Method and system for displaying the electric field generated on the brain by transcranial magnetic stimulation

Abstract

A visualization surface representative of a portion of the brain at a depth below the head surface of the subject is generated by combining an actual representation of the head surface with an idealized representation of the head surface. The combining is a function of the depth and is performed to minimize in the visualization surface any irregularities existing in the actual head surface of the subject. A display shows the visualization surface overlaid on a volumetric image of the brain, the electric field induced on a region of the visualization surface by a transcranial magnetic stimulation (“TMS”) induction coil device positioned above the head surface and the TMS coil device. By viewing the display, a user of the TMS coil device can interactively position the TMS coil device in relation to the head surface and, for a target site on the brain at a selected depth, determine the position at which the TMS coil device induces a maximum E-field on a visualization surface corresponding to the selected depth.

1 . A method for generating a visualization surface of a portion of a brain for use in transcranial magnetic stimulation (“TMS”) comprising:
 generating an actual image of a head surface of a head; 
 generating an model of the head surface; 
 forming a visualization surface, which is a combined configuration of the actual image and model, wherein the visualization surface is representative of a portion of the brain within the head at a depth beneath the head surface; and 
 displaying the visualization surface. 
 
     
     
         2 . The method of  claim 1  further comprising:
 computing, based on the visualization surface, an electric field (“E-field”) induced by a TMS induction coil device on a portion of the brain at the depth; and 
 displaying the induced E-field on the visualization surface. 
 
     
     
         3 . The method of  claim 2  further comprising:
 displaying the TMS coil device positioned in relation to the visualization surface, a configuration of the head surface corresponding to the actual image of the head surface and a volumetric configuration of the head. 
 
     
     
         4 . The method of  claim 1 , wherein the combining further comprises:
 weighting the actual image and model with first and second weightings, respectively, wherein the first and second weightings are a function of the depth;   combining the actual image and model weighted by the first and second weightings, respectively, to generate an unsealed visualization surface; and   scaling the unsealed visualization surface based on the depth to generate the visualization surface.   
     
     
         5 . The method of  claim 4 , wherein the first and second weightings are determined by a linear function of the depth, wherein the sum of the first and second weightings equals one and each of the first and second weightings is not less than zero or more than one. 
     
     
         6 . The method of  claim 5 , wherein the first weighting decreases as the depth increases and the second weighting decreases as the depth decreases. 
     
     
         7 . The method of  claim 1 , wherein the actual image is a polyhedral surface mesh defined by a plurality of polygons and generated from a volumetric image of the head, the method further comprising:
 generating a smallest convex surface mesh containing the surface mesh;   eliminating polygons, from the convex surface mesh, corresponding to a lower portion of the head, said polygons having an area exceeding the area of a predetermined large polygons and said polygons sharing vertices with the large polygons, to generate a scalp mesh; and   generating the model based on the scalp mesh.   
     
     
         8 . The method of  claim 1  further comprising:
 comparing the visualization surface with the actual image of the head surface; 
 computing a differential between the visualization surface and the actual image based on the comparing; and 
 generating a new visualization surface by combining the actual image and model with an adjustment, if the differential exceeds a predetermined threshold. 
 
     
     
         9 . The method of  claim 8 , wherein the combining for generating the new visualization surface further comprises:
 weighting the actual image and model with first and second weightings, respectively, wherein the first and second weightings are a function of the depth; and   wherein the combining for generating the new visualization surface further comprises adjusting the weightings of the first and second weightings as a function of the depth.   
     
     
         10 . The method of  claim 1  further comprising:
 generating control information for controlling position and orientation of a TMS coil device in relation to the head, based on an actual position of the TMS coil device in relation to the head surface and a computation of an electric field induced on the visualization surface, to maintain a constant amplitude electric field induced on the visualization surface. 
 
     
     
         11 . An apparatus for generating a visualization surface of a portion of a brain for use in transcranial magnetic stimulation (“TMS”) comprising:
 a processor for performing a process encoded on a computer readable medium, wherein the process includes the steps of: 
 generating an actual image of a head surface of a head; 
 generating an model of the head surface; 
 forming a visualization surface which is a combined configuration of the actual image and model, wherein the visualization surface is representative of a portion of the brain within the head at a depth beneath the head surface; and 
 providing for display of the visualization surface. 
 
     
     
         12 . The apparatus of  claim 11 , wherein the process further comprises:
 computing, based on the visualization surface, an electric field (“E-field”) induced by a TMS induction coil device on a portion of the brain at the depth; and   providing for display of the induced E-field on the visualization surface.   
     
     
         13 . The apparatus of  claim 12 , wherein the process further comprises: providing for display of the TMS coil device positioned in relation to the visualization surface, a configuration of the head surface corresponding to the actual image of the head surface and a volumetric configuration of the head. 
     
     
         14 . The apparatus of  claim 13 , wherein the process further comprises:
 generating information representative of the position of the TMS coil device in relation to the head surface for a target site on the visualization surface stimulated by the TMS coil device.   
     
     
         15 . The apparatus of  claim 12 , wherein the process further comprises: providing for display of a maximum and a minimum of the induced E-field on the visualization surface. 
     
     
         16 . The apparatus of  claim 11 , wherein the combining further comprises:
 weighting the actual image and model with first and second weightings, respectively, wherein the first and second weightings are a function of the depth;   combining the actual image and model weighted by the first and second weightings, respectively, to generate an unsealed visualization surface; and   scaling the unsealed visualization surface based on the depth to generate the visualization surface.   
     
     
         17 . The apparatus of  claim 16 , wherein the generating of the model based on the scalp mesh further comprises:
 computing a centroid of the brain based on vertices of all polygons in the scalp mesh, wherein the model is formed around the centroid and fitted to the vertices,   
     
     
         18 . The apparatus of  claim 16 , wherein the model is formed from a plurality of n model idealizedn, wherein each of the idealizedn models has a second weighting wideal-n and wherein the weightings wideal-n are a function of a position of a projection of a vector V from a point on the surface mesh onto the corresponding idealizedn representation, in relation to a predetermined portion of the brain. 
     
     
         19 . The apparatus of  claim 11 , wherein the process further comprises:
 comparing the visualization surface with the actual image of the head surface;   computing a differential between the visualization surface and the actual image based on the comparing; and   generating a new visualization surface by combining the actual image and model with an adjustment, if the differential exceeds a predetermined threshold.   
     
     
         20 . The apparatus of  claim 11 , wherein the processor generates control information for controlling position and orientation of a TMS coil device in relation to the head, based on an actual position of the TMS coil device in relation to the head surface and a computation of an electric field induced on the visualization surface, to maintain a constant amplitude electric field induced on the visualization surface.