I think there are a couple of problems here. Firstly, I think you need to change the way you're assigning into dcm_stack because as written, you are not satisfying the "sliced variable" requirement for a parfor loop output. I think you can fix this by storing an additional sliced output containing the the ct_location, I think you should apply the slope and intercept inside the parfor loop. Here's a sketch:
ct_list = dir(fullfile(path_, '*.dcm'));
num = length(ct_list);
temp_img_size = dicomread(fullfile(ct_list(1).folder, ct_list(1).name));
dcm_stack = zeros(temp_img_size(1), temp_img_size(2), num);
% Additional output for the parfor loop:
ct_locations = zeros(1, num);
parfor iter2 = 1: num
temp_img = dicomread(fullfile(ct_list(iter2).folder, ct_list(iter2).name));
temp_info = dicominfo(fullfile(ct_list(iter2).folder, ct_list(iter2).name));
% Store information from temp_info
ct_locations(iter2) = temp_info.InstanceNumber;
% Note we must store in stack indexed using "iter2":
dcm_stack(:,:,iter2) = (temp_img .* temp_info.RescaleSlope) + temp_info.RescaleIntercept;
end
% You can now re-order the "pages" in dcm_stack like this:
dcm_stack = dcm_stack(:,:,ct_locations);
