Problem with 3D printing myocardium only

I created an stl file for the heart myocardium only using the tutorial :

However when i try to 3D print it , it still comes up as a solid 3D model instead of a hollowed out heart. Could you kindly help? I am using the “Preform” software for the 3D printer.

You can use Hollow effect in Segment Editor module to create hollow segments.

Note that if you use a Formlabs printer then you need to add drain holes to allow excess resin to escape from cavities.