Multi-sphere head model for dipole localization

Abstract

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.

What is claimed is: 
     
         1 . A method implemented on a computer system, the computer system executing instructions to execute a method for correcting a multi-ellipsoid head model used in dipole localization for a set of magnetic field sensors (MEG sensors) that senses magnetic fields produced by a brain, the method comprising:
 accessing the multi-ellipsoid head model, the multi-ellipsoid head model comprising an ellipsoid corresponding to each MEG sensor of the set of MEG sensors;   automatically identifying an individual ellipsoid of the ellipsoids as a ghost ellipsoid, if (a) the individual ellipsoid has a center outside the brain's volume and encloses the brain's volume, (b) the individual ellipsoid has a volume that is at least fifty percent non-overlapping with the brain's volume, or (c) the individual ellipsoid lies entirely outside the brain's volume; and   replacing the ghost ellipsoid with a replacement ellipsoid that are not ghost ellipsoids.   
     
     
         2 . The computer-implemented method of  claim 1  wherein the individual ellipsoid is identified as the ghost ellipsoid, because the individual ellipsoid has a center outside the brain's volume and completely encloses the brain's volume. 
     
     
         3 . The computer-implemented method of  claim 2  wherein the individual ellipsoid is identified as the ghost ellipsoid, because the individual ellipsoid has a volume that is at least fifty percent non-overlapping with the brain's volume. 
     
     
         4 . The computer-implemented method of  claim 1  wherein the individual ellipsoid is identified as the ghost ellipsoid, because the individual ellipsoid lies entirely outside the brain's volume. 
     
     
         5 . The computer-implemented method of  claim 1  wherein the replacement ellipsoid is a global sphere that is fit to an entire surface of the brain. 
     
     
         6 . The computer-implemented method of  claim 5  wherein the global sphere is determined using location of only the e MEG sensor corresponding to the ghost ellipsoid. 
     
     
         7 . The computer-implemented method of  claim 1  wherein, for the replacement ellipsoid:
 a center of the replacement ellipsoid lies on a line through (a) the location of the corresponding MEG sensor, and (b) a point on the brain's surface closest to the corresponding MEG sensor; 
 the replacement ellipsoid includes the point on the brain's surface closest to the corresponding MEG sensor; and 
 a major axis of the replacement ellipsoid is not larger than a diameter of a smallest sphere that completely encloses the brain volume. 
 
     
     
         8 . The computer-implemented method of  claim 1  wherein, for the replacement ellipsoid:
 a center of the replacement ellipsoid lies on a line through (a) the location of the corresponding MEG sensor, and (b) a center of a global sphere that is fit to an entire surface of the brain; 
 the replacement ellipsoid includes a point on the brain's surface closest to the corresponding MEG sensor; and 
 a major semi-axis of the replacement ellipsoid is not larger than a distance between said point and the center of the global sphere. 
 
     
     
         9 . A method implemented on a computer system, the computer system executing instructions to execute a method for correcting a multi-ellipsoid head model used in dipole localization for a set of magnetic field sensors (MEG sensors) that senses magnetic fields produced by a brain, the method comprising:
 accessing the multi-ellipsoid head model, the multi-ellipsoid head model comprising an ellipsoid corresponding to each MEG sensor of the set of MEG sensors;   identifying ellipsoids that are ghost ellipsoids, wherein the ghost ellipsoids are not suitable for modeling dipole localization in the brain's volume; and   replacing the ghost ellipsoids with replacement ellipsoids that are not ghost ellipsoids;
 wherein at least one replacement ellipsoid is selected from a family of candidate replacement ellipsoids. 
   
     
     
         10 . The computer-implemented method of  claim 9  wherein the family of candidate replacement ellipsoids all have centers that lie along a line. 
     
     
         11 . The computer-implemented method of  claim 10  wherein the line traverses through the location of the corresponding MEG sensor. 
     
     
         12 . The computer-implemented method of  claim 10  wherein the line traverses through the region of the brain's surface closest to the corresponding MEG sensor. 
     
     
         13 . The computer-implemented method of  claim 10  wherein the line is normal to the brain's surface. 
     
     
         14 . The computer-implemented method of  claim 1  wherein the replacement ellipsoid is selected from a family of candidate replacement ellipsoids, wherein the family of candidate replacement ellipsoids all have centers that lie along a line that traverses through at least two of the following three regions: (i) a location of the MEG sensor corresponding to the ghost ellipsoid, (ii) a region of the brain's surface closest to the MEG sensor corresponding to the ghost ellipsoid, and (iii) a location of the brain's volume. 
     
     
         15 . The computer-implemented method of  claim 14  wherein the family of candidate replacement ellipsoids all have centers that lie within an area or volume defined by at least two of the following three regions: (i) the location of the corresponding MEG sensor, (ii) the region of the brain's surface closest to the corresponding MEG sensor, and (iii) the location of the brain's volume. 
     
     
         16 . The computer-implemented method of  claim 14  wherein the family of candidate replacement ellipsoids all have major axes that do not exceed a maximum that is one of: (i) a diameter of a largest sphere in the family that is enclosed by the brain's volume, (ii) a diameter of a smallest sphere in the family that completely encloses the brain's volume, and (iii) a diameter of a smallest sphere in the family that has center inside the brain's volume. 
     
     
         17 . The computer-implemented method of  claim 14  wherein the family of candidate replacement ellipsoids all include a closest point on the brain surface to the corresponding MEG sensor. 
     
     
         18 . A method implemented on a computer system, the computer system executing instructions to execute a method for correcting a multi-ellipsoid head model used in dipole localization for a set of magnetic field sensors (MEG sensors) that senses magnetic fields produced by a brain, the method comprising:
 accessing the multi-ellipsoid head model, the multi-ellipsoid head model comprising an ellipsoid corresponding to each MEG sensor of the set of MEG sensors;   identifying ellipsoids that are ghost ellipsoids, wherein the ghost ellipsoids are not suitable for modeling dipole localization in the brain's volume; and   replacing the ghost ellipsoids with replacement ellipsoids that are not ghost ellipsoids;
 wherein at least one replacement ellipsoid is a global sphere that is fit to an entire surface of the brain. 
   
     
     
         19 . The computer-implemented method of  claim 18  wherein at least one ghost ellipsoid has a center outside the brain's volume and completely encloses the brain's volume. 
     
     
         20 . The computer-implemented method of  claim 18  wherein the ghost ellipsoid has a volume that is at least fifty percent non-overlapping with the brain's volume.