghost spheres to form. By taking out the unconstrained radius and center positioning for local spheres, and replacing it with constraints tied to the global sphere, the method eliminates the possibility of ghost spheres while maintaining model completeness for all sensors.
Data Source
AI summary
In one aspect, a computer-implemented method corrects a multi-sphere head model used in dipole localization for a set of magnetic field sensors (MEG sensors) by replacing ghost spheres with replacement spheres that are not ghost spheres. One type of ghost sphere completely encloses the brain volume but is so large that a center of the sphere is outside the brain volume. Another type of ghost sphere lies entirely outside the brain volume. Various approaches for correcting ghost spheres are disclosed.