You haven't really described your planned workflow for the CFD, but I would assume you want a DICOM to 3D format (like .stl). There are many many tutorials in this area. From there, it depends on the software you are using for flow analysis.
Yueh Z Lee ! Thanks a lot for your response. Actually, I want to analyze the flow of particles mixed air. and I want to use Ansys (Fluent) for this purpose.