save as nii file

Hi there! Here is the description o f the problem I am getting into:

I have a multidimentional matrix m1(288,288,80)and I also have a 2 collumn, 1266 rows matrix: m2 , smthing like this: cortex_reg1 1 cortex_reg2 2 cortex_reg3 3 cortex_reg4 4 (...) How do I save that m1 matrix as a nii file with cortex_regx name? Stm like cortexreg1.nii It is not mandatory to be reg1, as I have other functions running that use different (288*288*80) matrixes and save them as nii files . Regards