build nii file from .mat file

I use class dicom23d to build a .mat file, and now I have one struct with these fields: volume_image, slice_data, image_meta_data, and the type of the volume_image is double. How could I build nii file from it?

a=load('123de.mat'); z=a.volume_image; n12=make_nii(z); save_nii(n12,'N_144_R2star.nii'); When I use n12=make_nii(z);, I have an error that I can't use this code for double. So I cast it to unit8 but still have similar error for unit8. What should I do?