Manual segmentation results

I was trying to manually segment a left atrium, using the segmentation tool I have developed (C++/VTK/FLTK/ITK). The results are rather crude, and the segmentation is not that great. Here is a segmentation I obtained by segmenting parts of the atrium separately and stichting them together (performing a union of segmentations):

Each of the pulmonary trunks (4 in total for this atrium) have been segmented separately. Some of the trunks are over-segmented. It is very hard to tell sometimes where the trunks finish, whereby drawing out a bounding-box (for region-grow) which is bigger than it's necessary.

Here's a glimpse of a part of the tool I use:

The segmented result is super-imposed on the 3D volumetric view of the MRI scan which uses movable orthogonal slices. This is useful in locating deficiencies in the segmentation.