MATLAB: Dicomwrite function help needed

dicomdicomwriteImage Processing Toolbox

I have over 100 ct images. With your help I was able to make a 3d box variable. In my work space it shows a variable imstack of 512*512*143.
m=143;
imstack=zeros(512,512,m);
for z=1:m;
n=dicomread([num2str(z),'.ima']);
imstack(:,:,z)=n;
end
dicomwrite(imstack,'X.ima');
How do I write that imstack into a file, say some 'X.ima'? I am having problem with the dicomwrite function.
??? Error using ==> dicom_prep_ImagePixel>getPhotometricInterp at 98
Cannot determine photometric interpretation.
This error pops up when I try to execute the code.

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