Segmenting outer heart walls + septum

Hi all!

I am searching for a method to segment the outer heart walls + septum from the CTACardio data without heaving to segment all seperate heart chambers and heart walls. I was thinking to first segment the solid body of the heart. Then to extract the boundaries of this segmented solid region such that this results in 3mm thickness outer heart walls. And finally to segment the septum, such that I obtain a hollow heart with anatomically correct outer walls of 3mm thickness in which the left and right heart chambers are seperated by the septum. However, I cannot find a way to extract the surface boundaries from a solid segmented region and set these to a certain thickness. Does anyone has experience with this? I am very gratefull for every step forward! Thank you in advance!

Kind regards

Research student, KU Leuven, Belgium