The Mimics Innovation Suite by Materialise is a nice solution for this purpose. It is a dedicated medical image post-processing tool for 3D printing. It imports and processes traditional 3D imaging (CT/MRI) in addition to several 3D US formats. Exports variety of 3D formats including STL. It is not meant to be licensed to individuals as there is significant cost associated with the license. Intended for commercial, clinical and academic research use.