R help - Sep 2011 - How to find regions enclosed by edges within a TIF image

I am processing grayscale TIF images that represent X-rays of a sand- and
gravel-filled tube.  Each image represents a plane through the porous medium,
and I need to classify each pixel in the image as void or solid.  For the most
part, voids are dark and solids are bright, but there are many gray regions.  In
particular, there are obvious pebbles (larger sized solids) that have darker
spots within them, which get erroneous classification as voids.  Can anyone
suggest packages/functions that I could use to identify pebbles by their edges
(roundish areas outlined by voids), so I could then classify those regions as
solid?  So far I am using just the adimpro package, to read the TIF files.

Thanks,
Scott Waichler
Pacific Northwest National Laboratory
scott.waichler at pnnl.gov