Volume rendering without clothes obstruction

The image is shown in axial view.
I am a beginner in VTK. I have achieved to visulize the airway and lung with VTK by using volume rendering. I set some brightness indexes to change opacity and gradientopacity. The surfaces of airway and lung can be observed. But the outer clother and the bed at the back also can be seen. These will reduce visualization effects. How can I make the clothes and bed invisible? Thank you!

The bed is always separated from the patient by a sufficiently radiotranslucent layer, so removing (masking out) the table is an almost trivial ooeothat can be performed fully automatically using thresholding and connected component filtering. (see for example the Remove Patient Table module in Sandbox extension of 3D Slicer).

Patients wear very light clothing, usually gown provided by the hospital that does not interfere with imaging. Buttons sometimes can be somewhat dense, but they are not connected to the body, so they can be removed the same way as the table.