3d volume analysis of pores

[mariella.dentini]

Hi everyone!
I'm trying to analyze some samples of polymeric porous materials with ImageJ...I'm new in this, so please forgive me if my questions may seem stupid to you...

i've got a 200 images (more or less) stack collected with microtomography. They are slices of the material in the vertical direction. I'm posting an example of one of this images.

The parameters i need to calculate are the mean size and distribution of pores; when i try to use the particle analyzer i can measure the characteristic of the pores in each slide, but i don't know how to measure the diameter of the pore, since i measure its "local" diameter in each slide...i also tryed to invert the image, and to use not the porous material, but just the geometry of the pores to make my calcultions but still i can't figure out how to proceede....
can someone please help me??
thanks, mariella.