A 3D model of a human heart I extracted from a CT Aortocardiogram using the free 3DSlicer software for windows. Cleaning the mesh up to get it 3D print ready was a little bit of a nightmare (I used thresholding to try and get a mesh of the heart, but this also included a lot of other bits of nearby tissue which were of a simmilar density).
Here's the 3D printed end product (would like to try printing it again but using an SLA printer instead) https://imgur.com/gallery/NzVyR
Neat Tutorial by Thomas Flynn on how to extract 3D models from CT scans using Osirix and 3DSlicer: https://www.youtube.com/watch?v=m7dtqj0KcI0&ab_channel=ThomasFlynn