What is the z axis in 3D images (MRI) in dicom files ?

Hi all,

I have a question, I have sequences of Dicom images What is the z-axis in each slice of Dicom images? I really do not understand when I want to calculate derivatives for z-axis what is the meaning of that? How can we consider z coordinate for each voxel?

For example in slice 1 , a point in 263 and 63: Is [x,y,z] = [263 63 1]? and the next one in the same row is [263 64 1]?

what is the same pixel position in the next slice? is it [263 63 2]?

Could you please help me and let me know how you have considered the z axis?

I will be thankful if you guide me

Kind regards