How to align PD weighted images to EPI

Dear AFNI experts,
I want to align a PD-weighted image to the MidWarped_Forward EPI generated by unWarpEPI.py. When I use align_epi_anat.py, there must be something wrong with the cost functions which are designed for T1 images. So, what programs should I choose to align PD images to warped EPI, or I just need to change the cost function?

Many thanks for your helps.
Yazhu