FFTN - why do we not specify the variable we wish to carry out the fft wrt?

Hi there,

I am attempting to write a code to for DSI. I want to carry out the Fast Fourier transform on my q space signals stored in a matrix. My information is along the x, y and z directions and so I have chosen to use the FFTN function, ie fftn(MAT) (MAT = my matrix of numbers) however, I am a little confused. Obviously I want to carry out the fast fourier transform on my signals with respect to the variable q but I see that we only put in the matrix and do not specify the variable wrt when using fftn. Why is this?

Any help you can offer would be greatly appreciated Many thanks Bran