TTField treatment with optimization of electrode positions on the head based on MRI-based conductivity measurements
When electrodes are used to impose an electric field in target tissue within an anatomic volume (e.g., to apply TTFields to treat a tumor), the position of the electrodes can be optimized by obtaining electrical conductivity measurements in an anatomic volume and generating a 3D map of the conductivity directly from the obtained electrical conductivity or resistivity measurements, without segmenting the anatomic volume into tissue types. A location of the target tissue is identified within the anatomic volume, and the positions for the electrodes are determined based on the 3D map of electrical conductivity and the position of the target tissue.
1. A method of optimizing positions of a plurality of electrodes placed on a subject's body, wherein the electrodes are used to impose an electric field in target tissue within an anatomic volume, the method comprising the steps of:
obtaining electrical conductivity or resistivity measurements in the anatomic volume;
generating a 3D map of electrical conductivity or resistivity of the anatomic volume directly from the obtained electrical conductivity or resistivity measurements, without segmenting the anatomic volume into tissue types;
identifying a location of the target tissue within the anatomic volume; and
determining positions for the electrodes based on the 3D map of electrical conductivity or resistivity generated in the generating step and the location of the target tissue identified in the identifying step.
2. The method of claim 1 , further comprising the steps of:
affixing the electrodes to the subject's body at the positions determined in the determining step; and
applying electrical signals between the electrodes subsequent to the affixing step, so as to impose the electric field in the target tissue.
3. The method of claim 1 , wherein the measurements obtained in the obtaining step represent the diffusion of molecules.
4. The method of claim 1 , wherein the obtaining step comprises acquiring MRI data using diffusion weighted imaging.
5. The method of claim 1 , wherein the obtaining step comprises acquiring MRI data using customized multi echo gradient sequences.
6. The method of claim 1 , wherein the obtaining step comprises acquiring MRI data using diffusion tensor imaging.
7. The method of claim 6 , wherein the step of acquiring MRI data using diffusion tensor imaging comprises a direct mapping method that assumes a linear relationship between eigenvalues of diffusion and conductivity tensors, σ v =s·d v , where σ v and d v are the v th eigenvalues of the conductivity and the diffusion respectively.
8. The method of claim 6 , wherein the step of acquiring MRI data using diffusion tensor imaging comprises a volume normalized method in which a geometric mean of conductivity tensors eigenvalues in each volume element in the anatomic volume are matched locally to specific isotropic conductivity values of a tissue type to which the volume element belongs.
9. The method of claim 1 , wherein the anatomic volume comprises white matter and grey matter of a brain.
10. The method of claim 1 , wherein the anatomic volume is a brain, and
wherein the determination of positions for the electrodes is based on a composite model in which the 3D map of electrical conductivity or resistivity of the brain is surrounded by a model of a first shell having a first constant conductivity.
11. The method of claim 10 , wherein the model of the first shell represents a scalp, a skull, and CSF, taken together.
12. The method of claim 10 , wherein the model of the first shell represents CSF,
wherein the composite model further includes a second shell that represents a skull, the second shell having a second constant conductivity, and
wherein the composite model further includes a third shell that represents a scalp, the third shell having a third constant conductivity.
13. The method of claim 10 , wherein the step of determining positions for the electrodes comprises adding a dipole to the composite model at a location that corresponds to the target tissue and selecting external positions at which a potential attributable to the dipole is maximum.
14. The method of claim 1 , wherein the step of determining positions for the electrodes comprises calculating positions for the electrodes that will provide a maximum intensity of the electric field in the target tissue.
15. The method of claim 1 , wherein, in the generating step, the 3D map has a resolution that is higher than 1 mm×1 mm×1 mm.
16. The method of claim 1 , wherein the step of generating a 3D map comprises generating a simple geometric object representing the anatomic volume.
17. The method of claim 1 , wherein the step of generating a 3D map comprises classifying a tissue type for each volume element based on a fractional anisotropy.
18. The method of claim 1 , wherein the step of generating a 3D map comprises classifying a tissue type for each volume element based on a mean conductivity.
19. The method of claim 1 , wherein the step of generating a 3D map comprises matching geometric means of conductivity tensors' eigenvalues to specific isotropic reference values.