Bounding boxes in manual segmentation

From my experience with manual segmentation, for a left atrium which has 4 PV branches, we need to define a minimum of 4 bounding boxes, with a seed-point in each bounding box. Region-growing segmentation at each of these bounding boxes are then merged together to give the final segmentation result.

Above is a dirty hand-drawn illustration of the bounding boxes together with the region they segment.