Interpolation and extrapolation of MRI images

Hi,
I am trying to reconstruct a heart model from MRI scans.
I used the Segmentation Editor to create a 3d model and managed to get the following result:

image

Which feature can I use to fill the holes of the outer layer? interpolation is not ideal due to the large distance between the contours.
Furthermore, I am missing the slice of the tip of the apex. Is it suitable to make an extrapulation? if so how?

image

thank you.