No difference when generating image with show_slices for 3d using imdl.meas_icov = meas_icov_rm_elecs( imdl, 1);

At the moment i am able to generate different images when using 2d by changing the value from 1 to other numbers (1-16 electrodes) imdl.meas_icov = meas_icov_rm_elecs( imdl, 1) as the code shown below

% Forward solvers $Id: forward_solvers01.m 3790 2013-04-04 15:41:27Z aadler $
% 2D Model
imdl= mk_common_model('d2d1c',19);

% Create an homogeneous image
img_1 = mk_image(imdl);
h1= subplot(221);
show_fem(img_1);

% Add a circular object at 0.2, 0.5
% Calculate element membership in object
img_2 = img_1;
select_fcn = inline('(x-0.5).^2+(y-0.4).^2<0.1^2','x','y','z');
img_2.elem_data = 1 + elem_select(img_2.fwd_model, select_fcn);

h2= subplot(222);
show_fem(img_2);

img_2.calc_colours.cb_shrink_move = [.3,.8,-0.02];
common_colourbar([h1,h2],img_2);

imdl.meas_icov = meas_icov_rm_elecs( imdl, 1);

img = mk_image(img_1,1);
vh = fwd_solve(img);
vi = fwd_solve(img_2);
ring = inv_solve(imdl, vh, vi);
show_fem(ring)

But when using 3d GREIT, all the images are the same. I know there might be a different when using 2d and 3d. At the moment i am still unsure.

% $Id: mk_GREIT_matrix01.m 3354 2012-07-01 21:42:05Z bgrychtol $
n_elecs = 16;
fmdl = ng_mk_cyl_models([2 2 0.2] ,[n_elecs,1],[0.1]);
fmdl.stimulation =  mk_stim_patterns(n_elecs,1,[0,1],[0,1],{'no_meas_current'}, 1);
fmdl = mdl_normalize(fmdl, 0);
img = mk_image(fmdl,1); % Homogeneous background

show_fem(fmdl); view(0,70);
print_convert mk_GREIT_matrix01a.png '-density 60'

% $Id: mk_GREIT_matrix02.m 2473 2011-03-02 20:32:23Z aadler $
opt.imgsz = [32 32];
opt.distr = 3; % non-random, uniform
opt.Nsim = 1000;
opt.target_size = 0.05; % Target size (frac of medium)
opt.noise_figure = 0.5; % Recommended NF=0.5;
imdl = mk_GREIT_model(img, 0.25, [], opt);
imdl.meas_icov = meas_icov_rm_elecs( imdl, [1,2,3]);

% $Id: mk_GREIT_matrix03.m 2474 2011-03-02 20:38:51Z aadler $

img = mk_image(fmdl,1);
vh = fwd_solve(img);

select_fcn = inline('(x-1).^2+(y-1).^2+(z-1).^2<.03','x','y','z');
img.elem_data = 1 + 0.1*elem_select(img.fwd_model, select_fcn);
vi = fwd_solve(img);

show_fem(img); view(0,70);
print_convert mk_GREIT_matrix03a.png '-density 60'


ring = inv_solve(imdl, vh, vi);
show_fem(ring)