emse_mri2elec

 EMSE_MRI2ELEC - Convert mri coordinates to points in head frame
 
 [Velec] = emse_mri2elec(vert,reg)
 
 vert   - the Nx3 (X,Y,Z) MRI coordinates to be converted
 reg    - a structure containing coordinate transform matrices,
          which is read using emse_open_reg
 
 Given a point P(x,y,z) in MRI frame (eg, an fMRI activation 
 overlayed onto a high res T1 volume) this function will find 
 the corresponding location in the head space of the electrodes.
 Symbolically we have P(voxel) and want to find P(head).
 
 1.  Use the offset between the MRI coordinate frame and 
     the MRI volume coordinate frame to find P(MRI-voxel).
 2.  Given P(MRI-voxel) and the voxel size, we can find 
     P(MRI-mm), which is the MRI coordinates expressed in mm
 3.  The registration file contains the matrix ImageToHeadMatrix,
     so P(head) = P(MRI-mm)*reg.mri2elec, where P(MRI-mm) is the 
     point in MRI coordinates.
 
 This function performs the last calculation.
 
 See also: EMSE_OPEN_REG, EMSE_ELEC2MRI

0001 function [V] = emse_mri2elec(vert,reg)
0002 
0003 % EMSE_MRI2ELEC - Convert mri coordinates to points in head frame
0004 %
0005 % [Velec] = emse_mri2elec(vert,reg)
0006 %
0007 % vert   - the Nx3 (X,Y,Z) MRI coordinates to be converted
0008 % reg    - a structure containing coordinate transform matrices,
0009 %          which is read using emse_open_reg
0010 %
0011 % Given a point P(x,y,z) in MRI frame (eg, an fMRI activation
0012 % overlayed onto a high res T1 volume) this function will find
0013 % the corresponding location in the head space of the electrodes.
0014 % Symbolically we have P(voxel) and want to find P(head).
0015 %
0016 % 1.  Use the offset between the MRI coordinate frame and
0017 %     the MRI volume coordinate frame to find P(MRI-voxel).
0018 % 2.  Given P(MRI-voxel) and the voxel size, we can find
0019 %     P(MRI-mm), which is the MRI coordinates expressed in mm
0020 % 3.  The registration file contains the matrix ImageToHeadMatrix,
0021 %     so P(head) = P(MRI-mm)*reg.mri2elec, where P(MRI-mm) is the
0022 %     point in MRI coordinates.
0023 %
0024 % This function performs the last calculation.
0025 %
0026 % See also: EMSE_OPEN_REG, EMSE_ELEC2MRI
0027 %
0028 
0029 % $Revision: 1.1 $ $Date: 2004/11/12 01:32:34 $
0030 
0031 % Licence:  GNU GPL, no express or implied warranties
0032 % History:  06/2002, Darren.Weber@flinders.edu.au
0033 %                    EMSE details thanks to:
0034 %                    Demetrios Voreades, Ph.D.
0035 %                    Applications Engineer, Source Signal Imaging
0036 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
0037 
0038 fprintf('EMSE_MRI2ELEC: In development. Be careful!\n');
0039 
0040 % elec input is Nx3 matrix that should be represented
0041 % in homogenous coordinates:
0042 vert = [ vert ones(size(vert,1),1) ];
0043 
0044 Velec = vert * reg.mri2elec;
0045 
0046 % Note reg.Melec ~= Velec(:,1:3) due to floating point rounding only.
0047 
0048 % reg.mri2elec is a 4x4 matrix, eg:
0049 %
0050 %  -0.9525    0.0452    0.3012         0
0051 %  -0.0522   -0.9985   -0.0154         0
0052 %   0.3000   -0.0304    0.9534         0
0053 %  -0.1295    0.1299    0.0756    1.0000
0054 %
0055 % The first 3x3 cells are the rotations,
0056 % the last row is the translations,
0057 % the last column is projections (usually 0),
0058 % and the value at 4,4 is the homogenous
0059 % coordinate scale unit, usually 1.
0060 
0061 % In homogeneous coordinates, the last column
0062 % is the scale factor, usually 1, but in case
0063 % it is ~= 1
0064 V(:,1) = Velec(:,1) ./ Velec(:,4);
0065 V(:,2) = Velec(:,2) ./ Velec(:,4);
0066 V(:,3) = Velec(:,3) ./ Velec(:,4);
0067 
0068 return