Start Ansys SpaceClaim then File, Open and set the file type to STL. Then you will have access to the faceted body. SpaceClaim has a lot of capabilitiy to edit STL data and convert it into geometry that can be meshed in Ansys Mechanical.
I have made several videos of the process of converting STL data into geometry for meshing. See the STL File Editing playlist on my YouTube channel.
No, Ansys cannot segement MRI image data, other software is used for that job. A free software package that does segment MRI image data is called Segment.
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