Hi Ji,

The '-Fourier' option is the default in 3dvolreg, so using it
should have no effect.

So heptic produced larger movement values than Fourier, is
that what you are saying? What sort of value changes are you
seeing?

Note that it hard to say what is better or worse. Though it
is easy to say that better means getting closer to reality,
the trouble is we can only estimate what we think reality is.
A larger motion value is not necessarily worse, it might even
be more accurate.

Anyway, there is no significant recommendation between the
choices, as far as I know. Each interpolation function has
pros and cons (of course). Fourier is generally considered
the most accurate, but can have a ringing effect. High order
polynomials can fluctuate a bit quickly, but low order
polynomials can truncate local peaks. On the other hand, all
of the methods produce reasonable results.

The current processing stream that we teach from uses Fourier
interpolation. But any of cubic, quintic or heptic would seem
fine, too.

I'm glad to be of service in not providing any concrete response
whatsoever. :)

- rick