free autoCAD software compatible with Abaqus ?

Dear all,

 I need a free autoCAD software to model the complex 3D geometry of human brain.  It is desirable that this software should generate output files readable to Abaqus/CAE for mesh generation.  I wonder whether these is such a software available.  An alternative way is to handle the geometrical modeling myself but I need to know the details of the file format used by Abaqus/CAE.  Does anyone have a clue?

Thank you very much!

Yasheng